FORMATION, COMPOSITION OF SOIL
Definition
The soil is that portion of the surface of the land which is essential for plant growth. Plants are anchored in the soil by their roots which spread in all directions and which, by holding on to the soil, keep the plant in position.
Plants draw all their water and most of their food, or nutrients, from the soil. Soil is therefore the source of food for plant, animal and man, and the study of agriculture should properly begin with the study of soil.
THE PROCESS OF SOIL FORMATION
Rocks which are exposed to the surface of the earth are broken up and changed physically and chemically to form a major part of the soil. There are many rock types with various mineral components. The number of different types of soil which can be formed from rocks is therefore enormous.
Weathering: is the first stage of the formation of soils. The term weathering is used to describe the processes which change a rock into the parent material or rock meal which later becomes soil.
Weathering can be divided into (a) physical, (b) biological and (c) chemical weathering. Physical or mechanical weathering, which is the breakdown of rock into smaller fragments, while biological occurs as a result of the pressure inflicted on rocks by animals such as man, elephants and cattle moving about cause small fragments of rocks to disintegrate and chemical weathering, which describes the changes in the chemical composition of the rock minerals.
Physical and chemical weathering occurs simultaneously, but it is convenient to discuss each separately.
(a) Physical Weathering/Mechanical Weathering
The breakdown of rocks by physical or mechanical means is achieved primarily by temperature. The actions of temperature, wind, ice, pressure and water are important in this respect.
(i) Temperature
The various minerals which rocks consist of expand when they are heated and contract as they cool. Not all minerals expand and contract at the same rate. Some minerals respond to temperature more than others. As the various minerals in one rock react to atmospheric temperature, they expand and contract at different rates. This produces stresses within the rock which, over a long period of time, causes disintegration.
(ii) Winds
Wind carries away dust particles which they are blown against rock surfaces, can cause wearing away. Also, sand dunes were carried by the wind and deposited in this way. Frequently, wind removes loose materials from slopes and exposes new surfaces.
(iii) Ice
Water turns into ice if the temperature is at or below 0°C. As water is converted into ice it expands, and increases in volume by about 9 percent. The force of this expansion is very strong, which is about 1,700 tonnes to the square metre. This contributes to rock weathering in this way, as the temperature reaches freezing point, water in the crevices and cracks of rocks freezes. The force of expansion of the ice causes the rock to crack further and shatter. When the temperature rises again the ice thaws (melts) and the pieces of rock shattered by the ice are carried away by the melting ice. The movement of these pieces of rock causes further disintegration.
(iv) Pressure
Plants growing in the cracks and crevices of rocks send their roots down into the cracks. As the roots grow larger they cause cracks and breakage in the rock. When the pressure of upper layers of rock is removed or lessened, deeply-buried rock expands slightly. This expansion can cause many cracks in rock above it, even in rock such as granite. This is called release pressure.
(v) Water: Water is one of the most powerful weathering agents, either as rain or as streams, rivers, seas and lakes. Running water carries away disintegrated materials, partly by dissolving them and partly due to the rate at which it flows. Rock fragments carried in suspension grind against one another and also rub against the sides and beds of streams. This further helps to erode the surface of the fragments. Openings of various sizes, called fissures or crevices, occur in rock surfaces and water is often trapped in these openings. When the temperature is very low, water in these openings freezes and the ice formed in the cracks expands.
(b) Biological weathering
The pressures inflicted on rocks by living organisms (plants and animals) cause small fragments of rocks to disintegrate.
(i) Plants: The roots of growing trees may penetrate small cracks in rocks and exert considerable pressures which eventually split some rocks.
(ii) Insects and earthworms: Burrowing animals such as earthworms and termites eat their way into soft-textured rocks, creating space for the entry of air and water which are the main agents of disintegration. Their burrowing can also physically promote the disintegration of rocks.
(iii) Algae: Algae are the dominant plant form in some mountain areas and arctic regions where few other plants can survive. They grow into the rocks surface by producing acids which dissolve the mineral parts of the rock and assist in its disintegration.
(iv) Animals: The pressure inflicted on rocks by animals such as man, elephants, horses and cattle moving about cause small fragments of rock to disintegrate. The hoofs of animals walking on the rocks break off many tiny particles. Likewise, animals like rats and rodents which make holes in the ground, help in this disintegration of rocks into small particles.
(c) Chemical weathering
This chemical dissolution of minerals in water and the action of atmospheric gases, such as carbon (IV) dioxide and oxygen, constitute chemical weathering which is much more important than physical weathering. The action of water on minerals in rocks can cause simple carbonation, oxidation, reduction, solution, hydrolysis and hydration.
Carbonation: This is the process by which certain atmospheric gases react with water molecules to form weak acids leading to the disintegration of rocks.
(i) Carbon (IV) oxide reacts with water to form weak trioxocarbonate (IV) acid.
CO₂ + H₂O → H₂CO₃
(ii) Sulphur dioxide reacts with water to form weak trioxosulphate (IV) acid.
SO₂ + H₂O → H₂SO₃
Oxidation: This is the addition of oxygen to a rock mineral or the process of electron loss. During weathering, the most significant example of oxidation is the conversion of Iron II to Iron III. Rock mineral — Siderite is changed to Haematite.
4FeO + O₂ → 2Fe₂O₃
Iron (II) Oxide → Iron (III) oxide.
Reduction: This is the addition of hydrogen to a rock mineral or the removal of oxygen from a rock mineral or the process of electron gain. Typical example is that of Iron (III) to Iron (II).
2Fe₂O₃ → 4FeO + O₂
Solution: This is the process by which a solvent (water) dissolves a solute (rock minerals or salts) to form solution. The substances formed will be carried away.
H₂O + CO₂ → H₂CO₃
water + carbon (IV) oxide → carbonic acid
Hydrolysis: This is the process by which the hydrogen ions from water molecule are used to replace the metallic elements in a rock minerals. A strong alkaline is formed which disintegrates the rock.
KAlSi₃O₈ + H₂O → HAlSi₃O₈ + KOH
Aluminium silicate → Kaolinite
Hydration
Hydration: This is the rigid attachment of water molecule to a rock mineral e.g. the changing of haematite — (red) to limonite (yellow). The newly formed rock will disintegrate due to instability.
Fe₂O₃ + 3H₂O → Fe₂O₃·3H₂O
Red (Haematite) → Yellow (Limonite)
Iron (III) oxide → hydrated iron (III) oxide
Addition of organic matter to the weathered rock
Small animals, together with soil bacteria, also play significant roles in the decomposition of plant and animal tissues to form the humus of the soil. The decomposition of the organic material is brought about by the bacteria while earthworms, termites and other soil organisms assist in mixing soil from different layers with organic materials normally found nearer the surface.
The thin crust formed by the death of these primitive plants forms a suitable substrate for the growth of a higher group of plants, known as liverworts and mosses, whose spores germinate on the layers of dead algae and the weathered surface crust.
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