Soil is the surface of the earth’s crust where plants have their roots and where many small animals make their home. It is also a result of the breakdown of rocks.
Formation of Soil
Soil is formed when rocks break down into very small particles by various processes such as weathering. There are two types of weathering: Physical Weathering and Chemical Weathering. Physical Weathering arises from the heating and cooling, the wetting and drying, and the freezing and thawing of the land. These actions cause alternate expansion and contraction of rocks which eventually cause the rocks to crack. Factors which play important role in physical weathering include: * Erosion by wind and water.
* The action of plants, for example, cracks caused by penetrating roots. * The action of animals, for example, worm casts formed by earthworms. * Human interference.
Chemical Weathering is the breakdown of rocks using a solvent that is able to dissolve the mineral structure. Chemical weathering is due largely to the oxidation and reduction of the elements in the soil. Oxidized rocks break up to form soil easily. Solution of minerals in the rocks helps in weathering. For example, when carbon dioxide dissolves in rain water it forms a weak solution of carbonic acid which can dissolve rocks which contain lime. Hydrolysis is another chemical action which causes rocks to break up into soil. The physical and chemical weathering agents and the living organisms in the soil determine the characteristics of the soil.
Components of Soil
Soil is composed of different components. However there are main components which each soil contains. The proportion of each component determines the soil type. The main components include, soil water, soil air, soil organisms, humus, rock particles and mineral salts. Soil Water:
Soil water exists as a thin film around soil particles and occupies the air spaces between soil particles whenever there is rainfall or addition of water to the soil. Soils need the water to support plant roots living in the soil and to supply the plants and small animal organisms living in the soil. Soil Air:
Soil air is present in the air spaces between the soil particles. The larger the particles of soil, the larger the air spaces and this means that the soil has good drainage and aeration. The smaller the particles of soil, the larger the air spaces and this means that the soil has poor drainage and aeration. Soil Organisms
There are organisms living in the soil. These include bacteria, fungi, worms, insects, burrowing animals and plant roots. Soil organisms, such as earthworms, burrow through the soil, therefore improving the aeration and drainage of the soil as the soil particles are moved farther apart. Earthworms also pull organic matter such as leaves through the soil and this adds to the humus content, hence, improving fertility of the soil. Also, their waste matter increases the humus content of the soil and enriches the soil with nutrients. Other soil organisms include plants. The roots of plants hold soil particles together and help to prevent against soil erosion. Humus:
Humus is made up of dead and decaying remains of plants and animals in the soil. It can also be referred to as ‘dead organic matter’. Humus is dark and sticky and is primarily found in the topsoil. Humus gives the soil the brownish colour. Its glue-like characteristic helps to stick or holds soil particles together in clumps. This binding of soil particles helps in preventing erosion. Soils with good humus content are highly suitable for planting or gardening because humus adds mineral salts to the soil. Mineral salts are taken up by plant roots and used by the plant. This is an example of recycling nutrients (The plants use nutrients from the dead organic matter and these nutrients are used by the plants in growth and food production). Rock Particles:...