AGRICULTURALSOIL SCIENCE TECHNOLOGY
Soil Science and Management
Updated soil Science Notes
CHELAL ABRAHAM
REVISEDJUNE 2017
Course Objectives.
By the end of the course unit ,the students will be able to:-
i) Describe geology of rocks and minerals.
ii) Explain weathering and soil formation and their relationship to agriculture.
iii) Identify features of various types of soil.
iv) Identify various plant nutrients and their availability to plants.
v) Calculate percentage base saturations.
vi) Explain importance of good soil management in crop production.
Recommended Text books.
i) Edward J Plaster (2008); Soil Science & Management ,4/e; International Thomson
Computer Pres
ii) Brady, N.C (1999); The nature and properties of soils; Macmillan, New York.
iii) Hassett, J.J Wayne , L. banwart (1992); Soils and their environment, Prentice Hall, inc . Eaglewood Cliffs, New Jersey.
Reference books for soil science
a) Donahue Roy L
Raymond W. Miller (1983) Soil .An introduction to soils and plant growth. New jersey Englewood cliffs.
John C. Shicluna
b) Brady Nyle C. (1984 or 1978) The nature and propertied of soils New York
Macmillan publishing.
c) Foth Henry D. (1978) Fundamentals of soil science
U.S.A John Wiley & Sons,Inc
SOIL GENESIS AND CLASSIFICATION
Topic objectives.
i) Define soil
ii) Explain soil geology.
iii) Outline various phases of soil.
iv) Describe various types of rocks.
Definition; Soil is a dynamic natural body composed of mineral and organic materials, living forms as well as air in which plants grow.
ELEMENTARY GEOLOGY
Geology is the scientific study of the earth’s crust and formation of rocks and minerals in relation to the sphere factors e.g. climatic factors and volcanic eruptions.
The sphere is divided into four regions;
-Atmosphere (gases)A Atmoshere
Hydrosphere(surface and underground water) Hydroshere
-Lithosphere (rocks and minerals)Lithospere Lithospere
Lithosphere (rocks and minerals)
Barispere
Barisphere (Fluid magma)B
-Lithosphere (rocks and minerals)
-Hydrosphere (surface and underground water)
-Atmosphere (gases)
-Barisphere is the source of fluid magma which is forced into the lithosphere through volcanic force. Cooling of the larva occurs at lithosphere or at hydrosphere when volcanic force is insufficient to pu8sh the magma to the earth’s surface. Cooled magma forms the rocks (igneous rocks).
SOIL PHASES
Soil is made up of three phases;
- Solid phase
- Liquid phase
- Gaseous phase
CLASSIFICATION AND PROPERTIES OF THE ROCKS
-Rocks are classified by three major divisions;
- Igneous rocks
- Sedimentary
- Metamorphic
1. Igneous rocks
-When molten magma from under the earth’s crust is exposed on the surface or at different depths in the earth, igneous rocks are formed from it as it cools. The fastest cooled expelled igneous rocks have a glossy texture while those slowly cooled have small crystals in the rock mass.
-Extrusive igneous rocks form above the earth’s surface while intrusive or plutonic igneous rocks form below the earth’s surface and have large crystals.
-Porphyries are igneous rocks having mixed large and small crystal particles due to changes in cooling rate.
-Examples of igneous rocks include granite, diorite and biotite, hornblende, augite, gabro and basalt. Gabro and basalt weather easily.
2. Sedimentary rocks
-They result from the deposition and cementation of weathering products of other rocks e.g. sandstone (formed from quartz) shale, (cemented clays), dolomite [CaMg (CO3)2], limestone which is composed mainly of calcite (CaCO3) mineral and the conglomerate.
-Resistance to weathering by sedimentary rock is determined by the dominant minerals and the cementing agent. The cementing material often makes part of the name for instance ferruginous for iron oxides, Siliceous for silica (SiO2) and Calcareous for carbonates or lime as in calcareous sandstone.
-Sedimentary conglomerates and breccias are made up of various sized fragments of rocks cemented together.
-Sandstones are formed mainly from quartz mineral, shades from consolidated clays and silts, limestone forms from calcium carbonate or mixtures of calcium and magnesium carbonate, clays, silts and sand.
-Dolomites are similar to limestone but have more magnesium carbonate. Quartzites are silica cemented sands.
3. Metamorphic rocks
-These are rocks formed by the metamorphism or change in or change in the form of other rocks. Igneous and sedimentary rocks are subjected to tremendous pressures and high temperatures form metamorphic rocks.
-Examples; Gneiss formed mainly from granites, rhyolites, aridesites among other minerals. Slate from shale or siltstone, schist from micas, quartzite, and marble from limestone or dolomite and are Primary minerals are mainly found in igneous and metamorphic easily decomposed.
SOIL GENESIS
-Soils are formed from hard (solid) rock masses, loose unconsolidated transported materials and organic residues. Organic soils develop mainly from plants that have fallen into stagnant water where decomposition is slow.
-Minerals are inorganic substances which are homogenous, have a definite composition and have characteristic physical properties such as shape, colour, melting temperature and hardiness. Minerals may be primary or secondary.
- rocks. They are formed from cooling the molten larva to rock.
a)Important primary minerals
-Quartz (SiO2), Feldspar; are hard major materials in sands. They weather to give clay and important minerals
-Micas (Muscovite and Biotite); Source of potassium and clay
-Augite and hornblende; they are good clay formers and weather moderately faster
-Apatite, albite, anorthite and olivine; they are common minerals supplying phosphorous
b)Secondary minerals
-Are mainly found in sedimentary rocks. These minerals are formed from precipitated soluble substances.
Examples;
-Calcite and dolomite; are slightly soluble minerals in limestone or dolomite rock common in arid soils. Are sources of calcium or magnesium.
-Gypsum; is soft, moderately soluble mineral found in arid-region soils
-Iron oxides; yellow to red coloured group of minerals with different amounts of water giving their soils yellow-red colours. Provides iron.
-Kaolinite, montmorillonite; are silicate clays
-Geothite; most resistant to weathering
-Gibbsite; resistant to weathering
- Hermatite
STAGES IN SOIL FORMATION
1 .Weathering
It is the process through which the parent materials are broken down by agents of weathering. It can be mechanical (Physical), Chemical and Biological weathering.
2 .Organic matter formation
The formation of a dark – colored surface horizon due to accumulating of organic matter of the second stage in soil formation.
Hemitation is the process by which organic matter is decomposed to form humus. This is a complex mixture of dark brown amorphones and colloidal substances. Both the accumulation of organic matter and the formation of humus are governed by climate.
3. Leaching
This is a process through which minerals are corroded down the soil profile and deposited in different layers down the soil horizons. This leads to the formation of layers of different colors called horizons.
FACTORS INFLUENCING SOIL FORMATION
- Parent material
-This could be consolidated or unconsolidated. It may also be organic or inorganic.
-Inorganic parent materials produce mineral soils. The organic matter is usually composed predominantly of unconsolidated material, dead decaying materials while the inorganic material can either be consolidated or unconsolidated.
-Depending on the materials in the parent material, the resulting soil may be acidic or alkaline. E.g. granite rocks may give reddish brown oxisols which are acidic also trachyte rocks.
-Parent material also contributes to soil colour, permeability, and specific surface area (texture).The chemical and mineralogical compositions of parent material often control the natural vegetation.
-Parent material determines the type of clay minerals in the soil profile e.g. volcanic tuff of Nandi hills produce acidic soils but produces black clays in the Nyando-Chemilil zone.
- Topography
-This refers to the outline of the earth’s surface and is synonymous with relief. It includes the dramatic mountain ranges and flat featureless plains. Topography forms dynamic system, the study which is known as geomorphology.
-Topography may hasten or delay the work of climatic forces in soil formation. Rolling topography encourages soil erosion which if extensive will reduce formation of a deep soil. Soils in in rolling places will thus differ from the ones in the plains (maturity).
-Medium to high altitudes is associated with low evaporation rate. The rate of decomposition of litter decreases with increase in altitude due to low temperature among other factors. This means soils with different profiles are found along the landscape and this association is known as catena.
- Living organisms
-Living organisms are responsible for breaking down the organic matter to produce proteins, celluloses, hemicelluloses, suberins, waxes, pectin, alcohols and lignin among higher plants. All these form humus which binds the soil particles together. The microbes also die to produce protein.
-Coniferous trees have lower metallic cations than deciduous when decomposed. The former encourages leaching of cations.
-Organic matter in grassland soils is higher than in forest soils especially in subsurface horizons. This gives the soil darker colour, higher moisture and cation holding capacity as compared to the forest soil.
-Human activities of deforestation, cultivation, fertilizer use have effect on soil formation while ploughing affects soil profile.
- Climate
-This includes the influence of temperature, humidity, evapotranspiration, rainfall and duration of sunshine. Climate is the principal factor governing the rate and type of soil formation as well as being the main factor determining vegetation distribution and the type of geomorphological processes; therefore it forms the basis of many classifications of natural phenomena including soils.
-Temperature and precipitation affect the rates of chemical and physical process responsible for profile development. For every 10˚C rise, the rate of chemical reaction doubles. This also affects microbial activities in the soil.
-The amount of sunshine is influenced by the cloud cover frequency so that most of the lowlands have greater amount of sunshine. The greater the sunshine, the more the evaporation rate and less moisture level in the soil. Also in high altitudes particularly in the sun facing slopes, soil formation is faster than slopes facing away from the sun.
-Wind influences the temperature and precipitation.
5. Time
-The length of time the material is subjected to soil forming processes and factors is crucial in soil formation e.g. glaciated regions have their soil profile developing quite slowly. Maturity of the soil can be determined by the quantity of the clay within the soil profile.
COMPONENTS OF THE SOIL
(SOIL CONSTITUENTS)
SOIL
Liquid Solids Gases
25% (50%) 25%
Organic components Inorganic components
Fine Fractions < 2mm in diameter Coarse materials
gravel & rocks >2mm in diameter
Sand Silt Clay
Primary Min Secondary Minerals
Volume of a silt loam surface soil in good condition for plant growth shows the following component quantities;
Soil air 20-30%
Soil minerals 45%
Organic matter 5%
Water 20-30%
-The term mineral in soil science can be taken to mean soils dominated by constituents and also to describe distinct minerals found in nature such as in nature such as quartz and feldspars.
a) Soil air
-Soil air is important in aerobic decomposition of organic matter by bacteria. Soil air has more moisture than that of atmospheric air. High water or moisture in the soil displaces soil air. Soil air has more carbon-dioxide content than that found in the atmosphere and less oxygen than that in the atmosphere.
-Larger pores are the first to loose water while small pores are the last. This is why clay soils with small air pores are poorly aerated.
b) Soil water
-Soil water is held within soil pores with varying degrees of pressure, thus we have;
- Hygroscopic water; it’s held strongly in the soil and is not available for plants
- Gravity held water ; it’s not available for plants
- Capillary water; it’s available for plants
-Water is the medium for nutrient absorption. Too much water in the soil for along period of time leads to anaerobic condition since air pores are all filled with water while microbes utilise the remaining oxygen.
c) Minerals (inorganic) constituents in soils
Regolith refers to the unconsolidated mantle of weathered rock and soil material on the earth’s surface i.e. loose earth materials above the solid rock. Primary minerals often resist weathering thus dominate the coarser fractions of the soil where as secondary minerals are prominent in the finer materials especially in clays.
-Inorganic component is the carrier of cations in the soil.
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Size fraction
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Common name
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1. very coarse- >200mm diameter
- 200-20mm
2. Coarse- 20-2mm
2-0.2mm
0.2-0.02mm
3. Fine- 0.02-0.002mm
4. Very fine- <0.002
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-stones
-gravel
-Fine gravel
-Coarse sand
-Fine sand
-Silt
-Clay
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d) Soil organic matter
This is comprised of partially decayed and animal residues. The soil micro-organisms are active in this medium. It is the main source of nitrogen and an important source of phosphorous and sulphur.
-Organic matter gives the soil dark colour hence enhances soil temperature and moisture retention.
-Organic matter holds soil together and also forms source of energy for soil organisms.
-Organic matter has more agricultural quantities than the inorganic (clay) components
WEATHERING OF ROCKS.
Weathering refers to the physical and chemical changes produced in rocks at or near the earth’s surface leading to disintegration and decomposition of the rock matter.
-Soil formation comprises of two different processes; first, the disintegration of solid rock and secondly, the changes occurring within the loose material as time passes. This later process is also called soil development but the two processes occur simultaneously.
-Soil formation is therefore used to mean production of unconsolidated material by weathering process and soil profile development which are the changes involved in development of horizons. Horizons tell much about the characteristics of a soil.
AGENTS OF WEATHERING
There are three main agents of weathering;
· Physical/mechanical
· Chemical
· Biological
The above agents exhibit themselves in various processes (forces) and therefore we the following processes under each;
1. PHYSICAL PROCESSES/AGENTS
These processes are designated as disintegration. Disintegration results in a decrease in size of rocks and minerals without appreciably affecting their composition.
a) Aggregation
This is a process whereby a number of particles are held or brought together to form units of varying but characteristic shapes (cementing together).The type and degree of aggregation is referred to as structure. Aggregation can be facilitated by humus, mesofauna e.g. earthworms, plant roots, exchangeable cations etc.
b) Translocation
This refers to the reorganization and redistribution of material mainly in the upper 2m or so of the earth’s surface. This can be achieved through suspension, solution, thawing among others.
c) Frost action (freezing and thawing)
In high altitudes, low temperatures lead to freezing of water found in rock cracks. At 4˚C, water attains maximum density of 1.0g cm-3, therefore it expands slightly until it reaches 0˚C and changes into ice where upon there is a 10% increase in volume corresponding to a density of 0.9g cm-3. With further reduction in temperature, ice behaves like any normal solid and decreases in specific volume but has a very high coefficient of linear expansion which is 51x 10-6K-1. Freezing can exert pressure of 146kg cm-2. Expansion of water in cracks will lead to slight crack enlargement. During high temperature periods, the frost will melt and run out as water, the process known as freezing thawing. Repeated freezing and thawing results into deep cracks which further weaken the rock structure
d) Solifluction
This is movement ‘en masse’ of the material down the slope forming stratified slope deposits that are known as solifluction deposits. This is due to repeated freezing and thawing on sloping land.
e) Expansion and contraction (wetting and drying)
These processes are common with high clay-containing soils like montmorillolite. Contraction on drying leads to large cracks and the top soil falls in the cracks and during rains there is expansion of these soils leading to bulging out, a phenomenon known as gilgai.
f) Exfoliation
Boulders or outcrops of rocks experience a wide temperatures fluctuation which leads to ‘scaling’ of rocks.
g) unloading
Removal of the weathered material from the rock surface giving it more surface area for the agents of weathering like temperature changes to manifest on the rocks.
h) Wind effect
Wind is a major transporting agent. It grinds together materials on transit via cyclonic actions. Fine soils often found in the desert called Loess are as a result of wind deposits and they are fertile soils.
i) Man activities, ants, termites moles
j) Water movement and action
2. CHEMICAL PROCESSES
-This is faster than physical processes
-These processes include;
i.) Solution
-Its dissolving of a solid in liquid. Solid material is separated to independent soluble ions. Each ion is surrounded by water. The driving force behind solution is ionization and when an element is ionised then it’s soluble.
E.g. NaCl + H2O → Na+ Cl- H2O
-Sodium chloride is soluble in soil and the ions become ready to react in chemical reactions
ii.) Hydrolysis
-This deals with water molecule being split to its ions and then reacting with other substances.
E.g. KALSi3O8 + HOH → HALSi3O2 + KOH
(Ordthodase) (Silicate clay) very soluble
Not very soluble more soluble
iii.) Carbonation
-This is the reaction of compound with carbonic (weak acid produced by carbon dioxide dissolving in water)
E.g. CaCO3 + H+ +HCO3 → Ca (HCO3)2 + HCO3
Slightly soluble readily soluble
NB: Hydrolysis and carbonation are the most effective processes in chemical weathering.
iv.) Reduction
-Is the process in which electrons are gained. Negative charge increases as positive charge decreases. It occurs when there is lack of oxygen i.e. anaerobic condition especially in stagnant water. It results in;
Electrically unstable compounds
More soluble compounds
Increased atom size- internally charged atom is stressed and therefore reduction in faster decomposition
v.) Hydration
-It’s the combination of solid chemical with water. They interchange mineral structure. There increase in volume and the solid become softer due to internal stresses and therefore more easily weathered.
3. BIOLOGICAL PROCESSES
-This important in soil formation and development and not in weathering.
Translocation
Churning or other soil disturbances by earthworms, termites among others which keep microbiological population into contact with fresh food supply hence breakdown of organic matter.
Humification
Is the breakdown of organic matter to yield humus
Nitrification
Is the process during which nitrate is formed by soil organisms (autotrophs). Nitrogen is converted to ammonia then to nitrites and finally nitrates.
-Ammonia is oxidised by Nitrosomonas and Nitrococcus bacteria and the nitrite by the Nitrobacter bacteria in an aerobic condition which is moist.
-Nitrogen is constantly being recycled from the soil to plants and then back to the soil via the litter and its’ decomposition products.
Nitrogen fixation
There are a number of free living chemo-heterotrophic bacteria including Azobacter, Clostridium and other algae varieties capable of fixing atmospheric nitrogen. The microbes utilise atmospheric nitrogen to form their cells protein which upon death of the organism, is decomposed to ammonia to form part of nitrogen available to plants.
Ammonification many
Amino Acids from NH4 + (Ammonium Ion)
Different degradation of proteins bacteria
Benefits of Soil Organisms
1. Organic matter decomposition
Plants residues are broken down releasing organically held nutrients for use by plants.
Stabilizes soil aggregate since decay products form humus.
2. Inorganic transformations
Organically bound forms of nitrogen, phosphorous and sulphur are converted by microbes into plant- available forms.
- Presence of nitrates, phosphates, sulphates in soils are due to microbes
-They oxidise ion and manganese into their higher valence states whose solubilities are quite low and thus non-toxic.
3. Nitrogen fixation
- Nodule organisms of legumes fix elemental nitrogen into forms usable by plants.
NH4 + Nitrosomonas
NO3-
Ammonium ions Nitrate ions
FACTORS AFFECTING WEATHERING OF MINERALS
1. Climatic conditions
-These will control the kind and rate of weathering. High moisture enhances chemical as well as mechanical changes. High temperatures accelerate weathering.
-Climate also affects vegetation hence indirectly affecting the weathering process e.g. soils developing under conifer trees whose needles are low in metallic cations are more acidic than soils developed under grassland or most deciduous trees.
-Large diurnal range will also enhance exfoliation.
2. Physical characteristics
Particle size, hardness and the degree of cementation affect weathering.
-Rocks with large crystals have varying quantities of different minerals which have differing expansion –contraction coefficient hence large sized particles will weather more readily than small sized ones. Finer particles will however react more readily with chemicals due to the enhanced surface area thus high rate of chemical weathering.
-Compact cemented quartz is hard and difficult to weather while porous volcanic ash or coarse limestone are readily broken down.
3. Chemical and structural characteristics
Gypsum (CaSO4.2H2O) is easily dissolved in adequate water. Calcite and dolomite are easily dissolved by carbonic acid, Ferro-magnesium which are dark-coloured primary minerals are more susceptible to chemical weathering than feldspars and quartz due to reaction of iron in ferromagnesian minerals.
SOIL PROFILE
This refers to a vertical section of a soil in the field showing distinct horizontal layers. The individual layers in the soil profile are referred to as horizons. These horizons above the parent material are collectively referred to as solum. Every well developed, undisturbed soil has its own distinctive soil profile characteristics which are utilised in soil classification and survey. A profile is vital in soil judgement.
-Top soil is darker due to high accumulation of organic matter.
-Subsoil especially in humid region mature soil has two belts; the upper belt with higher organic matter and the lower belt with higher mineral components i.e. gypsum, clays, aluminium oxides and calcium carbonates.
-For convenience, the layers resulting from the soil forming processes are grouped into five main categories; O, A, E, B, and C. These capital letters designate the master horizons in soils. Lower case letters are used to give subordinate distinctions of these master horizons (specific details). Numbers 1, 2, 3 can also be used to give some distinctions of the horizons.
-A pedon is a three dimensional body of soil of 1-10m2 top areas, whose lateral dimensions are large enough to permit study of horizons both physical and chemical composition.
-Many pedons (polypedons) are used in thorough study of a given soil for classification.
-Solum is the upper most weathered part of the soil profile (A and B horizons). A transition zone lies between two main horizons and has characteristics of both. The first letter e.g. B/A means the transition zone has more than 50% characteristics of B than A.
O Horizon
A Horizon
E Horizon
B Horizon
C Horizon
D or R Horizon
O Horizon
Soil Surface consists of dry decaying organic matter.
A Horizon
It is the top soil
It is underlying layer. It is darker in color than the lower horizon. It results from decayed plants and animal residues. Seeds germinate in this layer.
E Horizon
It is eluviation (Leaching) layer.
It is light in color. It is beneath A Horizon made up of mostly sand and silt. It has lost most of its minerals and clay as water drips through the soil (the process of eluviation).
B Horizon
It is the subsoil .
It is characterized by: clay accumulation of varying amounts of silicate, iron, aluminum oxides, gypsum, calcium carbonates that are washed from the upper layers or through weathering.
C Horizon
It is also called regolith.
It is a layer beneath B Horizon and above D or R Horizon. It has slightly broken up bedrock. Plants roots do not penetrate this layer very little organic matter is found in this layer.
D or R Horizon
It is the unweathered rock.(Bedrock)
It is the layer beneath all other layers. It is the parent rock.
N/B Various soil horizon have characteristics and properties that influence the use of soils and also differ from soil to soil.
Stages of soil development
-Soils are constantly undergoing change. Soil is a three phase system, consisting of solids, liquids and gases.
-The life cycle of soil includes the following stages;
- Parent material
- Immature soil e.g. Entisol
- Mature soil e.g. Mollisol
- Old soil e.g. Alfisol
-The mature stage is attained with development of the B horizon. Old soil e.g. clay-pan soils are highly differentiated and have low fertility.
-Mohr and Barren have recognised five stages in the development of tropical soils;
- initial stage; the un-weathered parent material
- Juvenile stage; involves the start of weathering
- Virile stage; large proportion of the rock has weathered
- Serile stage; final stage of decomposition and only most resistant minerals have survived.
- Final stage; soil decomposition has completed
Terminologies used in the process of soil formation
-Solodization; process of the removal of sodium ions from clay humus complex usually leading to solodized solonetz leached of metal cations and is dominant with hydrogen ions resulting in an acid soil.
-Alkalization; excess Na+, Ca+, Mg+ hence basic horizon
-Salinization; enrichment of soil with salts e.g. in arid regions
-Leaching; process by which clay-humus complex and substitute hydrogen ion in their place
-Eluviation; is loss in suspension of material from a soil horizon
-Podzolization; The development of an extremely acid humus known as mor in which decomposition of organic matter is slow, this is common in cool humid soil
-Terrallitzation/latolization/laterization; is characteristic of soil; formation in the humid tropical regions of the world. It involves the relative accumulation of sesquioxides of iron and aluminium accumulation with the loss of silica.
-Pedoturbation; is mixing of soil naturally through contraction shrinking and expansion (dry and wet) as in montmorillonite (vertisols).
-Faunal pedoturbation; mixing of soils by soil living animals.
-Illuvial horizon; is horizon that receives material in solution or suspension from some other parts of the soil.
-Illuviation; The process of movement of material from one horizon and its deposition to another horizon of the same soil usually from upper horizon to middle or lower horizon in the pedo-unit (also lateral movement).
-Alluvial soil; soils developed on fairly recent alluvium; usually they show no horizon development.
-Alluvium; sediment deposited by streams and varying widely in particle size (fertile in nature).
-Catena; segment of soils developed from similar parent material under similar climatic conditions but whose characteristics differ because of variations in relief and drainage.
-Relief; the difference between the high and low areas of a landscape (topography, elevation difference, earth surface contour).
-Sesquioxides (metal oxides); minerals containing 1.5 atoms of oxygen per atom of the metal, particularly Al2O3 and Fe2O3 and at time TiO2 although it doesn’t strictly fit the meaning of sesqui (=1.5 times).
-Colloid; the organic and inorganic material with very fine particles thus high surface area which usually exhibits exchange properties (-charge).
-Micelle; highly charged clay-humus particle
-Eluviations means leaching (washing out)
-Illuviation means accumulation (washing in). Horizon A and B together constitute the solum.
NB: All the stated horizons are not always present in very soil. Some might not be discernible.
-Additional features of the horizons are indicated with the use of lowercase letters e.g. A1 and part of the A2 may be mixed together hence having a layer designated AP to indicate disturbance through cultivation or pasturing.
MASTER HORIZONS/LAYERS
-All soil profiles contain two or more master horizons. The descriptions of these horizons are summarised below;
Ø O- these are organic horizons of mineral soils
O1 horizon forms at the upper part mainly composed of un-decomposed organic matter
O2 horizon is dominated by partly decomposed organic material
Ø A- mineral horizons
A1; organic matter is accumulating here but characterised by severe leaching. The finely decomposed organic matter leads to dark colour
A2; a light coloured leached horizon representing a layer of maximum leaching. It
is prominent inmany acid forest soils and faintly developed or even absent in most
grassland soils
A3; is a transition layer similar to A2
Ø B horizons are characterised by accumulation of leached minerals e.g. iron, aluminium and silicon (illuvial concentration).
B1; transition layer similar to A2
B2; layer of maximum accumulation of silicate clay minerals and development of blocky structures
B3; transition to C
Ø C horizon shows little effect from podogenic processes and lacks properties diagnostic of A and B.
Ø R is underlying consolidated bedrock such as granite, sandstone or limestone
DIAGNOSTIC HORIZONS
-Are combinations of specific soil characteristics that are indicative of certain classes of soils. Characteristics of different horizons are due to soil forming processes thus diagnostic horizons for separating soil units is based on resultant quantitative morphological properties that have identified value.
-Diagnostic horizons are more distinct and allow for specific classification of soil layers in soil taxonomy. They differentiate among soil orders, sub-orders, great groups and subgroups.
-The A, B, and C nomenclature referring to master horizons are still used as the standard for describing and defining soil horizons (general). But the need for more strict definition of soil horizons led to the development of diagnostic horizons
-Two kinds of diagnostic horizons are identified;
ü Surface diagnostic horizons (epipedons)
ü Sub-surface diagnostic horizons (endopedons)
-The two above have peculiar distinguishing features. The major features are used in defining most of the soil orders.
-Diagnostic horizons are identified from the general master horizons. The former are used in soil taxonomy e.g. by USDA and UNESCO/FAO.
-Diagnostic horizons are therefore combinations of specific soil characteristics that indicate certain categories of soil.
Derivations and major features of diagnostic horizons
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Horizon
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Derivation
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Major features
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Surface diagnostic horizons
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Mollic
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L. mollis, soft
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Thick, dark, high base saturation, not hard when dry
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Umbric
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L. umbra, shade
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Same as mollic but may be hard when dry and highly saturated
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Ochric
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Gr. Ochros, pale
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Thin, light coloured, low in organic matter
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Histic
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Gr. Histos, tissue
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Very high in organic matter, water saturated unless artificially drained
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Anthropic
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Gr. Anthropos, man
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High phosphate due to fertilizer application, mollic-like
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Plaggen
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Ger. Plaggen, sod
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Very thick over 20inches due to long continued manuring
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Sub-surface diagnostic horizons
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Argillic
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L. Argilla, white clay
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Found in B horizon. It is an illuvial horizon of silicate clay accumulation
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Natric
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L. Natrium, sodium
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As argillic, high exchangeable sodium, columnar or prismatic structure. It is white and has permeability problem
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Spodic
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Gr. Spodos, wood ash
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Illuvial accumulation of free iron, aluminium oxides and organic matter. It is ash-white
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Oxic
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L. oxide
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Hydrated iron oxides, aluminium oxides and 1:1 clays
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Cambic
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L. cambiare, to change
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Altered horizon due to frost, roots, animals activities
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Agric
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L. ager, field
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Illuvial horizon of clay and organic matter accumulation just below the ploughing level due to long continued cultivation
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INTRODUCTION TO SOIL CLASSIFICATION
-To develop a useful classification and to establish the kinds and ranges of properties that are to characterise given soil units, the soil must be studied in the field.
-A pedon is a three dimensional body of soil whose lateral dimensions are large enough to permit study of horizons and the related physical and chemical composition. A pedon can be 1-10m2.
Soil orders; the order is the highest category and there are 10 orders each ending in ‘sol’ meaning soil. These are;
- Entisols; these are very recent soils, low humic clayey soils
- Vertisols; have high clay content characterised by alternate swelling and shrinking.
- Inceptisols; are young soils. Brown forest, low humic soils
- Aridosls; common in arid regions, reddish solonichalk and solonetz
- Mollisols; grassland soils with thick dark coloured surface
- Sopdosols; soils with spodic horizon
- Alfisols; leached but with high Al3+ and Fe2+
- Ultisols; are extremely leached and very low in bases
- Oxisols; are red tropical soils rich in iron oxide and aluminium oxide (also 1:1 clay)
- Histosols; big soils composed mainly of plant tissues
Sub-orders; these are differentiated largely on soil properties and horizons resulting from differences in soil moisture and soil temperature (47 sub-orders).
-Sub-orders are distinct to each other and are not interchangeable to other orders.
Soil types; this refers to the lowest unit in the natural system of soil classification; a subdivision of a soil series and describes soils that are alike in texture mainly horizon A.
COMMON SOILTYPES OF EAST AFRICA
The soils are differentiated on bio-climatic zones and they include;
1. HYDROMORPOHIC SOILS (GLEYSOLS)
-Poor drainage can be observed in the soils of most regions of the world. This leads to the formation of gleysol or hydromortphic soils
Definition; hydromorphic soils refers to suborder of intrazonal soils (calcimorphic, hydromorphic, halomorphic and andozols) all formed under conditions of poor drainage in marshes, swamps, seepage areas or flats.
-Thus they are soils developed in the presence of excess water. Gleying is the reduction of iron in an anaerobic environment leading to the formation of grey or blue colours and occurs when water saturates a soil filling all the pore spaces hence anaerobic.
-Hydromorphic soils can be found in association with all zonal soils (podzols, grey soils, ferralitic and vertisols) where water can gather in sufficient volumes and time to produce the effects of gleying
Characteristics of hydromorphic soils
Ø Water saturated by ground water (water-logged) through capillarity
Ø Anaerobic where even the remaining oxygen is consumed by the microbiological population
Ø In the reducing conditions brought about by the absence of oxygen and in the presence of organic matter, iron compounds are chemically reduced from the ferric to the ferrous state. The ferrous form of iron is very much soluble hence leached leaving behind the colourless minerals which gives gley soils their characteristic grey colouration
Ø Some are permanently saturated (gleysols) others temporarily saturated hence called pseudo-gleysols
Ø Peaty surface horizon (semi-decomposed organic matter) are saturated in water
Ø Hydromorphic soils develop from fine grained parent material (clay skin)
Ø When dry they become aerated hence ferric iron compounds colour (rust)
Ø Are formed from permeable parent materials e.g. alluvial sands and gravels which overlie an impervious substratum upon which water accumulates
Ø They occur in lower parts of the landscape
Ø Mottling when artificially drained
-There are seven subdivisions of gleysols; eutric, calcaric, dystric, mollic, plinthic, gellic and humic gleysols
Reclamation of hydromorphic soils
Land drainage;
i). Surface field drains. Used to remove surface water. Ditches are dug with gentle side slopes. They are constructed across the slope of the land and across the direction of cultivation e.g. open ditches
ii). Subsurface underground drains. Systems of underground channels to remove water from the zone of maximum water saturation e.g. mole channel underground channels, perforated plastic pipes and clay tile system.
iii). Ridging.
2. HALOMORPHIC SOILS
-These are soils comprising a suborder of intrazonal soil order consisting of saline and alkali soils formed under poor drainage in arid regions including solochak or saline soils, solonetz and soloth soils.
-The salts which affect the soils are chiefly the sulphates, chlorides and carbonates of sodium and magnesium. Excessive evaporation of arid regions where underground water moves up together with the dissolved salts leads to deposition of these salts on the surface of the ground. This results to three main groups of soils;
Ø Saline soils; contain a composition of neutral soluble salts sufficient to seriously interfere with the growth of most plants. pH of about 8.5 though salts are neutral
Ø Saline-sodic soils; have high adsorbed sodium ions with relatively high concentration of neutral soluble salts
Ø Sodic soils; have low neutral soluble salts but Na+ and OH- are in toxic quantities
3. SOLONCHAKS (SOLORTHIDS)
-This is an intrazonal group of soils with high soluble salts concentration, usually light coloured occurring in sub-humid or semi-arid climate
Characteristics/properties of solonchaks
Ø High content of salts which are usually highest near to or at the surface decreasing with depth
Ø The most common ions are chloride, sulphate, carbonate, sodium, bicarbonate, calcium and magnesium.
Ø Fine textured soils have a higher retentivity
Ø therefore they hold more saline water upon evaporation leaving high amount of salts
Ø Loess, alluvium and pedi-sediments are the principal parent materials
Occurrence of solonchaks
There are four subdivisions of solonachaks
i) Orthic solonchaks; have ochric A horizon and lack hydromorphic properties within 50cm of the surface
ii) Mollic solonchaks; have mollic A horizon and lack hydromorphic properties within 50cm of the surface
iii) Takyric solonchaks; have takyric feature and lack hydromorphic properties within 50cm of the surface
iv) Gleyic solonchaks; have hydromorphic properties within 50cm of the surface
-The common occurrence of solonchaks is in mid-latitude and tropical arid and semi-arid areas where evaporation is much greater than precipitation mainly in NW Africa.
-They develop in alluvial terraces and beds of old lakes
-Colour; very dark-grey clay loam within 20cm depth and pH of 8.3
-Dark-grey clay loam within 20-50cm depth with Ph of 8.5
-Olive-grey clay loam at 50-120cm depth of pH 8.4 from the ground surface
4. SOLONETZ
-This is an intrazonal group of soils having surface horizons of varying degrees of friability underlay by dark hard soil, ordinarily with columnar structure (prismatic structure with rounded tops)
Characteristics of Solonetz
Ø They are formed under better drainage than solonchaks and under a native vegetation of halophytic plants
Ø The most conspicuous property is the abrupt and large increase in clay passing from upper into the nitric B horizon
Ø Organic matter in the surface mineral horizon varies but it is usually less than 10%
Ø There is high humification as the C:N ratio is less than 12 (decomposition of organic matter leading to the formation of humus)
Ø The pH varies between 6.0 and 7.5 at the surface and 8.5 in the lower horizons
Ø Upper horizons are non-saline but salinity increases with depth
Occurrence
-Clay minerals emanate from parent material and are dominated by micas but kaolinites and montmorillonite can also be present
-Solonetz are formed by the progressive leaching of solonchaks which are deficient in calcium but have a large amount of sodium ions
-Some crops can grow in these soils but used for grazing in tropics and sub-tropics since these regions are too dry
-Solonetz are confined to flat or gently sloping land while solonchaks occur in depressions with close water table
-The soils occur in the arid and semi-arid regions of the world mainly N and S Africa, Australia, W. Pakistan and SW USSR
-In all cases Solonetz are deficient in plant nutrients but this can be enhanced by reclamation through liming and irrigation
-Has same subdivisions like solonchaks
5. VERTISOLS
-These are dark swelling soils formally called black cotton soils or tropical black clays
Characteristics
Ø Dark brown or black clays which are fine textured
Ø When dry they shrink and crack widely so that top soil material can fall down the cracks so that after sometimes the soils invert themselves
Ø They swell when wetted forming micro-relief on the soil surface e.g. montmorillonite
Ø Have over 30% clay (swelling type) to a depth of 1m
Ø The excessive shrinking, cracking and shearing make them unstable hence pose a problem when used for building foundations, highway bases and even for agricultural purposes
Ø Sometimes mica and kaolinite may be present but the amounts of these two minerals are fairly small
Ø The C:N ratio is about 10-14
Ø The cation exchange capacity is high varying between 25-80me%.
Ø Salinity is low since salts seldom appear to accumulate in vertisols and when they do it is usually below about 30cm.
Ø The pH ranges between 6.0-8.5 but an increase in case of accumulation of sodium
Distribution
- Vertisols are found mostly in sub-humid to semi-arid climates with moderate to high temperatures e.g. sub-Saharan Africa, NE Australia, India and Sudan
Utilisation of vertisols
-Generally in tropical areas, the natural grassland is grazed
-Irrigation in vertisols is also practised e.g. Gezira in Sudan
-It is not possible to exploit full potential of these soils due to narrow moisture range for cultivation
-They are deficient in many macro and micronutrients thus fertilizer application is necessary
-They are susceptible to erosion even a slope of 5% or less may develop deep gullies in a very short period. A characteristic form of erosion is landslides where large area will move as a unit
-They are sticky and plastic when wet and hard when dry thus less suitable for crop production. These soils are heavy and when wet are poorly worked by both machines and hand tools
-Sorghum, corn, millet and cotton are crops commonly grown though the yields are generally low. Modern management is required for productivity of these soils.
6. ACRISOLS (ULTISOLS)
-These are soils having argillic B horizon (illuvial accumulation of clay) of low base saturation or low base status
Characteristics
Ø They are moist soils that develop under warm to tropical climates
Ø Have low bases
Ø It is not at an advanced stage of humification as the C:N ratio is 15:1
Ø The cation exchange capacity has two maxima, 35me% in the A horizon due to presence of organic matter and about 45me% in the argillic B horizon due to the greater amount of clay
Ø Has pH of about 5.5 which is relatively uniform but increases with depth due to high surface leaching (acidic soil)
Ø Subsurface horizons are commonly red or yellow in colour
Occurrence
-The soils develop on stable sites that range from flat to steeply sloping water so long as water can percolate freely but their most common occurrence is on flat or undulating landscape
-They are also formed on old land surfaces normally under forest vegetations or savannah
-Beside EA Africa and the tropics the soils are common in SE USA, NE Africa, SE Australia and other humid tropical areas
-Liming of the soil is necessary and also application of nitrogenous and phosphate fertilizers
-The crops commonly grown in these soils are fruits, maize, tobacco and sweet potatoes.
Distribution and land use
-Beside E. Africa and the tropics, these soils are common in SE USA, NE Africa, SE Australia and other humid tropical areas.
-Liming the soil is necessary also nitrogenous and phosphatic fertilizers
-The crops commonly grown in these soils are fruits, maize, tobacco and sweet potatoes
7. ANDOSOLS (VOLCANIC ASH BLACK, LOW DENSITY)
Properties/characteristics
Ø Soils formed from materials rich in volcanic glass and commonly having a dark surface horizon
Ø Have high organic matter (20%) developed in volcanic ash deposits
Ø Weakly developed soils rich in allophone (dust-like clay from volcanic eruption) and having a low bulk density. The clay content is usually low and does not exceed 20-25%. Much of the clay occurs in the upper horizon and decreases with depth to <5% in the un-weathered parent material
Ø High porosity
Ø Very acidic (pH 4.5) in the upper surface which decreases to pH 6 down the unaltered ash
Ø The C:N ratio is 15:1
Ø In some andosols manganese dioxide is a product of weathering
Ø Have high water retaining capacity due to high content of allophone
Occurrence and distribution
-The formation of andosols is a very rapid process resulting from the high surface area of the volcanic ash parent materials which behaves in a unique manner under humid conditions
-Hydrolysis process weathers the volcanic ash initially to yellow, brown or orange palagonite. These changes quickly into allophone. There is also partial humification of organic matter
-The soil develops under aerobic conditions
-These soils are common in tropics but under humid conditions because volcanic ash doesn’t give rise to andosols in dry or very dry climates
-They are common in E Africa (Engasura, Langalaga), East Indies and Hawaii
Utilization of andosols in agriculture
These are relatively infertile soils but respond well when sufficient fertilizers and lime are applied
-Phosphorous is necessary because the allophone has very high capacity for absorbing and fixing this element
-It is advantageous in warm climates to plant certain crops such as sweet potatoes which show little response to phosphorous. Rice will do well due to the soils’ high water holding capacity
8. LUVISOLS (ALFISOLS)
-Latin meaning to wash; connotative of illuvial accumulation of clay
Characteristics/properties
Ø Soils having an argillic B horizon which has a base saturation of 50% or more at the lower part of the B horizon
Ø They lack mollic A horizon
Ø Can be a mixture of vertisols, ferric, calcic, orthic, albic, gleyic or chromic
Ø They develop material of medium texture with a high proportion forming on loess (material transported and deposited by wind and consisting of predominantly silt-sized particles)
Ø The content of clay (<2μm) is at a minimum in the two uppermost horizons increasing to maximum in the middle horizon
Ø pH values have an interesting pattern; they vary from 5.5-6.5 in the upper horizon decreasing to about 4.5-5.0 where there is the clay maximum then increasing to 7.5
Ø have organic matter at the uppermost layer of about 5-10%
Ø have moderate C:N ratio of 12:18 reflecting the partially decomposed state of organic matter
Ø have CEC maximum at the surface due to high organic matter and at the middle horizon where the clay content is also at its maximum
Distribution of luvisols
-Luvisols are widespread in NC USA, EC USA, E Africa and Central Europe
-They occur under moist conditions
Utilization
-The potential of these soils for agriculture varies from moderate to good. They are used for mixed farming, dairying, horticulture, wheat, maize and oats
-Fertility is maintained by the normal procedure of liming and fertilizer application
-Erosion is common feature and rigorous control methods must be maintained at all times
9. FERRASOLS (OXISOLS)
Derived from Latin meaning iron and aluminium connotative of high content of sesquioxides
Characteristics/properties
Ø Soils having an oxic B horizon which is highly weathered with primarily a mixture of iron and aluminium oxides
Ø pH of 4.5-5.5
Ø Have a thick oxic B horizon to 16m. The thicker the oxic B horizon the older the soil
Ø High clay content but of non-sticky type
Ø Deficiency of micronutrients
Ø 80% kaolinite hence low CEC
Ø Low silt: clay ratio due to the profound weathering that has taken place
Distribution
-Common in forests
Utilization
-Require phosphorous fertilization but can be highly productive if well managed
-Liming is necessary
10. NITOSOLS (ALFISOLS AND ULTISOLS)
These are sufficiently weathered Alfisols from ultisols. This class has been created recently and has not yet been fully categorised
General characteristics/properties
Ø They are strongly weathered kaollinitic soils. The clay increases with depth to a maximum in the middle horizon
Ø Soils have very deeply developed argillic (an alluvial horizon which clays have accumulated) B horizons and showing features of strong weathering
Ø Nitosols combine the characteristics of Alfisols and Ultisols
Ø Alfisols are moist mineral soils having no mollic (organic matter) epipedon
Ø They are grey to brown on the surface horizons, medium to high base status
Ø Have alluvial horizon in which silicate clays have accumulated
Ø More than 15% saturated with sodium
Ø Have prismatic or columnar structure
Ø Alfisols are more fertile than Ultisols
Ø High amounts of exchangeable aluminium are usually present ultisols occur in the warmer parts of the world with mean annual soil temperature is 8˚C or more where rainfall is considerably in excess of evaporation
Ø Fertilizers are necessary for better output with ultisols since the soils have low organic matter
Ø Ultisols have low pH. Ultisols are commonly red or yellow in colour due to accumulation of free oxides of iron. Ultisols are sometimes found mixed with oxisols
Agricultural use of Nitosols and distribution
-Fertilizer application can be used to alter the pH or slightly liming
-The importance of lime to raise pH and to reduce the aluminium toxicity often increases the cost of production
-Alfisols are common in S. Africa
-In general Alfisols are quite productive soils. Usually ultisols have developed in humid climates, tropical to subtropical temperatures and forest or grassy forests (savannah) vegetation
-Insects are abundant and fungus diseases are prevalent in the warm humid climate
-Ultisols account for 12.5% of the land area of the US and occupies 5.5% of the world’s land area
SOIL FAMILIES AND SERIES
-The family category of classification is based on properties important to the growth of plant roots mainly textural classes, mineralogical classes, temperature classes and soil temperature.
-Terms used to describe the mineralogical classes include; montmorillonitic, kaolinitic, siliceous, gypsic and mixed.
-For temperature we have frigid, mesic and thermic.
-For textural (particle size); loamy, clayey (fine, coarse).
-The name of the soil series has no pedogenic (soil formation) significance but represents a prominent geographical name or a river, town, or area near where the series was first recognised i.e. basis of observable and mappable soil characteristics e.g. colour, pH, texture and structure.
-Soil phases are sub-divisions of soil series. Soil phases describe soil surface texture, thickness, stoniness, saltiness etc. Phases are used to describe mapping units. All mapping units are polydpedons. Soil phases are technically not included as a class in soil taxonomy.
-Soil catena refers to a sequence of soils of about the same age derived from similar parental material and occurring under similar climatic conditions but having different characteristics due to variation in relief and drainage.
-Soil catena is essentially a group of soils derived from similar parent materials occurring in a complex in which the individual members are differentiated by topographical and hydrological conditions. The catena concept apart from its value directing attention to the relationships of different soils derived from similar parent materials is a convenient mapping unit in areas of great complexity where detailed mapping might be too demanding in time and labour.
LAND CAPABILITY
-Land capability classification is the grouping of soil into special units, classes according to their capability for intensive use and the treatments required for sustained use (commonly used by USDA).
- US soil conservation service (1997) identified eight land capability classes as illustrated below;
Definitions
Land Capability Classes and Subclasses:
Capability class is the broadest category in the land capability classification system. Class codes 1, 2, 3, 4, 5, 6, 7, and 8 are used to represent both irrigated and nonirrigated land capability classes.
Class 1 soils have slight limitations that restrict their use.
Class 2 soils have moderate limitations that reduce the choice of plants or require moderate conservation practices.
Class 3 soils have severe limitations that reduce the choice of plants or require special conservation practices, or both.
Class 4 soils have very severe limitations that restrict the choice of plants or require very careful management, or both.
Class 5 soils have little or no hazard of erosion but have other limitations, impractical to remove, that limit their use mainly to pasture, range, forestland, or wildlife food and cover.
Class 6 soils have severe limitations that make them generally unsuited to cultivation and that limit their use mainly to pasture, range, forestland, or wildlife food and cover.
Class 7 soils have very severe limitations that make them unsuited to cultivation and that restrict their use mainly to grazing, forestland, or wildlife.
Class 8 soils and miscellaneous areas have limitations that preclude their use for commercial plant production and limit their use to recreation, wildlife, or water supply or for esthetic purposes.
Capability subclass is the second category in the land capability classification system. Class codes e, w, s, and c are used for land capability subclasses.
Subclass e is made up of soils for which the susceptibility to erosion is the dominant problem or hazard affecting their use. Erosion susceptibility and past erosion damage are the major soil factors that affect soils in this subclass.
Subclass w is made up of soils for which excess water is the dominant hazard or limitation affecting their use. Poor soil drainage, wetness, a high water table, and overflow are the factors that affect soils in this subclass.
Subclass s is made up of soils that have soil limitations within the rooting zone, such as shallowness of the rooting zone, stones, low moisture-holding capacity, low fertility that is difficult to correct, and salinity or sodium content.
Subclass c is made up of soils for which the climate (the temperature or lack of moisture) is the major hazard or limitation affecting their use.
The subclass represents the dominant limitation that determines the capability class. Within a capability class, where the kinds of limitations are essentially equal, the subclasses have the following priority: e, w, s, and c. Subclasses are not assigned to soils or miscellaneous areas in capability classes 1 and 8.
SOIL SURVEY
-This refers to the systematic examination, description, classification and mapping of soils in an area. Soil surveys are classified according to the kind and intensity of field examination.
-Soil surveying thus involves studying and mapping the earth’s surface. A soil map and description of the area of study are shown
Soil mapping
-Mapping consist of walking over land at regular intervals and taking notes of soil differences and all related surface features e.g. slope gradients, evidence of erosion, land use, vegetation, cultural features etc.
-Sketches with clear boundaries are drawn. Aerial base maps are common when a large area is being studied. Thus soil series and phases can be easily identified.
-Scale may vary depending on the required details. Atlas world maps have a scale of 1:80000000 but a detailed map of scale 1:50000 are convenient.
Uses of soil surveys
-To facilitate research in crop production, land evaluation and zoning, and human settlements, also in engineering works.
SELECTING LAND FOR IRRIGATION
In choosing land for irrigation, a careful examination should be made of the soil to determine;
ü Texture of the soil to depth of several feet
ü Presence of impermeable stratum or gravel within a depth of 5-6 feet
ü Accumulation of soluble salts in injurious quantities
ü Land terrain (slope and evenness of the soil surface)
ü Behaviour of soil under irrigation
NB:
· A good desirable soil is readily permeable to water and yet is moisture retentive. Infiltration rates should be in the range of 0.1-3 inches per hour and wet soil to a depth of 2-3 feet in 24 hours, but some soils have varying wet depths.
· Desirable slope should be 10-20 feet to one mile, but it should be noted that irrigation water should be able to follow in all directions in the farm for uniform supply of water.
SOIL TAXONOMY
-This refers to classification of soils according to natural relationships among the soil characteristics. Taxonomy relates to natural (climatic, living organisms and modified relief) soil forming factors only as they have imparted observable and measurable characteristics of the soil.
-These characteristics are horizons, soil moisture regimes, organic matter content, soil pH, texture, mineralogy, colour, structure and soil temperature regimes. A minimum soil volume (pedon) that will represent an individual soil measuring about 1m2 is studied. It is as deep as roots grow.
-Soil taxonomy places emphasis on soil properties as they are (observable and measurable even in laboratory), rather than the pedogenesis or the soil genesis as was in earlier classification. Other advantages of this recent (taxonomy) classification are;
ü Focuses more on soil rather than related sciences e.g. geology and climatology
ü It permits classification of soils of unknown genesis so long there are known
ü It permits greater uniformity
-There are several systems of classification/taxonomy used in the world;
a) United States Department of Agriculture (USDA) Soil Taxonomy (1975)
-USDA system fits all soils into 10 orders, 47 sub-orders, 230 great groups, 1200 subgroups, 6000 families and 13000 series, thus there are six main classes (soil phase is included to specify the series but this is often ignored).
-The 1975 USDA system has designated 23 specific horizons called diagnostic horizons;
Ø An epipedon; is a surface is a surface horizon which is darkened by organic matter and includes eluvial horizons which makes up the surface horizons and these are;
· Mollic epipedon; dark-coloured, thick surface horizon with over 50% of the exchange capacity saturated by base cations
· Anthropic epipedon; similar to mollic but has high amount of phosphate accumulated by long continued farming
· Umbric epipedon; dark surface horizon less than 50% of the exchange capacity saturated by base cations
· Plaggen epipedon; man-made surface horizon more than 50cm thick with characteristics that depend upon original soil from which it was derived
· Histic epipedon; a thin surface horizon saturate with water for part of the year and with large amount of organic carbon
· Ochric epipedon; this is too light in colour and too low in organic carbon
Ø Sub-surface diagnostic horizons
These occur below the surface horizons. They include;
- Argillic horizon; an illuvial horizon in which clays have accumulated to a significant extent
- Agric horizon; compact horizon formed by cultivation which has been enriched by clay and/or humus
- Natric horizon; its argillic horizon with columnar structure and more than 15% saturated with exchangeable Na+
- Spodic horizon; has accumulation of free sesquioxides and/or organic carbon but not clay
- Cambic horizon; a change horizon including structure formation, liberation of free iron oxides and clay formation
- Oxic horizon; very low content of weatherable minerals in which clay is composed of kaolinite and sesquioxides having a low cation exchange capacity and poorly dispersible in water.
- Calcic horizon; enriched horizon with calcium carbonate in the form of secondary concentrations more than 15cm thick
- Gypsic horizon; enriched horizon with calcium sulphate more than 15cm thick
- Salic horizon; enriched horizon with more soluble than gypsum more than 15cm thick
- Albic horizon; horizon in which clay and free oxides have been removed so the colour is determined by the colour of sand and silt and not the coatings on these particle
Ø Others
· Duripan; horizon cemented by silica or aluminium silicate
· Fragipan; loamy sub-surface with platy structure and high bulk density, brittle when wet and hard when dry
· Petrocalcic; its cemented with calcium carbonate
· Plinthite; rich in sesquioxides, highly weathered and poor humus
Soil taxonomy based on USDA for soil orders
These are;
- Entisols; these are very recent soils, low humic clayey soils
- Vertisols; have high clay content characterised by alternate swelling and shrinking.
- Inceptisols; are young soils. Brown forest, low humic soils
- Aridosls; common in arid regions, reddish solonichalk and solonetz
- Mollisols; grassland soils with thick dark coloured surface
- Sopdosols; soils with spodic horizon
- Alfisols; leached but with high Al3+ and Fe2+
- Ultisols; are extremely leached and very low in bases
- Oxisols; are red tropical soils rich in iron oxide and aluminium oxide (also 1:1 clay)
- Histosols; big soils composed mainly of plant tissues
Soil taxonomy based on FAO/UNESCO (soil map of the world).
- Fluvinols; weakly developed soils from alluvial deposits in active flood plains
- Regosols; formed from collection of loose material (blanket)
- Arenosols; strongly weathered sandy soils of tropical and subtropical areas
- Gleysols; is formed from mottled layers resulting from excess of water. They are soils in which hydromorphic processes are dominant
- Rendzinas; soils developed in surface horizon enriched in organic matter over highly calcareous materials
- Rankers; soils developed in surface horizon enriched in organic matter over siliceous materials
- Andozols; soils formed from materials rich in volcanic glass and have dark surface horizon. The soils are weakly developed rich in allophone and have low bulk density
- Vertisols; are surface soils that are self-mulching. The soils are black and cracking clay soils
- Yermosols; are soils of desert environment
- Xerosols; soils of semi-arid environments
- Solonechaks; soils showing strong salinity
- Solonetz; soils developed under the influence of high sodium saturation
- Planosols; soils developed in level or depressed topography with poor drainage
- Greyzems; soils of the forest-steppe transition rich in organic matter and having a grey colour caused by white silica powder on structure faces
- Kastanozems; soils of the semi-arid steppes showing an accumulation of organic matter in the surface horizons often calcareous throughout.
- Chernozems; soils of the grassland steppes showing strong accumulation of organic matter in the surface horizons and an accumulation of calcium carbonate at shallow depth
- Phacozems; soils of the forest-steppes showing a strong accumulation of organic matter in the surface but a deep leaching of calcium
- Cambisols; soils formed from a weak alteration of the parent material
- Luvisols; soils having an argillic B horizon of medium to high base status
- Acrisols; soils having an argillic B horizon of low base saturation
- Podzols; soils with a strongly bleached horizon having B horizons with iron or humus accumulation or both
- Podzoluvisols; soils having an argillic B horizon but also showing features of podzols (combined podzol and luvisol)
- Nitosols; soils having very deeply developed argillic B horizon features of strong weathering
- Ferrasols; strongly weathered soils consisting mainly of kaolinite, quartz and hydrated oxides
- Histosols; are organic soils
- Lithosols; are shallow soils over hard rock
SOIL PHYSICAL PROPERTIES
Physical properties of soils include texture, structure, density, porosity, consistency, temperature, stability, colour and water content. These are dominant factors affecting the use of the soil. These physical properties are important in that;
Ø They determine the movement of water movement in the soil (hydraulic response)
Ø They determine the availability of oxygen
Ø They facilitate ease of root penetration
Ø They determine the thermo response i.e. how the temperatures respond in different soils in terms of how they influence germination and growth of a plant.
Ø Temperatures influence soil microbial activity
1 .SOIL TEXTURE
-This is the relative proportions of various ultimate soil particle sizes and how they are distributed i.e. the grain size in the soil in terms of sand, clay, silt and gravel.
-The soil particle-size groups (sand, silt and clay) are also called soil separates. If particles are more than 2mm in diameter then it is gravel, if 2-0.2mm its sand, 0.2-0.02mm its fine sand, 0.02-0.002mm is silt and less than 0.002mm in diameter is clay.
Importance of soil texture
-Soil texture will in part determine;
v Water intake rates (absorption)
v Water storage in the soil
v Ease of tilling the soil
v Root zone aeration and will influence soil fertility in general
2. SOIL STRUCTURE
This refers to the relative proportion of sands, silt and clay in soil i.e. how these particles are grouped together into stable collections or aggregates.
-It can also be defined as the arrangement of individual soil particles in respect to each other into a pattern or shape.
-Natural aggregates are called peds while artificial aggregates are called clods
Soil structural classes
-Soil structural units (natural aggregates or peds) are described by three characteristics;
i) Shape –It gives types of Structure
v Granular; small particles that are separate.
v Crumby; porous than granular
v Prismatic; vertical axis is longer than horizontal axis. The tops are flat or pointed
v Blocky; 5-50mm in size. They are in B horizon. Can be angular or sub-angular
v Spheroidal; has a size of 1-10mm. they are found in A horizon and often in granules
v Platy; is 1-10mm; found in all horizons. Horizontal axis is longer than vertical axis
v Columnar; vertical axis is longer than horizontal axis and the tops are curve-smoothened
ii) Classes of structure
These are the ped sizes i.e. very fine, fine, coarse and very coarse.
iii) Structure grades
These are evaluated by the stability or strength of the peds i.e. weak, moderate and strong.
-Structure-less soil is single grained and shows no observable aggregation
-Soil structure influences many important properties of the soil such as the rate of infiltration of water. Both granular and single-grain (structure-less) have rapid infiltration rates. Platy and massive soil conditions have slow infiltration rates
Some processes which cause the formation of soil structure
Ø Freezing and thawing of the soil
Ø Wetting and drying of the soil (swelling and shrinking)
Ø Plant roots and animal activities in the soil
Ø Human activities
3. SOIL STABILITY/STABILIZATION
Once the structure is formed, it needs to be stabilized. Soil stabilizers can be;
Organic matter; this has a temporary or permanent influence on soil stability
Inorganic binders; oxides and hydroxides, carbonates
Adsorbed cations
The stability of structure
-Structural stability relates to the permanent of soil aggregates and is as important to the physical character of the soil as is the initial development forms.
-Generally the degree of stability is a reflection of the chemical and biological state of the soil
Factors influencing the stability of soil structure
· Cations Kind of ions (cations) adsorbed countering the negative charge of clays. Maximum bonding and stability is afforded by ions of high valence, principal among them being Ca2+, Mg2+ or Al3+
-Monovalent ions especially Na+ provides relatively weak bonding forces between particles. Such a structure breaks easily hence erosion. The soil formed is also poorly aerated because of the lack of large, structure-related pore spaces. To improve this condition, Na+ is replaced by Ca+ (puddling: destroying soil structure when one works on a very wet soil leading to a massive state)
· Cementation; the chief cementing inorganic agents are lime, silica and sesquioxides. Aggregates stability is especially desirable in soils of humid tropics; otherwise the heavy rains would results to immense erosion problems
Cementation can also be aided by
· Organic matter such as gums, wax, polysaccharides etc. (cementation can also be enhanced by combined action of organic and inorganic matters). Fungal mycelia can be a binding agent
-Cementing action of the intermediate products of microbial synthesis and decay such as gums certain polysaccharides
-The cementing action of the more resistant stable humus components aided by inorganic compounds such as oxides of iron
NB: Aggregate (soil) stability is not entirely an organic phenomenon but there exists continued interaction between organic and inorganic components such as Ca2+, Mg2+ and Al3+
-Films of clay called ‘clay-skins’ often surround the soil peds and help to provide stability
-The noted stability of aggregates in red and yellow soils of tropical and subtropical areas is due to the hydrated oxides of iron. The larger the aggregates present in any particular soil the lower is their stability.
-Soil stability is of great importance. Some soil aggregates readily succumb to the rains and man’s activities like cultivation. Others resist disintegration thus maintaining suitable soil structure.
4. SOIL DENSITY
-Density is the mass of an object per unit volume. Water is the conventional reference for density measurements;
-Densities of some materials
Material g/cm3
Water 1
Pine wood 0.1
Lose sand 1.6
Quartz mineral 2.6
Common steel 7.7
Lead metal 11.3
Mercury metal 13.5
i) Bulk Density(Pp)
-Bulk density is the weight of a dry soil as it exists naturally in the field. It includes any air space and the organic matter materials. Moisture is not included.
-It is calculated by getting the dry weight of soil divided by the volume of soil
Bulk density (Pp) = dry weight (oven dry soil at 105˚C)
Soil volume
-High bulk density of about 1.6mg/cm3 has low porosity, poorly aerated and compact. Soils with high percentage of loam have bulk density of 1.6mg/cm3, and those with high sand percentage have a bulk density of 1.3mg/cm3. But sandy soils have high bulk density if organic matter content is low.
-The cultivation and management employed on a given soil is likely to influence its bulk density especially of the surface layers.
-Soils with high proportion of pore space have lower bulk densities.
Factors Affecting Bulk Density Of Soil
i) Organic matter content.
High Organic matter content leads to low bulk density because organic matter have a lot of air pores that reduce the density.
ii) Compactness of the soil.
Soil can be compact naturally or can be compacted through human activities. The more the compact, the higher the soil density.
iii) Porosity It is the quantity of the pores spaces. It is due to structure, presence of mesofauna such as ants, earthworm etc. The higher the porosity, the lower the bulk density.
iv) Texture of the soil.
Coarse textured soils have low bulk density.
v) Soil depth.
Bulk density increases with soil depth.
vi) Soil structure
Different soil types exhibit different structures .Certain soil structures are associated with high bulk density eg. Crumby structure implies low bulky density.
ii) Particle Density (Ps)
-This is mass or weight of a unit volume of soil solids only. The pore spaces are not considered. The dominant minerals are quartz, feldspars, micas and clay minerals which average approximately 2.65g/cm3
Ps = Dry weight of solids
Volume of solids
1. POROSITY (f)
Pore spaces (voids) in a soil consist of that portion of the soil not occupied by solids (organic and mineral). It is the volume of the empty space
f = volume of voids
Volume of soil
-The percentage of a given volume of soil occupied by pore space may be calculated from the formula
Ep = % pore space
= 100 - % solid space
= 100 – (Pb/Ps x 100)
= 100 (1- Pb/Ps)
Significance of soil porosity
-In green houses and in nursery beds, water infiltration, retention and drainage are very important. These factors plus costs have transformed much of the container grown plant industry into one using mixtures of ground bark, expanded sericulture, peat and partite.
-These minerals will hold about 15-20% water against drainage.
NB: The primary methods to increase large pores are the use of high proportions of sand and/or organic materials. The advantage of these large pores is that they provide for adequate drainage and aeration.
6. SOIL COLOUR
-Dark soils absorb more heat than lighter soils. Some black coal mining wastes and dark-coloured oil-shale residues reach 65˚C-70˚C which is lethal to life forms.
-Dark coloured humic soils hold more water. This water requires more heat to warm and hence the net result is that many dark coloured soils may not be warmer than adjacent lighter coloured soils.
-Soil colour may indicate the following;
Mineral origin of the soil
Level of aeration, spots (mottles), rust coloured may indicate inadequate aeration
Bluish, greyish and greenish sub-soils (gleying) indicate water-logging and low aeration
Dark colour may indicate presence of organic matter but this differs with climatic conditions
White colours are common with carbonates (lime) and silica
Munsell colour charts
Soil colour determination is standardised and determined by the comparison of the soil colour to Munsell colour charts
-Soil colour notation is divided into three parts
Hue; the dominant spectral or rainbow colour (red, yellow, blue or green)
Value; the relative blackness or whiteness, intensity of a colour or the amount of reflected light
Chroma; purity of the colour, strength or saturation of a colour. It is inversely related to greyness.
7. SOIL WATER
Classification of water relates to plant growth and is classified as;
i). Gravitational (drainage) water; is the portion of soil water that will drain freely from the soil by the force of gravity. It is held at a potential of -1/3 bar.
ii). Plant-available water;/capillary water; is the portion of stored soil water that can be absorbed fast enough by plat roots to sustain life. It has water potential between -1/3 and 15 bars. The water held within theses potentials make up most of the storage water used by plants. Crop plants wilt if only -15 bar water is present because the water loss through transpiration is faster and greater than that amount absorbed by roots at these low moisture potentials.
iii). Permanent wilting point (wilting point); is the percentage of soil water held with water potential less than -15 bars. It is held so strongly that plants are not able to absorb it fast enough for their needs. In contrast to temporary wilting, the wilting point indicates low moisture availability in such conditions wilting plants do not recover except when additional water is added to the soil.
iv). Field capacity; is the percentage of soil moisture that is held with water potential less than -1/3 bar and is a measure of the greatest amount of water that a soil can hold or store under conditions of complete wetting followed by free drainage. Field values are used to determine the amount of irrigation water needed and the amount of stored soil water available to plants
Factors affecting moisture holding capacity of soils
ü Soil texture; large air pores to drain by gravitational flow. Small pores retain water by capillary force. Soils with swelling montmorillonite clays and hold more water than a soil with similar amounts of kaolinite and sesquioxides clays which do not admit water between clay layers. Medium textured soils have the unique combination of pores small enough to hold large amounts of water at high water potentials (held loosely) and relatively small amount of total surface which holds low amounts of water at low water potentials (held tightly in clays). Because of these conditions the largest amount of plant –available water is found in silt loams and other high silt soils
ü Organic matter; the higher the humus content in soil the larger the amount of water the soil can store
ü Soil structure;
-Tensiometers are more useful in measuring moisture in sandy soils than in the fine-textured ones because the matric potentials of most of the plant available water are higher in sandy soils than in clayey soils.
i). Resistance blocks
This is based upon the principle that electrical conductivity decreases with decrease in soil moisture.
-Soluble salts (ions) in water carry electrons which form an electrical current; the more ions present the more electrical current carried. The thicker the water films the more ions between the block electrodes. For use, the conductivity or resistance reading for various moisture contents must be calibrated for each soil that the blocks would be used in.
Low; infiltration of 0.25-1.25 cm/hour. Soils are shallow, high in clay or low in organic matter
Medium; infiltration rates of 1.25-2.5 cm/hour. Soils in this group are loams and silts.
High; rates of infiltration greater than 2.5 cm/hour. These are deep sands, deep well-aggregated silt loams and tropical soils with high porosity.
Unsaturated flow
The flow of water is held with water potential of lower than -1/3 bar. Water will move towards the region of lower potential. In a uniform soil this means that water moves from wetter to drier areas. The rate of flow is greater as the water potential gradient increases and the size of water-filled pores also increases.
-Unsaturated water movement is rapid through fine sand or well-aggregated loams (larger loams) and slower through very fine and poorly aggregated clayey soils (small pore sizes)
NB: Any type of land use that adds organic matter, living or dead results in a soil that is more open, porous and easily penetrated by water. Planting trees, leaving crop residues, adding manure and rotational grazing contribute to more porous soil structure increasing infiltration and reducing runoff.
Moisture availability to plants
Water in plants is absorbed by the following mechanisms;
- Passive absorption; this is the pulling force on soil water by the continuous water column up through the plant cells as water is lost by transpiration.
- Root extension; because the water movement to roots by unsaturated flow occurs only over very short distances of a few millimetres maximum per day, root extension is important in aiding the plant’s absorption of moisture, particularly when the soil root zone holds no gravitational water.
- Active transport; is absorption in which the plant must expend considerable energy. The selective accumulation of soluble ions, which increases a plant’s soluble salt content (osmotic potential) which can lead to active absorption. Active absorption accounts for considerable portion of absorbed water during low water needs.
SOIL CHEMISTRY
1.SOIL AGGREGATES
Nature of soil aggregates
-Soil aggregate refers to a group of soil particles joined together so as to behave mechanically as a unit. Aggregation in soils depends primarily on the cohesive nature of the finer particles and on natural forces that organise and retain them in specific natural units (peds) of definable shape and size.
-Soil aggregates are stabilised by;
Ø Cations involved in countering the negative charge of clays or soil colloids. Principal among these are Ca2+, Mg2+ or Al3+. Maximum bonding and stability is afforded by ions of high valence (Ca2+, Mg2+ or Al3+). Mono-valence ions especially Na+ provides relatively weak bonding between particles. Na+ leads to unstable and easily eroded soil, poor aeration due to lack of large structure related pore spaces.
Ø Cementation; the chief cementing agents are lime, silica and sesquioxides. The latter is very vital more so in tropical regions
Ø Organic matter; the mucilaginous materials or organic colloids bind clay particles in a stable union.
Ø Fungi mycelium
Dispersion
-This refers to the process of breaking up of soil aggregates into small individual component particles and the distribution of such particles.
Factors favouring dispersion of colloids
ü High state of hydration forming suspension
ü Absence of oppositely charged colloids or particles in the same system
ü Mechanical movements
-Dispersion is disadvantageous since it makes the soil system unstable
-Expansion and contraction associated with wetting and drying, freezing and thawing and particularly with gain and loss of material through migration, leaching and primary mineral alteration contribute to the formation of the bulk density profile.
Cementation and de-cementation
-The term cementation is used to the consolidation of soil material by means of cementing materials such as FeCO3, Al2O2 and/or SiO2
-De-cementation is the removal of the cementing agents resulting to desegregation of a material
Other properties of colloids and aggregates
ü Plasticity; soils containing more than 15% clay exhibit plasticity. This is the pliability and capacity to be moulded. The particles easily slide over each other much like planes of glass with films of water between them. Plasticity is exhibited only when soils are moist or wet. Excess water makes the soil to cease to be plastic. Plasticity of the soil is used to determine strength of the soil in engineering work e.g. in buildings or highway bed. It also determines soil structure hence tillage operations. Fine textured soil when it is too wet will limit aeration and drainage e.g. in montmorillonite
ü Cohesion; this refers to attraction of the colloidal particles for each other. This is closely related to plasticity. Montmorillonite and illite exhibit cohesion to a much more degree than does kaolinite or hydrous oxides. Humus by contrast tends to reduce the attraction of individual particles for each other. Adhesion is the attraction of unlike particles.
ü Swelling and shrinkage; silicate clays exhibit swelling and shrinkage properties. Expanding crystal lattice as in the case with montmorillonite, extreme (expanding) swelling may occur upon wetting. Kaolinite and most hydrous oxide with static lattice do not exhibit the phenomenon to any extent, illite and vermiculite are intermediate. After prolonged dry spell soils high in montmorillonite often experience wide deep cracks.
ü Dispersion; this refers to repulsion of particles by each other thus each act independently. This is caused by the colloidal net negative charge.
ü Flocculation; is the coming together of micelles when there is an increased concentration of cations in the system. This stabilises the soil.
2. SOIL COLLOIDAL PROPERTIES
The colloidal state refers to a two phase system in which one phase in a very finely divided state is dispersed through a second phase which is a bit coarse. Examples of colloidal states are milk and cheese, clouds and fog, soil etc.
-In nature colloids are found as emulsions, aerosols or gels.
-Emulsions is where a liquid is dispersed in a liquid e.g. fat globules in water
-Aerosols is where a solid or liquid is dispersed in a gas e.g. smoke (carbon is solid), fog (liquid in a gas)
-Gels is where a solid is dispersed in a liquid
-The most active portions of the soil are those in the colloidal state. There are two types of colloidal matter; organic and inorganic
-Inorganic are almost exclusively of clay minerals of various kinds
-The organic colloids are represented by humus
· SOIL COLLOIDS
THEIR NATURE AND PRACTICAL SIGNIFICANCE
The colloidal state refers to a two phase system in which one material/s in a very finely divided state is dispersed through a second. Examples of colloidal states are milk and cheese, clouds and fog, soil etc.
-The upper limit in size of the mineral colloidal particles is less than 0.001mm or one micron or even 0.5-0.2μ. The maximum size limit of the clay fraction of a soil is considered to be 0.002mm or 2μ thus not all clay is strictly colloidal
-The most active portions of the soil are those in the colloidal state. The two distinct types of colloidal matter are;
Inorganic matter; includes mainly clay minerals
Organic matter; includes mainly humus
These two exist in intimate intermixture
-Colloidal particles regardless of their composition are made up of a complex negative radical called the micelle and the adsorbed cations. Micelles are minute silicate-clay colloidal particles ordinarily carrying negative charges
-Adsorption is the attraction of ions or compounds to the surface of a solid. Soil colloids (negative) adsorb large amounts of ions (cations) and water.
Organic soil colloids-humus
A highly charged anion (micelle) is surrounded by adsorbed cations
Comparison between organic (humus) and inorganic micelles (silicates)
Ø The complex humus micelle is composed basically of carbon, hydrogen and oxygen rather than of Aluminium, silicon and oxygen as are silicate clays
Ø Cation exchange capacity (CEC) of humus per unit weight by far exceeds any of the silicate clays (even montmorillonite)
Ø Humus micelle is not considered crystalline as in silicate but the size of individual particles although extremely variable may be at least as small as montmorillonite
Ø Humus is not as stable as silicate and is thus somewhat more dynamic, being formed and destroyed much more rapidly than clay
Ø Humus is not a specific compound nor does it have a single structural make-up as does silicate
Ø The major sources of negative charge in humus are thought to be partially neutralised carboxylic (-COOH) and Phenolic ( OH) groups associated with central units of varying sizes and complexity
-Carboxyl groups (-COOH-) and phenolic hydroxyl groups ( O-) ( O-) attracted to aromatic rings) are the major source of negative sites for adsorption of cations
NB: -COOH and OH lose (exchange) H+ for other cations as shown above
-Under strongly acid conditions hydrogen is tightly bound and not easily replaceable by other cations. The organic colloid therefore exhibits a low negative charge. With the addition of bases and consequent rise in alkalinity (rise in pH), first hydrogen from the carboxyl groups and then the hydrogen from the phenolic groups –COOH and OH respectively ionises and is replaced by Ca2+, Mg2+, Na+, NH4+ etc.
-On the basis of solubility in acids and alkalis, humus is thought to be made up of three classes;
Fulvic acid humus, soluble in acids and bases
Humic acid humus, soluble in alkali only
Humin, highest in molecular weight and darkest in colour (followed by humic in both properties) is soluble in acids and alkalis
-In humid regions, Ca2+, Al3+ and H+ are by far the most numerous. The order of strength of adsorption of metallic cations when they are present in equivalent quantities is Al>Ca>Mg>K>Na (H+ which is a non metallic cation will rank second in the above order)
4. Inorganic colloids (clays)
Two groups of clays are recognised;
i). Silicate clays; which are common in temperate regions e.g. kaolinite, illite, montmorillonite, vermiculite (micas). It is important to note that some are expanding and others are non-expanding clays in the examples above.
ii). Iron and aluminium hydrous oxide clays (sesquioxides). This class is less dominant in the most developed agricultural regions of the world.
-Allophane and other amorphous minerals are in significant quantities in some soils. They are generally non-crystalline colloidal matter but made-up of sesquioxides and silicate clay components. They therefore may be a separate class of clays in some authorities.
Silicate clays
Shape: they are made up of layers of plates or flakes. Their individual sizes and shapes depend upon their mineralogical organizations and the conditions under which they have developed. Micas are hexagonal e.g. kaolinite, montmorillonite and illite are irregularly plate or flake-like
Surface area: clays have large surface due to their fineness
Electronegative charge and adsorbed cations: the minute silicate-clay colloid particles referred to as micelles (micro-cells) ordinarily carry negative charges. The positively charged ions (cations) are attracted to each colloid crystal. This results to ionic double layer. Inner being
SILICATE CLAYS
ü Shape: they are made up of layers of plates or flakes. Their individual sizes and shapes depend upon their mineralogical organizations and the conditions under which they have developed. Micas are hexagonal e.g. kaolinite, montmorillonite and illite are irregularly plate or flake-like
ü Surface area: clays have large surface due to their fineness
ü Electronegative charge and adsorbed cations: the minute silicate-clay colloid particles referred to as micelles (micro-cells) ordinarily carry negative charges. The positively charged ions (cations) are attracted to each colloid crystal. This results to ionic double layer. Inner being negatively charged and the outer positively charged. The adsorbed cations often coming in hydrated form brings with them water.
ü Adsorption of cations: in humid regions, adsorption of cations in order of their numbers are H+, Al3+, Ca2+, Mg2+, K+ and Na+. For well drained, arid and semi-arid region soils, the order of exchangeable cations is usually Ca2+ > Mg2+ > Na+ > K+ > H+
NB: the exchangeable bases exclude H+ and Al3+ which are acidic
5. FUNDAMENTALS OF SILICATE CLAY STRUCTURE
X-rays, electron microscopy and other techniques have shown that the silicate clay particles are definitely crystalline
Silica tetrahedral and alumina octahedral layers
-Most silicate clays are aluminosilicates i.e. there are both aluminium and silicon components of the clay structure
-One silicon structure surrounded by four oxygen atoms makes up the silicon tetrahedron, so called because of its’ four sided configuration
-One aluminium atom surrounded by six hydroxyls or oxygen(s) is an eight sided structure called octahedron.
-A chain of silica tetrahedron units will form a silica layer while a chain of aluminium octahedron units will form an aluminium layer of clay or sheet of clay (Tetrahedral sheet and octahedral sheet). These two basic layers in different stacking arrangements and combinations provide the fundamental structural units of silicate clays. The layers are bound (together) to each other within the clay crystals by shared oxygen atoms
NB: The silicon in the tetrahedral layer and the aluminium in the octahedral layer can be replaced by ions of comparable size but of lower charge. Silicon with four charges Si4+ is replaced by trivalent aluminium (Al3+) by a process called isomorphous substitution. Other atoms of higher valence can also be substituted by those of lower valence.
-Isomorphous substitution of three-valence aluminium for four-valence silicon is responsible for the net negative charge in an otherwise neutral silicate layer.
-The ionic radius will determine substitution as the two ions should be almost of the same size
Mineralogical organization of silicate clays
Silicate clays differ in their properties depending on the number and arrangement of the tetrahedral (silica) and octahedral (alumina) layers contained in the crystal units. These layers are represented in ratio of each other.
i). 1:1 CLAYS/MINERALS CRYSTAL LATTICE eg. KAOLINITE
-The units are made up of one silica (tetrahedral) layer alternating with one alumina (octahedral) layer. Oxygen atoms are shared by the silicon and aluminium atoms in their respective layers. Hydrogen in hydroxyls of alumina sheet form hydrogen bonding between the two sheets. This tight bonding is so rigid that the lattice can not expand when wet. The cations and water can not penetrate between the structural units of the micelle.
-Due to this bonding, contrary to other clays, plasticity, cohesion, shrinkage and swelling properties are very low in kaolinite clay
OH OH OH OH OH OH OH
Al Al Al Al Al
O O OH O O O OH
Si Si Si Si
O O OO O O O OO O O
ii). 2:1 SILICATE CLAYS (EXPANDING CLAYS egMONTMORILLONITE AND VERMICULITE )
-The crystal units of these minerals are characterised by alumina (octahedral) sheet sandwiched between two silica (tetrahedral) layers e.g. montmorillonite and vermiculite.
-the crystal units are loosely held together by very weak oxygen to oxygen linkages. Molecules of water and cations are attracted between crystal units causing expansion of the crystal lattice.
-Isomorphous substitution of magnesium for the aluminium in the octahedral sheet and to a lesser extent substitution of aluminium for silicon in the tetrahedral sheet leaves montmorillonite crystals with a high net negative charge hence attracts H+, Al3+, Ca2+, K+ etc. this makes montmorillonite to have high cation adsorption capacity (perhaps 10-15 times that of kaolinite)
-The clay is signified by plasticity, highly cohesive and marked shrinkage on drying. The montmorillonite clays dry to very hard clods, making tillage difficult. It is also very sticky when wet.
NB: Vermiculite has 2:1 crystal lattice but there is octahedral layer is dominated by magnesium rather than aluminium. Three magnesium being in place of two aluminium atoms (thus the charges are equal), this is due to isomorphous substitution.
-In the tetrahedral layer of vermiculites there is considerable substitution of aluminium for silicon. This often results to net negative charge in vermiculite.
-Vermiculite has limited expansion when wet.
-Vermiculite has the highest cation adsorption capacity over all other silicate clays including montmorillonite. This is due to the net negative charge in the tetrahedral layer.
2:1 CLAY NON-EXPANDING MINERALS (eg ILLITE)
-These are the hydrous micas. Illite is the most common and important in soils.
-About 15% of the tetrahedral silicon sites are occupied by aluminium atoms. This results to a high net negative charge in the layer. K+ are attracted to satisfy the negative charges
2: 2 CLAYS MINERALS
This group is represented by chlorites. These are basically silicates of magnesium with iron and aluminium present.
-Magnesium dominates the octahedral layer
-The crystal unit contains two silica tetrahedral sheets and two magnesium (replaces aluminium) octahedral sheets, hence 2:2 clay.
-It is non-expanding clay
NB: the specific clay groups do not occur independent of each other in any soil but occur in an intimate mixture. Hence there are chlorite-illite, illite-montmorillonite etc.
2:1:1 MINERAL CLAYS
This group is similar to 2:2 clay minerals. With 2:1:1 clay minerals, two silica tetrahedral sheets are attached to one alumina sheet and a magnesium sheet hence 2:1:1 crystal lattice.
=the genesis of silicate clays or the sources are minerals such as feldspars, amphiboles and pyroxenes
5. CATION EXCHANGE CAPACITY (CEC)
This is the amount of exchangeable cations per unit weight of dry soil.
-It can be seen as the number of cation adsorption sites per unit weight of soil or sum total of exchangeable cations adsorbed expressed in milli-equivalents per 100grams of oven-dry soil
-An equivalent (quantity) weight is that quantity that is chemically equal (equal charges) to 1gm of hydrogen. The number of hydrogen atoms in an equivalent weight is the Avogadro’s number (6.03 x 1023) i.e. hydrogen which has a valence of one its 1gm has 6.02 x 1023 which is one equivalent.
One milli-equivalent = equivalent/1000 (for hydrogen = 1gm/1000=0.001gm)
-An equivalent is the weight of a substance that will replace or be displaced by one gram of hydrogen.
-CEC can be expressed in terms of milli-equivalents (M.e/100gm) or cent-moles (cmol/kg)
One milli-equivalent per 100gm of soil = 1cmol of positive or negative charge/kg of soil
But the mass number is used in the calculation of these equivalents
Therefore one mole of hydrogen will give 1gm while one mole of calcium will give 40gms
1milli-equivalent of hydrogen = 0.001gm
1milli-equivalent of calcium= 40/2 x 1000 = 0.02g (since calcium is a bi-charge we divide by two). This means that 0.001g of H+ will exchange with 0.002g of Ca2+ in 100gm soil.
Factors affecting the cation exchange capacity
ü Fineness of soil particles; fine textured soils tend to have higher CEC than sandy soils
ü Organic matter content
ü The type(s) of dominant clay types 1:1, 2:1 etc.
ü High pH increases CEC.
6. PERCENTAGE BASE SATURATION OF SOILS
-Hydrogen and aluminium tend to dominate acid soils, both contributing to the concentration of hydrogen ions in the soil solution.
-Adsorbed hydrogen contributes directly to the H+ concentration in the soil solution. Al3+ contributes indirectly through hydrolysis as illustrated below;
Al3+ + H2O Al(OH)2+ + H+
Al(OH)2+ + H2O Al(OH)2+ + H+
Al(OH)2+ + H2O Al(OH)3 + H
-Most of the other cations called exchangeable bases neutralises soil acidity
-Percentage base saturation is the portion of the cation exchange capacity occupied by exchangeable bases. The higher the percentage base saturation in the soil the higher the pH.
-Exchangeable bases are calcium, magnesium, zinc, sodium and potassium.
-Exchange acidity is the titratable hydrogen and aluminium that can be replaced from the adsorption complex by a neutral salt solution
-Zeta potential refers to the general net charge (negative or positive)
-The total cation exchangeable capacity of the soil is the total of exchange sites of both the organic and mineral colloids. The CEC then would be the total number of milli-equivalents of cations that the soil is capable of adsorbing.
-Exchangeable cations form hydroxyls as follows;
[Micelle]K + H2O [Micelle]H + K + OH
In solution
% base saturation = sum of exchangeable cations (Na, K, Mg, Ca) x 100
CEC
Some average CEC of soils
a) Clay minerals
· Hydrous oxides 4 M.e/100g of soil
· Kaolinite 10 ,,
· Illite 40 ,,
· Montmorillonite 100 ,,
b) Organic matter 200 ,,
c) Sandy soil >5 ,,
-Fine sand loams 5-10 ,,
d) Clay loams 15-30 ,,
Example
An experiment on determination of exchangeable bases revealed that at a depth of about 20cm a soil contained 12.3 M.e of calcium/100gm of soil, 1.8 M.e of Mg/100gm of soil and 10 M.e of K/100gm of soil.
Assuming that one hectare to plough to a depth of 20cm has a soil weighing 2,000,000 kg.
a) Calculate the amount of calcium, magnesium and potassium contained in the soil in kg/hectare
b) If the minimum calcium supply for optimum crop growth is 5200kg Ca/ha, comment.
Solution
a) Calcium has a relative mass of 40 and has a valence of two;
Therefore its equivalent is 40/2 = 20g
Its milli-equivalent = 20/1000 =0.02g
-The weight of soil 1ha is 2,000,000kgs
If 100g of the soil gives 0.02g of Calcium what about 2,000,000kg of the same soil? (Change the units to be in kg values)
0.1kg = 0.00002kg Ca
2,000,000kg = 0.00002 x 2,000,000 = 400kgs Ca
0.1
But there are 12.3 M.e Ca/100g of soil
400 x 12.3 = 4920kg Ca/ha
b) More calcium should be supplied (5200-4920) =280kg calcium per hectare.
Assignment: Calculate the weight of magnesium and potassium.
ANION EXCHANGE CAPACITY (for negatively charged ions)
-The word negative charge implies that positively sites are also present although in lesser numbers. Certain soil colloids have a net positive charge and attract exchangeable anions. Such soils exhibit anion exchange instead of cation exchange.
-Soluble anions such as nitrates, chlorides and sulphates are held exchanged on positively charged surfaces where the cations are repelled and remain in the solution. H2PO4- is also attracted and held more tightly on the surfaces of iron, aluminium and calcium bearing minerals by mechanism that operates in either positive or negatively charged soils.
-Soils that show more positive surface charges are characteristically acidic and their colloidal component is high in kaolinite, iron and aluminium oxides and hydroxides but low in humus and expanding silicate clays. They are undesirable for crop production because they contain enough aluminium or manganese to be toxic to plants. Many of these soils have the ability to fix phosphorous in a form that is not available to plants. Certain tropical acid soils contain colloids with a net positive charge.
-From economic point of view, positive soils may be better than negative soils in that nitrates are not easily leached.
Example 1
Suppose 20g of soil was extracted with 200ml KCl solution and the concentration of NH4+ in the filtrate was found to be 270ppm. Calculate the CEC of the soil in both m.e./100g of soil or cmol/kg of soil.
Solution
1cmol/kg soil = 1m.e./100g soil
Ppm – parts per million (mg/litre)
1000000mg = 1000g = 1kg
1ppm = 1part/1000000 parts, 270 is amount of mg/litre
1000ml = 1litre thus in 1000ml there is 270mg NH4+
We extracted 20g soil with 200ml KCl
1000ml = 270mg
200ml = 270 x 200 =
1000
To convert to milliequivalent/100g of soil divide by equivalent weight of NH4+
Equivalent weight of NH4+ = molar weight of NH4+
Valence of NH4+
=14+4 = 18 = 270 x 200 x 1 m.e of NH4+
1 1000 18
For 100g soil = 270 x 200 x 1 x 100 =15m.e/100g or 15cmol/kg of soil
1000 18 20
Example 2
Using NH4+, the following ion concentrations were determined for 20g of soil in 200ml.
Ca2+ 100ppm
Mg2+ 30ppm
K+ 78ppm
Na+ 23ppm
Determine
i) Concentration of ions in m.e/100g
ii) Percentage base saturation if CEC is 15m.e/100g
Solution
Ca2+ = 100ppm implies that 1000ml of solution contains 100mg of Ca2+
200ml = 200 x 100
1000
Equivalent weight of Ca2+ = 40/2 = 20
M.e of Ca2+ in 20g soil = 200 x 100 x 1
1000 20
100g soil = 200 x 100 x 1 x 100 = 5m.e Ca2+/100g
1000 20 20
Calculate for Mg2+ (2.5m.e/100g), K+ (2m.e/100g) and Na2+ (1m.e/100g)
ii) Total bases = 5+2.5+2+1 =10.5m.e/100g soil
% base saturation = Total bases x 100 = 10.5/15 x 100 = 70%
CEC
Example 3
The cation exchange capacity of a given soil is made of 0.06g Mg2+, 0.054g NH4+, 0.078g K+, 0.08g Ca2+ and 0.08g H+
a) Express the CEC In m.e/100g soil
b) Calculate total exchangeable cations in m.e/100g of soil (CEC)
c) Calculate the percentage base saturation
d) Calculate exchangeable acidity in m.e/100g of soil
Solution
Change the grams to mg
0.06 x 1000mg Mg2+ = 60mg
Equivalent of Mg2+ = 24/2
M.e/100g = 60/12 = 5m.e/100g of soil
K+ = 2m.e/100g
Ca2+ = 4m.e/100g
H+ = 8m.e/100g
b) Total cations = 5+3+2+4+8 = 22m.e/100g
c) Percentage base saturation = total exchangeable cations x 100
CEC
Total bases = 5+3+2+4 = 14
= 14/22 x 100 = 63.64%
d) Total exchangeable acidity
8/22 x 100 =
6. IONS IN THE SOIL SOLUTION
Forms of elements used by plants
-There are two general sources of readily available nutrients in the soil
ü Nutrients adsorbed on the colloids
ü Salts in the soil solution
-In both cases the essential elements are present as ions
-Cations are mostly adsorbed by colloids where as the negatively charged ions (anions) are found in the soil solution.
-The most important ions present in the soil solution or in the soil colloids are;
Ø Nitrogen NH4+, NO2-, NO3-
Ø Phosphorous HPO42-, H2PO4-
Ø Potassium K+
Ø Iron Fe2+, Fe3+
Ø Calcium Ca2+
Ø Magnesium Mg2+
Ø Sulphur SO32-, SO42-
Ø Zinc Zn2+
Ø Chlorine Cl-
Ø Boron BO33-
Ø Copper Cu+, Cu2+
Ø Manganese Mn2+, Mn4+
Ø Molybdenum MoO42-
Ø Water H+, OH-
-Nutrient availability is markedly affected by root exudates and by microbial activity in the vicinity of the roots (the rhizosphre)
7. SOIL BUFFER CAPACITY
This is the ability of the soil to resist change in pH.
-The buffering capacity depends on percentage of clay and organic matter. The higher the clay/organic matter the higher the buffering capacity
- The CEC gives the soil most of its buffering capacity
-The colloids contain basic and acidic cations adsorbed on their surfaces of clay and organic colloids
Adsorbed H+, Al3+ soil solution active acidity leads to increase in basic ions
-This will cause equilibrium to shift to the right. The bases will neutralise acidity leading to more ions to move to the soil solution from adsorbed surfaces. There will be little change in pH until enough calcium ions has been added to deplete reserve acidity.
Soil buffering capacity
1.)The soil buffering capacity refers to ability of the soil to supply ions to the soil solution from the adsorptive complex
2.)This includes when plants nutrients are utilized by plants or lost, the ions will adsorb from the exchange site to replace them
3.)The buffering capacity is about the ratio of adsorbed ions to the ions in the soil solution
4.)The soil buffering capacity increases with increasing organic matter and CEC
5.)The soils with fine texture have higher buffering capacity than the coarse textured soils
6.)Addition of fertilizers to the soils increases ability of the soils to buffer against nutrient deficient due to plant uptake or leaching
7.)The plants nutrients availability and supply increases with the soil buffering capacity
8.)Ability of the soil to resist change in PH
9.)The buffering capacity also assists the soils to resist change in soil PH
8. SOIL ACIDITY
The presence of Al3+ and H+ are responsible for soil acidity. The ions can be in soil solution or adsorbed on soil colloids.
-Total soil acidity is the total quantity of H+ that may be produced in soil when equilibrium is continually shifted by introduction of hydroxyl (OH-) ions to end point
-Total acidity consists of active acidity and reserve acidity
-Residual acidity is the acidity that remains when active and reserve acidity are removed
Reserve acidity Active acidity
H+, Al3+ (colloids) replaces
Adsorbed on ion colloids ions of Al3+, H+ in solution
-Active acidity arises due to the presence of Al3+ and H+ in solution
-Reserve acidity (exchangeable) arises due to Al3+ and H+ adsorbed on soil colloids
Sources of soil acidity
i). Hydration of aluminium ions
Al3+[micelle] Al3+ + [micelle]
Adsorbed Al3+ soil solution
-Aluminium is hydrolysed by combining with water to form aluminium hydroxyl ions and aluminium hydroxide
Al3+ + H2O Al(OH)2+ + H+
Al(OH)2+ + H2O Al(OH)2+ + H+
Al(OH)2+ + H2O Al(OH)3 + H+
H+ + H20 H30+
-The Al3+ and H+ are able to generate acidity that can make soil strongly acidic (pH 3.8-5.2)
ii). Microbial acid production
Nitrification of organic matter or ammonium ions applied through fertilizers by bacteria.
-Nitrification is the process of enzymatic oxidation by bacteria. Autotrophic bacteria obtain energy by these chemical reactions
NH4+ + 3O2 2NO- + 2H2O + 4H+ + Energy
Nitrosomonas
The hydrogen ions formed during this reaction will increase soil acidity
-Use of certain fertilizers will lead to increased acidity e.g. ammonium sulphate, urea and DAP which have acidic residues in the soil
iii). Kind of soil forming materials
Materials like pyrite can undergo weathering by oxidation to form sulphuric acid
4FeS2 + 5O2 + H20 Fe2(SO4)3 + 2H2SO4 (pH <3.8)
Pyrite
-Soils formed from silica become acidic under humid conditions (argillic clay)
-Other materials include acidic igneous rocks. The materials can affect acidity and intensity of the acid
iv). Decomposition of organic matter
The acidity can arise from decomposition and composition of leaves and plant parts (plants like pine, oak are acidic). Micro-organisms under favourable conditions act on organic matter during decomposition giving rise to organic and inorganic acids (H2SO4, HCl, HNO3 and –COOH which are moderately acidic)
-Strong organic acids are formed by processes like podzolizations (humus, coniferous vegetations). Fungi aid in decomposition of organic matter
v). Ionization of carboxylic groups of organic matter
Organic matter contains soil colloids that have carboxylic groups (COOH) and phenolic groups on central unit of humus
O O
R-C-OH R-C-O- + H+ (pH 5.2-6.5)
-The ionization of carboxylic groups and phenolic groups yields H+ (acidity)
vi). Formation and ionization of carboxylic acid
-Carboxylic acid is formed from reaction between water and carbon dioxide
H2O + CO2 H2CO3
Carbonic acid ionizes to produce H+
H2CO3 CO32- + 2H+
-The acid is weak but can contribute to soil acidity directly and indirectly. H+ can lead to solution of bases and leaching. This acidity from carbonic acid however account for low pH values.
NB: Processes like leaching/high rainfall contribute increased soil acidity. Bases are removed through leaching. The percolating water removes basic elements from the soil leaving behind Al3+, H+ thus low base saturation.
9. SOIL REACTION/SOIL PH
This refers to the degree of soil acidity, alkalinity or neutrality. The soil can be classified as acid, neutral or alkaline depending on amount of H+ or OH-. This can be measured by pH.
-pH is the negative logarithm of the hydrogen ion concentration in solution
pH = -Log [1/H+] log [H+]
-Hydrogen ions are concentrated at near colloidal surfaces and become less numerous on the outer portions of water films
OH- vary in numbers inversely with H+
i.e. at pH 7 OH- = H+ = 10-7
-pH ranges for soils is between 3.5 and 10. Humid regions have soils with pH ranges of 5-6 while arid soils have pH of 6-9.
-Soil colloids are involved in controlling soil reaction e.g. the relative proportion of Al3+, H+ and exchangeable bases indicate percent base saturation
-The low PBS indicates acidity while high PBS (>80-100) indicates alkalinity or near neutral. This depends on different micelles/colloids. Organic colloids have strong acid site thus low pH, low PBS (ionization is rapid with pH 4.5-5.0
-Iron and aluminium hydrous oxides have low dissociates of adsorbed H+ thus high pH of 6-7
-Silicate clays are an intermediate between organic colloids and aluminium iron hydroxides with pH of 5.2-5.8.
-Type of adsorbed bases influence the pH of the soil, sodium saturated soils have higher pH than those accumulated by calcium and magnesium. The bases can arise from weathering (releases cations from minerals making them available in solution-adsorption), addition of fertilizers/lime, through irrigation water. Increases in exchangeable bases contribute to high pH values. The bases contribute to increases in or high pH values by generating hydroxides
Ca[micelle] + H2O 2H+[micelle] + Ca2+OH-
-The increase in alkalinity is favourable by conditions like arid and semi-arid, low precipitation (less leaching), weathering bases which replace Al3+, H+ fr4om the soil solution.
Importance of soil pH on plant nutrition
- pH influences plant nutrient absorption and growth. It affects solubility, exchangeability of nutrients to plants for absorption
- It affects availability of plant nutrients. Most plant nutrients are available at pH of 6.0-7.0. phosphorous is available at pH of 6-7 but become fixed acidic Ph
H2PO4
Al(OH)3 + 2H2PO4- Al-H2PO4 + 2OH-
OH
Other nutrients whose availability is affected by pH include;
Nitrogen is available at pH above 5.5
Calcium and magnesium are available at 6-8.5
Potassium and molybdenum at 6-9
Boron at 5-7
- At pH below 5.5; aluminium, iron, manganese, zinc, cobalt and copper are found in quantities that are toxic to plants. These elements are found in high concentration. At low pH soils are also poor in bases like calcium, magnesium, potassium due to leaching
- At very high pH bicarbonates (HCO3-) are present in high concentration that can interfere with uptake of other ions. This is detrimental to plants growth. At high pH phosphates are also precipitated thus insoluble.
- Low pH (below 5.5) will also affect soil micro-organisms like bacteria and actinomycetes
Measurement/determination of soil pH
- Electrometric method- makes use of standard glass electrode with pH meter where H+ is determined. This method is accurate.
- Dye method- it involves use of indicators and accompanying colour chart. The method is less accurate.
RIFT VALLEY INSTITUTE OF SCIENCE AND TECHNOLOGY
SOIL FERTILITY AND PLANT NUTRITION(MODULE2)
DILOMA IN GENERAL AGRICULTURE
Keya Evusa Kennedy
SOIL FERTILITY AND PLANT NUTRITION
TERMINOLOGIES
- An element: is a fundamental substance that consists of atoms of one type which are defined by the number and arrangement of electrons in the orbital.
- A mineral: is an inorganic substance usually made up of one or more elements that is formed at a given at a given temperature and pressure condition and it is characterized by unique set of chemical and physical properties.
- Soil productivity: this is the capacity of the soil to produce a certain amount of crop yield per unit area of land
- Soil fertility: Is the ability of soil to provide essential plant nutrients in adequate and balanced amount to support the set level of crop yield.
- Nutrient: is the element or a chemical compound required by an organism (plants) for metabolism and growth.
- Deficiency :Is the condition where an element is in low quantity in plant tissue such that the plant shows certain characteristic symptoms that leads to observable or measurable features
- Toxic element: is an element of which when taken up by plant in excess will impair the normal performance of the plant e.g. Al, Mn
- Beneficial element: Are mineral elements which either stimulate growth but are not essential or might not be essential to plant under certain conditions.
PLANT NUTRITION
-This is the process of absorption and utilization of essential elements for plant growth and reproduction.
-Plants need 17 elements for plant growth as listed below;
|
ELEMENT
|
% IN PLANT (alfalfa)
|
IONIC FORM AVAILABLE TO PLANTS
|
|
Carbon
|
41.2
|
CO2
|
|
Oxygen
|
46.3
|
CO2, O2-, OH- CO32-
|
|
Hydrogen
|
5.6
|
HOH, H+
|
|
Nitrogen
|
3.3
|
NH4+, NO3-
|
|
Calcium
|
2.1
|
Ca2+
|
|
Potassium
|
0.80
|
K+
|
|
Magnesium
|
0.42
|
Mg2+
|
|
Phosphorous
|
0.30
|
H2PO4-, HPO4-
|
|
Sulphur
|
0.085
|
SO42-, HSO4-
|
|
Chlorine
|
0.011
|
Cl-
|
|
Iron
|
0.0066
|
Fe2+, Fe3+
|
|
Boron
|
0.0045
|
H3BO3, H2BO3-
|
|
Manganese
|
0.0036
|
Mn2+
|
|
Zinc
|
0.0009
|
Zn2+
|
|
Copper
|
0.0007
|
Cu2+, Cu(OH)+
|
|
Molybdenum
|
0.000005
|
MoO42-
|
|
Cobalt
|
-
|
Co2+
|
Nutrient utilization by plants
Deficiency Optimal Toxicity
Milddeficiency
Mid toxicity
Extreme deficiency Extreme toxicity
Amount of nutrients
-Carbon, oxygen and hydrogen are mostly obtained from the air and water, while the rest from the soils.
-Nitrogen, phosphorous, potassium, calcium, magnesium and sulphur are required in large quantities and are called macronutrients. N, P, K are the primary nutrients and Ca, Mg and S are secondary nutrients. Primary nutrients can not be substituted with any other element.
-The rest are required in relatively low amounts and are called micronutrients (trace elements). Micronutrients are never deficient in many soils
-Plants are dependent on a favorable combination of some six environmental factors namely light, mechanical support, heat, air, water and nutrients. Soils can supply all of them with the exception of light. The condition that is least optimal will limit plants growth hence the law of the minimum which states that ‘the level of plant production can be no greater than that allowed by the most limiting of the essential plant growth factors’
Macronutrient cations availability
These cations are found in the soil in three forms
· Within the solid framework. In this, large quantities of silicon, oxygen, aluminium and iron are found in the case of inorganic materials, while carbon, hydrogen and oxygen in the case of organic materials. Essential macronutrients are also dominant but not readily available
· Associated assemblage of cations (adsorbed). These are held on the surface of clay and humus particles by negative charges that characterise these colloids. Theses cations are in small amounts compared with solid framework but they are crucial chemical properties of soils in supplying nutrients
· Cations in soil solution (soil water). These are readily available for plants and they also leach easily. Macronutrient cations are in smaller amount compared to the two above.
-Solid framework releases nutrients slowly over along period of time after weathering, the released nutrients now occur in associated assemblage. The later then move to soil solution upon exposure to water
NB: The above reaction can be reversible, thus minimising loss of nutrients through leaching. Adsorbed cations can exchange for cations in the solution a process called cation exchange e.g. exchange of H+ for Ca2+
Ca[colloid] + 2H2CO3 2H[colloid] + Ca(HCO3)2
The above exchange of H+ for Ca2+ and vice versa is of great practical significance in maintaining acid-base balance of soils. Colloid-sized particles are often negatively charged on their surfaces that are able to attract and hold (adsorb) the cations
Macronutrient anions availability
-Nitrogen, sulphur and phosphorous are often adsorbed in the form of anions (NO3-, HSO4- and H2PO4-), these are mainly sourced from the soil organic matter. Nitrogen, sulphur and phosphorous are critical components of organic compounds such as proteins, amino acids and nucleic acids. These compounds must be transformed to simple inorganic ions before they can be absorbed by plants. Biochemical reactions utilising enzymes and micro-organisms are involved
R-NH2 NH4+ NO2- NO3-
R-S HS- HSO3 HSO4-
Organic form [inorganic forms in solution]
-This process of simplification is called mineralization and the reverse whereby the inorganic forms are converted to organic compounds is called immobilization. Both processes are accomplished by micro-organisms
Mechanisms of nutrients uptake
Prior to the absorption into root cells, nutrients reach the surface of roots by three mechanisms
i). Mass flow; this is the movement of plant nutrients in the flowing soil solution. (water). It is the most important as it delivers the largest amount of nutrients of about 80%
ii). Diffusion; is the movement by normal dispersion of the nutrient from a higher concentration through the soil water to areas of lower concentration of that nutrient
iii). Root interception; this is the extension of root into new soil areas with untapped supplies of nutrients in the soil solution
The mechanisms of absorption into the root cells are;
i). Carrier transport (active process); for a nutrient to cross a cell membrane into the cell it is believed that each ion must be attached to a carrier. The carrier-nutrient complex can pass through the membrane into the cell. The carriers utilize energy and thus enable transportation of nutrients against concentration gradient
ii). Facilitated diffusion (active); as cations are absorbed, H+ are excreted into the soil solution or more organic acid anions are produced inside the cell to balance the absorbed cations. Likewise as nutrient anions are absorbed by the plant more compensating cations are absorbed and or HCO3- are excreted into the soil solution in order to maintain the electron balance in the cell
iii). Simple diffusion (passive process); plants also absorb nutrients through the stomata e.g. carbon in form of carbon dioxide and hydrogen in form of water but in small amounts
iv). Transpiration stream;
Factors affecting nutrient availability/uptake
The factors that affect metabolism and thereby the availability of respiratory energy will directly affect nutrient uptake. These include;
ü Temperature in the soil which affects the microbial activities
ü Oxygen supply which is usually altered by management practices like compaction which affects the amount of oxygen reaching the roots
ü Concentration of the nutrient; the higher the concentration the higher the likelihood of more uptake up to a given level
ü The moisture/water content; this influences the rate of nutrient movement
ü The density and distribution of roots; phosphorous uptake is favoured by root proliferation
ü Soil reactions (pH)
ü The source of the nutrient; the strength to which the nutrient is held (availability)
Antagonism
Some enzymatic and other biochemical reactions requiring a given micronutrient may be affected by the presence of a second trace element in toxic quantities e.g.
i). Excess copper or sulphate may adversely affect the utilization uptake of molybdenum
ii). Iron deficiency is encouraged by excess zinc, manganese, copper or molybdenum
iii). Excess phosphate may encourage a deficiency of zinc, iron or copper but enhances the absorption of molybdenum
iv). Heavy nitrogen intensifies copper and zinc deficiencies
v). Excess sodium or potassium may adversely affect manganese uptake
vi). Excess lime reduces boron uptake
vii). Excess iron, zinc or copper may reduce the absorption of manganese
-Some antagonistic effects may be utilised effectively in reducing toxicities of certain micronutrients
NB: Phosphate fertilizer when applied together with nitrogen enhances uptake of phosphorous than when the phosphate fertilizer is applied alone
MACRONUTRIENTS
These are needed by plants in large quantities for plant growth and development. They cannot be substituted by any other element.
-They are comprised of nitrogen, phosphorous, potassium called primary nutrients. Secondary nutrients are calcium, magnesium and sulphur. All these are found in the soil. From the air and water plants utilise hydrogen, oxygen and carbon
Roles of the essential mineral elements in plants
-Nutrient cations are absorbed by plants either from the soil solution or from the surface of clay and humus particles when they have been adsorbed
-The criteria for identifying essential nutrients is;
- A deficiency of the element makes impossible for a plant to complete reproductive or vegetative stage of its life
- The deficiency symptoms of the element in question can be corrected or prevented only by supplying that specific element. It cant be substituted
- The element is directly involved in the nutrition of the plant
-The elements which satisfy the first two criteria are referred to as beneficial elements. Beneficial elements do stimulate growth in some plants but actually may not be essential
NITROGEN
This is one of the major nutrients in the soil. It can be sourced from organic material which forms 90-98% nitrogen in the soil but it is not readily available due to poor decomposition.
-Another source of nitrogen is the inorganic forms which accounts for 1-2% and it is the most available. It occurs as nitrates (NO3-) or ammonium ions (NH4+) in the soil. The nitrates are easily leached since they are replaced by the negatively charged soil colloids. Also conversion by micro-organisms;
NO3- NO(g) this escapes to the atmosphere
High pH leads to conversion of ammonium ions to ammonia gas in the process called nitrification
NH4+ + 1½O2 Nitrosomonas NO2- + 2H+ + H2O + Energy
NO2- + ½O2 Nitrobacter NO3- + Energy
Nitrogen fixation occurs biologically to form ammonia
N2 NH3 NH4+
Functions of nitrogen
Ø Essential constituent of proteins,Nucleotides, enzymes and coenzymes
Ø Component of chlorophyll molecule
Ø Promotes vegetative growth
Ø Improves quality of leafy crops
Ø Increases the size of grains and their protein content
Ø Essential for carbohydrate utilization by vegetative plants
Ø Control developmental and hereditary processes (DNA nucleo protein)
Ø Governs the utilization of K,P,and other nutrients
Deficiency symptoms
Ø Stunted growth
Ø Chlorosis starting with old lower leaves
Ø Enhanced senescence of the older leaves (dropping of leaves)
Ø Reduced flower count
Ø Low protein content
Ø Premature ripening of crops
-Excess nitrogen leads to delayed maturity, low disease resistance e.g. rice blast, high succulence, lodging, non-broken down nitrogen in vegetables
PHOSPHOROUS
It occurs in organic form (80%) in the soil and inorganic form
-The inorganic forms that are readily available are H2PO4- and HPO2-. H2PO42- is more available since it has more carriers but pH is the main determinant in absorption
Functions of phosphorous
Ø Constituent of ATP
Ø Constituent of proteins
Ø Stimulates flowering
Ø Root development hence quick maturity of the crops
Deficiency symptoms
Ø Stunted growth hence delayed maturity
Ø Dark-green colouration in leaves due to the small-sized leaf against high chlorophyll content
Ø Purpling of leaves and stems e.g. in maize
POTASSIUM
This is found in mica, vermiculite, feldspars but its availability depends on these mineral rocks. It is absorbed in the form of K+
Functions of potassium
Ø Regulation of photosynthesis, translocation of photosynthetic materials and protein synthesis
Ø Strengthens stem and root development (cell extension)
Ø Stomatal control (opening and closing)
Deficiency symptoms
Ø Chlorosis along the leaf margin and premature ripening of fruits
Ø The tip becomes necrotic mostly in the older leaves
Ø Weak stalks e.g. maize, wheat and barley
Ø Small fruits and shrivelled seeds
CALCIUM
It occurs in the soil bound in feldspars, dolomite, hornblende, apatite, calcium sulphate and calcium carbonate. Vertisols have high calcium content
-Calcium is absorbed inform of Ca2+ and is quite available. It has low mobility in plants hence highly deficient in growing tips
Functions of calcium
ü Cell wall stabilization
ü Involved in mitosis
ü Controls the pH and enzymatic activations
Deficiency symptoms
ü Distorted leaf shape
ü Small-sized leaves
ü Impaired terminal ends (buds) due to low mobility hence supply
ü Weak structures and poor root growth
MAGNESIUM
It is absorbed as Mg2+. The K+, NH4+, Mn2+ and H+ hamper the uptake of Mg2+
Functions of magnesium
ü Activates enzymes
ü Constituent of chlorophyll
ü Regulates pH in the cells better than Ca2+ due to its high mobility
Deficiency symptoms
ü Inter-veinal chlorosis and also veins in extreme conditions occurs as spots
ü Weak twigs
SULPHUR
Itis taken by plants as sulphate (SO42-) mainly from the soil or as sulphur dioxide through the leaves
-Sulphates are reduced in water-logged soils to hydrogen sulphide gas (H2S) and the element sulphur. Sulphur reacts similarly to nitrogen
Functions of sulphur
ü Protein synthesis e.g. cystine and methionine
ü Vitamin formation
ü Stabilization of protein through the sulphur-hydrogen bonding
Deficiency symptoms
ü Chlorosis evenly distributed but mostly in upper younger leaves due to low mobility
ü Restricted shoot growth
ü Stems become woody, stiff and small in diameter
FUNCTIONS OF MICRONUTRIENTS IN HIGHER PLANTS
-BORON
It is believed to be important in sugar translocation
-It is important in component of enzymes and synthesis of nucleic acids
-It occurs in the soil as borate (oxides of boron) and as borosilicate
-It is taken up by plants as H3BO3 (boric acid)
-Its deficiency leads to inter-veinal chlorosis in younger leaves due to its low mobility. The leaves become bleached in severe deficiency
MOLYBDENUM
It occurs in the soil as sulphides and molybdate (oxides of molybdenum)
-It is taken up by plants as MoO42- or HMoO4-. The higher the pH the higher the availability of molybdate
-It is associated with nitrogen fixation by nitrogenase
-Deficiency of molybdenum leads to inter-veinal chlorosis in the lower leaves in legumes
CHLORINE
It ever available in large amounts in most soils
-It occurs as chlorides (mainly NaCl) in dry soils and is absorbed as Cl-
-Chlorine is involved in cell expansion and enzyme activation
-It influences water holding capacity of plant tissues
-It balances the positively charged ions e.g. K+ in the cells
-Deficiency is most not common but wilting of leaflet tips followed by necrosis may ensure in case of deficiency
ZINC
-It is found in the form of oxides, sulphides and silicates. It is taken in the form of Zn2+. The pH very much affects the uptake of zinc, the lower the pH the more it is available and vice versa.
-It is important in protein synthesis, assists in the uptake of nitrogen and phosphorous. It is also involved in pollination process and enzyme synthesis
-Deficiency of zinc is associated with high pH and they include;
ü White bands on sides of the midrib in members of graminae family
ü Shortened internodes and stunted growth e.g. in citrus
ü Yellow spots on older leaves e.g. rice
IRON
It is found in the soil in the form of oxides, sulphides and silicates
-It is mainly taken in the form of Fe2+ (ferrous) but some grasses can take it as Fe3+ (ferric) but they must converted it to Fe2+ because Fe3+ is hard to transport
-Iron is also found in organic compounds e.g. chelates are organic compounds that bond strongly to metal atoms through more than one bond but there are also soluble chelates mainly artificially made like Fe-EDTA
-Iron is important in the synthesis and maintenance of chlorophyll and functioning of nucleic acids. It is also a component of enzymes
-Deficiency is associated with high pH and includes chlorosis in the younger leaves and inter-veinal chlorosis in broad-leaved plants
MANGANESE
-Occurs in the soil as oxides, silicates and carbonates
-Manganese has many oxidation types i.e. manganese two, three and four but plants take it in Mn2+ i.e. oxidised MnO
-It is a component of enzymes and their activation
Deficiency symptoms include;
ü Grey spots on leaves of cereals
ü Inter-veinal chlorosis
Factors influencing availability of nutrients
i). The natural supply of the nutrient within the soil. This is closely related to the parent material of the soil and the vegetation under which soil developed
ii). Soil reactions; this is the degree of acidity or alkalinity of any soil. It is expressed as pH value. This affects the rate of nutrient release from the soil
iii). Relative activity of soil micro-organisms. They are biogegraders
iv). Fertilizer addition to enhance nutrient supply
v). Soil temperature; it has high influence on micro-organisms and also plant roots (physiological)
vi). Soil moisture; most nutrients are absorbed in solution form. Thus low or absence of moisture reduces availability of nutrient elements
vii). Soil aeration; many microbial activities are aerobic
SOIL CHARACTERISTICS ESSENTIAL FOR CROP PRODUCTION
-Soil is important for crop production but is not essential (mandatory). This is because hydroponic solutions have been used as artificial nutrient media and other artificial media like vermiculite have been used to offer support to the plants thus making it unnecessary to have soil for crop production, but the materials are expensive and not common
-For good crop production in terms of yield, economy and quality of produce a soil should have the following qualities
- Adequate levels of plant nutrients (not little or excess)
- Well aerated to enhance aerobic activities
- Free from toxic elements, pathogens and pests
- Soil colour mainly dark for better heat absorption
- Good water holding capacity for both irrigation and rainfall water
- Easy to operate manually and use of machines
- Good soil structure so as to resist erosion, severe leaching, good drainage etc
-No soil can meet all the above criteria thus the need for modifications like fertilizer applications, manures, irrigation etc
-Salinity is the accumulation of soluble salts to such high levels as to affect growth. The soil is considered saline when the soil electrical conductivity (EC) is four millimoles per centimetre
-Sodicity is the accumulation of sodium to high levels such as to cause dispersion of soil particles when the exchange sodium percentage is more than 15%.dispersion is due to the high positive charges making soil colloids positively charged. Dispersion leads to blocking of pores thus affecting drainage and aeration
ORGANIC FERTILIZERS (MANURES)
These are materials derived from plant and animal parts/droppings or residues which are applied to fertilize the soil. These include farm yard manure, green manure and compost manure
- Farm yard manure
This includes the accumulation of animal droppings where the animals are enclosed during the night or in zero grazing unit plus the beddings. These manures can be important source of plant nutrients such as nitrogen, phosphorous and potassium.
-The quality of FYM is influenced by
ü Type of bedding used
ü Type of animal
ü Type of feeds given to the animal
ü Age of the animal
ü Health status of the animal
ü Physiological status of the animal
ü Method of preparation
ü Period of preparation
ü Storage
Preparation of FYM and application
-FYM is prepared by collection of the farm animals’ waste products and their beddings.
-when manure is to be stored to await the planting season, it should be gathered into piles which are kept under a suitable shelter or covered with a layer of soil, dry grass or crop residues to prevent nutrients from being washed away in runoff water or being leached down wards as rain water percolates through the soil. Covering also reduces loss of nitrogen to the air (volatilization)
-The manure should be incorporated into the soil as soon as its spread to reduce nutrient losses through volatilization. Incorporation of FYM should be done at least two weeks before planting to allow the process of nutrient release to be initiated to benefit the early stages of crop growth.
- Compost manure
This involves plant and other organic waste which are decayed
-the compost is prepared by piling plant residues either in a heap or in a pit. The material should be turned over at intervals to facilitate even decay. With low nitrogen vegetable matter it is sometimes necessary to add a nitrogen source e.g. ammonium sulphate to encourage bacterial decomposition.
-It is essential that temperatures are controlled during composting so as to ensure production of good quality manure. Too high temperatures can be lowered by sprinkling water on the compost. Once ready compost manure must be protected from the sun and rain in order to conserve nutrients
-Compost is important because it improves soil colour, aeration and water retention properties of the soil.
Preparation of compost manure
-Pits measuring 150 x 150cm are excavated to a depth of around 60cm.
-The materials are then placed in the pits in layers
-Loosen the soil at the bottom of the pit and place a layer of dry crop residues like maize stover or grass at the bottom
-Add 10cm of fresh manure and the bedding or 30cm of green material preferably legume cover crop or kitchen wastes
-Cover with a thin layer of top soil to ensure presence of micro-organisms to break down the compost
-Add a thin layer of wood ash and then water adequately
-Repeat the above steps until the heap is 1.5m high
-Cover the pile with soil and finally with grass, maize stalks or banana leaves to prevent drying.
Use a pointed stick to monitor the temperature and moisture of the pile. Add water as soon as the stick feels dry
-Turn the pile into another pit two or three weeks then into the third pit after two weeks
-Three weeks after the second turning use the stick to check the temperatures. When the stick feels cool then the pile is ready for use
Composting process
The process takes place in four stages (Lampkin 1990)
- Mesophilic stage; the micro-organisms start to decompose the materials. Temperature rises and pH falls as organic acids are produced
- Thermophilic stage; it takes place at temperatures above 40˚C and rises up to 60˚C at which the fungi become deactivated. The more readily degradable substances such as sugars, starches, fats and proteins are rapidly consumed. The pH becomes alkaline as ammonia is liberated from the proteins
- Cooling-down stage; this is characterised by fall in temperatures. The more resistant materials like cellulose are attacked
- Maturing stage; this is the final stage that leads to breakdown of organic matter to produce humus. It can take several unless hastened. There is intense competition of food among the micro-organisms. Antagonism and antibiotic formation occurs and the heap is invaded by macro-fauna (mites, ants, worms etc) that contribute to further breakdown of the organic material by physical maceration of the particles.
3. Green manure
-These are made from green vegetative plant materials that are both cut and spread on a field or just ploughed in depending on the plant to be used as manure
Characteristics of plants used for green manure
· Should be fast growing
· Should have elaborate vegetative or leafy section
· Should be broad-leafed
· Should be leguminous, the roots should be short or less elaborate
· The plant should be generally succulent, less woody or stemmy to ease quick decomposition
NB: The stage of harvesting or ploughing in should be considered because the plant which has flowered or form woody stem will take long to decompose therefore the plant should be cut or ploughed in before flowering. Ploughing in should be done in soil that has fairly low moisture to ensure optimal microbial activity (decomposition)
Advantages of green manure
Ø They are cheap to make
Ø Less skilled labour is required for preparation
Ø They maintain soil structure, soil stability, porosity, pH, better root penetration etc
Ø Plants which are mainly leguminous fix nitrogen into the soil
Disadvantages of green manure
Ø They take long to prepare
Ø The plants may deplete moisture from the soil which may compromise the development of the main crop
Ø The plants may carry pests and diseases that eventually attack the main crop
INORGANIC FERTILIZER
-The term inorganic fertilizer is adopted because the fertilizers are in mineralised form. Most soils need to be replenished with plant nutrients for higher yields from intensive cultivation
Causes of soil nutrient losses
i). Harvesting and translocation of crops
ii). Leaching
iii). Immobilization; this is the conversion of an element from the inorganic form to organic form in microbial tissues in or plant tissues thus making the element unavailable to other organisms or to plants
iv). Denitrification; this refers to the biochemical reduction of nitrates or nitrite to gaseous nitrogen either as molecular nitrogen or as an oxide of nitrogen
v). Fixation; is the process in the soil by which chemical elements are converted from a soluble or exchangeable form to a much less soluble or to a non-exchangeable form e.g. phosphate fixation,
vi). Volatilization; is the loss of mainly nitrogen in form of gas from the soils e.g. through denitrification whereby nitrates are reduced to nitrogen oxide compounds and the element nitrogen
vii). Erosion
Terminologies used in fertilizers
Ø Fertilizer grade; is a guaranteed minimum percentage of NPK nutrients. Commercial fertilizer analysis is the actual tested percentage in each sampled batch. A fertilizer grade gives the percentage by dry weight of the three key nutrients listed in order as total nitrogen, available phosphorous pent-oxide and water soluble potassium oxide in a complete fertilizer e.g. 17-17-17
Ø Fertilizer ratio; it is the relative percentage of N, P2O5 and K2O in a fertilizer e.g. if the fertilizer grade is 17-17-17 then the fertilizer ratio is 1-1-1
Ø Fertilizer material; is any substance that contains one or more of the essential elements. A minimum quantity must be there for a material to qualify to be a fertilizer material
Ø Mixed compound fertilizer; is a combination of two or more fertilizer materials which contain one or more essential elements
Ø Complete fertilizer; is a material that contains the three major plant nutrients (NPK). The proportion of each may vary due to the different soil needs
Ø Filler material; is material added to mix fertilizer to make up the difference between the weight of the added ingredients required to supply the plant nutrients.
Ø Conditioners; materials added to improve the physical conditions of the fertilizer. This is done mainly to maintain quality e.g. coating urea to prevent caking
Calculations of fertilizer formulas
-In calculating formulas for mixtures it is necessary to decide first what percentages of nitrogen, phosphorous and potassium are desired in the fertilizer mixture. Then what materials are to be used to supply the nutrients N, P and K.
Example 1
Calculate the amount of the following fertilizer carriers required to prepare one tonne of a mixed fertilizer of grade 10-5-5.and the Filler material The carriers are;
Ammonium sulphate (NH4)2SO4- 20-0-0
Super-phosphate 0-20-0
Potassium nitrate 13-0-41
Steps
i)Start with the grade given to find out the amount ofN,P2O5 andK2O in the required 1 ton(in our case10-5-5)
ii) Identify the compound fertilizer in the fertilizer materials provided (in our case KNO3)
iii) Identify the nutrient in the compound fertilizer that is not found in the other fertilizer materials provided (in our caseK2O)
NB: The ratios are in the unconventional form i.e. N, P2O5 and K2O
N is supplied by two carriers in this case
Step1
i)Amount of N in 1 ton(from fertilizer grade given is10-5-5)
100kg = 10kg N
1ton(1000kg) = 1000kg x 10kg = 100kg N
100kg
ii)Amount of P2O5 in 1 ton(from fertilizer grade given is10-5-5)
100kg = 5kg P2O5
1ton(1000kg) = 1000kg x 5kg = 50kg P2O5
100kg
iii) Amount of K2O in 1 ton(from fertilizer grade given is10-5-5)
100kg = 5kg K2O
1ton(1000kg) = 1000kg x 5kg = 50kg K2O
100kg
Step2
1)Amount of KNO3 needed
41kg K2O=100kg KNO3
50kg K2O = 50 x 100 = 122kgs KNO3
41
But in 100kg KNO3 there is 13% N therefore in 122kg of KNO3
100kg KNO3 = 13kg N
122kg KNO3 = 122 x 13 = 15.9kg N
100
2)Amount of (NH4)2SO4 Needed
In 100kg of (NH4)2SO4 there are 20kg N.
Therefore what amount of (NH4)2SO4 will be needed to supply (100-15.9kg N) =84.1kgN
20kg N = 100kg (NH4)2SO4
84.1 kg N = 84.1 x 100 = 420.5 kg (NH4)2SO4
20
3)Amount of super phosphate required
20kg P2O5 = 100kg Super Phosphate
50kg P2O5 = 50 x 100 = 250kg Super Phosphate
20
4) Calculate the filler material
1ton- ( KNO3+ of (NH4)2SO4+ Super Phosphate)
1000 - (122 + 420.5 + 250)
1000- 792.5kg
Filler material = 1000 – 792.5 = 207.5kg
Example 2
A farmer intends to apply sulphate of Ammonia21%N in his 100m2 plot at a rate of 60kgN/ha. Calculate the amount of Sulphate of Ammonia fertilizer the farmer must apply?
Solution
21kgN= 100kgSA
60kgN = 60 x 100 = 286kgsSA/ha
21
10,000m2 =1ha.
10,000m2 = 286kgSA
100m2 == 100 x 286 = 2.86kgSA
10,000
Fertilizer conversion (for phosphorous and potassium)
-Accuracy of the true ratio of the major nutrient elements N, P and K in a fertilizer requires that the elemental basis be used.
-Reference to a 10-5-5 fertilizer tends to make one believe that the ratio of N-P-K is 2-1-1 but this is not true. 10-5-5 represents the ratios of N, P2O5 and K2O and not N-P-K
Formula for conversion of P2O5 to P and K2O to K
P x 2.29 = P2O5
P2O5 x 0.44 = P
-K x 1.2 = K2O K2O x 0.83 = K
The multiplying factors (2.3, 0.44, 1.2 and 0.83) are derived from the relative atomic masses of the elements P, K and O (31, 39 and 16)
-P2O5 = (2 X 31) + (5 x 16) = 142
Ratio of P = 62/142 = 0.44
K2O = (39 x 2) + 16 = 94
Ratio of K = 78/94 = 0.83
Example 3
To prepare a tonne of a mixed fertilizer grade 8-16-16, a farmer was provided with the following carriers;
MAP 11: 52: 0
AN 33: 0: 0
KCl 0: 0: 60
a) Calculate the amount of ammonium nitrate (AN) carrier that the farmer will be
required to have
b) Calculate the quantity of the filler material needed Step1
i)Amount of N in 1 ton(from fertilizer grade given 8-16-16)
100kg = 8kg N
1ton(1000kg) = 1000kg x 8kg = 80kg N
100kg
ii)Amount of P2O5 in 1 ton(from fertilizer grade given 8-16-16)
100kg = 16kg P2O5
1ton(1000kg) = 1000kg x 16kg = 160kg P2O5
100kg
iii) Amount of K2O in 1 ton(from fertilizer grade given 8-16-16)
100kg = 16kg K2O
1ton(1000kg) = 1000kg x 16kg = 160kg K2O
100kg
Step2
1)Amount of MAP needed
52kg P2O5=100kg MAP
160kg P2O5 = 160 x 100 = 308kgs MAP
52
But in 100kg MAP there is 11% N
therefore in 308kg of MAP
100kg MAP = 11kg N
308kg MAP = 308 x 11 = 33.9kg N
100
2)Amount of AN Needed
In 100kg of AN there are 33kg N.
Therefore what amount of AN will be needed to supply (80-33.9kg N) =46.1kgN
33kg N = 100kg AN
46.1 kg N = 46.1 x 100 = 139.67 kg AN
33
=140kgAN
3)Amount of KCL
60kg K2O = 100kg KCL
160kg K2O = 160 x 100 = 266.7kg KCL
60
=267kgKCL
4) Calculate the filler material
1ton- ( MAP+ of (AN+ KCL)
1000 - (308 + 140 + 267)
1000- 715kg
Filler material = 1000 – 715
= 285kg
Example 4
A farmer in Njoro wants to apply 520kgs of SA per hectare and has a 13 hectare farm to apply the fertilizer. How much nitrogen will he have applied in the 13 hectares? (SA contains 20% N)
1ha =520kgSA
13ha =13 x 520 = 6760kg SA
1
100kgSA =20kgN
6760kg SA = 6760 x 20 = 1352kg N
100
Example 5
From a stockists shop a farmer buys 4 bags of urea, 8 bags of triple super phosphate (TSP) and 2 bags of murate of potash. If each bag is 50kg, analyse the grade attained from mixing the fertilizers
Urea: 4 x 50 = 200kg @ 46:0:0
TSP: 8 X 50 = 400 @ 0:46:0
KCl: 2 x 50 = 100 @ 0:60:0
NB: Grade = weight of fertilizer x % element grade
Total weight
a) Determination of the fertilizer grade
50×4bags Urea =200kg
50×8bags TSP =400kg
50×2bags MP 100kg
Total weight of fertilizer materials
= 200 + 400 + 100
= 700kg
Calculating amount of N, P2O5 and K2O in the quantities of fertilizer materials given
N :46/100 X 200kg N = 92kg N
P2O5:46/100 X 400 kg P2O5
= 184kg P2O5
K2O:60/100 x 100kg K2O = 60kg K2O
Total weight of fertilizer materials
= 200 + 400 + 100
= 700kg
Determination of % of N, P2O5 and K2O as % of total weight
% N = Quantity of N x 100
Total weight
= 92/700 x 100 = 13.14
% P2O5= Quantity of P2O5x 100
Total weight
= 184/700 x 100 = 26.28
% K2O = Quantity of K2O x 100
Total weight
= 60/700 x 100 = 8.57
Fertilizer grade = 13: 26: 9
(N = 13%, P2O5 = 26%, 9%)
Example 6
A researcher intends to carry out an observation trial of fertilizers on maize crop with recommendations of 70kg N/ha and 60kg P2O5/ha. The available fertilizers are triple super-phosphate (0.46.0) and sulphate of ammonia (21.0.0). The plot to conduct the trials is 25m x 50m. The phosphate fertilizer is applied at planting with 1/3 of nitrogen fertilizer and the rest applied as top dress.
a) How much T.S.P is needed for the trial?
b) Calculate the amount of S.A applied at planting
c) Calculate the amount of S.A applied as top dress
Solution
1ha =10,000m2
10,000m2 =70kgN
10,000m2 =60kgP2O5
a) Amount of N in( 25×50)m2 plot
10,000m2 =70kg
(25×50)m2 = 1250 x 70 = 8.5kg N
10,000
Amount of P2O5 in ( 25×50)m2 plot
10,000m2 = 60kgP2O5
1250m2 = 1250 x 60 = 7.5kg p2o5
10,000
Amount of TSP in ( 25×50)m2 plot
46 P2O5 = 100kg TSP
7.5kgP2O5 = 7.5 x 100 = 16.3kg TSP
46
Amount of SA in ( 25×50)m2 plot
21KgN 100Kg SA
Therefore 8.7Kg N = (
)
= 41.42Kg N
At planting 
× 41.42
=13.8Kg N
Top dressing 
× 41.42
=27.6KgN
Example 7
It is recommended to apply Nitrogen at the rate of 60Kg per ha. The following materials are available.
S.A 21% N
Urea 46% N
Ca (No3)2 26 % N
Calculate the amount of each fertilizer material in Kgs that will supply the 60Kg Nitrogen and the No. of bags of each fertilizer in each case, if the fertilizer are packed in 50Kg bags.
21kg N = 100kg SA
60 kg N = 60 x 100 = 285.7 kg SA
21
= 285.7 = 5.72 bags SA
50
= 5.72 bags SA
46kg N = 100kg Urea
60 kg N = 60 x 100 = 130.43 kg Urea
= 130.43 = 2.6 kg Urea
50
= 3 bags Urea
26kg N = 100kg Ca (NO3)2
60 kg N = 60 x 100 = 230.7 kg Ca (NO3)2
26
= 230.7 = 4.614 kg Ca (NO3)2
50
= 5 bags Ca (NO3)2
Example 8
Mr. Manyatta was recommended to apply 60Kg of P2 os to his plot. At the local shop: 10:10:5 was available in 50Kg bag.
i) Name the other nutrient available in this fertilizer.
ii) Calculate the amount of each nutrient available in the 50Kgs bags of fertilizer.
iii) Calculate the no. of bags of fertilizer he should buy for the plot.
iv) Calculate the amount of inert filler material which will be applied to the plot with the fertilizer
Solution
Nitrogen and Potassium
10kg P2O5 = 100kg (From10:10:5)
60kgP2O5 = 60 x 100 = 600kg
10
100kg = 10kg N
600kg = 600 x 10 = 60kg N
100
100kg = 5kg K2O
600kg = 600 x 5 = 30kg K2O
100
Number of bags = 600 = 12bags
100
INERT MATERIAL =600kg - (60 + 60 + 30)kg
= ( 600- 150)kg
=450kg
Example 9
It was discoverd that to get higher crop yields, 120 Kg N, 60Kg P2 0s and 80Kg K20 per ha were applied. The basic material available are:,
SA 21% N
SSP 15% P2 O5
Muriate of Potash 60% K2O
Calculate the amount of fertilizer needed to supply the nutrients for higher crop yields from each of the basic materials.
21kg N = 100kg SA
120 kg N = 120 x 100 = 571 kg SA
21
=571kgSA
15kg P2O5 = 100kg SSP
60 kg P2O5 = 60 x 100 = 400 kg SSP
15
=400kgSSP
60kg K2O = 100kg AN
80 kg K2O = 80 x 100 = 133.33 kg Muriate of Potash
60
=133kg Muriate of Potash
Example 10
Determine the fertilizer grade of the compound fertilizer formed when the following materials are mixed. Show each step of your work.
1000Kg of (NH4)2 SO4 – Ammonium Sulphate 21%N
2000Kg of SSP - 20% P2O5
667Kg of KCL 60% K2O
333Kg filler material.
Determination of the fertilizer grade
Calculating amount of N, P2O5 and K2O in the quantities of fertilizer materials given
N :21/100 X 1000kg N = 210kg N
P2O5:20/100 X 2000 kg P2O5
= 400kg P2O5
K2O:60/100 x 667kg K2O = 400.2kg K2O
Total weight of fertilizer materials
= 1000kg
Determination of % of N, P2O5 and K2O as % of total weight
% N = Quantity of N x 100
Total weight
= 210/1000 x 100 = 21%
% P2O5= Quantity of P2O5x 100
Total weight
= 400/1000 x 100 = 40%
% K2O = Quantity of K2O x 100
Total weight
= 400.2/1000 x 100 = 40.02%
= 1: 1.9:1. 9
=Fertilizer grade 1:2:2
Example 11
A farmer who blends own mixed fertilizer material with grade 12-20-12(NPK). The materials available are:
NH4NO3 33.5% N
TSP 46% P2 O5
KCL 60% K2O
i) Calculate the amount of each fertilizer material required to prepare the compound fertilizer.
iiHow much of the filler material will be needed?
Example 12
Using the following fertilizer material silicon, ammonium nitrate 33% N, Superphosphate 20% P2 O5 and muriate of Potash 60% K2O. Calculate the amount material that can be used to prepare 50Kg of 6-12-12 (NPK) mixed fertilizer.
Example 13
An experiment revealed that a soil sample weighing 100g required 8 me to raise the PH from 5.8 to 6.8. Assume the weight of the ploughed soil to a depth of 10cm is 2000 tonnes and the relative atomic mass of calcium is 40. Calculate the amount of calcium in tonnes that will be required to raise the PH of the ploughed soil in per ha by per unit.
Types and manufacture of inorganic fertilizers
- Nitrogenous fertilizers
Most chemically manufactured nitrogenous fertilizers come from ammonia. Ammonia is manufactured by the Haber-Bosch process which requires high amount of energy (200 atm and heat) and presence of iron oxide catalyst
H2 + N2 2NH3
-Nitrogen is extracted from the air while hydrogen is got by reacting carbon-monoxide with steam
-The production of ammonia is expensive due to the high heat requirement and hydrogen production which is expensive
General characteristics of nitrogenous fertilizers
· Easily leached
· Have a scorching effect
· Are soluble in water
· Are hygroscopic
· Highly volatile
Categories of nitrogenous fertilizers
Ø Ammoniacal fertilizers
-These are the main carriers of nitrogen in form of ammonium ion
-The fertilizers can be retained well than other forms of nitrogen in the soil i.e. not easily leached
-They undergo the nitrification process which acidifies the soil by release of the hydrogen ions (H+). With time the pH goes down hence continuous supply of lime is recommended
2NH4+ + 4O2 2NO3- + 4H+ + 2H2O
i). Ammonium nitrate (NH4NO3): AN
-It is manufactured by passing compressed ammonia gas through nitric acid
-It supplies nitrogen as NH4+ and NO3- in almost equal amounts
-Has high nitrogen content of about 33-34%
-Its most popular in USA Europe
Disadvantage; has poor physical condition (hygroscopic)
-Has risk of explosion especially if stored with fossil fuels (oil)
-The fertilizer is banned in Kenya unless with permission
ii). Calcium ammonium nitrate (CAN) (NH4NO3 + CaCO3)
-It is manufactured by mixing ammonium nitrate with lime in the ratio of 1:3
-It has lime that eliminates explosion
-Has low amount of nitrogen of about 26%
iii). Ammonium sulphate (NH4)2SO4
-It was originally produced as a by-product of caking process where coal was heated with other ingredients to produce fuel
-It is now produced by bubbling ammonia gas through sulphuric acid which is an easier method
Advantage; has low hygroscopicity
Disadvantage; has high acidifying effect because it has ammonia causing acidity in the sulphate. Sulphate is converted to sulphurous acid hence high acidity
-Has 21% nitrogen
-Its used for crops that require low pH e.g. tea
-It supplies nitrogen and sulphur thus good for soils low in sulphur
-Its has white crystalline appearance almost like sugar
iv). Urea (NH2)2CO
-This is an important source of source of nitrogen
-it is manufactured by combining ammonia gas with carbon-dioxide gas at 140˚C and high pressure
2NH3(g) + CO2(g) 140˚C NH2CONH2 + H2O
-When applied in the soil some plants take urea directly but in most cases urea is decomposed through microbial action
NH2CONH2 + H2O Urease (NH4)2CO3 NH3 + CO2
-It behaves like ammoniacal fertilizer therefore acidifies the soil
-The fertilizer can be applied as foliar feed
-It is highly volatile (up to 40%) and hygroscopic
-Has 45-46% nitrogen
-Has very white granules, round and bigger than those of salt
v). Ammonium phosphate
Has two important forms; MAP (NH4H2PO4) providing 1.2% nitrogen thus more useful as phosphorous fertilizer and DAP providing 18% nitrogen [(NH4)2H2PO4]
-They provide both nitrogen and phosphorous to the soil
-They have good physical characteristic i.e. not very hygroscopic and volatile
vi). Ammonium chloride (NH4Cl)
-Has 25% nitrogen
-Its not suitable for plants due to toxic chloride ions thus mainly used in rice production and also used in palm and coconut production
-The ammonium form of nitrogen is used in waterlogged soils
Ø Nitrates
-These are fertilizers that provide nitrogen in the form of nitrates
Advantages
-They are non-acid forming because no hydrogen ions are in it and the accompanying ion is a base
Disadvantages
-They are very mobile hence prone to leaching hence not suitable in rainy areas
-They can be lost into the atmosphere in gaseous form (denitrification)
Examples
-Sodium nitrate
-Potassium nitrate (expensive and used for high quality crops like horticultural crops)
-Calcium nitrate (very hygroscopic)
- Phosphatic fertilizers
-They encourage early maturity of crops as the element phosphorous helps in root development
-Phosphatic fertilizers have residual effect
-Most of the sources of phosphatic fertilizers are through mining. The major ones are;
i). Rock phosphate
-Its an apatite (calcium phosphate). It is mined in USA, Morocco, Algeria, Tunisia and Jordan. Tanzania has some deposits that they sell to Kenya
ii). Basic slag
-It’s a by-product of steel industry
-Its used as liming agent and phosphatic fertilizer because its cheap
iii). Guano
-They are fossilified bird droppings
-Found mostly in pacific highlands
Examples
i). Single super phosphate (SSP)
-Contains 20-21% P2O5
-Supplies calcium, sulphur and phosphorous
-Has white-cream granules
-Its slightly soluble in water
-Its applied during planting
ii). Double and triple super phosphates
-They have two to three molecules of phosphates respectively
-They contain 40-45% P2O5
-They are grey granules
-They have slight scorching effect
- Potassic fertilizers
-They are manufactured from salts of potassium chloride and sodium chloride i.e. sulvinite salt
-The fertilizers help the plant resist diseases and make starch and proteins
-They have moderate scorching effect to crops
-Are moderately soluble in water
Examples
Muriate of potash
-Contains 50% K2O
-It supplies calcium, magnesium, sulphur and other trace elements
-Its highly hygroscopic
-Has cream-white granules
Potassium sulphate (sulphate of potash)
-Is formed by combining muriate of potash with sulphuric acid
-Has 50% K2O
-Has a scorching effect
Fixation of phosphorous in the soil (retention)
Phosphorous is retained in the soil by combining with certain soil components. This is called phosphorous fixation or retention
-There are different types of chemicals that cause phosphorous retention in the soil
a) Fixation by Fe2+, Al3+ and Mn2+
-Soluble phosphorous combine above components to form an insoluble form
-Acidic soils have high amount of iron, aluminium and manganese. Volcanic soils also have high aluminium amounts
Al3+ + H2PO4- + H2O 2H+ + Al(OH)2H2PO4
Insoluble precipitate
b) Fixation by Ca2+ in calcium carbonate
-High amounts of calcium are found in alkaline and calcium carbonate soils
Ca(H2PO4)2 + 2Ca2+ Ca3(PO4)2 + 4H+
Insoluble precipitate
c) Fixation by hydrous oxides
-These are oxides of Aluminium, manganese and iron in the soil
-The soils are not necessarily acidic but have above oxides
Al(OH)3 + H2PO4- Al(OH)2H2PO4 + OH-
-Soils with high hydrous oxides are highly weathered soils (e.g. oxisols
i) (NH4)2 HPO4 - Diammonium Phosphate
(18 – 46-0)
D A P
ii) Ca (NH4)2 NO3 Calcium ammonium Nitrate
(C A N)
iii) K2 SO4 Sulphate of Potash/ Potassium Sulphate
ii) Advantages of NH4+ions
- Held on cation exchange sites, does not leach directly
- In expensive
- Ready in state required for plant metabolism (3mks)
Disadvantages
- Loss through volatilization
- Acidify weakly buffered soils
- Plant toxicity possible
- Used by micro-organisms – volatilization
- Less preferred by plants like NO3-
Benefits of slow-release nitrogen fertilizer
- Efficient use of N by the crop. Pattern of release is synchronized with the growth rate of the crop. Recovery of the fertilizer by the crop is much higher.
- Reduction of salt and / or ammonia toxicity of crops. Crops are free from burning effect or toxicity.
Reduction of N losses and environmental pollution. N uptake is increased. Nitrate contamination of water surfaces or volatilization of ammonia is reduced.
Ionic forms in which Nitrogen, Phosphorus and Potassium are available for plant uptake:
- Nitrogen- (NO3-)Nitrate ion
-(NH4+)Ammonium ion
· Phosphorus -(H2PO4-) Primary Orthophosphate
(HPO42-) Secondary Orthophosphate
· Potassium- (K+) Potassium ion
Ways that ensure efficient utilization of nitrogenous fertilizers
- Apply optimum amount of nitrogenous fertilizers depending on specific crop requirements
- Timing of nitrogen fertilizer application should coincide with specific crop requirements need i.e. stage of growth is important
- Apply the nitrogenous fertilizers in splits
- Incorporate the nitrogenous fertilizer in the soil to avoid losses due to volatilization
- Use slow release nitrogen fertilizers
- Use urea and nitrification inhibitors
- Apply balanced doses of N.P.K to the soil
- Apply the fertilizer during the right weather conditions to avoid excessive leaching during heavy down pour
- Apply the fertilizer during the right stage of plants growth
- The fertilizer placement should be within the reach of plants roots
Factors affecting Mineralization of Organic matter
· Moisture content: High MC of 60-75% favours mineralization
· Temperature : Warm temperature of 25-35oC favours mineralization
· Soil PH : Low acidity/Neutral PH favours mineralization
· Type of plant materials : Age of plant materials,stage of grouth,determines decomposition rate.
· Time/duration of decomposition depends on conditions of storange but 3 to 6 months is average time.
· Aeration: supply of co2 to micro-organisms favor an aerobic fermentation .
·
Factors that influence nutrient availability
- Organic matter: High humus content determines soil fertility, organic colloids supply plant nutrient and retains the adsorbed nutrients.
- Soil texture: silicate clay colloids favored in clay soil have high nutrient or cation adsorptive capacity.
- Type of silicate clay: vermiculite has higher CEC compared to chlorite,illite and montmorillonite
- Soil pH: Optimum pH for nutrient availability is 6-7.0.Higher pH or low pH leads to nutrient fixation.
- Soil moisture: Soil moisture acts as a medium for absorb or dissolving nutrients making them in the form available.
- Soil temperature: soil temperature affects nutrient solubility absorption and plants activity. It also affects microbial actitivities for instance the decomposition.
Behavior of urea fertilizer in the soil
Ø Urea hydrolyzes to form ammonium carbonate under the the influence of enzyme urease.
CO(NH2)2 +2H2O (NH4)2CO3)
Urea Ammonium carbonate
Ø Ammonium carbornate is unstable compound and decomposes to form ammonia and carbon dioxide.
Ø The volatilization of NH3 and CO2 takes place
Ø Condition tha favors volatilization includes placement on the soil surface and temperature
Ø Some ammonium ions dissolves in water to form ammonium ions(NH4+).
Ø Ammonium ions get adsorbed on soil colloids
Ø Ammonium ions also undergo nitrification process to form H+ and NO3-
Ø Ammonium ions are also absorbed by plants or leached
Ø Placement and mixing urea with the soil can reduce volatilization
Ø Cold and dry soils reduce volatilization
Ø Placement with seeds or seedlings should be avoided:Low germination rate or injury to seedlings
Importance of CEC
1.)CEC serve as the storage of plant nutrients, indicator of soil fertility
2.)Adsorbed cations are held by forces reducing loss due to leaching
3.)Gives the soil buffering capacity which reduces change in soil PH or shift in equilibrium
4.)Reduces contamination of soil water ,holds toxic substances in the soil by filtering
Factors affecting the cation exchange capacity
ü Fineness of soil particles; fine textured soils tend to have higher CEC than sandy soils
ü Organic matter content
ü The type(s) of dominant clay types 1:1, 2:1 etc.
ü High pH increases CEC.
Calculate the amount of Calcium carbonate
Volume of hfs=
Soil density 1m3 =2000kg
1000m3 == 2000 x 1000 = 2000000m3
2x106 kg =2x109 g
1
Functions of Micronutrients
- Structural components of enzymes and coenzymes.
- Involved in enzymes activation and regulation.
- Electron carriers in reactions
- Osmotic solute charge balance.
- Formation of cell wall structure (B)
- Involved in non-metabolic reactions
Impact of No3 – N fertilizer on environment:
- NO3 – N is not adsorbed by soil colloids thus most likely fertilizer element leached out into surface. Water or ground water causes pollution.
- Loss to the atmosphere by denitrification and volatilization (NO2- gas)
- Loss to atmosphere as (NO2- gas) – green house gas which is environmental hazard.
Nitrogen Management
· Matching total N applied to attainable yield goals.
· Timing of N application to fit crop needs.
· Use of appropriate or slow- release N fertilizers.
· Split application of nitrogen to fit high N demands periods by crops.
· Soil NO3- monitoring devices to measure what is already present so as to adjust fertilizers rates appropriately.
· Use of N stabilization techniques to slow formation of NO3.
· Application of N- fertilizer with irrigation water for controlled plant uptake.
· Balanced fertility: Other nutrients must also applied in sufficient amount to maximize Nitrogen – Use efficiency
Biological Nitrogen Fixation
· Occurs when atmospheric Nitrogen is converted toAmmonia by bacterial enzymes.NH2is converted into ammonium( NH4- )ions by Rhizobium bacteria
· Replenish soil nitrogen reserve through through symbiotic relationships with legumes
· Improves soil nitrogen thus productivity
Factors affecting availability of K to plants
· Amount of exchangeable K
· Capacity of soil to fix K
· Soil aeration
· Soil temperature
· Ca /Mg concentration
· Cation Exchange capacity C.E.C
· Soil P H
Types of ammonim phosphate fertilizers.
- Monoammonium Phosphate (MAP): 11-52-0
- Diammonium Phosphate (DAP): 18-46-0
- Ammonium Polyphosphates (APP): 10-34-0
- Ammonium Sulphate Phosphate (ASP)
- phosphate Properties of ammonium
· High analysis PIncrease acidity
· High water solubility (heat 100%)
· Have initial basic reaction to soil
· Solid granular, ease to use
Reduced shipping costs
Nitrification process
Completing and balancing the chemical equation
(NH4)3SO4 + 3O2 2HNO2+
H2SO4 + 2H2O + Energy
2HNO2 + O2 2HNO3 + Energy
Discuss nitrification process
1.)Nitrification process refers to enzymatic oxidation of ammonium ions by bacteria to form nitrates
2.)The ammonium ions applied through fertilizers are acted on by the bacteria like nitrosomonas and nitrobacter species to produce hydrogen ions, nitrates and energy
3.)The process may also involve mineralization of organic matter where proteins are decomposed to produce nitrates
4.)The hydrogen ions produced results in increased soil acidity
5.)The use of ammonium containing fertilizers results in increased soil acidity
Soil buffering capacity
1.)The soil buffering capacity refers to ability of the soil to supply ions to the soil solution from the adsorptive complex
2.)This includes when plants nutrients are utilized by plants or losts, the ions will desorb from the exchange site to replace them
3.)The buffering capacity is about the ratio of adsorbed ions to the ions in the soil solution
4.)The soil buffering capacity increases with increasing organic matter and CEC
5.)The soils with fine texture have higher buffering capacity than the coarse textured soils
6.)Addition of fertilizers to the soils increases ability of the soils to buffer against nutrient deficient due to plant uptake or leaching
7.)The plants nutrients availability and supply increases with the soil buffering capacity
8.)Ability of the soil to resist change in PH
9.)The buffering capacity also assists the soils to resist change in soil PH
Explain how Aluminium ions contribute to soil acidity
Aluminium ions increase soil acidity through hydrolysis of water molecules
Al3+ + H2O Al(OH)2+ + H+
Al(OH)2+ + H2O Al(OH)2+ +H+
Al(OH)2+ ++ H2O Al(OH)3 + H+
Overall Al3+ + 3H2O Al(OH)3 + 3H+
The hydrolysis of water molecules by Al3+results in production of hydrogen (H+) ions that contribute to increased soil acidity
The hydrogen ions can undergo hydration to form hydroxonium ions
H+ + H2O H3O+
b) Determination of the fertilizer grade
· Calculating amount of N, P2O5 and K2O in the quantities of fertilizer materials given
N :20/100 X 500kg N = 100kg N
P2O5:20/100 X 350 kg P2O5
= 70kg P2O5
K2O:60/100 x 150kg K2O = 90kg K2O
Total weight of fertilizer materials
= 500 + 350 + 150
= 1000kg
· Determination of % of N, P2O5 and K2O as % of total weight
% N = Quantity of N x 100
Total weight
= 100/1000 x 100 = 10%
% P2O5= Quantity of P2O5x 100
Total weight
= 70/1000 x 100 = 7%
% K2O = Quantity of K2O x 100
Total weight
= 90/1000 x 100 = 9%
Fertilizer grade = 10: 7: 9
a) Anhydrous ammonia
i) Manufacture process
· Anhydrous ammonia is a gaseous fertilizer material that is manufactured by Haber/Haber –Bosch process
· Anhydrous ammonia is manufactured from hydrogen gas from petroleum refinery or water and nitrogen from the atmosphere
· The nitrogen gas and hydrogen gas are mixed in the ratio of 1:3 by volume
· The nitrogen and hydrogen gases are reacted to form anhydrous ammonia
N2 + 3H2 2NH3
(g)(g)(g)
The conditions that favour the reaction between H2 gas and N2 gas include:
- High pressure (200 – 1000 atmospheres)
- High temperature (200 – 5000C)
And Catalysts – Iron and osmium
ii) Advantages of anhydrous ammonia
· High nitrogen content (82%N)
· Its manufactured from basic materials that are available and can be easily obtained e.g. nitrogen and hydrogen
· It temporarily sterilizes the soil after application
· It increases biological activities of micro-organisms like bacteria in the soil
· It can be easily applied to the soil through festination
· It is used to manufacture other nitrogenous fertilizers
· It is not easily leached/slow acting
iii) Disadvantages
· It is a gaseous fertilizer and it is difficult to store or handle e.g. it requires strong high pressure tanks
· Its easily lost through volatilization (high evaporation rates)
· High concentration of anhydrous ammonia is phytotoxic to seedlings (cause damage to seedlings)
· Anhydrous ammonia causes irritation to eyes and nasal passages in human beings
· Anhydrous ammonia requires special equipment for application e.g. injection/pumping
· Anhydrous ammonia is unsuitable for hot tropical climates
Factors that influence phosphorous fixation in the soils
· Oxides of aluminium and iron
Oxides of aluminium and iron in acid soils react and fix phosphorous by forming hydroxyl-phosphate ions that are unavailable to crops
· Silicate clays
The 1:1 silicate clays like kaolimite fix phosphorous by adsorbing it
· Calcium ions
The calcium ions at high PH of 8.5 react with phosphorous ions to form insoluble substances
· Organic matter content
The organic matter form complexes with organic phosphorus (chelation)
· Soil temperature
The phosphorous adsorption on soil colloids increases with temperature
The soil temperature increases mineralization and soil biological activities
· Soil flooding
Soil flooding increases availability of phosphorous. This through conversion of Fe3+ into soluble ions and minerals
· Fertilizer application
The fertilizer application increases amount of phosphorous even if phosphorous fixation occurs. The mode of application will determine the level of phosphorous fixation
b Chelation
· Complexation of metal ions by organic molecules eg Zn, Fe ,Chelates
· Increases availability of micro-nutrient cations: Fe ,Zn, Mn, Cu
· Protect cations from precipitation/adsorption reactions
· May induce deficiency or reduce availability eg Cu in Organic soils
Chelation and Fe uptake by plants
- Chelated Fe diffuses to plant roots where Fe3+ is released at root surface.
- Free chelate diffuses back to the bulk solution and complexes with another to form Fe3+
- Chelation removes free Fe solution thus decreases Fe concentration
- Causes release of adsorbed Fe or dissolution of Fe minerals to replenish solution Fe.
c) Phosphate solubilization
- Microbial process that increases the nutrient availability to plants.
- Phosphorous solubilizing micro organism occur in soils.
- Ability of solubilizing micro organism to mobilize phosphorous is attributed to their ability to reduce P H by releasing organic acids.
- Organic acids dissolve mineral phosphorous and increase availability to plants.
- Insoluble phosphorous is converted to soluble monobasic (H2 PO4 -) and dibasic (HPO4-2) forms.
H3PO4 H+ + H2PO4- 2H+ + H PO4-2
Role of Potassium in plant body
- Forms plant structure material
- Involved in transport mechanism of other nutrients across cell membrane
- Essential in synthesis of amino acids- Proteins
- Aids efficient , Photosynthesis pathway
- Fruit ripening, quality
Effects of flooding on nutrient availability
· Roots of plants rot and decay, this affects uptake of nutrients
· Affects aerobic bacteria such as Rhizobium activity and thus does not fix nitrogen to be available to plants
· Leaching soluble nutrients such as nitrogen are leached in solution form
· Oxidation reactions, oxygen diffuses from plant roots and combines with nutrients to form an oxidized environment, affects nutrient availability.
Problems associated with foliar application of nutrient:
· Low penetration rates especially in leafy thick cuticles
· Run off from hydrophobic surfaces
· Washing off by rainfall
· Rapid drying of spray solutions
· Limited rates of translocation of certain mineral elements e.g Calcium from site of uptake
· Limited amounts of micronutrients which can be supplied by foliar spray.
· Leaf damage.
Factors that affect potassium ( K) fixation
- Amount and type of clay
- Presence of ammonium ions
- Moisture cycles
- Concentration of added potassium
- Influence of P H
Straight fertilizer supply only one of the major nutrients
Compound fertilizer = Supply two or more nutrients
E.g Straight – Sulphate of Ammonium, Urea
Ammonium nitrate, Muriate of Potash
Compound= N.P.K – 10:30:0 or 17:17:17
Advantages of compound fertilizers
- Ease of application apply more than one nutrient element in one operation.
- Saves problem of home mixing of straight fertilizers to get desired mixture (mixing may not be thorough )
- Have superior physical properties than straight fertilizers and hence better to handle.
Disadvantages
- Higher unit cost
- Difficult to eliminate unwanted element if present in the mixture. 2Mks
1 ton = 1000Kg
Fertilizer analysis 12-20-12
Nitrogen 100Kg = 12N
1000Kg= 12x1000=120Kg
100
Phosphorous
100Kg = 20 P2O5
1000Kg = 20 x 1000 = 200Kg
100
Potassium
100Kg = 12K20
1000Kg = 12x1000 = 120Kg
N- 33.5 in 100kg
120 – 100 x 120 = 358.2 Kg NH4 N032
33.5
P2 05 – 46 in Kg 100Kg
200 – 200X100 = 434.8 TSP
46
K20 – 60 in 100Kg
120 – 120x100 = 200Kg KCL
60
Total = 358.2 = 434.8 +200 = 993Kg
ii) Filler Material = 1000-993 = 7Kg
b) Filler Material – Non nutrient material such as sand, Limestone etc added to the available fertitizer, carriers to bring them to the specified, analysis or quantity.
Factors that determine quality composition of farm yard manure (F.Y.M)
i) Kind of animal
- Different animals provide manure of different quality poultry and rabbits provide manure rich in nitrogen unlike cattle manure.
ii) Age of the animal
- Very young and very old will give dug not fully digested. This will result into poorer manure.
iii) Food consumed
- Food eated contains different nutrients
- The level of digestibility influences nutrients available
iv) Litter used as animal beddings
- Fast decomposing litter provides quality farm yard manure
- Different plants will have specific nutrient content.
v) Condition and individuality of the animal
- A sick animal will provide different manure quality as compared to a healthy one.
vi) Storage
vii) Age of the animal
viii) Process or procedure of preparation
Difference between Peat from Muck
Peat refers to partially decomposed materials mixed with unconsolidated soil.
It is found in perennially wet or dump soils with low microbial activities
Peat has unavailable nutrients but can release them upon further decomposition. Peat is lighter in colour
Muck is a highly decomposed organic matter.
The material is of plant origin and due to high mineralization state the nutrients within are available for plant absorption’
It is usually dark in colour
Five factors that affect potassium availability to plants
i) Kind of clay minerals
Soil containing vermiculite and montmolironite clay, contain more potassium than soils containing kaolinite clays.
ii) Presence of Ca and Mg
- Excess of Ca and Mg increases competition with K for uptake
- If Ca and Mg are more in solution, K is found in low concentration
iii) Amount of exchangeable Aluminum and Manganese
- This occurs in very acidic soils which creates unfavorable environment for K absorption.
- Raising PH to 5-5-7.0 will favor K uptake
iv) Soil Moisture and aeration
- With low soil moisture, water failure around soils particles are thinner and discontinues slowing the movement of potassium by diffusion
- With higher moisture content and increased level of K, diffusion is accelerated.
Preparation of basket compost and the use where the piece of land to put under a crop is small, such as a kitchen garden, or where there is not enough FYM, then the ‘basket’ method can be used to make compost.
The following materials are needed:
- Banana fibres – long strips 15 cm wide or alternative
- Stick – at least 60 cm long
- Kitchen garbage farm and garden wastes and plant materials from leguminous crops
There are several steps in making basket compost as follows:
i) Prepare the garden keep leaves and plant materials of leguminous plants for use during composting. These are important as they contain useful plant nutrient.
ii) Dig holes along centre of plots. The holes should be at least 12 cm deep with a diameter of 0.5m and should be spaced at 1 m apart along the centre of the plot.
iii) Drive 5-9 sticks into the ground around each hole. An uneven number of sticks are good for weaving the basket
iv) Amount of exchangeable potassium-Potassium availability in the soil and the need in plants will influence plants absorbs it.
v) Use the long strips of banana fibres to weave around the sticks to make a basket. If banana fibres are not available, use a longer number of sticks at closer spacing.
vi) Place rotten garbage and animal manure into the basket first.
vii) Add fresh materials like been pods and leaves.
viii) Fill to the brim with other organize waste and include some ash and water the materials
ix) Plant seedlings around the basket at a distance of 15-20 cm from the basket to prevent scorching by the organic matter concentrated in the basket.
x) Water the seedlings while they are young. Eventually, just water the basket. The plant roots will grow towards the source of moisture. The basket serves as a moisture and nutrient reservoir for the plants.
xi) After harvesting and when the compost is used up remove the decomposed material and incorporate it into the soil during cultivation.
xii) Put new composting material into the basket to start the process again
xiii) Repair the basket as necessary
Characteristics that make CAN a popular top dress fertilizer
It does not have residual effect
- It is chemically stable thus does not react readily with air or other soil elements
- it does not exhibit caking property, it thus remain in the original form
- Does not affect the PH of the soil after continuous use because it is balanced in reactions
- It is in form of granules that makes it easy to apply
- It is cheap/ affordable to most farmers because of its cost.
- Provides the two available forms of Nitrogen NH4+ and NO3-
-It is generally safe
Significance of liming
- Liming reduces soil acidity by raising the PH. This improves availability of some plant nutrient such as P
- It also promotes microbial activities for instance the Rhizobia. Common beans as well as other close related Legumes will do well at high PH, thus limit will improve the production of common beans among others. Liming agents will increase Cation Exchange capacity the soil. Calcium and Magnesium are the most common active ingredients in agricultural limes. They are macro elements thus improve soil fertility through liming.
Nitrification- Refers to biological oxidation of ammonia ( NH4+) to nitrates (NO3-)
Factors affecting nitrification process
- Supply of ammonium ions – substrate for reaction
- Population of nitrifying organisms – population and ability to multiply rapidly depend on NIC, Temp, Carbon supply.
- Soil PH. Optimum PH is 8.5 but range between 4.5 -10- nutrients + micronutrients
- Soil aeration – bacteria are aerobic and depend on oxygen.
- Soil temperature- optimum at field capacity
- Soil temperature – optimum at 25-350c
Importance of soil test information
- To Provide an index of nutrients available in the soil.
- To provide the basic for development of fertilizer and lime recommendations
- To determine profitable response to fertilizer or lime application
- To determine suitable crops for a particular soil
Phosphorous – purple color accumulate of anthoayamin
- P H – low PH/acidic conditions, only H2PO4- ions are present, HPO4- is fixed.
- Soluble Fe, AL and Mn ions precipitate phosphorous ions rendering them insoluble.
- Hydrous oxides of AL, Fe,and Mn react with phosphorous to form insoluble compounds.
- Fixation by silicate clays – contain OH and AL3+ on their surfaces react with phosphate to become fixed.
- Ca and Ca containing compounds.
- React with phosphorous to form insoluble compounds.
- Organic matter – reacts with phosphorous to form resistant complex substances.
- Activities of mircro- organisms with phosphorous, tying it.
Factors influencing availability of micronutrients
- Soil PH – Most micronutrients are available at low PH
- Oxidation States – various oxidation states are favoured by aeration and high MC.
- Inorganic reactions – clays fertilizer and lime bound micronutrients (rendering unavailable).
- Organic reactions – Organic compounds react to form complexes which may make nutrients unavailable.
i) Active acidity – refers to the concentration of Aluminium and Hydrogen ions in the soil solution while potential acidity refers to the concentration of AL3+ and H+ ions adsorbed on soil colloids.
ii) Table 1
|
Liming material
|
Molecular Weight
|
Equivalent
|
% CCE
|
% Calcium Content
|
|
Calcium Oxide
(Cao)
|
56
|
MW/charge=56/2
= 28
|
100/56 X 100
178.57
|
40/56 X 100
71.42
|
|
Calcium Hydrogen Ca(OH)2
|
74
|
MW/change
74/2 = 37
|
100/74 X 100
= 135.14
|
40/74X100
54.05
|
|
Calcium
Carbonate
CaCo3
|
100
|
MW/change
100/2 = 50
|
100/100 x 100
= 100
|
40/100x 100
40%
|
iii) CaCo3 – neutralizing soil acidity
Acidity in the soil solution
CaCo3 + 2H+(aq) == Ca 2+(aq) + Co2(g) + H2O(l)
Exchangeable acidity
Al3+ ca2+
Al3+ + 3CaCo3 == ca2+ +Al(OH)3(aq) + 3CO2(g) + H2O (l)
ca2+
The effect of organic manures on soil water retention capacity.
--Manures improve soil water retention capacity by binding soil particles and causing granulation.
--Granulation improves the soil pore spaces that hold air and water molecules.
-- Organic manures also contain organic colloids that are capable of hydration.
-- Manures are spongy and capable of water retention.
Chemical forms of fertilizers
i) Ammonium Nitrate - NH4 NO3
ii) Ammonium Phosphate – NH4 H2 NO3
ii) Sodium Phosphate – NA2PO4
Available ions of Phosphorous
Phosphorous is one of the primary macro nutrients
The available forms of phosphorous are H2PO4 and HPO42-i.e Primary Orthophosphate ion and secondary orthophosphate ion respectively. At lower PH, the availability of H2PO4- higher than that of HPO42- .However the primary orthophosphate (H2PO4-) ion is more acceptable by the plants over the secondary orthophosphate (HPO42-) ion. The implication of this is that at lower PH slightly below 7 phosphorus will be highly available. While at pH above 7 the plants will not be able to harness the form of phosphorus that is abundant, this may results to deficiency of phosphorus even if the ions of the secondary orthophosphate are available.
Reasons for soil testing/analysis
- It provides information on the soil fertility status of an individual farm.
- It helps to know the type and rate of fertilizer to apply.
- Provide information on the right time to apply a certain fertilizer and the appropriate method to apply fertilizers.
- Provide information on the appropriate crops to be grown
- Provide information on the appropriate land management whether short or long term eg liming, enhance drainage
Soil Sampling Methods
a) Zigzag
Samples are taken along the zigzag shaped path
Impact of excessive rates of fertilizer
- Wasteful of plant nutrients which are not utilized
- It negatively impacts the environment whereby excessive phosphate and nitrates contaminate water consumed by human being, livestock and wildlife.
- The nitrogen oxides resulting from dentrification affects upper atmosphere which adversely influences the global climate.
- Some nutrients causes toxicity to plants when in excess e.g potassium.
- Alteration of soil PH.
Fertilizer recommendations
70Kg N/ha
60 Kg P2O5/ha
Available fertilizers
TSP - 0:46:0
SA - 21:0:0
Plot area = 25M x 50M = 1250M2
P205 applied at planting with 1/3 of N fertilizer.
TSP is applied at planting
TSP needed for 1 ha.
46kg P2O5 100kg TSP
60kg P2O5 100 X60
46
= 130 -4 kg TSP /ha
130.4 Kg TSP is applied in 10,000M2
130.4 X1250 1.25m2
10,000 = 16.3kg of TSP
ii) The amount of SA applied at plaintiff
Amount of S.A in 1 ha.
21kg N is found in 100 kg SA
70Kg N 100 X 70
21 = 333.3kg of S.A
Amount used at planting = 42 X 1/3
14kg S.A
iii)Amount of SA applied as top dress
Total Amount of S.A used for the amount used at planting
42-14 = 28kg S.A
Determination of available soil nitrogen by Kjeldahu method.
i) Soil Sample is collected from the form
ii) Drying is carried out at 40oc temperature or air dried.
iii) The Sample is weighed and passed through sieves of known diameter.
iv) The sample is digested in strong sulphuric acid in the presence of the catalyst.
v) During digestion amine nitrogen is converted into ammonium ions.
vi) The ammonium ions are then converted into ammonium gas by heating and distillation.
vii) The ammonium gas is led into a trapping solid where it is dissolved.
viii) The dissolved ammonium gas becomes an ammonium ion once again.
ix) The amount of the ammonia that has been trapped is determined by titration with a standard solution
x) The results of titration are used to determine the nitrogen amount through calculations.
Factors considered when choosing fertigation
1 PPM = 1
1,000,000
1 PPM = Mg = 1Mg = 1g
Litre 1,000 1,000,000
270PPM = 1000Ml
200Ml = 270x 200 =54Mg
1000 NH4+
Equivalent Weight = MW of NH4+
Valency of NH4+
= 18
1
Convert 54 Mg into equivalent wh
54 = 3 me for 20g of soil
i.e 100 g of soil = 100 x3 me
20
= 15me / 100g of soil
METHODS OF FERTILIZER APPLICATION
Factors influencing the type and amount of fertilizer to apply
a) Crop characteristics
-Different crops take up nutrients differently, hence consider nutrient utilization characterization, rooting system (fibrous vs tap roots) and time plant requires a given nutrient e.g. one can require potassium than nitrogen
b) Soil characteristics
-Soil characteristics affects the rate and method of fertilizer application e.g. soil fertility status, interaction of a fertilizer with the soil (acidic with phosphatic fertilizers) and history of management of the soil (how much was applied before)
c) Salt index
-Salt index of a fertilizer is a ratio of the increase in osmotic pressure produced by the material in question to that produced by an equal weight of sodium nitrate based on a relative value of 100 e.g. DAP (34), MAP (30), KNO3 (74), TSP/SSP (15), Urea (75) and NaCl (154).
Methods of application
a) Broadcasting
-The fertilizer is applied uniformly over the surface before planting
This involves uniform and random spreading of fertilizers.It is normally done by hands.
Advantages
you can apply large amounts of fertilizer without the danger to plants i.e. fertilizer burn as the its well spread in the soil
the distribution of nutrients through ploughing encourages deep rooting
its labour saving during planting
its only practical method of applying maintenance fertilizer to established forage stands
Disadvantages
· contact of fertilizer with soil hence chances of fixation
· effect of fertilizer is diluted due to mixing with large volumes of soils
b)Banding
Apply fertilizers at planting time in a band near the seed to stimulate early growth of the crop.Fertilizer is placed to the side or below the seed.
-Fertilizer is applied as a band at a suitable distance from the seed e.g. side banding where the fertilizer is applied to a side of seed but deeper or ring banding
Advantages
· Early stimulation of seedling as it is placed near plant roots. The fertilizer is concentrated in few lines hence high nutrient concentration near the seedling
· It results in limited contact of fertilizer with the soil. The reduction in contact with the soil has advantage in that it reduces phosphorous fixation
· Since nutrients are placed within easy reach of the roots, the effect of fertilizer is not diluted by mixing with huge volumes of the soil
c)Combined drilling
-Its also called inter-row application
-The fertilizer is applied in immediate contact with the seed i.e. by grain drill or open furrow then place seed and fertilizer
-This method is for certain crops and certain fertilizers at a given rate. High rates result to toxicity and salt injury
-Its applicable for fertilizers like TSP and SSP because they have low salt index
d)Hill placement
Application of fertilizer at planting time in the planting hole/hill
e ) Row application
Application of fertilizer a long the planting rows
f) Ring placement
A type of topdressing done around a plant, a voiding contact with the plant, common in wide spaced crops ,mainly perennials
g) Top dressing
-Top dressing refers to application of fertilizers on the surface after the crop has established or is growing. Common in pastures, cereals,
Requires soil moisture at the surface
h) Side dressing
Placement of fertilizers between rows after the crop is growing.
Common with Nitrogenous fertilizers to reduce loss
i) Foliar application
-Fertilizer is applied to the leaf/ foliage in solution form
-The fertilizer gives more rapid response than solid fertilizer and offers effective mode of applying the micronutrients e.g. 98% of copper added to the soil forms a complex hence its applied through foliar
-Its also important for high cost green house crops
--If its applied at high rates then it can cause fertilizer burn therefore the recommended solution should be 3.5%
-Its useful for soils with problems of acidity, alkalinity, water-logged and very sandy soils
--Its also applied in dry land farming where soil moisture is not sufficient enough for root absorption
-Its also used when spraying thus does not involve additional spraying expenses
j) fertigation
Application of fertilizer at root zone together with water.
This is the application of fertilizers in irrigation waters. It is therefore a combination of irrigation system with fertilizer application. The nutrients are applied in solution form
Advantages
Ø It saves time because both water and nutrients are supplied at the same time
Ø It is economical in terms of resources since a single system will supply both water and fertilizer
Ø Through drip irrigation system water and nutrients are supplied directly to the root zone
Ø Wastage of fertilizer of fertilizer is reduced
Disadvantages
Ø Clogging of the pipes (lumen) and also the nozzles as a result of precipitation of calcium
Ø High initial capital base is required
Ø Skilled labour I formulation and installation is mandatory
Ø Blockage may result to unequal distribution of irrigation water
Ø Water quality must be high and levels of calcium, iron, sand organic matter must be insignificant
Ø Frequent soil testing is necessary to ensure correct levels of pH, electrical conductivity etc.
Limitations of Drip Fertigation system
1) Choice of the crop. Not all crops can economically in this system
2) Require skilled labour. The entire maintenance and operations of the system require trained personnel particularly the formulation step
3) Cost. The system requires high initial capital, the daily running of the system is an expensive venture.
4) Clogging of the pipe. This is brought about by the formation of scum ,an insoluble precipitate formed as a result of chemical reaction
5) Quality of water. Water must be free from particles that can block the nozzles. The water should be soft to a void reaction with the elements in the fertilizers. Hard water is one that has many soluble minerals
The universal soil-loss equation
A=RKL SCP
Where:
A=The computed soil loss per unit area
R=Rainfall that is experienced by vulnerable soil to erosion
K=Soil erodibility, that is the ease of being eroded
L=Slope length. This refers to the length of the field that is under consideration whose gradient makes it prone to erosion.
S=Slope gradient. This refers to the sloppiness or degree of slope of the agricultural land under consideration.
C=crop or vegetable cover that is planted or exists in the field under consideration. The higher the density of the crop cover the lower the soil loss through erosion
P=Erosion control practices. This refers to the deliberate effort by man to control soil erosion.
The universal soil loss equation describes the major factors that influence erosion losses. This equation explains how these factors determine water retention by the soil, water run-off and the rate of its removal.
Problems of acidity that necessitate liming of agriculture land
1) Phytotoxicity of some elements has been evident in low PH ,the concentration of aluminium,iron and manganese ions is normally high and also their availability. The root zone environment is thus occupied by these ions and as they are absorbed by plants in large amounts they injure the plant cells hence impairing plant physiology .
2 )Phosphorus fixation occurs at pH below 6.The form of phosphorus as that is adversely affected by the low pH is the secondary orthophosphate ions (HPO42-)
3) At low pH the activities of micro-organisms are interfered with. The condition is unconducive for the survival of these organisms. At high pH also the life of micro-organisms is threatened. A pH of around neutral is normally ideal for optimal microbial activity.
4 )Low pH is mainly due to active acidity free (H+) hydrogen ions in the soil solution. High concentration of active acidity injures delicate root hairs. These affect the absorption of other nutrients as well as water.
5) Mineralization of organic matter takes longer time than normal as low pH.This is because the process of mineralization is facilitated by micro-organism that operate well at about neutral pH.This makes the use of organic matter as nutrient suppliers to the plants to be of no consequence.
6) Availability of micro-nutrients such as molybdenum is interfered with by high acicidity.Low pH will favour uptake of some nutrients but not others as seen in antagonistic in micro nutrients.
LIMING
This is the addition of calcium oxide, magnesium oxide or their compounds or any other material that is capable of increasing pH (reducing acidity) to a level required for growth (optimal) of certain crop plants
-Lime refers to materials that are capable to raise pH thus agricultural limes are used to increase soil pH.
-Liming is aimed at altering (raising) the soil pH to meet agricultural production efficiency
Common liming materials
i). Calcium oxide (CaO) or oxides of lime e.g. quick lime is produced from burned limestone to remove carbon dioxide
CaCO3 Heat CaO + CO2
ii). Calcium hydroxide Ca(OH)2 . It is also called hydrated or slaked lime i.e. addition of water
CaO + H2O Ca(OH)2
iii). Marl (CaCO3) or chalk
iv). Calcium-magnesium carbonate CaMg(CO3)2 or dolomite
v). Calcium silicates CaSiO3. It is cheaper it is mined directly but weaker in alkalinity
Effects of lime in the soil
-Limestone is a carbonate form of lime with calcium and magnesium carbonates as major components. The adsorbed hydrogen ions on the soil colloids are replaced to form water as shown below;
CaCO3 + [micelle]2H [micelle]Ca + H2O + CO2
Ca(OH)2 + [micelle]2H [micelle]Ca + H2O
-All the above reactions increase the soil pH
Other effects of liming include
- The raised pH also reduces soluble manganese and iron by causing them to form insoluble hydroxides
- If dolomite (CaMg(CO3)2) is used Ca2+ and Mg 2+ which are plant nutrients are added into the soil
- It makes phosphorous in acidic soils available
- In case of soil with high potassium content, plants absorb it in high quantities than needed. Lime introduces Ca2+ and thus reduces uptake of unnecessary K+
- Liming creates favourable environment for microbial activities like decomposition and nitrogen fixing as micro-organisms especially of agricultural importance in the soil thrive well at slightly at basic soils
- Some heavy metals are available at pH lower than 6.5 e.g. lead, nickel, cadmium and strontium
Factors to consider when selecting choice of a liming material
Ø Chemical guarantees of the lime under consideration (percentage of active ingredients)This refers to the concentration of the active ingredients contained in the lime
Ø Cost of the material, lime is purchased in large quantities ,the cost must be affordable and economical
Ø Rate of reaction with the soil
Ø The crop to be grown
Ø Kind and fineness of the lime used. Fine liming materials ensure high rate of reaction
Ø The soils’ pH, organic matter, texture and structure
Ø Bulkiness. The lighter the material the better
Ø Handling and storage properties of the liming agent. Some materials are hygroscopic and will cake after sometime if not well stored, others will cake regardless of the storage facilities
Neutralizing power/value
-To express one lime in chemically equivalent amounts of another, the molecular weight ratio is used as the reference
-Consider CaO and MgO as liming material substitutes;
Molecular weights; CaO = 40 + 16 = 56, MgO = 24.3 + 16 = 40.3
-The CaO equivalent of MgO = 56/40.3 = 1.3895
-CaO equivalent of CaCO3 = 56/100 = 0.56
-Mg equivalent of MgCO3 = 24.3/84.4 = 0.288
-CaCO3 equivalent of MgCO3 = 100/84.3 = 1.186
Effectiveness of liming material
The neutralising value (CaCO3 equivalent or CCE) neutralising power is an expression of how much more effective is a liming material compared to the same amount of CaCO3.
NB: CaCO3 is taken as the standard measure in most cases i.e. 100% effective or 1.
· The effective liming material has little weight required to offset a low pH
· Relative neutralising values are expressed as percentage of the standard material
· Some liming materials can be over 100% effective
Example
Assume CaCO3 is our standard liming material. Consider the following reactions
i). CaCO3 + 2H+ Ca2+ + H2O + CO2 One mole of calcium carbonate (100g) neutralises two moles of H+ as shown above
ii). Ca(OH)2 + 2H+ Ca2+ + H2O
Calculate the neutralizing value (N.V) of Ca(OH)2
Solution
CaCO3 x 100 = 100/74 x 100 = 135%
Ca(OH)2
Relative neutralizing values of different liming materials
Compound molecular wt(g) N.V (%)
CaCO3 100 100
MgCO3 84 119
Ca(OH)2 74 135
Mg(OH)2 58 172
CaO 56 178
MgO 40 250
CaSiO3 116 86
Measurement of liming requirement (LR)
This is done by determining the pH of the soil by the following methods;
- Use of calorimetric method; can be done by use of litmus paper i.e. blue and red litmus papers or by use of universal indicator and the colour chart
- Use of electrometric method; its done by means of pH meter which is more accurate. The soil water ratio of 1:2 is used and a buffer is used to standardize the pH meter rods
Assignment
Discuss the following methods of determining the liming reagents
- Buffer method
- Field method
- Incubation method
Factors to consider in deciding on a brand of lime to apply
Ø Chemical guarantees of the limes under consideration
Ø Cost per tonne applied to the land
Ø Rate of reaction with soil the burned and hydrated limes (caustic materials) react more rapidly than do the carbonate forms
Ø Fineness of the limestone material
Ø Handling properties
Ø Storage qualities
NB: The loss of lime from the soil may be caused by; erosion, leaching and crop absorption
Factors determining amount of lime to apply
Ø Soil characteristics, pH, texture and structure
Ø Crops to be grown
Ø Kind and fineness of liming material
Ø Economic returns in relation to cost of lime
Negative influences of excess lime
Ø Deficiencies of available iron, manganese, copper and zinc may be induced
Ø Phosphate availability may decrease because of the formation of complex and insoluble calcium phosphate
Ø Uptake and utilization of boron may be hindered
Ø The drastic change in pH may in itself be detrimental
Positive chemical effects of lime
Ø The concentration of hydrogen ions will decrease
Ø The concentration of hydroxyl ions will decrease
Ø The solubility of iron, aluminium and manganese will decline
Ø The availability of phosphates and molybdates will increase
Ø The exchangeable calcium and magnesium will increase
Ø The percentage base saturation will increase
The availability of potassium may be increased or decreased depending on condition.
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