Agricultural Science
Topic: Soil: Types, Composition (Components), Classification And Properties Of Soil
Contents

• Composition (Components) Of The Soil
• Soil Types And Their Properties
• Laboratory Experiments On Soil
• Properties Of The Soil

Objectives
At the end of this article, readers should be able to:
1. State the composition of the soil and their importance.
2. List the various types of soil and their properties.
3. Define, sketch, describe and state the importance of soil profile.
4. Define and discuss soil texture, soil structure, soil temperature, soil pH.
5. Discuss basic principles of soil classification.
6. List the physical components of soil and state beneficial effects of each on Agriculture.
Composition (Components) Of The Soil
The soil is made up of five components which are:
1. Inorganic (mineral) matter
2. Organic matter
3. Soil water
4. Soil air
5. Living organisms.
Mineral or organic matter, organic matter, water and air are collectively referred to as Physical components of the soil, while living organisms are referred to as biological components of the soil.
Percentage of Soil Component by Volume
The percentage of the various soil components by volume is as follows:
1. Mineral matter = 45%
2. Water = 25%
3. Air = 25%
4. Living organisms = 5%
Total = 100%
1. Mineral or Inorganic Matter: The mineral matter represents small rock fragments of the soil. It forms the bulk of 45% of total volume of the soil. It consists of gravel, stones, sand, silt and clay.
Importance/Effects of Mineral Matter on Agriculture
1. It forms the solid part of the soil and provides support for plants.
2. Mineral matter is the main source of plant nutrients such as nitrogen, calcium, magnesium and iron.
3. It represents the home or habitat of all soil living organisms.
4. It hold water and air for both plants and animasl activities.
5. It also affects soil porosity.
6. Mineral matter has moderating effecrs on soil temperature.
2. Organic Matter: The organic matter represents the remains of the decomposition of plants and animals. It is about 5% of the total volume of the soil. Leaves, roots of plants, the residue of cropsm animal dung etc, when deposited on the soilm decay to form a dark colour on the upper part of the soil known as organic matter or humus.
Importance/Effects of Organic Matter on Agriculture
1. It is very rich in plant nutrients.
2. It is the habitat of many soil micro organisms.
3. It prevents leaching in soil where it is present in adequate amount.
4. It also prevents soil erosion and evaporation of soil water.
5. It allows for good drainage and holds water in the soil for plant use.
6. It increases water-holding capacity of the soil.
7. It improves soil cation exchange capacity.
8. It improves soil aeration.
3. Soil Water: Soil water refers to the water in the soil which is usually obtained either from rain or irrigation. Water represents 25% of the total volume of the soil. It is usually found in the soil within the pore spaces. When water is too much in a soil (covering the soil surface), the soil is said to be waterlogged. A waterlogged soil can, however, be improved by drainage to make such soil more productive. On the other hand, a situation where there is lack of water in the soil for a very long time, to the extent that plants cannot absorb water even when supplied again, results in a condition called permanent wilting point. The plant at this stage can die.
Types of Soil Water
There are four major types of soil water. These are:
1. Hygroscopic water: This water is tightly held by the soil particles such that it is never available to the plant.
2. Field capacity: This is the type of water left in the soil after excess water has been drained off, following heavy rainfall. This water is available to the plants.
3. Capillary water: This is the water which rises above the water table in the soil and it is held in the fine and medium pores of soil particles by surface tension. Capillary water is easily available to plants.
4. Gravitational water: This is the water which can drain from the soil under the influence of gravity. It is available to plants but is often pulled down beyond the reach of the roots.
Importance/Effects of Soil Water on Agriculture
1. Water is an important agent of weathering of rocks in the soil.
2. It helps to dissolve plant nutrients into solution form which can easily be absorbed by plant roots.
3. Water is an essential raw material for photosynthesis.
4. Presence of water aids easy tillage of soil and also helps to improve the soil structure.
5. It aids the turgidity of cells.
6. It protects plants from injurious effects of high temperature.
4. Soil air: This refers to the gases present in the soil pores found between the soil particles. The amount of soil air varies, depending on the amount of soil water, the sizes of the pore spaces, the type of soil and the amount of living organisms in the soil. The percentage of air is about 25% of the total volume of the soil. The ability of air to circulate freely in the soil is called aeration.
Importance/Effects of Soil Air on Agriculture
1. Soil air, especially oxygen, is necessary for the growth and development of plants.
2. Oxygen in the soil promotes easy germination of seeds.
3. Soil organisms require oxygen for respiration.
4. Excess of carbon dioxide in soil, when combined with water can cause acidity and aid weathering of rocks.
5. Some plant-disease organisms such as fusarium which causes damping-off are favoured by poor aeration.
6. It is needed in soil reactions, particularly carbon and nitrogen cycles.
5. Living Organisms: These refer to plants and animals which inhabit the soil. They range from microscopic organisms to bigger organisms. Some are beneficial while others are harmful to crops and livestock. The most commonly found groups of soil organisms include bacteria, fungi, virus, nermatodes, insects (e.g, termite, soldier ants), millipede, centipede, earthworm, snails, reptiles, mammals (e.g. Rats and rodents).
Importance/Effects of Living Organisms on Agriculture
Soil organisms are very useful in many ways, especially in soil formation and improving the soil for the growth of crops. Their effects are:
1. Soil organisms improve soil structure and granulation.
2. They also improve the aeration of the soil.
3. They help to decompose organic materials in the soil to form humus.
4. They improve soil water percolation or drainage.
5. They stabilize soil pH through the increase in soil organic matter and buffering.
6. They increase the mineral or nutrient status of the soil.
Soil Types And Their Properties
There are three main types of soil. These are sandy soil, clay soil and loamy soil.

1. Sandy Soil
Definition: Soil is said to be sandy if the proportion of sand particles in a sample of the soil is very high.
Properties Of Sandy Soil
1. Sandy soil is coarse, grained and gritty.
2. It is loose with large pores spaces.
3. It absorbs and loses water easily.
4. It is well aerated with low water holding capacity.
5. Sandy soil has grey or brownish colour.
6. It supports leaching. Hence, it is low in plant nutrients.
Methods of Improving Sandy Soil
Sandy soil can be improved through the following agricultural practices:
1. Planting cover crop: Cover crops help to provide shade, prevent erosion and add more nutrients to the soil.
2. Application of compost manure: Compost manure helps to bind the sand particles together and also add humus (nutrients) to the soil.
3. Application of farm yard manure: This will also improve the structure of the soil as well as add nutrients to the soil.
4. Mulching the soil: Mulching sandy soil prevents water loss through evaporation and nutrient loss by water erosion.
5. Avoidance of bush burning: Bush burning promotes soil erosion, kills soil organisms and removes organic matter which can contribute to the fertility of the soil.
Economic Importance of Sandy Soil
1. It is good for the cultivation of few crops such as cotton, groundnut, cassava, etc.
2. It is also useful in building construction, especially its combination with cement and water in block-moulding.

2. Clay Soil
Definition: Soil is said to be clayey if the proportion of clay in a sample of the soil is very high. The relative size of a clay particle is less than 0.002mm in diameter. It is a heavy type of soil because it is difficult to work on or cultivate.
Properties of Clay Soil
1. Clay particles are fine, powdery and smooth when dry.
2. The particles are sticky and mouldy when wet.
3. The particles are tightly bould together with little pore (air) spaces.
4. The structure is granular and does not lose water easily.
5. It supports water-logging and erosion.
6. It is hard when dry and sticky when wet.
Differences Between Sandy soil and Clay soil
1. Sandy soil is coarse-grained while Clay soil is fine grained.
2. Clay soil has high capillarity while Sandy soil has low capillarity action.
3. Sandy soil has high percolation rate while Clay soil’s percolation is low.
4. Sandy soil is gritty while Clay soil is smooth to the touch.
5. Sandy soil’s leaching is high while Clay soil is not easily leached.
6. Sandy soil has large pores while Clay soil has small/fine pore spaces.
7. Sandy soil is easy to work while Clay soil is difficult/heavy to work.

3. Loamy soil
Definition:Loamy soil is a mixture of sand and clay particles with high proportion of organic matter. Loamy soil is more fertile than either clay or sandy soil. If a type of soil is described as sandy loam, it means that the proportion of sand is high and if it is clay loam, it shows that the proportion of the clay is high, while that of sand is low.
Properties of Loamy Soil
1. Loamy soil is moist, loose with moderate-sized pores spaces.
2. The structure breaks easily when wet and friable when dry.
3. It can be easily be worked on or cultivated.
4. Is has non-powdery and non-sticky texture.
5. It is dark-brown or black in colour.
6. It is well aerated and it can hold water.
Laboratory Experiments On Soil
1. Experiment 1
Title of experiment: To determine the moisture content of a moist sample of soil.
Materials Required:
Soil sample, weighing balance, evaporating dish, stirring rod, wire gauze, bunsen burner tripod stand.
Method: Weigh the mass of the empty evaporating dish. Place a sample of soil into the dish and reweighed​. Heat the dish with the soil with a bunsen burner for two to three hours. Stir the soil constantly with the glass rod to aid evaporation. At the end of the period, the dish with the soil is cooled in a desiccator and reweighed.
Conclusion: Water is one of the components of soil, but the quantity varies with different types of soil. It is higher in clay soil than in sandy soil.
Experiment 2
Title of experiment: To compare or determine capillary action of soil types.
Materials Required: Three long glass tubes, cotton wool, trough, stop clock, water, dry sand, dry clay and dry loam.
Method: Close one end of the three glass tubes with cotton wool and then fill each of the tubes with clay, sandy and loamy soils seperately. Each of the tubes containing different grounded soils are immersed at one end into the trough containing water. The end containing cotton wool is the area to be immersed into water.
The experimental set-up is allowed to remain for three to six hours.
Observation: At the early stage of the rise of water in the different tubes, it is observed that the rise of water (capillary action) is faster in sandy soil than the other two. But after many hours or at the end of the experiment, clay and loamy soil experienced higher rise in water but very low rise in sandy soil.
Conclusion: Clay and loamy soils have greater capillary action due to their tiny pore spaces. In addition, loamy soil contains some organic matter while sandy soil has poor capillary action due to large pore spaces and large particle sizes.
Importance of Water to Crop Plants
1. Water is an important agent of weathering of rocks in the soil.
2. It helps in dissolve plant nutrients into solution form, which can easily be absorbed by plant roots.
3. Water is an essential raw material for photosynthesis.
4. It aids the turgidity of cells.
5. It provides the medium for soil reactions.
6. It protects plants from injurious effects of high temperatures.
3. Experiment 3
Title of Experiment: To demonstrate the presence of living organisms in the soil.
Materials Required: Rubber cork, string, conical flasks, fresh moist soil, heated soil, lime water and cellophane bag.
Method: Place some fresh garden soil in a piece of cellophane bag or a piece of cloth and another heated soil taken and placed in another cellophane bag. Both bags are tied up with a string and placed inside conical flask containing lime water. The other end of the flask is corked to make sure that no gas or air enters the flask. Allow the set-up to remain for four to six hours. The experiment which contains heated soil serves as the control experiment. The sample is heated to kill all organisms in soil sample.
Observation: The lime water in the fresh garden soil turns milky which shows that carbon dioxide is released from the soil sample into the flask through respiration by the living organisms in the soil sample. The lime water in the heated soil remains the same because al the living organisms have been killed by heating the soil and therefore, no carbon dioxide is given out.
Conclusion: The soil also contains living organisms.
Importance of Soil Living Organisms to Farmers
1. Living soil organisms help to improve the aeration of the soil.
2. They help to decompose organic materials in the soil to form humus.
3. They aid percolation of water.
4. They improve the structure of the soil.
5. Soil micro-organisms produce acidic substances which help to break down rocks.
6. Some organisms like bacteria helps to fix nutrients in the soil.
4. Experiment 4
Title of experiment: To determine the percentage of organic matter content in the soil.
Materials Required: Soil sample containing organic matter, evaporating dish, bunsen burner, tripod stand, wire gauge, stirring rod, weighing balance and desicator.
Method: Weigh the empty evaporating dish, place some soil sample into the dish and reweigh. After that, the dish containing the soil is heated strongly with bunsen burner to remove water. During the second heating, stir with the rod until the soil is completely burnt during which the organic matter is converted into gases and escapes into the atmosphere. After heating the dish and the content is cooled in the desicator, reweigh.
Conclusion: The soil contains some quantity of organic matter (humus).
Importance/Effects of Organic Matter on Agriculture
1. It is very rich in plant nutrients.
2. It is the habitat of many soil micro-organisms.
3. It has buffering effect that moderates pH values.
4. It prevents leaching in soil where it is present in adequate amount.
5. It moderates the soil temperature which stimulates good development and growth of roots.
6. It increases water-holding capacity of the soil.
Properties Of The Soil
Properties of the soil include soil texture, soil structure, soil temperature, porosity, soil colour, water-holding/retaining capacity and soil pH.
The properties of the soil are grouped into two:
1. Physical properties of the Soil: These properties include soil texture, soil structure, soil temperature, porosity, soil colour and water holding/retaining capacity.
2. Chemical properties of the Soil: These include the soil pH and cation exchange capacity.
Soil Profile
Definition: Soil profile is defined as the vertical section of the soil, showing series of horizontal layers of different types of soil. These horizontal layers are called horizons.
Horizons of Soil Profile
The major horizons of soil profile are:
1. The A-Horizon: Also called the top soil, represents the surface of the soil profile. This horizon contains more organic matter than other horizons. Most food crops, especially shallow-rooted crops derive their nutrients from this top soil, e.g., vegetables, legumes, etc.
2. B-Horizon- The B-horizon, also called the sub-soil, is the next horizon immediately after the top soil. It is rich in minerals which are carried or leached down by percolating water. The B-horizon is suitable for cultivation of deep rooted crops like cocoa, rubber, orange, oil palm, etc.
3. C-Horizon: The C-horizon, also called parent materials, represents the type of material from which top-soil and sub-soil are derived. It is the parent materials which are small fragments of rocks that are unweathered and found at the bottom of the soil profile.
4. D-Horizon: The D-horizon is also called the bedrock, It represents the unweathered rock materials. This horizon is found at the bottom of the profile and they are usually of large soil particles.
Importance of Soil Profile
The suitability of a soil for agriculture is determined by looking at the soil profile. The importance of soil profile includes:
1. The level of fertility: Soil profile determines the level of soil fertility. A thick top soil represents high level of soil fertility.
2. To know the type of crop to grow: It helps the farmers to know the type of crop to grow. For example, shallow-rooted crops like cowpea, groundnut, etc are grown in the top soil, while deep-rooted crops are grown where the sub-soil is thick.
3. Penetration of roots: A loosely packed sub-soil allows for easy penetration of roots of crops.
4. Level of drainage and aeration: A loosely-packed sub-soil also allows for easy drainage and aeration.
5. Easy percolation: A loosely-packed sub-soil also ensures easy percolation of water, thereby preventing the occurrence of erosion.
Soil Texture
Definition: Soil texture refers to the relative proportion (sizes) of the various particles of the soil. In other words, it refers to the degree of fineness or coarseness of the various soil particles.
The particles that make up a soil sample include gravel, sand, silt and clay. Sand, silt and clay are usually referred to as the primary particles of the soil.
Determination of Soil Texture:
Soil texture can be determine by various methods. These methods include:
1. By feeling: Take a little sample of soil and rub them between the fore-finger and the thumb. A sharp feel represents the presence of sand while a smooth or powdery feel represents the presence of clay.
2. By mechanical analysis through sieving: The various sizes/fractions present in sample of dried soil can be separated by putting the sample into series of various measured mesh diameters and shaken vigorously.
One starts with the sieve which has the smallest mesh diameter and progresses up the table to the sieve with the largest. The particles which can pass through a particular mesh belongs to the corresponding grade of soil..
3. By sedimentation: A sample of soil is placed inside a glass jar and large volume of water is added and the mixture is vigorously shaken and allowed to settle. At the end large particle like coarse sand and gravel settle at the bottom, while the organic materials float on top of water in the glass jar.
4 By moulding: Mix a sample of soil with little water and dry to mould the mixture. If the mixture is sticky, it shows that clay is present but if it is not sticky and cannot form a ribbon or cast, it shows that sand is present.
Importance of Soil Texture
Soil texture is very important, especially to farmers in the following ways:
1. It is useful in the evaluation of soil ability to supply mineral nutrients.
2. It supports soil micro-organisms essential for plant growth.
3. It determines the type of crop to grow on the land.
4. It enables the farmer to know the type of soil in his farmland.
5. It determines the relative proportion of air and water in the soil.
6. It determines the movement of air and water.
7. It helps to determine tillage practices to be adopted by the farmer.
Soil Structure
Definition: Soil structure refers to the ways in which the different particles of the soil are packed or arranged. It also refers to the shape and arrangement of primary particles to form compound.
Soil structure has a direct effect on crop yield. If the soil structure is good, air will circulate well while waterlogging, erosion and leaching will be reduced. The structure of the soil can be preserved in the following ways:
1. Planting cover crops.
2. Mulching.
3. Application of manure (green manure and lime).
4. Avoidance of over-grazing and erosion.
5. Avoidance of clean clearing with machines.
Types of Soil Structure
The different types of soil structure include:
1. Single-grained structure: In this structure, the primary particles exist in single form and are not cemented together. It is found in sandy soil.
2. Crumb structure: There exist large gravels or stones embedded within the primary particles which are cemented together. It is found in the top soil.
3. Plate-like structure: The primary soil particles are arranged horizontally and flat, resembling plates or leaflets on the top of each other. It is commonly found in the sub-soil.
4. Spheroidal structure: This is also referred to as granular​ structure. The particles are cemented together in a circular​ form with lots of air spaces. It is commonly found in top soil.
5. Prismatic structure: This could be columnar or prismatic. They are just like spheroidal structure with air spaces. When the top ones are round, they are columnar, but when they are flat just like a prism, they are prismatic. It is found in sub-soil.
6. Block-like structure: The aggregates are like blocks whose edges are irregular and may be either sharp or rounded. It is commonly found in the sub-soil.
Importance of Soil Structure
1. It determines the level of fertility of the soil.
2. A good soil structure supports aeration.
3. It also prevents erosion and waterlogging.
4. A good soil structure promotes the activities of soil micro-organisms.
5. A good soil structure has a good water-retaining capacity.
6. It supports the growth of crops.
Soil Temperature
Soil temperature refers to the temperature within the soil. In other words, it is the temperature usually different from the temperature of the air above the soil.
Importance of Soil Temperature to Crop Growth
1. It determines the rate of formation and decomposition of soil organic matter.
2. Low temperature causes a decrease in metabolic activities and reduction in enzymatic reactions in plants.
3. Extreme temperatures will impede the activities of micro-organisms.
4. It affects the level of soil moisture.
5. It affects the absorption of water and nutrients by roots.
6. Optimum temperature promotes seed germination.
7. High temperature may be harmful to crops by causing premature dropping of fruits.
8. It determines the population of the microbes.
Soil pH
Meaning: The word pH (pondus de Hydronium) is defined as a measure of the degree of acidity or alkalinity of the soil. It can also be described as a measure of concentration of hydrogen ions (H+) in the soil. A higher hydrogen ions (H+) concentration indicates soil acidity while a lower concentration of hydrogen ions (OH+) indicates soil alkalinity.
Causes of Soil Acidity
Soil acidity can be caused by any of the following:
1. Leaching: The washing down of plant nutrients below the soil beyond the reach of the plant roots, leaving behind hydrogen ions.
2. Use of acid fertilizers: The use of acid fertilizers like ammonium nitrate can easily cause acidity in the soil.
3. Presence of acid parent materials/ the presence of acid parent materials results in the easy dissolution of the rocks, leaving behind materials rich in hydrogen ions.
4. Nutrient uptake by plants: The absorption of soluble minerals by plants results in the accumulation of hydrogen ions which cause soil acidity.
5. Presence of sulphur in the soil: Sulphur undergoes oxidation and dissolution to form weak acid in the soil.
Removal of Soil Acidity: Soil acidity can be removed by the application of liming materials which are rich in calcium.
Effects of Soil Acidity
1. Low pH or high acidity causes the disintegration of clay minerals which are leached from the soil.
2. Low pH causes the accumulation of aluminium and manganese which may be toxic to plant roots.
3. It reduces the formation of pods in legumes.
3. Humus can easily be leached out of the soil, if the pH is high.
4. It reduces the activities of micro-organisms.
5. It causes reduced growth/yield of crops.
6. There will be lower rate of decomposition of organic matter due to its adverse effect on soil microbes.
Soil Alkalinity: Causes and Removal of Soil Alkalinity
Soil alkalinity is caused when there are excessive quantities of soluble minerals in the soil. Soil which is alkaline is usually found in arid or semi-arid areas like deserts and Sahel savanna.
Soil alkalinity can be removed by the:
1. Application of irrigation to dissolve some of the salts.
2. Application of some acid fertilizers like ammonium sphate.
3. Application of sulphur to the soil.
Crops that Can Do Well in each of the Soil Types
1. In acid soil, iron and aluminium are present in large quantities. Thus, crops which use these elements in large quantities do well. They are said to be acid-tolerate. Examples are: oil palm, swamp rice, sugar cane, maize, rubber, cocoa, banana and plantain.
2. In alkaline soil, very few crops do well. Examples are: millet, sorghum, coconut, cotton, groundnut, cowpea and onion.
3. In neutral soil, only few crops do well. Examples are: cassava, yam, maize, potatoes and oranges.
Soil Classification
Soil is classified into three major groups, namely: zonal soil, intra-zonal soil and azonal soil. The classification is based on climate, vegetation, topography, nature of soil profile, presence of salt and soil types.
1. Zonal soil: This is matured soil which has recognisable soil profile as a result of the influence of climate and vegetation. This soil is further sub-divided into:
a. Pedalfers: This is non-lime accumulating soil.
b. Pedocals: This is lime-accumulating soil. Examples of zonal soil include podsol soil, grey brown soil, chemozem soil, sierozem soil, latosol or red soil, such as laterite.
2. Intra-zonal soil: This soil is formed under special circumstances and conditions such as inadequate drainange which results in waterlogging or salt accumulation, leading to alkalinity. This may result in different types of soil Intra-zonal soil is of three types:
i. Hydromorphic soil: This is characterised by excess of soil moisture which results in waterlogging that may give rise to peat soil.
ii. Holomorphic soil: This type of soil has a high concentration of salt. It is found in drier areas of low rainfall.
iii. Calcimorphic soil: This is also called calcisols. This type of soil has high lime content, e.g, limestone.
3. Azonal soil: This is soil formed by the nature of its parent material rather than climate. It is young soil and does not exhibit any profile. There are three types of azonal soils:
i. Lithosols: These are types of soil formed around mountainous area.
ii. Regosols: These are types of soil formed on deep, soft, unconsolidated area with deposit of mineral matter.
iii. Alluvial soil: These are soil types formed through the deposition of materials by rivers.

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