Soil formation is greatly controlled by five major factors, which are (i) Climate, (ii) Parent materials, (iii) topography, (iv) biotic factors (living organisms), and (v) time

1. Climate:

Elements of climate, such as rainfall, temperature, wind, and pressure, are all very important in soil formation.

a. Temperature: The alternating heating and cooling of rocks results in the continual expansion and contraction, which eventually result in cracks in the rocks and their consequent breakdown into small pieces to form the soil. Temperature affects the rate of chemical weathering of rocks

b. Rainfall: The action of running water from rainfall causes the gradual wearing away of rocks during erosion to form soil. Rainfall provides water for hydrolysis.  Also, raindrops may break down some parent rocks to form soil. Rainfall enhances the vegetative growth of plants whose roots cause further breakdown of rocks, while the rainwater transports rock particles after disintegration.

c. Wind: High-wind velocity in deserts carries with it other tiny rocks which collide with one another or other rocks, resulting in the breaking of rocks into tiny pieces to form soil. Wind also removes weathered materials, thereby exposing parent materials to further breakdown.

d. Pressure: High pressure on a hanging rock may cause such a rock to fall and break into tiny pieces, resulting in the formation of soil.

2. Parent material:

Parent materials constitute the major materials from which soil is formed. They are igneous, sedimentary, and metamorphic rocks.  Parent materials determine the chemical composition of the soil that is formed.  It also contains different minerals, which account for differences in the fertility of the soil formed from each of the different types.  Parent materials determine the physical characteristics of the soil.

Hardness of parent material affects the rate of soil formation.

a. Topography: The shape of the ground about the underlying rock of the earth’s surface is known as topography. Topography affects the rate of run-off and erosion.  Steep slopes encourage erosion and retard soil formation.  Soil formation is faster in the valley than on slopes.  Steepness of the slope affects the rate of abrasion of rocks; hence, soil is formed.

3. Biotic Factors (Living Organisms):

The activities of living organisms help to speed up the process of soil formation.

Termite, earthworm, and rodent mix the mineral and organic matter, and this results in the formation of the soil.

• They also allow water and air into the soil, which eventually react with rocks to cause their breakdown into soil.
• The activities of man during tillage and other farm operations indirectly help to break rocks into tiny pieces to form soil.
• The activities of microorganisms that promote the decomposition of organic materials aid soil formation.
• The roots of plants penetrate rocks and break them into tiny pieces to form soil
• They influence the organic matter content of the soil
• Organisms produce carbon dioxide, which forms carbonic acid with water and enhances the weathering of rocks.
• Microbes also improve the soil aeration and water percolation. This enhances chemical and physical weathering.
• Microbes help in the decomposition of organic matter in the soil.
• The decay of fallen leaves of trees with the aid of bacteria results in the formation of humus, and this is rich in plant food.

Time: Time also plays an important role in soil formation. It takes a long time for mature soil to be formed.

• It takes a long time for small pieces of rock to disintegrate into grains of soil.
• It also takes a long time for plants to decay and become part of the soil
• It also takes time for rainfall to leach chlorides, sulphates, and carbonates from the soil.
• It takes a short time for the formation of immature soil.
• Time also determines whether or not the soil is well-developed.

 PROCESS OF SOIL FORMATION

The process of soil formation is called weathering. Weathering is defined as the disintegration or breakdown of rocks into tiny pieces to form soil.  In other words, weathering can also be defined as the breaking down of rock masses (rock minerals) into simpler forms through the agents of physical, chemical, and biological processes. The Processes of soil formation include:

Physical process: Agents of physical weathering are temperature, ice, wind, water, and pressure.

• Temperature: The alternating heating and cooling of the rocks produces pressure within the rocks and causes them to break down into pieces.

Wind: As a result of the grinding of rock surfaces by solid materials carried by wind, water, and moving ice (glacier), rocks break down to form soil.

• Ice: The conversion of water inside cracks in rocks into ice results in an increase in volume. This increase in volumes results in more pressure being exerted on the rock walls, which eventually break into smaller pieces.
• Water: Running water carries some fragments of rocks along its course and these rub against the surface of rocks in the river bed, thus breaking off small pieces of rocks.

2. Chemical Process: Agents of Chemical weathering include solution, carbonation, hydration, hydrolysis, and oxidation.

• Solution: This occurs when water dissolves and soluble minerals present in the rock are carried from place to place while water flows.
• Carbonation: Atmospheric carbon dioxide mixes with rainwater to form weak carbonic acid. This acid dissolves rocks, resulting in their breakdown.
• Hydration: This is the attachment of water to rock minerals. This results in chemical alteration of the minerals, e.g, the conversion of iron II rocks to hydrated rocks.  Hematite rock is also changed to limonite.
• Hydrolysis: This is the reaction of water with rock minerals to produce a rock, entirely different from the original one. For example, olivine rock is changed to serpentine.
• Oxidation: This is the reaction of rocks with oxygen from the atmosphere. This reaction eventually weakens and breaks down the rock to form soil.

3. Biological Process: This involves the activities of plants and animals in breaking down rocks to form soil.

• It is caused by the action of animals like earthworms, termites and other soil organisms.
• Movements of these organisms cause small fragments of rocks to disintegrate.
• Earthworms and termites burrow into the rocks and break off fragments of rocks.
• The roots of growing plants penetrate rocks through crevices, exerting pressures that split some rocks.
• The activities of man during farm operations, such as ploughing and harrowing, also break down rocks into tiny pieces.

COMPOSITION (COMPONENTS OF THE SOIL)

The soil is made up of five components, which are (i) Inorganic (mineral) matter, (ii) Organic water, (iii)soil water, (iv) soil air  (v)living organisms.

Mineral or inorganic 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.

1. Mineral or Inorganic Matter: The mineral matter represents small rock fragments of the soil. It forms the bulk of about 45% of the 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 and 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 plant and animal activities.
• Mineral matter has moderating effects on soil temperature.
• It also affects soil porosity.

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 e.tc, when deposited on the soil, 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

• It is very rich in plant nutrients.
• It is the habitat of many soil microorganisms
• It prevents leaching in soil where it is present in an 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 of coarse-texture soil together.
• It increases the water-holding capacity of the soil
• It moderates the soil temperature, which stimulates the good development and growth of roots.
• It has a buffering effect that moderates pH values.
• It improves soil cation exchange capacity
• It improves soil aeration.
• It is responsible for the loose, friable condition of loamy soil.

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 a 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
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 that 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 that can drain from the soil under the influence of gravity. It is available to plants, but it is often pulled down beyond the reach of the roots.

Importance/Effects of Soil Water on Agriculture:

• Water is an important agent of the weathering of rocks in the soil.
• It helps to dissolve plant nutrients into a 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 reaction.
• It aids the turgidity of cells.
• It protects plants from the 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

• 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 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 favored by poor aeration.
• It is needed in soil reactions, particularly carbon and nitrogen cycles.

5. Living Organisms: These refer to plants and animals that inhabit the soil. They range from microscopic organisms to larger organisms.  Some are beneficial while others are harmful to crops and livestock.  The most commonly found groups of soil organisms include bacteria, fungi, viruses, nematodes, and insects eg, termites, 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:

• Soil organisms improve soil structure and granulation
• They also improve the aeration of the soil.
• They help to decompose organic materials in the soil to form humus
• They improve soil water percolation or drainage.
• They also increase the collocidal properties of the soil
• Some organisms, like bacteria, help to fix nutrients into the soil.
• Some soil microorganisms produce acidic materials that help to break down rocks.
• Soil organisms enhance the cation exchange capacity of the soil.
• Through burrowing, they leave holes for crop root penetration.
• They increase the mineral or nutrient status of the soil.
• They stabilize soil pH through the increase in soil organic matter and buffering.