(a) What is soil?
Soil is the uppermost loose layer of the earth’s crust in which plants grow. It is made up of weathered rock particles (mineral matter), humus or organic matter, water, air and living organisms. It provides plants with support, water and nutrients.
(b) Process of soil formation
Soil formation is the gradual development of soil from parent rock and organic materials. The main process involved is weathering, together with the addition and decomposition of organic matter. Weathering is the breaking down of rocks into small particles by physical, chemical and biological agents.
The processes of weathering involved in soil formation are:
(i) Physical weathering:
This is the breaking down of rocks into smaller pieces without changing their chemical composition. It is caused by agents such as heating and cooling, running water, wind, ice, pressure and abrasion.
(ii) Chemical weathering:
This is the decomposition or breakdown of rocks through chemical changes in their minerals. It is caused by water, oxygen, carbon dioxide and weak acids. Chemical processes include solution, oxidation, carbonation, hydrolysis and hydration.
(iii) Biological weathering:
This is the breakdown of rocks by living organisms. Plant roots grow into cracks in rocks, while burrowing animals, earthworms, rodents and human activities help to break and loosen rock materials. Micro-organisms also decompose dead plants and animals to form humus.
As weathered rock particles mix with humus, water, air and living organisms over a long period, soil develops. Rainwater may wash fine particles and dissolved minerals from the upper layer to lower layers, leading to the formation of soil horizons.
(c) Soil profile
A soil profile is a vertical section or cut through the soil showing the arrangement of its different layers. These layers are called soil horizons. A typical well-developed soil profile has the following main horizons:
A Horizon (Topsoil):
This is the uppermost layer of soil. It is dark in colour because it contains humus formed from decomposed plant and animal remains. It contains many plant roots, insects, earthworms and micro-organisms. It is rich in nutrients and is the main layer in which crops grow. Fine clay particles and soluble minerals may be washed downward from this layer; therefore, it is also called the zone of leaching or eluviation.
B Horizon (Subsoil):
This is the layer below the A horizon. It contains less humus and fewer living organisms than the topsoil. Clay particles, iron compounds and minerals washed down from the A horizon accumulate here. It is therefore called the zone of accumulation or illuviation. It is usually lighter in colour than the A horizon and is relatively compact.
C Horizon (Parent-material horizon):
This layer consists mainly of partly weathered rock fragments and weathered debris. It contains little humus and few organisms. It is the source of the parent material from which the soil develops.
R Horizon (Bedrock):
This is the hard, solid and unweathered rock beneath the C horizon. It is the original parent rock from which the overlying soil materials are formed.
