Soil is defined as the thin surface of the uppermost layer of the earth’s crust on which plants grow.

Components of the soil

The soil is made up of five components which are:

  • Mineral or Inorganic Matter.
  • Organic matter.
  • Soil water.
  • Soil air.
  • Living organisms.


  1. Mineral or Inorganic Matter: The mineral matter represents small rock fragments of the soil. It forms the bulk of about 45% of total volume of the soil. It consists of gravel, stones, sand, silt and clay.

Importance/Effects of Mineral Matter on Agriculture

  • It forms the solid part of the soil which provides support for plants.
  • Mineral matter is the main source of plant nutrients such as nitrogen, calcium, magnesium, iron, etc.
  • It represents the home or habitat of all soil living organisms.
  • It holds water and air for both plants and animal activities.
  • Mineral matter has moderating effects on soil temperature.
  • It also affects soil porosity.


  1. 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 crops, animal dung, etc. when they are deposited on the soil, decay to form a dark colour on the upper part of the soil. This becomes organic matter which is also called

Importance/Effects of Organic Matter on Agriculture

  • It is very rich in plant nutrients.
  • It is the habitat of many soil micro-organisms.
  • It prevents leaching in soil where it is present in adequate amount.
  • It also prevents soil erosion and evaporation of soil water.
  • It allows for good drainage and holds water in the soil for plant use.
  • It improves the structure of the soil by binding the particles pf coarse texture soil together.
  • It increases water holding capacity of the soil.
  • It moderates the soil temperature which stimulates good development and growth of roots.
  • It has buffering effects that moderates pH values.


  1. 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 A waterlogged soil can however be improved by drainage to make such soil more productive. On the other hand, when there is complete lack of water in the soil for a very long time such 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:

  • Hygroscopic water: This water is tightly held by the soil particles such that it is never available to the plant.
  • 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.
  • 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.
  • 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

  • Water is an important agent of weathering of rocks in the soil.
  • It helps to dissolve plant nutrients into solution form which can easily be absorbed by plant roots.
  • Water is an essential raw material for photosynthesis.
  • Hydrolysis of many food nutrients and enzymatic activities are promoted by the presence of water.
  • Loss of water from plant through a process called transpiration is helpful in the cooling of the plants.
  • Presence of water aids easy tillage of the soil and also helps to improve the soil structure.
  • It also promotes the activities of soil organisms.
  • It is needed for the germination of seeds.
  • It provides the medium for soil reactions.
  • It aids the turgidity of cells.
  • It protects plants from injurious effects of high temperatures.


  1. 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

  • Soil air, especially oxygen is necessary for the growth and development of plants.
  • Oxygen in the soil promotes easy germination of seeds.
  • Soil organisms require oxygen for respiration.
  • Excess of carbon dioxide in the soil, when combined with water can cause acidity and aid weathering of rocks.
  • Some plant disease organisms such as Fusarium which causes damping-off are favoured by poor aeration.
  • It is needed in soil reactions, particularly carbon and nitrogen cycles.


  1. 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, nematodes, insects (e.g. termite, soldier ant), millipede, centipede, earthworm, snails, reptiles, mammals (e.g. rats and rodents).

Importance/Effects of Living Organisms on Agriculture

  • They help to improve the aeration of the soil.
  • They help to decompose organic materials in the soil to form humus.
  • They aid percolation of water.
  • They improve the structure of soil.
  • Some organisms like bacteria help to fix nutrients in the soil.
  • Some are pests and parasites while some cause diseases of crops.
  • Some soil micro-organisms produce acidic materials which help to break down rocks.



There are three main types of soil. These are:

  • Sandy soil.
  • Clay soil.
  • Loamy soil.


  1. Sandy Soils
  • A soil is said to be sandy if the proportion of sand particles in a sample of soil is very high.
  • The particles are mainly quartz (Si02).
  • They have the size of 2.0mm in diameter.
  • Sandy soil is not very good for farming because, it is poor in plant nutrients.

Properties of Sandy Soils

  • Sandy soil is coarse, grainy and gritty.
  • It is loose with large pore space.
  • It is well aerated and cannot hold water.
  • Percolation is high but capillarity is low.
  • It is not sticky when wet.


  1. Clay Soils
  • A soil is said to be clayey if the proportion of clay in a sample of soil is very high.
  • The relative size is less than 0.002mm in diameter.
  • It is a heavy soil because it is very difficult to work on or cultivate.

Properties of Clay Soils

  • Clay soil is fine grained and smooth.
  • Particles are tightly packed with little pore spaces.
  • It is poorly aerated and can hold water.
  • Percolation is low but capillarity is high.
  • It is sticky when wet and hard when dry.
  • It is usually waterlogged.
  • Clay soil is impermeable.
  • It cracks when dry.
  • It could be red or whitish in colour.
  • It has poor drainage.


  1. Loamy Soil
  • Loamy soil is a mixture of sand, clay with high proportion of organic matter.
  • Loamy soil is the most fertile and the best soil for farming.

Properties of Loamy Soil

  • Loamy soil is moist and loose.
  • It contains lots of organic matter (humus).
  • It does not support erosion and waterlogging.
  • It is well aerated and can hold water.



Certain characteristics are used to identify or describe soil sample. These characteristics are:

  1. Soil colour: The first observation made when a soil profile is dug is soil colour. Colour varies from one soil to another. There are brown, black, red and yellow. Black and dark brown soils are known to contain more nutrients while red or yellow soils are poor in plant food.
  2. Soil texture: This refers to the relative proportion (or sizes) of the various particles of the soil. In other words, it refers to the degree of fineness or coarseness of the various particles. The particles that make up a soil sample include gravel, sand, clay and silt.
  • Clay (below 0.002).
  • Silt (0.002 – 0.02).
  • Fine sand (0.02 – 0.2).
  • Coarse sand (0.2 – 2.0).
  • Gravel (above 2.0).
  1. Soil Structure: Soil structure refers to the ways in which the different particles of the soil are packed or arranged. The various types of soil structure include single grain, crumb, platelike, spheroidal, prismatic, block like and columnar structures.
  2. Permeability: This is the ability of water to pass through a sample of soil. While sandy soil is permeable, clay is not.
  3. Porosity: This refers to the presence of pore spaces in a sample of soil. While sandy soil is porous, clay soil is not.
  4. Capillarity: This is the upward movement of water in the soil. While sandy soil has poor capillarity because of its large pore spaces, clay soil has good capillarity because of its tiny pore spaces.
  5. Soil pH: This refers to the degree of acidity and alkalinity of the soil. The pH scale is graduated from 1 – 14. Values between 1– 6 are acidic, 7 is neutral while 8 – 14 is alkaline. Soil acidity is caused as a result of the presence of abundance of hydrogen ions (H+) while soil alkalinity is caused due to the presence of hydroxyl ions (0H-) in the soil.


Process of Soil Formation

The process of soil formation involves the following stages:

  1. Weathering of parent materials: The process of soil formation starts with weathering of parent materials (rocks). Weathering is the gradual breaking down of rocks into tiny pieces by physical, chemical and biological factors in situ.
  2. Action of chemicals: The presence of agents of chemical weathering like reaction of carbon dioxide, water, oxygen etc. also acts on the broken rock particles.
  3. Further disintegration: These agents of weathering present in the soil also help to weaken and further break the rocks particles into smaller and tiny pieces.
  4. Appearance of biological activities: With time, bacteria including other soil organisms and plant life begin to appear in the soil. Increased activities of these organisms further promote the formation of soil.
  5. Formation of humus: When soil organisms and plants die, they decay with the aid of bacteria and other micro-organisms to form organic matter called humus.
  6. Final soil formation: The combination or mixture of these weathered rock particles and the dead plants and animals (humus) results in the final formation of the soil.


Factors of Soil Formation

The rate, type and the quality of soil formation in a particular place depend on the following factors:

  1. Parent Materials
  • The type, texture and mineral composition of soil are influenced by the nature of the parent materials.
  • Parent material influences the depth of soil.
  • It affects the texture of soil.
  • It affects the water-holding capacity of the soil.
  • It is responsible for the soil chemical content.
  • It is the main source of mineral particles in soil.


  1. Topography or Relief
  • The topography or shape of the land has direct effect on soil formation.
  • Soils are thinner and less developed on steeper slopes.
  • Deeper and more developed soil in low-lying areas.
  • Relief produces differences found in the soil.
  • Relief influences the distribution of organic matter and the parent materials.
  • It also affects the soil solutions.


  1. Climate: Elements of climate which aid soil formation include rainfall, temperature and wind.
  • Temperature: The alternate heating and cooling of rock results in rock breakdown.
  • Rainfall: The action of running water results in wearing away of rocks.
  • Winds: The action of wind erosion, especially in arid and semi-arid areas also helps to breakdown rocks.
  • Climate: Determines the extent of weathering, influences soil profile development e.g. leaching.
  • Excessive water: Leads to formation of water-logged soil.
  • Major soil zones: Are influenced by major climate zones.


  1. Vegetation
  • The roots of plants penetrate rocks which break them down into tiny pieces.
  • The decaying of fallen leaves results in the formation of humus in the soil.
  • Different types of vegetation are associated with different types of soils.


  1. Biotic or Living Organisms
  • Termites, earthworm, rodents, etc. penetrate rocks and break them down into tiny pieces.
  • The activities of micro-organisms during decomposition also aid soil formation.
  • The activities of man during mining, tillage, grazing, etc. do aid soil formation.


  1. Time
  • It takes a long time for small pieces of rocks to disintegrate into grains of rocks.
  • It also takes a long time for plants to decay and become part of the soil.



Soil profile is defined as the vertical section through the soil to the underlying rocks, showing series of horizontal layers. These horizontal layers are called Horizons.

A typical soil profile has three layers or horizons which are A, B and C horizons.


  1. A – Horizon is called Top soil
  • It represents the surface layer of the soil profile.
  • It contains more humus or organic matter.
  • It is the zone of elluviation (elluvial horizon) where minerals are leached down the profile.
  • It has maximum biological activities of plants and soil micro-organisms.


  1. B – Horizon is called Sub-soil
  • It is the zone of illuviation (illuviation horizon) where soil minerals accumulate leached from the A – horizon.
  • It contains lots of mineral matter.
  • There is the conversion of organic matter from A – horizon into inorganic matter.
  • This zone may contain hard pan or is leached.


  1. C – Horizon is also called zone of parent materials
  • It contains weathered parent rocks.
  • It represents materials from which top and sub-soils are formed.
  • It underlay C – horizon solid parent rocks called


Importance of Soil Profile

  1. It determines the level of fertility of the soil e.g. a thick top soil represents high level of soil fertility.
  2. It helps the farmers to know the type of crops to grow.
  3. It determines the level of root penetration, drainage and aeration.
  4. It determines the rate of percolation of water and the level or degree of soil erosion.

Importance of Soil

  1. Agriculture: The soil provides the only medium through which agriculture is practiced to provide food for man.
  2. Building: All building are sited and erected on the soils.
  3. Construction: Various forms of construction like roads, industries, railway, etc. are constructed on the soil.
  4. Vegetation: All vegetation types form their basis on soils. Without soil, plants cannot get support and nutrients for growth.
  5. Storage of nutrients: Soil provides plants and crops with all the nutrients for their continuous existence.