SOIL PROPERTIESThese are divided into two categories i.eSoil physical properties

Soil texture Soil structure Soil colour Soil temperature Bulk density Particle density Soil drainage Soil capillarity Soil aeration

Soil chemical properties Soil PH Cation exchange capacity Buffer capacity Soil nutrients

SOIL TEXTUREThis is the proportion of sand, silt, and clay particles in a particular soil. Soil texture affects the following:-

a. The circulation of air in the soil (soil aeration)In fine textured soils, there is limited movement of air due to the small spaces.b. Water holding capacity,This is higher in fine textured soils like clay than coarse textured soil like sand.c. Root penetration

This is higher in coarse textured soils and lower in fine textured soils since the fine particles resist root penetration.

d. Response of plants to fertilizersPoor root penetration means limited response to fertilizers hence fine textured soils are not good when it comes to movement of nutrients.

e. Rate of chemical reaction in the soilPoorly textured soils would limit soil reaction like carbonation, hydration and hydrolysis.Importance /influence of soil texture on crop production

It influences soil aeration i.e coarse textured soils have a good aeration where as smooth textured soils have a poor aeration

It influences movement of water into the soil i.e fine textured soils tend to be poorly drained while coarse textured soils tend to be well drained

It influences the water holding capacity of the soil i.e coarse textured soils like sand tend to have a low water holding capacity while fine textured soils have a high water holding capacity

It influences root penetration into the soil i.e loose soils like sand tend to facilitate proper root penetration as opposed to compacted soils

It influences the temperature of the soil e.g clay soils with fine particles that hold water for a long period of time tend to be cooler than sand which holds little water.

It influences the susceptibility of the soil to erosion It influences the ability of the soil to hold nutrients It influences the ease of tillage or workability of the soil.Fine textured soils

have a tendency to stick together hence difficult to till while corase textured soils donot stick together hence easier to till.

SOIL TEXTUAL CLASSESThis is done according to the United States Department of Agriculture and the international soil science system.USDA classification

Soil separate Particle diameter (mm)Very course sand 2.00 – 1.00Course sand 1.00 – 0.50Medium sand 0.50 – 0.25Fine sand 0.25 – 0.10Very fine sand 0.10 – 0.05Silt 0.05 – 0.002Clay below 0.002

SOIL TEXTUAL CLASSES (SOIL TYPES)There are three main textual classes (types) i.e. clay soil, Sandy soil and loam soil.These three give rise to other classes depending on the percentage of sand silt and clay present in a particular soil e.g. sandy clay, clay loam, silty clay loam, sandy clay loam, loamy clay sand and silty clay.

PROPERTIES OF TEXTUAL CLASSES

Clay soil It has a high water holding capacity It has a high nutrient holding capacity. It has small air spaces It is not gritty It is not smooth It forms extremely cohesive balls and long threads which bend into rings easily

when wetted.

Sandy soil It is extremely gritty It is not smooth Its not plastic It forms non cohesive balls which collapse easily It has a low water nutrient holding capacity It has big air spaces It has big soil particles.

LOAM SOIL It is moderately gritty It is slightly smooth It is slightly sticky It is slightly plastic It forms moderately cohesive balls It forms long threads which bend into rings with difficulty. It has a moderate water holding capacity It contains 5 – 10% organic matter Loam soil is an optimum mixture of sand, silt and clay.

Experiment to determine the different sizes of particles present in the soil(Sedimentation/Mechanical analysis)

Place 40g of soil in 100cm3 measuring cylinder Fill the cylinder with water up to ¾ of it Cover the open end firmly with land and shake the content of the cylinder

vigorously Stand the cylinder on a land flat surface and observe how the suspension settles

down.

Diagram

Results Some bubble escape from the container indicating the presence of air. After shaking, the largest particles fall to the bottom (sand) followed by fine

sand, silt, clay and finally organic matter which floats on the surface

BULK DENSITYThis is the ratio of the weight/mass of the soil to the volume of the soil Bulk density = weight of soil (g) volume of the soil (cm3)It is calculated in g/cm3

Bulk density is calculated for dry soils and takes into account the volume or mass of the particles and spaces but excludes water.

Factors that influence Soil bulk Density Size of pores paces; Soils that are loose and porous have lower bulk

densities than compacted ones Number of pore spaces; The fewer the pore spaces, the higher the bulk

density Particle sizes; Soils with tiny particles have a higher bulk density than those

with large particles

Organic matter content; The higher the organic matter content, The lower the bulk density of the soil

Soil structure; The more compacted the soil structure ,the higher the bulk density

Tillage/Cultivation;Ploughing or cultivation of the soil decreases the bulk density since it increases the pore space

Machinery movement over the land; Use of heavy machines like tractors compacts the soil hence increasing the bulk density

PARTICLE DENSITYIt’s the ratio of weight of solids to volume of solids in soil .The solids in soil are organic matter, inorganic matter, and living organism. In the calculation of particle density, The weight of the water and pore spaces should be excluded.

Particle density = weight of soil solids (g) volume of the soil solids (cm3)

SOIL PLASTICITYThis is the capacity of the soil to be molded without breaking or rupturing.The terms used in describing the degree of plasticity are non plastic, slightly plastic, plastic and very plastic.

SOIL CONSISTENCYThis is the degree of cohesion of soil or the resistance of the soil to deformation and it is measured by filling and manipulating the soil by hand or pulling tillage equipment through it.

SOIL POSOSITYThis is the measure of the size of pore space in soils. In soils where the particles are closely packed together, there is limited pore space and therefore less air in such soils and poor plant root development.

SOIL STRUCTUREThis is the arrangement or grouping of soil particle in a particular soil. The arrangement of soil particles results into formation of the different shapes of soil.The individual soil particles join together to form aggregates

Importance of soil structure

It facilitates aeration/ easy movement of air and water which ensures that

there’s good balance between water and air in the soil

It allows adequate retention of water for plant use

It facilitates good drainage avoiding water logging

It determines workability/ the ease with which soil can be ploughed

Controls soil temperature through its control of soil aeration

It allow plant roots to grow and spread widely with in the soil

It prevents build up of Co2 in the soil to toxic levels through proper aeration

Factors That Influence Soil Structure Formation/Aggregation

Organic matter content;This is sticky and so binds soil particles together

thus stablising soil aggregates

Soil moisture content; Moist soils are more plastic than dry soils hence

easier to bind together. However too much water in the soil destroys soil

particles

Soil texture; Soils with large particles are less plastic hence the particles are

loosely attached to one another

Amount of living organisms in the soil; Some living organisms produce

substances that hold soil particles together

Amount of calcium in the soil; calcium has the ability to flocculate soil

particles hence stabilizing the aggregates

Ways through which soil structure is destroyed

Continuous cultivation, this leads to rapid breakdown of organic matter that binds particles and denies the soil structure the chance to recover;

Tillage of the soil with very high moisture content; this leaves the soil particles puddled together;

Overgrazing; this reduces the amount of vegetation cover present and encourages severe compaction of the soil, hence destroying the soil structure;

Soil erosion; encourages severe compaction of the soil, hence damaging the its structure;

Deposition of eroded soil materials; encourages clogging of soil pores, leading to puddled soils

Water logging; causes dispersion of soil particles, destroying the soil structure and creating puddled soils;

Leaching of bases; bases such as Ca have the ability to cause aggregation of soil particles (flocculation of soil colloids), and promoting favourable effects on the activities of soil living organism, hence when leached, these effects are lost;

Un controlled burning; leading to loss of vegetation cover and its effect on soil structure;

Frequent use of certain agro chemicals; these pollute the soil and affect soil structure eg fertilizers and pesticides;

Mining and quarrying; expose the soil to agents of erosion that destroy its structure;

Ways of maintaining the structure of the soil

Minimum tillage; This gives the soil a chance to recover and also avoid

compaction caused by tillage. It also reduces rapid breakdown of organic

matter

Growing of cover crops; These cover the soil and reduce the rate of erosion.

They also contribute to organic matter addition into the soil

Addition of organic manure; Organic matter has colloidal properties that

help to hold/bind soil particles together

Bush Fallowing; This reduces the rate of soil erosion and also adds organic

matter to the soil

Liming; This adds calcium ions into the soil that granulates and flocculates

soil colloids

Drainage;This reduces dispersion of soil particles and increases organic

matter content of the soil.

Mulching; The organic mulches add organic matter into the soil and also

reduce soil erosion

Agroforestry; The roots of trees bind soil particles together. Also the leaves

of trees that fall down form organic matter.

CLASSES OF SOIL STRUCTURE

a) Structure less/Single grained

These are single separate grains that do not stick together e.g. sand

b) Aggregated

This is an ideal structure where particles stick together to form

secondary and large particles (granules).There are spaces between

granules

c) Massive

This is found in very fine textured soils where soil particles unite with

others. They are almost without pores and the particles are more or

less cemented.

Types of Soil Structure(a) Plate like structure / Platy structure

In this type, soil particles are arranged in such away that they form

thin sheets

lying horizontally on top of the other.

Plate structure is found in top soils

Diagram:

(b) Blocky structure/ Cubical structure

In this type, soil particles are arranged like cubes, Blocks that can fit

together.

Some have sharp edges

It is found in the subsoils and Antihills soil

(c) Prismatic structure

In this type, soil aggregates are vertically arranged and some times in

distinct columns . It is found in sub soils

(d) Columnar structure

In this type of structure, the soil particles are arranged to form vertical

columns but the tops of these columns are round. Also found in the

subsoil

(e) Granular structure; here soil particles are formed together to form

small rounded aggregates. It is found in Top soils that contain a lot of

organic matter.

- found in surface soil surface

(f) Crumb structure

In this type, Aggregates resemble granular type except; that, in this type,

the particles/aggregates are bigger, more porous and they readily

crumble down shaken to give aggregates with granular structure.

-found in Topsoil

SOIL TEMPERATURE

This is the measure of coldness or hotness of a given soil

Effects/Importance of soil temperature

It controls the moisture content of the soil by affecting the rate of

evapotranspiration i.e at high temperatures the moisture content

is very low

It affects the aeration of the soil by affecting the moisture content

of the.

It controls germination of seeds i.e all seeds require a certain

critical temperature for activation of enzymes

It controls the uptake of water and mineral salts through its

influence on water movement and transpiration by plants

It influences microbial activities within the soil e.g optimum

temperature increases activity of microorganisms in the soil

It affects the rate of weathering by influencing the rate of

chemical reactions

It indirectly affects the availability of plant nutrients by affecting

the rate of breakdown of organic matter

Factors affecting soil temperature

Soil radiation

Conduction of heat from the atmosphere

Soil structure

Rainfall depending on its temperature

Soil texture

Ways of Maintaining soil temperature

Mulching

Irrigation

Cover cropping

Application of organic manure

Drainage

SOIL PH (SOIL REACTION)

Is the measure of Acidity/Alkalinity of the soil.

It is also the concentration of hydrogen ions in the soil.

A soil is Acidic if it has more Hydrogen ions concentration than Hydroxyl

ions concentration

Alkaline: Soil has more OH-- ions than H++

Neutral soils have equal amounts of HH and OH- Ions

PH of a soil is measured using the PH scale or meter which runs from 0 –

14.

PH 7 is at Neutral point

The lower the PH the more acidic the soil is while the higher the PH the

more Alkaline the soil is

Determination of soil PH

Soil PH can be determined by use of the indicator/dye called – Universal

indicator,

- Soil sample

- Distilled water

- Test tubes

- Barium sulphate

- Phcolour chart

Procedure

- Get a small amount of soil and place it in a test tube

- Add Barium sulphate to help in breaking of soil clods

- Add distilled water and shake the contents

- Wait for the contents to settle and then add Universal indicator

solution

- Hold the test tube against a PH colour chart on which PH values

correspond to different indicator colours are recorded

- The exact PH of the soil under investigation is read off the colour

chart

N.B:

• Red colour indicates acidic pH ( 1-2)

• Pink colour indicates moderately acidic pH ( 3-4)

• Yellow colour indicates weakly acidic (5-6)

• Green colour indicates neutral (7)

Deep blue/ Violet/ indigo colour indicates ( 9-14)

IMPORTANCE OF SOIL PH. It affects the presence of certain plant pathogens like bacteria and fungi are not

common at low PH but fungi are common At very low PH the concentration of certain nutrients such as iron and

Aluminium in the soil becomes toxic to plants. Soil PH has a strong influence on the availability of various plant nutrients. Very low or very high PH inhibits the activity of the soil micro-organisms more

especially the nitrifying bacteria.

ACIDITY IN SOILS.This is caused by the following:-

Soil formed from acid rocks: Rocks such as granite contain an excess of silica (Quartz) which combine with water to form acids.

Presence of humus: Humus may react with iron and aluminum to form complexes which under go hydrolysis to yield hydrogen ions responsible for acidity.

Presence of soluble salts: These may be acidic, neutral, or basic arising from fertilizers mainly.

Water lodging: This causes hydrolysis of some of cautions in the soil which may release hydrogen ions leading to soil acidity

Rain water: atmospheric carbondioxide combines with rain water to form a weak carbonic acid responsible for soil acidity.

Cultivation of crops: The growing of crops can cause acidity when the plant absorbs cations that can be replaced by hydrogen ions.

Fertilizers application: Application of fertilizers increases cations and anions content in the soil. When the nitrates are leached they move along with cations leaving the H+ behind

Biological activities in the soil: Anaerobic decomposition of organic matter leads to formation of weak acids in the soil e.g. lactic acids, ethanoic acids (Organic acids)

The presence of sulphides in the soil: The sulphides in the soil can also cause soil acidity due to the production of H+ ions.

Presence of sulphides : These can cause acidity to the production of H+

Causes of soil Alkalinity Weathering of limestone Addition of bases such as lime Irrigation using saline water

SOIL AMMENDMENTRising of the soil PH to reaction nearer neutral is done by addition of lime a process called liming.

Liming materials include; Calcium carbonate (lime) Calcium Sulphate Calcium carbonate Magnesium carbonate. Wood ash Sugar factory lime Calcium hydroxide Calcium magnesium carbonate(dolomite)

Qualities of a good liming material It should be cheaper to buy It should be easy to handle,store and transport It should have a mild alkanizing effect on the PH It should last long in the soil to avoid frequent applications It should have a favourable effect on the soil structure It should contain a desirable proportion of cations It should easily dissolve in the soil solution

Liming RequirementThis refers to the amount of liming material that should be added to raise the pH

of the soil to a particular level.Factors considered before liming

Soil pH; Lime should be applied when the pH of the soil is found to be very low

Buffer capacity of the soil i.e. ability to resist change in pH after addition of lime

Percentage base saturation; The bases have the capacity to replace hydrogen ions on the exchange sites and therefore soils with low bases need more liming materials.

Type of crops to be grown; Different crops have different pH requirements Fineness of the liming materials; Amount of manganese present in the soil;Mn and Fe are more available in

acidic soils and therefore become less available when the soil is limed Economic Returns; The cost of liming should be less than the benefits got

from liming

Importance of liming:- It increases the use of nutrients by crops leading to increased yields. It increases the microbial activity in the soil Organic matter decomposition in the soil is accelerated It increases the availability of plant nutrient and their supply like phosphates. Make soils easy to cultivate more especially clay soil Ensures sufficient utilization of soluble acidic manures such as phosphates Keeps the soil in good condition for crop growth. It improves root nodulation and hence nitrogen fixation

Negative effects of liming It can lower the yields of crops in later years if the PH is allowed to increase

continuously. It is expensive therefore can increase the production. It decreases the future supply of plant nutrients Accelerated organic matter (OM) decomposition causes a decline in organic

matter content of the soil.

Over limingThis occurs when the amount of liming material added is so high that it reduces the availability of plant nutrients.Effects of overliming

It reduces the availability of plant nutrients e.g. Mn,Zn,Cu,P and boron

Drastic change insoil pHaffects some plants adversely It induces spread of diseases e.g. potato scarb The uptake of phosphorous by plants is retarded It is expensive and therefore it increases the cost of production

SOIL FERTILITY AND PRODUCTIVITY

Soil fertility is the ability of the soil to supply the required plant nutrients in

their right amounts and proportions for crop growth.

Soil productivity; is the ability of the soil to produce and sustain high crop

yields

Factors contributing to soil fertility/Characteristics of a fertile soil

• Good soil depth: deep soils give plant roots a greater area for nutrient

absorption.

• Proper drainage: well drained soils are aerated and aeration promotes

healthy root development.

• Good water holding capacity: this ensures that enough water is

retained in the soil for plant use.

• Adequate nutrient supply: A good soil should have correct amount of

plant nutrients.

• Good soil structure: good soil structure ensures aeration and

respiration of plant roots and soil living organisms.

• Correct soil pH: different plant nutrients are available 0at specific pH

values.

• Good soil texture: a good soil texture encourages aeration, drainage,

root penetration and ability to retain nutrients.

• Good soil colour: soil colour influences soil temperature e.g. dark

colour absorbs heat which moderates soil temperatures

• Freedom from pests and diseases: weeds compete with crops for plant

nutrients and other growth factors. Pests make utilisation of nutrients

by plants ineffective.

• Presence of organic matter: a fertile soil should have large amounts of

humus because humus improves soil structure and adds plant nutrients

into the soil.

WAYS THROUGH WHICH THE SOIL LOSES FERTILITY

Excessive irrigation

This leads to loss of soil particles and nutrients.

Excessive drainage

This leads to loss of soluble nutrients which are drained off with water.

Water logging

This encourages acidity and leaching of nutrients, and poor aeration.

Mono cropping

This leads to exhaustion of a particular nutrient from the soil.

Soil CappingThis is the formation of an impervious layer on the surface of soil which prevents water infiltration. This stops the dissolution of plant nutrients and proper root development

Formation of Hard PansThis is an impervious layer formed just beneath the soil surface and can be caused by continuous ploughing at the same depth. It prevents water percolation and proper crop root development.

Over cultivationThis causes rapid oxidation of organic matter by micro organisms leading to loss of fertility.

Effects of over cultivation of arable land Destroys Soil structure making the land more prone to erosion Increases production costs by engaging more labour in cultivation

Can destroy crop roots Can increase evaporation of moisture from soil by increasing surface area

for loss. Increases oxidation of crop nutrient hence loss of fertility

Crop removal during harvestingThe removal of crops from the garden or their products from the garden carries away all the nutrients concentrated in their tissue and seeds.

Weeds over growthThese use a lot of nutrients and therefore cause loss of nutrients from the soil

LeachingThis is the washing out of soil nutrients in both solution and suspension to the deeper layers of the soil where plants cannot utilize it.

Soil erosionThis carries away the more productive top layer of soil leading to loss of fertility

Change of soil PHSome plants nutrients are available at low PH e.g. iron, Mg, A1, Zn, K and Boron while others are favoured by a high PH e.g. phosphorous and molybdenum

Build up of pests and diseasesThis may be caused by mono-cropping and therefore less products expected from such a soil.

MAINTENANCE OF SOIL FERTILITYThe fertility of the soil can be maintained through the following ways:-

1. Crop rotation: Is the growing of different crops on the same piece of land in a particular sequence season after season for proper utilization of nutrients

Principles to follow when planning a good crop rotation programme Legumes should alternate with other crops since they increase soil fertility by fixing

nitrogen Crops with high nutrient requirement should come first on a newly cultivated piece of

land to utilize a high content of nutrients present at this stage.

Deep rooted crops should alternate with the shallow rooted crops since deep rooters bring nutrients to the upper layers of soil for shallow rooters to use

Cover crops should be included in the rotation to control erosion. A fallow period should be included in the rotation to preserve soil structure and

restore the lost nutrients. Crops that are easy to weed should alternate with those that are difficult to weed. Crops attacked by similar pests and diseases should not succeed one another in the

rotation to reduce spread

Importance of crop rotationo There is maximum use of soil nutrients since different crops with different

nutrients requirements are involve in the rotation.o Pests and diseases are easily controlled by breaking their cycle and starvation.o Parasitic weeds like string spp in sorghum are easily controlled under crop

rotation.o The nitrogen content of the soil can be improved more especially if legumes are

included in the rotation.o Good rotation evens out Labour requirement throughout the year.o It spreads financial risks over several cropso Some deep rooted crops in the rotation will recycle nutrients.o Soil erosion can be controlled during the fallow period when grasses are

allowed to grow on the land or by the binding action of the plant roots.

An example of a four year crop rotation

Year Plot 1 Plot 2 Plot 3 Plot 41 Sweet potatoes Beans Cassava Millet2 Millet Sweet potatoes Beans Cassava3 Cassava Millet Sweet potatoes Beans4 Beans Cassava Millet Sweet potatoes

Fallow Fallow Fallow Fallow

NB After the fourth year the farmer can decide to have a fallow periodLimitations of crop rotationShortage of land due to an increasing human population

Presence of permanent building that cannot be rotatedIntroduction of perennial crops with a long gestation periodMerits of the practice cannot be easily recognized by farmers hence difficult to convince them2. Proper weed control: weeds compete with crops for soil nutrients. A high

population of weeds will extract a lot of nutrients from the soil making it infertile in the long run.

3. Mulching: Is the covering of top soil with dry plant materials or artificial substances like polythene papers. Apart from conserving the soil moisture and suppressing weeds, mulches also rot and add fertility to the soil.

4. Minimum tillage: Minimum disturbance of the soil will conserve its organic matter content and moisture hence maintaining fertility.

5. Soil pH control: At different pH some macro and micro elements are present while others are absent. The soil pH can be maintained through liming and addition of fertilizers.

6. Addition of manure: The addition of both organic and inorganic manure will increase the amount of soil nutrients therefore maintaining the fertility. However, over use of artificial manure can cause acidic conditions in the soil that may lower soil fertility.

7. Soil erosion control: The washing away of he most fertile top soil leads to loss of soil fertility as well. Methods of controlling soil erosion like terracing, mulching, contour ploughing etc should be used.

8. Improving on soil drainage: This will eliminate water logging with all its disadvantages like increased leaching

9. Improving on the water holding capacity of the soil: Water is needed by plants in the absorption of soil nutrients and in photosynthesis therefore water holding capacity of the soil should be improved to maintain the soil fertility through the addition of organic manures.

PLANT NUTRIENTSThese are chemical elements that are required by plants for proper growth and development.The nutrients are required by plants to perform their biochemical, physiological and biomass processes. The nutrients that must be supplied to plants to complete their lifecycle are called essential nutrients. A nutrient is regarded as essential if;

Its deficiency results into specific symptoms or growth abnormalities The deficiency symptoms or growth abnormalities can only be corrected by

supplying the nutrient in question i.e. cannot be substituted by another

The nutrient is directly involved in the metabolism of the plant and its effects can be demonstrated in a number of species

Categories of essential soil nutrients1. Macronutrients/Major

These are elements/nutrients that are required by plants in large quantities.Theyinclude;carbon,hydrogen,oxygen,nitrogen,phosphorous,calcium,magnesium,sulphur,potassium and sodium. They are sub-divided into;

i. Primary elements; These are usually lacking from the soil because plants use them in large amounts for their growth and these are N,P K

ii. Secondary nutrients; These are usually enough in the soil and therefore fertilizer application is not required to supply them and these include Ca,Mg,S,Na.Oxygen,hydrogen and carbon are obtained from the atmosphere and water.

2. Micro nutrients/Minor/TraceThese are nutrients or elements required by plants in small quantities.They include Fe,Mn,B,Cu,Zn,CL,Co etc.

Sources of Plant Nutrients From soil solution e.g. phosphorous is released into the soil solution from

where it is absorbed by plants From exchangeable ions in the soil.Nutrients such as Calcium and potassium

can be taken directly and absorbed by plant roots From decomposition of organic matter Application of artificial fertilizers and organic manures

NITROGEN

This is one of the most important elements needed by plants.

Uses of nitrogen to plants It’s necessary for the formation of chlorophyll It improves the quality and quantity of leafy crops such as cabbages It is a constituent of plant proteins. It helps in cell division and therefore responsible for growth Controls the use of phosphorus and potassium in the plants It promotes succulence of crops which is a desirable quality in some crops

such as vegetables and fruits It increases the size of grains in cereals

Sources of nitrogen Inorganic fertilizers Organic manure Atmospheric nitrogen from lightning Symbiotic nitrogen fixation Non symbiotic nitrogen fixation

Deficiency symptoms of Nitrogen in plants. There is restricted root development Plants become stunted. There is even yellowing of leaves There is pre-mature ripening of fruits. Premature loss of leaves

Signs/Effects of excess nitrogen in plants. Excessive leaf production/Excessive vegetative growth Delayed maturity Excessive succulence which leads to lodging of plants Scotching of leaves

Fate /loss of nitrogen from the soil Crop removal during harvesting Soil erosion. Through leaching Burning of crop residues Volatilization