Soil

This is a mixture of weathered rock materials and organic matter which are formed through physical, chemical and Biological processes.

Or

It is the outer most layer of the earth’s crust where plants grow and derive nutrients.

Or

It is a natural body of loose unconsolidated material which constitutes a thin layer of several meters deep on the earth’s surface.

Soil formation

Soil is formed through the process of weathering.

Weathering

This is the process of breaking down or the disintegration of rocks to form soil.

The rocks that form soil are of three types i.e. igneous rocks, sedimentary rocks, and metamorphic rocks.

Igneous rocks

These are rocks formed by the cooling and solidification of molten magma (Larva) extracted at high temperature from the interior regions of the earth’s crust near on the surface of earth.

The main types of igneous rocks are:-

Granite, diorite, and basalt and gabbro.  The minerals present in these rocks are mica, feldspar, quarts, iron oxides and biotite.

Sedimentary rocks

These are formed by the deposition of weather minerals which are derived from igneous rocks.  E.g. shales, sand stone, and limestone.  The minerals in these rocks are clay minerals, quartz, calcium, phosphate, dolomite, iron oxides.

Metamorphic rocks

These are formed by the action of heat, pressure and chemical changes on igneous and sedimentary rocks e.g. gneiss, schist, slate, and quartzite

Types of weathering

There are three main types of weathering i.e. physical weathering, chemical weathering and Biological weathering.

Physical weathering

This is the mechanical disintegration of rocks which is caused by heat, roots of trees, ice, wind and rain.

Temperature

When rocks are heated, they expand unevenly between their layers.  A change in temperature will set up stresses which will result into breaking of rocks. The alternative heating and cooling results into rapturing of rocks.

Ice

When water cools to form ice, it expands.  Therefore the presence of water in rocks cracks can lead to the breaking of rocks when it cools to form ice

Rain

Rainfall particularly that with hail stones falls on rocks surfaces crashing and removing some particles from them which are carried by the running water.

Wind

As strong wind, blows it carries away tinny rock particles to different places from the mother rock.

Plant roots

As roots penetrate through the rock cracks, they will cause further cracking as they increase in size through growth.

Chemical weathering

This is aided by physical weathering which increases a greater surface area of rock exposed to chemical weathering. Chemical weathering is the chemical transformation or decomposition of parent rock mineral materials into new mineral complexes.

Types of chemical weathering

This includes hydrolysis, hydration, oxidation, carbonation, reduction, and solution.

Hydration

This occurs when water combines with minerals, silicates and oxides of iron or aluminum to form hydrated compounds e.g.

Fe₂O₃ + 3H₃O → 2Fe₂O₃ . 3H₂O .

The hydrated compound are softer than the original rock and therefore easily worn away.

Oxidation

This is where atmospheric oxygen and free oxygen contained in rain water convert rock minerals to oxides.  These oxides usually take more space and thus help to break up the rock e.g. 4Fe(s) + 3O₂ (g).                                       2Fe (s) + 3O₂(s) →2Fe₂O₃(s)

Hydrolysis

This is the use of water to break up chemical bonds of a particular compound e.g

KAlsi₃O8 + H₂O →  HAlsi₃O₈ + KOH

Reduction

This occurs in wet, badly drained and poorly aerated sights such as deep zones of the earth’s crust.  It involves the removal of oxygen from minerals

e.g. Fe₂O₃ → 4FeO + O₂

Solution

Water is the most important chemical weathering agent in most kinds of rocks through its solvent action when it dissolves water solvent minerals of rocks, it participates actively in weathering.

Carbonation

Water and carbon dioxide combine to form a weak carbonic acid which reacts with oxides of calcium and magnesium to form carbonate and bi-carbonates.

CO₂ (g) + H₂O (l) → H₂CO₃ (aq)

Biological weathering

This is the weathering which is influenced by living organisms like bacteria, fungi, nematodes, lichens and mosses.  A part from decomposing dead animals and plants to form soil, they produce acids which act on rock minerals and weaken it.

Factors affecting soil formation

Climate

• The development of soil profile is largely controlled by temperature and precipitation (rainfall). Enough moisture in the soil encourages microorganisms to carry out decomposition while in the soil.
• It influences vegetation and therefore type of soils formed.
• High temperatures discourage microbial activities of organism in the soil.
• Varying environmental temperature can cause breaking up of rocks to form soil.

Living organisms

• Living organisms like bacteria and fungi carry out decomposition of dead plants and animals remain leading to soil formation.
• The termites are able to convert wood into soil because they have the cellulose enzymes in their guts which act on cellulose in wood.
• The vegetative cover protects the soil surface from soil erosion hence minimizing soil loss.
• Living organisms die and decompose to form soil
• Leaves from trees fall and provide organic matter

• Earth worms grind up mineral particles important in soil formation

Parent material

The main features of the parent materials here are texture, chemical and mineral composition of the soil.

Soils developing from limestone are usually fine textured and higher in inorganic matter than those formed from coarse textured material.

Topography (relief)

• This influences the amount of rainfall received in an area,
• Surface erosion which determines soil depth by removal and deposition of soil
• Water infiltration into the soil.
• It also influences vegetation through its influence on rainfall hence affecting soil

Time

It requires a lot of time for a soil to develop up to full maturity.  A mature soil will contain all the required nutrients needed by plants.  Conditions which speed up soil formation are; warm humid climate, flat topography, forest vegetation, the factor flowing down soil formation are cold or hot day climate, grass vegetation, slopping topography.

Human influence

Humans tend to disrupt soil formation through disturbing soil profiles during the construction of buildings, roads and dams.  Their practice of bush burning destroys organic matter and raises soil temperature, slowing down the process of soil formation.

Man can transform soil in the following ways;

• Natural vegetation is destroyed in getting land for agriculture
• Fertilizer application interferes with the chemical nature of soil
• Topography is altered through constructions of roads and buildings
• Application of pesticides changes chemical soil properties
• Soil cultivation destroys soil structure
• Irrigation may interfere with soil nutrient composition and structures

Soil formation involves 3 stages of disintegration, decomposition and translocation.

Disintegration; breaking up of parent material

Decomposition; process of decomposing organic materials

Translocation; removal of soil or vertical movement of particles and dissolved solids with in a profile and often into the ground

Properties of soil

These include soil drainage, water holding capacity, plasticity, aeration, fertility, structure, density texture, porosity, colour, soil fauna, PH, and productivity.

Soil texture

This is the proportion of sand, silt, and clay 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.

(c) 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 fertilizers

Poor 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 soil

Poor textured soils would limit soil reaction like carbonation, hydration and hydrolysis.

Soil	Appearance of particles	Behavior when moist	Feeling
Sand	Loose and single grained	No ribbon formed Not plastic	gritty
Loam	Soft clods	Weak ribbon formed Slightly plastic	Gritty
Clay	Hard lumps and highly cemented	Long flexible ribbon Formed Highly plastic	Smooth

Resistance to crushing

Loose; separate and non-coherent soil particles

Friable; lumps that can be crushed into small crumbs with gentle pressure

Compact; dense lumps of soil that can be crushed with a lot of difficulty

Slightly compact; lumps that can be crushed into fragment with moderate pressure

Cemented; soil materials that cannot be broken in hand

Plastic; soil that can be moulded when wet without breaking

Soil textual classes

This 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 – 1.00

Course sand                                                                       1.00 – 0.50

Medium sand                                                                    0.50 – 0.25

Fine sand                                                                             0.25 – 0.10

Very fine sand                                                                   0.10 – 0.05

Silt                                                                                          0.05 – 0.002

Clay 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.

 Bulk density

This is the mass per unit volume of un disturbed soil dried to consistent weight at 105⁰C.

Bulk density =   weight of oven dry  soil (g)

The volume of the oven dry soil (cm³)

This property affects

Water holding capacity

Soil aeration

Crop root development

Seed germination

Particle density

It’s the ratio of weight of solids to volume of solids in soil g/cm³. the solids in soil are organic matter, inorganic matter, and living organism. In the calculation of particle density, volume of air should be excluded.

Soil plasticity

This 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 consistency

This is the degree of cohesion of soil or the resistance of the soil to deformation is measured by filling and manipulating the soil by hand or pulling tillage equipment through it.  The classes of consistence include:-

Soil porosity

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

Soil structure

This is the arrangement or grouping of soil particle in a particular soil.  Soil structure affects water movement, heat transfer, aeration, bulk density and porosity.

Types of soil structure

i.             Platy structure.

Here the soil aggregates are arranged in a relatively thin horizontal plates or leaflets. This type of structure is more common in the surface layer of virgin soils.

ii.            Prismatic structure.

This includes columnar type and prismatic type.  Both types are usually found in sub soils in arid and semi arid regions.

iii.           Blocky structure.

This structure has two sub-structures like cube like and sub angular in heavy sub soils particularly those of humid soils.

iv.           Spheroidal structure

This consists of sub-structures like granular where the aggregates are porous and crumb where the aggregates are very porous.

(v) Crumb structure

This is commonly found in top soil and particles are granulated

Soil fertility

This is the ability of the soil to supply plant nutrients in adequate amount and right proportion for better plant growth.

Factors affecting soil fertility

Soil depth, soil structure, soil drainage, soil PH, soil aeration, water holding capacity, availability of plant nutrients, presence of pests and diseases, soil compaction, living organisms, accumulation of salts, soil capillarity, hard pans, soil capping and presence of polyethene materials in soil.

(i) Soil depth

Soil depth is associated with the maturity of the soil and it also influences the amount of water retained in it for plant use.

(ii) Soil structure

This affects the amount and movement of air and water within the soil and also the transfer of heat.  Therefore a good soil structure gives ideal conditions for plant growth.

(iii) Soil drainage

This refers to the ease with which excess water drains out of the water logged soils. There is poor aeration, low temperature, poor soil structure, and low PH in water logged soils.  All the above will interfere with normal crop growth.

(iv) Soil aeration

Adequate air in the soil particularly oxygen improved water and nutrients uptake and also encourages better root development.  The air is also needed by the soil organisms during the decomposition of plant or animal remains.

(v) Polyethene materials in soil

Affects soil aeration, root development and water infiltration which all affect soil fertility

(vi) Soil capping

This affects soil drainage which determines pest attack, soil Ph and root development in crops and hence soil fertility.

(vii) Hard pans

This impedes water infiltration hence affecting soil fertility.

(viii) Availability of plant nutrients

Plant nutrients are needed by plants and therefore a soil which contains most of the nutrients and can easily supply them is said to be fertile.

(ix) Soil pH (soil reaction)

This is the acidity or alkalinity of the soil.  It influences the (ability) availability of plant nutrients e.g. at lower PH (acidic) phosphorous and molybdenum are not available but iron, Mg, Zn, K and Boron are available yet unavailable again at high PH (alkaline)

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.

Testing for soil pH

Place a sample of soil in a dry test tube

Add barium Sulphate / ammonium Sulphate to  the soil sample

Add distilled water to the mixture and shake gently

Add 3 drops of the universal indicator and allow the contents to settle for about 45 minutes

Use the pH chart to find the pH of the soil by relating with the color of the contents in the test tube

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 undergo 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 carbon dioxide 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⁺
• Presence of sulphides : These can cause acidity to the production of H+

Soil amendment

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

Liming materials: –

• Calcium carbonate (lime)
• Calcium Sulphate
• Calcium carbonate
• Magnesium carbonate.
• Wood ash
• Sugar factory lime
• Calcium hydroxide
• Calcium magnesium carbonate

Factors considered before liming

• Soil pH; very high or low pH may require amending
• Buffer capacity of the soil i.e. ability to resist change in pH after addition of lime
• Amount of bases in the exchange complex.
• Type of crops to be grown i.e. each crop requires a different pH
• Fineness of the materials to be used for liming
• Amount of manganese present in the soil

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.

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.

Characteristics of a good liming material

• It should have a mild balkanizing effect
• It should have a favorable effect on soil
• Should be cheap and readily available
• Should not affect soil microbes
• Should not affect availability of crop nutrients

Loss of soil fertility

Soil can lose fertility through: –

Soil capping

Change of soil PH

Buildup of pests and diseases in the soil

Burning

Formation of hard pans

Soil erosion

Over cultivation

Crop removal during harvesting

Large number of weeds.

(a) Soil Capping

This 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

(b) Formation of Hard Pans

This 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.

(c) Over cultivation

This causes rapid oxidation of organic matter by microorganisms 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 labor 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

(d) Crop removal during harvesting

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

(e) Weeds overgrowth

These use a lot of nutrients and therefore cause loss of nutrients from the soil

(f) Leaching

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

(g) Soil erosion

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

(h) Change of soil PH

Some plants nutrients are available at low PH e.g. iron, Mg, A1, Zn, K and Boron while others are favored by a high PH e.g. phosphorous and molybdenum

(i) Buildup of pests and diseases

This may be caused by mono-cropping and therefore less products expected from such a soil.

Maintenance of soil fertility

The 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

 

Follow the guidelines.

• 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.
• Cro

Importance of crop rotation
ps attacked by similar pests and diseases should not succeed one another in the rotation to reduce spread

• There is maximum use of soil nutrients since different crops with different nutrients requirements are involve in the rotation.
• Pests and diseases are easily controlled by breaking their cycle and starvation.
• Parasitic weeds like string spp in sorghum are easily controlled under crop rotation.
• The nitrogen content of the soil can be improved more especially if legumes are included in the rotation.
• Good rotation evens out labor requirement throughout the year.
• It spreads financial risks over several crops
• Some deep rooted crops in the rotation will recycle nutrients.
• 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 4
1	Sweet potatoes	Beans	Cassava	Millet
2	Millet	Sweet potatoes	Beans	Cassava
3	Cassava	Millet	Sweet potatoes	Beans
4	Beans	Cassava	Millet	Sweet potatoes
	Fallow	Fallow	Fallow	Fallow

 

NB After the fourth year the farmer can decide to have a fallow period

Limitations of crop rotation

Shortage of land due to an increasing human population

Presence of permanent building that cannot be rotated

Introduction of perennial crops with a long gestation period

Merits of the crop rotation cannot be easily recognized by farmers hence difficult to convince them

2. 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 topsoil 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, overuse of artificial manure can cause acidic conditions in the soil that may lower soil fertility.
7. Soil erosion control: The washing away of the most fertile topsoil 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.

Components of soil

Soil is composed of living organisms (micro and Macro), air, mineral elements, organic matter and water.

(a) Living organisms

These include microorganisms like bacteria, fungi, protozoa, etc.  It also includes macro living organisms like earth worms and some insects.

Importance of microorganism

• They carry out decomposition of dead plants and animals remains to produce humus which is used by plants.
• Some bacteria like Rhizobia fix nitrogen into the soil
• After their death more especially the macro-organisms decompose and add fertility to the soil.
• Other living organisms like earthworms, excrete urea which adds fertility to the soil by providing nitrogen.
• They aerate the soil by making tunnels. The more tunnels, the more the aeration.

 

(b) Soil air

Soil contains air, the biggest portion being CO₂ and O₂.  Soil air is used by living organisms in respiration and also the plant roots.

N.B.       Water and air occupy the same position and therefore high level of water in the soil will reduce the amount of air in that particular soil.

Soil permeability

This is extent to which a soil allows water to pass and spread through it.  However sometimes soil permeability and soil drainage are considered to be the same.

Soil drainage

Refers to the relative ease by which water passes through the soil.

Soil productivity

This refers to the ability of the soil to produce and sustain high crop yields.

Characteristics of productive soil

• It should be well drained.
• It should be of a sufficient depth for good root penetration and growth.
• It should be well aerated.
• It should have a good water holding capacity.
• It should have enough nutrients which must be the right proportions.
• It should be fee from crop pests and diseases.
• It should have a right PH for the particular crop to be grown on it.
• It should have a good structure and texture.

(c) Plant nutrients

Plant nutrients can be divided into two major groups i.e. Macro and Micro nutrients.

Macro nutrients (major elements)

These are nutrients needed by plants in large quantities e.g. carbon, oxygen, hydrogen, nitrogen, phosphorous, potassium, calcium, magnesium and Sulphur.

Micro elements (trace elements)

These are nutrients needed by plants in small quantities though they are very important for plant growth.  E.g. iron, manganese, copper, molybdenum, zinc, chlorides and cobalt.

Macro nutrients

Nitrogen:

This is one of the most important elements needed by plants yet its deficient in most areas of East Africa.

Uses of nitrogen to plants

• Its necessary for the formation of chlorophyll
• It improves the quality and quantity of leaf crops such as cabbages, dodo, etc.
• 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

.Deficiency symptoms of Nitrogen in plants.

• There is restricted root development
• Plants become stunted.
• There is even yellowing and loss of leaves
• There is pre-mature ripening of fruits.

Signs of excess nitrogen in plants.

• Excessive leaf production
• Delayed maturity
• Leaf and stem logging
• Scotching of leaves
• Poor crop yields

Fate /loss of nitrogen from the soil

• Crop removal during harvesting
• Soil erosion.
• Through leaching
• Burning of crop residues
• Volatilization (denitrification; oxidation of nitrates to atmospheric nitrogen)

Sources of nitrogen

Commercial fertilizers e.g. NPK, Urea, CAN, Sulphate of ammonia, Diammonium phosphate (DAP)

Organic fertilizers like farmyard manure, compost manure and green manure.

Lightening.

Phosphorous

Importance of phosphorus to the plants/crops

• Encourages the formation, development and establishment of roots.
• It is necessary in the formation of fruits and seeds.
• It is needed for cell division
• Production of fats and proteins.
• It helps in nitrogen break-down during respiration
• It is a constituent of nucleic acid (DNA, RNA)
• It is important in the synthesis of nucleoprotein
• It is a constituent of phospholipids.
• It gives resistance to certain diseases in crops.
• Its essential part of all the sugar phosphate in photosynthesis and other metabolic processes.
• Improves the quality of crops more especially vegetables.

Deficiency symptoms of phosphorus to the plants

• Purple coloration of the leaf especially at the margin.
• Low yield of grains, fruit and root crops
• Slow growth rate resulting into late maturity of the crops.
• Red necrotic areas on the leaves, petioles etc.
• Distortion of the leaf shape
• Older leaves become dark brown.
• There is a general overall stunted ness and leaf fall.

Potassium

Importance potassium to the crops/plants

• It increases resistance to certain diseases
• It encourages root development and growth
• It is necessary for formation of starch and transport of sugar within the plant.
• It is essential for chlorophyll formation.
• It is needed in nitrogen metabolism and protein synthesis.
• It reduces lodging in plants by strengthening cellulose cell wall.
• It controls stomatal movement hence loss of water.
• Its important to folic metabolism
• It has been linked with carbohydrates metabolism.
• It regulates water in plant cells.

Deficiency symptom of potassium to the plants/crops

• Retarded root development
• Plants are easily attacked by diseases
• Leaves dry out at the edges
• Premature loss of leaves
• Chlorosis can also be experienced
• In cereals cell at the leaf tip and margin die first.

Factors affecting the availability of potassium in the soil

Soil moisture:  Too much moisture interferes with exchangeable moisture.

Soil PH: High soil PH favors potassium fixation

Temperature:  High temperature favors the level of exchangeable potassium

Types of colloids:  Potassium fixation is usually done in soil containing montimolironite.

Calcium

Importance of calcium to crops/plants

• Raises soil PH which favors nitrogen fixing bacteria
• Improves root development and growth
• It improves vigor and stiffness of the stem,
• It governs the availability of certain essential minerals like phosphorus and potassium.
• It is an activator of enzymes in plants
• It is associated with cell wall structure
• In increases carbohydrates content in crops like cotton.
• It increases the number of mitochondria in wheat plants.
• It protects plants from injuries due to the effect of hydrogen ions.

Deficiency symptoms of calcium to the plants/crops

• The roots become stunted
• Death of the leaves occurs
• Formation of weak stem
• Leaves become chlorotic and chlorosis occurs along the margin of younger leaves.
• Terminal buds and tips of roots do not grow well
• There is distortion of the growing shoot tip
• Cell walls become rigid and brittle

Sources:

• Crop residues
• Manure i.e. organic and inorganic
• Weathering of sol minerals
• Agricultural lime

Magnesium

Importance of magnesium to the plants

• It is a constituent of chlorophyll hence responsible for the green colour.
• It is important in the formation of oils in plants.
• It encourages the production and transportation of carbohydrates and proteins in growing plants.
• It maintains the integrity of chromatic fiber and ribosomes.
• It is necessary for full activity of two principal carbon dioxide fixing enzymes.

Signs of magnesium deficiency in crops

• Loss of green color in leaves
• Development of purple, orange, and red patches in horticulture crops such as cabbages.
• It causes extensive chlorenchyma development and scanty pith formation.

Sulphur

Importance of Sulphur to plants

• It is needed in protein synthesis as it’s a component of some amino acids.
• It is used in the production and activation of some enzymes.
• It increases the oil content of crops (plants)
• It is essential in the production (formation) of some vitamins like biotine.
• Sulphur together with iron form enzymes important in photosynthesis, respiration, and nitrogen metabolism.

Deficiency symptom of sulphur

• Lack of root nodules in legumes.
• Stems are thin and plants are extremely small and short.
• There is complete chlorosis
• There is rapid leaf fall
• Leaf tips and margins are rolled inwards.
• Terminal bud growth is inhibited and lateral buds develop pre-maturely.

 

Revision questions

1. The main disadvantage of broad casting fertilizers during seed bed preparation is tha

A. There is wastage of fertilizers where seeds will not land

B. The spread of fertilizers is not uniform throughout the field

C. Some of the fertilizers is leached before crop emerge

D. Weeds are also stimulated to grow with the crops at the same time

2. The reactions below represents a process of weathering

2Fe₂O₃ (s) → 4FeO(s) + O₂(g)

Which of the following processes in rock weathering is represented by the equation?

A. Reduction

B. Hydrolysis

C. Oxidation

D. hydration

3. What happens during rock weathering by the process of hydrolysis?

A. Metallic cations in the rock are replaced by hydrogen ions

B. Hydroxyl ions in the rock are replaced by metallic cations

C. Metallic cations in the rocks are replaced by hydroxyl ions

D. Hydrogen ions in the rock are replaced by metallic ions

4. Interveinal chlorosis of leaves in a crop is a symptoms of

A. Magnesium deficiency

B. Calcium deficiency

C. Phosphorus deficiency

D. Sulphur deficiency

5. Biological activities in the soil may cause acidity through

A. Microbes secreting acids into the soil

B. Anaerobic decomposition of organic matter

C. Plant roots secreting acids into the soil

D. Their interaction with fertilizers

6. Which one of the following may lead to increase in the population of earthworms in the soil?

A. Drainage

B. Application of artificial fertilizers

C. Cultivation to loosen the soil

D. Application of organic manure

7. Organic matter buffers soil pH because of its

A. High cation exchange capacity

B. Adsorption capacity

C. Neutral nature

D. Amphoteric nature

8. Which of the following factors least affects the efficiency of sub-surface drainage

A. Soil structure

B. Soil porosity

C. Volume of water

D. Type of land use

9. The purple coloration of leaf margins in cereals is a deficiency symptoms of

A. Phosphorus

B. Magnesium

C. Nitrogen

D. Calcium

10. Which of the following characteristics of water contribute least to weathering? It

A. Expand on freezing

B. Dissolves most natural materials

C. Has a high specific heat capacity

D. Seeps into narrow cracks and crevices

11. Which of the following characteristics makes clay and humus an essential component of arable land?

A. Dispersion effect

B. Cation exchange capacity

C. Ability to swell and shrink

D. Ability to obey the cohesion-tension theory

12. Leterisation in the tropics is likely to occur if the parent rock is

A. Sandstone

B. Oxisol

C. Red tropical soils

D. ferralsols

13. Which of the following is the biggest disadvantage of classifying soils as silt, clay and sand? It

A. give no information about mineral origin

B. relies too much on management practices

C. over emphasize the effect of climate

D. does not discuss forming process

14. Why is classifying soils as silt, clay and sand not appropriate method? It

A. does not discuss forming process

B. over emphasizes the effect of climate

C. gives no information about mineral origins of the soil

D. relies too much on management practices

15. Which of the following process occur when rocks disintegrate by hydration during soil formation?

A. Metallic cations in rocks take up water molecules

B. Metallic cations in rocks are replaced by hydrogen ions

C. Metallic cations in rocks are turned into hydroxides

D. Metallic cations in rocks form strong bonds with hydrogen ions.

16. Which one of the following best explains why plants of the same species may respond differently in deficiency of the same nutrient?

A. Uneven distribution of the same nutrient

B. Genetic variation of the plant

C. Uneven distribution of water in the soil

D. Different photosynthetic mechanism of the plant

17. Which of the following does not adversely affect the pH of the soil?

A. Absorption of bases by plants

B. Production of carbon dioxide by the roots

C. Leaching

D. Water logging

18. Clays soils retain more water than sandy soils because clays have

A. Tiny particles and long macro pores

B. Large particles and tiny micro pores

C. Tiny particles and tiny micro pores

D. Tiny particles and large micro pores

19. Moderate moisture in the soil is important because it;

A. Dissolves organic matter

B. Source of water to living organism

C. Regulates soil temperature

D. Neutralizes acidity of the soil

20. The purple coloration of leaf margins in gramminae is a deficiency symptom of

A. Magnesium

B. Calcium

C. Phosphorus

D. potassium

21. The term soil reaction refers to

A. mineralization of elements in organic matter

B. antagonism between soil nutrients

C. hydrogen potential of the soil

D. fixation of nitrogen in the soil

22. Sub-soiling is carried out in order to

A. mix the soil

B. encourage soil drainage

C. discourage soil erosion

D. expel soil organism

23. When a super phosphate fertilizer is applied to a clay soil, crop may not show a positive response because

A. Phosphate uptake by crops is suppressed y other minerals in clay

B. The acidity of clay soils reduces the solubility of phosphates

C. Phosphates get fixed into insoluble minerals, once in clay

D. Clay soils are usually waterlogged and dissolve the phosphates which are less leached

24. Which of the following is the reason why regular turning of material during composting is recommended

A. Allow even distribution of nutrients

B. Prevents loss of nutrients through seepage

C. Allow uniform decomposition of the compost materials

D. Allow adequate wetting for proper decomposition

25. Which one of the following may lead to increase in the population of earthworms in the soil?

A. Drainage

B. Application of artificial fertilizers

C. Cultivation to loosen the soil

D. Application of organic manure

26. A nutrient is essential for plant growth and development if

A. It is required by the plant at the critical stage of growth

B. Its presence in the plant does not cause a growth abnormality

C. Its deficiency results into a growth deformity

D. It is easily absorbed by the plant

27. Which one of the following is an advantage of using farm yard manure over inorganic fertilizers? Farm yard manure

A. Contains more nutrients

B. Adds organic matter which improves on soil structure

C. Does not alter soil pH

D. Supplies all the micro-nutrients required by the plant

28. Phosphatic fertilizers are normally applied at planting time because they

A. Do not scorch the seedling

B. Promote growth

C. Are required for seed germination

D. Promote root growth

29. Inorganic fertilizers are not usually applied on a sandy-silty because they

A. Rarely retain minerals

B. Hardly support crop growth

C. Contains sufficient minerals

D. Are acidic

30. What do you understand by ‘heavy soils’

A. Soil that is difficult to cultivate

B. Very dark colored soils

C. Soils very rich in mineral components

D. Soils with plenty of organic matter

31. In which of the following will terracing be most applicable?

A. Contour farming

B. Crop rotation

C. Mixed farming

D. Strip farming

32. In chemical weathering:

A. The weak carbonic acid dissolves the rocks slowly

B. Chemical elements in the rocks react and disintegrate

C. Chemical reactions are speeded up to break down rocks slowly

D. The rocks break down slowly soon after physical weathering

33. In which ionic form is nitrogen not available to plants?

A. NH₄⁺

B. NO₂^–

C. NO₃–

D. NH₄^–

34. The nutrient which causes short joints and unthrifty growth in plants is

A. Calcium

B. Nitrogen

C. Phosphorus

D. potassium

35. Which one of the following is not a disadvantage of sub-surface drainage?

A. They are expensive and require skilled labor

B. They are liable to be obstructed by roots

C. They leave the field free of obstruction

D. With heavy rainfall, nutrients may leach

36. Which one of the following does not affect the amount of nitrogen released during ‘nitrogen flush?’

A. Soil temperature

B. Organic matter content

C. Length of dry period

D. Number of cultivators

37. Mulching is known to reduce caking in

A. Loam soils

B. Clay soil

C. Sandy soil

D. Sandy loam soils

38. The severe nitrogen deficiency in the soil which follows that addition of fresh plant residues with high carbon : nitrogen ratio is due to

A. Less nitrogen content in the plant residue

B. Ammonia compounds being fixed

C. Less decomposition arising from the damage

D. Lack of competition between microorganisms and plants

39. The amount of fertilizer applied in a given field depends mainly on.

A. The type of fertilizer

B. Result of soil analysis

C. The type of crop grown

D. Soil structure and texture

40. For which of the following reasons is excess water never present in plants

A. Water loss is always greater than gain

B. When the cells become turgid no more water is absorbed

C. The greater the water content in the leaf space the greater the rate of transpiration

D. The root hairs are too short lived

41. Deflocculating is not a desirable process in soil because it

A. Leads to the destruction of soil structure

B. Disperse soil nutrients from the root zone

C. Leads to fixation of plant nutrients in the soil

D. Leads to leaching of plant nutrients.

42. Plants grown in clay soils are likely to show phosphate deficiency symptoms because

A. Phosphates is unavailable to the plants in poorly aerated soils

B. Plant roots are poorly developed in poorly aerated soils

C. Fixation of phosphorus occurs in clay soils

D. Phosphorus will get lost through leaching

43. The major reason for adding sand to clay soil for crop production is to

A. Decrease acidity

B. Improve drainage

C. Improve water retention capacity

D. Decrease alkalinity

44. Supply excess nitrogen to the plant may cause

A. browning of leaves and premature drop off

B. chlorosis

C. delayed maturity

D. production of pigments other than chlorophyll

45. Good drainage and ploughing of soil reduce the process of

A. Nitrification

B. Decomposition

C. Denitrification

D. Nitrogen fixation

46. The conversion of nitrates into proteins in the soil by microbe can be termed as

A. Mineralization

B. Immobilization

C. Denitrification

D. fixation

47. Root nodules formation is a result of infection of the root cortex by a group of bacteria called

A. Azotobacter

B. Nitrobactor

C. Rhizobia

D. Nitrosomonas

Answers to objective questions

1D	6D	11B	16B	21B	21B	26A	31A	36D	41A	46B
2A	7D	12A	17B	17B	22B	27A	32B	37B	42C	47C
3A	8D	13A	18C	18C	23B	28D	33D	38B	43B
4A	9A	14C	19C	19C	24C	29A	34D	39B	44C
5D	10C	15A	20C	20C	25D	30A	35C	40C	45C

48. Give three reasonS why kitchen refuses should be composted before application
    • to make nutrients available
    • to kill harmful organism in the kitchen refuse
    • to avoid burning plant roots when decomposing in the garden
    • to allow breakdown of any toxic substances in the kitchen refuse
49. (a) what is cationic exchange

This is interchange between a positively charge ion in the soil solution with the adsorbed positively charged ions on the soil colloids such as clay or humus.

(b) Outline three factors which affect the rate of cation exchange in the soil

• Concentration of cations in the soil solution. Concentrated cations in soil solution replace the less concentrated ones
• The speed of movement of hydrated ions: movement of hydrated ions is slow compared to those not hydrated
• The amount of charge: ions with more charges are more in replacing those with less number of charges.
• The type of colloid: humus has a higher cation exchange capacity than clay.
• The nature of charge on the exchange complex: monovalent charged elements have higher cation exchange than divalent charged elements.
• soil type, the soil pH and the soil organic matter content.

50. (a) Describe the methods of applying solid fertilizers to the soil
    • Broadcasting: the fertilizer is scattered uniformly over the field either by using hand or machine broadcaster
    • Band placement: fertilizers are placed in bands on one or both sides of the crop row, about 5cm below the seed and 4cm away from the seed or plant
    • Top-dressing: involves application of fertilizers to the soil after the crop is fully established.
    • Side dressing: the fertilizer is placed alongside the band besides the crop row after planting and establishment of the crops
    • Frill/dibble placement: a hole is made in the soil where the fertilizer is placed and then covered with the soil.
    • Application by plough: the fertilizer is placed in continuous bands at the bottom of the plough furrow. Each band is covered as the succeeding furrows are turned over.
    • Application through irrigation system: fertilizers are mixed with water for irrigation
    • Spraying on leaves of the crop especially urea
    • Contact placement: the fertilizer is mixed with planting seed so that the seed and fertilizer are simultaneously placed in the soil.
    • Ring placement: the fertilizer is placed in a circular manner around the plant.
    • Basal placement: the fertilizer is placed directly into the planting hole, covered with soil before the seed.

(b) Discuss the factors affecting the response of crops to the fertilizer

• Age/stage of the crop: different nutrients in the fertilizers are required at different stage of the crop
• Soil texture: this affects nutrient retention and cation exchange capacity of the soil. Coarse soils have low adsorption of nutrients and encourage leaching
• Type of the soil for instance clay soil fix nutrients making them unavailable to the plants.
• Soil pH: extreme pH reduced availability of some nutrients to the plants because it affects solubility and ionization of fertilizers.
• Method of placement of fertilizer: fertilizers placed within reach of the plant roots are readily absorbed
• Amount of water in the soil: water dissolves and helps absorption of fertilizers.
• Plant density: proper plant spacing reduces competition for the fertilizer and help their optimal used by plants
• Amount of fertilizer applied: optimal amounts of fertilizers are recommended; too little may not illicit proper response while too much is poisonous
• Rooting habit of a crop: plants with roots near the surface easily take up fertilizer and show fast response.
• Amount of bioactivity in the soil: some organism help in the transformation of nutrients into absorbable form e.g. mycorrhiza.
• Health status of the crops: healthy crops respond optimally to the fertilizers
• Level of weed infestation: weeds compete with plants for nutrients thus limiting utilization of fertilizers by the plants.
• Solubility of the fertilizer: soluble fertilizers are easily utilized by the plant and lead to fast response
• Good crop management practices such as thinning, pruning produce good results from application of fertilizers
• Weather conditions: wet and warm weather conditions increase solubility and absorption of fertilizers.

51. (a) State the major forms of nitrogen in the soil

Nitrates (NO₃^–)

Nitrites (NO₂^–)

Ammonium compound (NH₄⁺)

(b) Outline uses of nitrogen to crops

• Increases the sizes of grains
• Encourages growth of vegetables
• Formation of plant proteins
• Promotes formation of chlorophyll
• Regulates the availability of potassium and phosphorus to the plants
• Required in fruit formation e.g. pineapple, cucumber, water melon

(c) State the factors that affect leaching in the soil

• Soil structure; weak structure encourages leaching
• Soil texture; coarse texture encourages leaching
• Rainfall intensity; high rainfall water encourages leaching
• Vegetation cover minimizes leaching
• Soluble nutrients are easily leached
• Tillage removes vegetation cover and encourages leaching
• Cation exchange capacity of the soil; high exchange capacity increases leaching
• Level of humus in the soil; high content of humus reduces the rate of leaching

52. Give reasons to explain why crops continue to show potassium (K) deficiency symptoms even after applying the right amount of Sulphate of potash in a given field
    • Perhaps there was a lot of soil moisture that reduced the exchangeable potassium elements
    • Drop in soil temperature which lowered the level of exchange potassium making it unavailable to the plants
    • Perhaps the field has 2:2 expanding clay soil which fixes potassium and make unavailable to the plant
    • Presence of antagonistic nutrient elements in the soil preventing use of potassium by the plants
53. (a) Explain the factors that affect the rate of decomposition of organic matter in the soil
    • Age of the plant- young plant materials decompose faster than old ones
    • Amount of water in the material: wet materials decompose faster than dry ones
    • Carbon: nitrogen ration – legumes decompose faster than cereals because they have high percentage of nitrogen.
    • Presence of putrefying organisms; for instance sterilizing soil with heat delays decomposition
    • Soil temperature: rate of decomposition increases with soil temperature
    • Soil aeration: adequate oxygen in the soil encourages growth of microorganism and promotes decomposition.
    • Soil pH between 6 and 8 promotes growth of microorganisms and thus promotes decomposition
    • Pollution kills microorganisms and delays the rate of decomposition
    • Climatic factors such as rainfall that provides water necessary for rotting

(b) Define the following terms as used on nutrient deficiency

• Chlorosis: this is general yellowing of the leaf tissue. It is very common deficiency symptoms since many nutrients affect the photosynthesis process directly or indirectly.
• Coloration abnormalities: some deficiency lower the amount of photosynthesis and chlorophyll which is produced by the plant
• Firing: yellowing followed by rapid death of lower leaves moving up the crop.
• Interval chlorosis: yellowing in between leaf veins, but with the veins themselves remaining green in grasses, this is called stripping.
• Necrosis: severe deficiencies result in death of the entire plant or part of it.
• Stunting: many deficiencies result in decreased growth. This result in shorter height of the affected plant
• Die-back of shoots in plants: refers to death of death tips or meristems
• Mobility of nutrients in plants: mobile nutrients can be translocated from old tissue (bottom of the plant) to new tissues (top of the plant). Deficiency symptoms occur on the lower older leaves; nutrients that are mobile include nitrogen, phosphorus, potassium, magnesium etc.
• Immobile nutrients: these are nutrients which are not easily translocated in plants e.g. iron, calcium, copper, manganese, molybdenum etc. Deficiency symptoms occur in new leaves.
• Volatilization refers to sublimation of ammonium compounds to form nitrogen that escapes to the atmosphere. The process is favored by high temperature, high soil reaction, leaving fertilizers on the soil surface and soil with low cation exchange capacity or when urea is applied on grass or soil surface.

54. (a) suggest conditions that encourage soil aggregation

• • Soil organic matter binds the soil particles together
  • Soil water: moist soils are more plastic than dry ones
  • Liming: calcium has a capacity to flocculate soil colloid
  • Living organisms produce substance such as mucus that binds soil particles together
  • Compaction lead to formation of platy structures
  • Soil texture: soils with large particles are not plastic enough so their particles easily detach.

(b) Explain the importance of soil structure in crop production.

• It controls the passage of water through the soil i.e. granular structure enables are more rapid downward flow of water than planty structure
• It controls aeration of soil
• It controls soil temperature
• It regulate water holding capacity
• Controls soil pH by controlling air passage
• It controls workability of the soil; single, loose grained soil is easier to work than sticky heavy soils.
• It control root soil penetration
• Control soil erosion; loose soil is easily eroded.
• Controls the ability of soil to hold nutrients

(c) Outline the effects of soil erosion in farming

• Leads to loss of nutrients
• Wash away/dislodges crops and planting materials
• Creates gullies that make movement of machineries and livestock difficult and also reduce available land for farming.
• Creates sand deposition to cover fertile soil.
• Cause silting of water bodies and drainage tunnels
• Gullies reduce the size of the farm
• It may bury crops
• It promotes spreading of water borne disease
• Spreads pests by dispersing crop debris.
• Spreads weeds
• Lead to landslides and destruction of life

55. (a) Give the reasons for the increase in soil nitrate level soon after the first rains after a dry season
    • Accumulation of organic matter from plant tissues in the soil during dry season. These decompose and release nitrates.
    • Rapid multiplication of decomposers
    • Rapid increase of nitrifying bacteria that oxidize ammonium compounds into nitrates
    • Rapid increase of the population of nitrogen fixing bacteria
    • Nitrogen fixation by lightening

(b) Explain the factors affecting nitrification in the soil

• Level of ammonia: nitrification takes place when there is a source of ammonia.
• Aeration: nitrification process requires air.
• Temperature: high temperature favors nitrification and nitrogen fixation
• pH: nitrification and nitrogen fixation require favorable pH
• chemical kill nitrifying and nitrogen fixing bacteria and reduce the level of nitrification
• moisture content of the soil: nitrification is slowed by very high and very lower soil moisture

56. (a) Give four ways in which soil organism contribute to soil fertility
    • Decompose plant and animal remaining into humus
    • They are used for nitrogen fixation
    • Help in soil aeration e.g. earthworm
    • Mix up soil particles
    • Produce mucus causing aggregation of soil particle e.g. earthworms
    • Improve soil drainage
    • Improve soil aeration

(b)  State four harmful effects of soil organism to crop

• Some organism destroy plant tissue e.g. millipedes
• Some organisms such as fungi cause disease
• Termites burrow deep and bring to the surface poor unfertile soil which reduces soil fertility.
• Soil organism e.g. nematode suck plant sap and block the transport tissues thereby hampering growth.

57. (a) Distinguish between bulk density and particle density in soil.

Bulk density is the weight per unit volume of a dry sample of soil in its natural structure intact while particle density is the mass per unit volume of soil solids.

(b) Explain factors that influence bulk density

• Organic matter content: organic matter is very light and spongy and so increase in organic matter content lowers bulk density of the soil.
• Soil particle size: clay soil with tiny particles have higher bulky density than sand with large particles
• Porosity of the soil: porous soil has lower bulky density than non-porous soil
• Soil depth: bulk density tends to increase the deeper one goes into the soil profile due to lower organic matter and high density of the underlying rocks.
• Level of cultivation: intensive cultivation increases bulk density because it causes rapid break down of organic matter and cause compaction of the soil
• Systems of soil management: addition of manure in large amounts to soil lowers its bulk density.
• Cropping systems: continuous cropping reduces the amount of organic matter in the soil and increases the bulk density.
• Amount of air in the soil: the higher the air in the soil, the higher the bulk density
• Soil structure: the more compacted the soil structure, the higher the bulk density
• Mechanization: use of heavy machines such as tractors during cultivation compact the soil increasing its bulk density.

(c) Suggest activities that can lead to destruction of soil structure

• Pollution of the soil such as application of fertilizers and insecticides.
• Ploughing soil at high moisture content results into mingling of the soil especially those that contain high clay content
• Excessive leaching especially calcium and iron oxides that bind soil particles
• Continuous cultivation of the soil break up soil aggregates
• Bush burning destroy the organic matter and expose the soil to agents of erosion
• Soil erosion erodes the aggregates destroying their shape
• Use of heavy machinery compact the soil and destroy soil structure.
• Water logging prevents organic matter from decomposing encouraging crumbling of soil structure

58. (a) Explain ways through which soil becomes acidic
    • Application of organic manure: decomposition of organic manure produces acidic substances
    • Application of Sulphur and nitrogen containing inorganic fertilizers
    • Leaching of bases such as calcium, magnesium and potassium leaving aluminium ions.
    • Water logging cause anaerobic respiration that produce carbon dioxide and lowers pH
    • Acidic rain
    • Acidic parent rock from which soil is formed
    • Uptake of bases by the plant roots and replaced by hydrogen ions from the plants.
    • Release of acidic industrial and domestic wastes.
    • Water logging causes acidic hydrolysis of salts such aluminium salts.

(b) What is the importance of lime to the soil?

• Improve soil structure by binding soil particles
• Neutralizes acidic soils
• Supplies calcium to the soil
• Binds soil particle to make aggregates that improves soils aeration and drainage.
• Prevents some plant diseases such as fungal diseases
• Makes availability of plant nutrients such phosphorus and nitrogen
• Make clay soil less sticky and easy to till
• Promotes soil organism’s activity and nitrogen fixation in root nodule
• Promotes decomposition of organic matter.
• Reduces toxicity of aluminium

(c) Explain factors to be considered before applying lime to the soil

• pH of the soil; the amount of lime required depends on the acidity of the soil
• size of particles of limestone or carbonates: this affects the solubility
• type of crops grown; different crops require different pH
• the amount of manganese and aluminium present
• The texture of the soil: fine textured soils have high cation adsorption capacity and less leached than coarse textured soil so more lime is applied.
• The amount of organic matter in the soil. Soil with much organic matter requires more lime because it has high cation adsorption capacity.
• Depth of the soil: deeper soil experiences high rate of leaching and thus requires larger amounts of lime.
• Frequency of application of lime; infrequent application of lime requires high amount per application.
• Fineness of limestone or carbonate used

59. (a) Distinguish between soil texture and soil structure (02marks)

Soil structure refers to the fineness or coarseness of the mineral particles of the soil while refers to the general arrangement of soil particles within a soil mass to form aggregates.

(b) Explain how soil structure can be destroyed.

• Continuous cultivation of the soil breaks soil aggregates
• Ploughing soil with high moistures breaks soil aggregate
• Soil erosion washes away top soil
• Poor sol drainage limits soil organisms that would otherwise enhance soil structure
• Excessive leaching of calcium and iron that bind the soil aggregate
• Pollution that kill microorganism that would otherwise enhance soil structure
• Use of heavy machines that deform/compact soil
• Poor harvesting practices that do not add organic matter to the soil

(c) Explain how soil structure can be maintained.

• Minimum tillage
• Growing cover crops to reduce soil erosion
• Mulching to reduce soil erosion, maintain soil moisture and add organic manure to the soil.
• Bush fallowing with grasses so that fibrous roots bind soil particles
• Proper drainage to remove excess water that cause dispersion
• Liming to bind soil particles
• Controlled irrigation to provide soil moisture
• Controlled grazing to prevent soil compaction and erosion

60. (a) What are the causes of low soil productivity/fertility?
    • Soil erosion leads to loss of crop nutrients
    • Leaching leads to inaccessibility of soil nutrients
    • Poor drainage may lead to water logging and hence poor soil aeration and unfavorable soil temperature.
    • Poor tillage destroys soil structure leading to leaching
    • Unfavorable pH
    • Soil capping i.e. development of impervious layer on the soil surface
    • Development of hard pans or impervious layers below the soil surface
    • Lack of required soil nutrients
    • Presence of pests and disease causing organisms in the soil
    • Presence of weeds which compete which compete with plants for nutrients
    • Lack of adequate soil moisture.
    • Mono-cropping cause soil exhaustion and buildup disease causing organisms
    • Burning of vegetation destroys soil living organisms
    • Excessive irrigation cause leaching
    • Excessive drainage lead to loss nutrients
    • Salinity of the soil

(b) Explain how soil productivity can be improved

• Through application of manure and fertilizers
• Practicing crop rotation to promote usage of different nutrients
• Irrigation to provide water in the soil.
• Drainage of excess water
• Modifying soil pH
• Prevention of soil erosion
• Control pests and diseases
• Proper cropping like preventing overcrowding
• Applying effective method of water conservation

61. (a) Explain the benefits of draining agricultural land
    • Improves soil aeration
    • Prevents buildup of toxic substance from decomposition of organic matter.
    • Cause favorable soil temperature that promote germination and plant growth
    • Frees soil of excess water
    • Make the soil lighter and easy to till
    • Helps control pests and diseases
    • Discourage leaching
    • Facilitate growth of plants that do not require water logged contions
    • Reduces soil erosion
    • Prevents rotting of roots.
    • Prevents accumulation of toxins

(b) What problems may result from draining agricultural land?

• May lower water table
• May lead to death of water loving organisms such as frogs
• Lead to loss of dissolved nutrients
• May dry up soil
• May cause unfavorably high temperature
• Salinization of the soil surface
• Upset ecosystem

(c) Outline the disadvantages of using surface drainage channels in draining land.

• Waste land since channels are constructed on the soil surface
• Ditches interfere with agricultural operations and livestock movement
• The open channels are prone to silting
• Expensive to maintain
• Require leveling before water can flow into the drains.

62. (a) Explain what should be considered when designing a crop rotation
    • Nutrient requirement of the crops
    • Rooting system of crops: deep and shallow rooted crops should alternate.
    • Botanical families to which the crops belong: crops should be rotated from one family to another.
    • Cover crops: the rotation should include cover crops to control soil erosion
    • Pest and disease control: crops affected by the same pests and diseases should not be grown in succession.
    • Rest phase should be included in the rotation
    • Growth habits of the crops: crawling crops should be alternated with erect crops.
    • Water requirement of the crops: crops that required a lot of water should be grown during wet seasons and those that require less water in dry seasons.
    • Ease of weeding: crops which are easy to weed should alternate with crops difficult to weed.
    • Inclusion of legumes such as beans, peas, groundnuts in the cycle to add nitrogen

(b) Explain the role of crop rotation in maintaining soil fertility

• Rotating crops of different rooting system enables plants to absorb nutrients at different soil layers
• Different crops prefer different nutrients which prevent soil exhaustion.
• Some plants like legumes fix nitrogen to the soil
• Crops rotation improves soil structure such as those with fibrous roots.
• Crop rotation controls pests and diseases by breaking their life cycles
• Crop rotation controls some weeds such as striga
• Resting helps regain fertility
• Nutrient recycling i.e. deep rooter against shallow rooter
• Conserve soil moisture when cover crops are planted
• Control soil erosion by maintain soil structure or planting cover crops,

63. Explain how each of the following affects crop growth

(a) Soil pH on crop production

• • Affects availability of plant nutrients e.g. low pH makes phosphorus and molybdenum unavailable while high pH makes manganese and potassium less available.
  • At low pH, iron and aluminium become excessively available and become toxic to the plants.
  • Very low pH inhibits nitrogen fixation
  • pH determines the types of crops that can grow in an area e.g. tea and pineapples prefer low pH.
  • Control prevalence of disease causing organisms, e.g. fungal disease are common in acidic soils.
  • It influences the type of fertilizer to be applied e.g. sulphates of ammonia should not be applied to acidic soil.

(b) Soil temperature on crop production

• • warm temperature encourages decomposition of organic matter to release plant nutrients
  • low temperature discourages germination
  • high temperature encourage drying of the soil
  • increase in temperature increase cell wall permeability
  • low temperature lowers the rate of photosynthesis and respiration
  • very high temperature cause wilting and death of the plant
  • affects solubility of nutrients
  • affects absorption of water and nutrients i.e. warm temperature encourages absorption
  • alternate hot and cold temperature promotes weathering
  • Influence nutrient loss from the soil for instance volatile ammonium compounds are lost on hot weather.

(c) Soil organisms on crop production

• • fix nitrogen to the soil
  • decompose organic matter to release nutrients
  • some cause diseases to the crops
  • denitrifying bacteria reduce nitrogen from the soil
  • compete for oxygen in the soil with roots
  • some organisms produce toxic substances that cause disease to the crops

64. Explain the factors to consider when deciding on which crop to grow.
    • Soil type; different crops prefer different soils
    • Rainfall
    • Temperature
    • Prevent weed
    • Labor requirement
    • Food value
    • Taste preference
    • Soil fertility
    • Cost of production
    • Available size of land
    • Government policy – the government may dictate crops to be grown
    • Altitude
    • Land ownership may determine whether to grow annual or perennial crop
    • Price of the crop
    • Purpose of the crops either for animals or human or drug
    • Gestation period
    • Culture for instance Baganda prefer growing bananas

 

65. (a) Discuss factors that affect soil porosity
    • Soil texture: very fine soil texture such as clay soil lead less porous soil
    • Level organic matter in the soil: high organic matter content increase soil porosity by promoting soil aggregate
    • Frequent tillage promotes soil porosity by breaking up soil particles
    • Soil organisms like earth worms burrow and levels tunnels in the soil improving soil porosity.
    • Soil depth: soil porosity tends to decrease and the soil depth increase.

(b) Means by which plant root take up nutrients

• Simple diffusion
• Facilitated diffusion
• Active transport
• Root interception

 

66. (a) Outline six properties of humus
    • Dark brown
    • Odorless
    • Colloidal
    • Insoluble in water
    • soluble in dilute alkali giving a dark colored solution
    • contains modified plant residues and synthesized microbial compounds
    • has high cation exchange capacity
    • has high water absorbing capacity about 5-7 that of clay
    • has high relative molecular mass
    • light amorphous material with very low bulk density(0.2 -0.5mg/g)

(b) Give five uses of humus in the soil

• it is a source of N, P, S and most micro-nutrients
• it increases cation exchange capacity (CEC) of the soil because organic matter is negatively charged.
• Source of food for soil organisms
• It increases water holding capacity especially of sand soil
• It improves soil structure by binding soil particles together
• Ti reduces the impact of compaction and capping in soil
• It buffers pH change
• It promotes root penetration into the soil because it keeps soil soft

(c) Describe any appropriate test that can be used to find humus content of the soil

Procedure

• Dry the soil sample (w g) in an oven at 105⁰C to a constant weight x g
• Heat the soil until red hot to a constant mass y.
• Percentage of humus =

67. Suggest the factors which influence soil erosion
    • The amount/density of rainfall: in areas which receive heavy rainfall, the rain drops hit the ground with much force causing splashing of the soil and the soil is carried in running water
    • Steep relief encourages soil erosion
    • Type of soil: loose soil particles are easily carried away by the agents of soil erosion.
    • Vegetation cover minimize soil erosion

 

Thank U

Dr. Bbosa Science