Write a short note on Soil.

Soil is the top layer of the earth on which plants and animals grow. It is defined as the layer of re-consolidated particles derived from weathered rock, organic matter (humus), water and air and form the upper surface of the earth and supports plant growth. Soil plays an important role since it produces food materials on which the survival of mankind depends. It is also a vital component of the environmental Biogeochemical cycles.

The formation of soil depends on the parent material (the original material from which soil is derived), the climate and the topography of the area, the organisms present in the soil, and the time in which the soil has been developing

The process of breakdown and alteration of rocks on the earth’s surface by mechanical or chemical processes is called weathering. Mechanical (physical) weathering is the splitting of rocks through the action of heat and extreme temperature changes. Physical weathering elements originate from climate. e.g. temperature, ice, gravity and wind. Due to sun’s heat, a rock becomes warm, which makes it to expand.

During night the rock cools more quickly from outside than form inside. This results in the development of cracks in the rock. The cracked layer of rock breaks into small fragments as pieces. Water may run into cracks, which freeze with the change in temperature. The freezing of water causes the cracks to become deeper.

Chemical weathering includes solution (the dissolving of solid materials by water) Carl ovation (the dissolving of soluble rocks and minerals by a weak carbonic and formed by the combination of water with atmospheric carbon dioxide) oxidation (the combination of atmospheric oxygen with rock materials) and hydration (the chemical combination of rock material).

Organic or biological weathering may involve both chemical mechanical processes caused by both plants and animals e.g, burrowing animals and plant roots may physically break up rocks, lichens, which can grow on bare rock surfaces, cause decomposition. Also lichens, have acid present in them which cause chemical changes and make the rocks crumble. When glaciars move, the crumbled rocks are pushed ahead of them.

This pushing help the rock to crumble. Water and chemicals present in the water, dissolve and wear away some kind of rocks. Once this process starts the remaining rock crumbles. The land slides due to earthquakes are also responsible for fragmentation of rocks by forces of impact and aversion. The wind help in carrying small fragmented particles rocks to the surface of the earth.

The rocks are subjected to a continuous process of physical and biological weathering. Along with physical weathering, chemical weathering also occurs by water, oxygen carbon dioxide. Water soluble minerals like lime-stone, gypsum etc. from weathered rocks increase the presence of CO₂, and organic acids formed as a result of decay of remains of animals and plants. Water is also responsible for hydrolysis along with CO₂. Thus the silicates weathered rocks are hydrolyzed.

K₂Al₂Si₆O₁₆ + 2H₂O + CO₂  → Al₂O₃ SiO₂ 2H₂O + 4SiO₂ + K₂CO₃

This process releases Ca, Mg, K, Na and silicates in the soil. All these are responsible for increasing the fertility of soil. The CO₂, and H₂O combine to from carbonic acid (H₂CO₃) which is also responsible for the weathering of rocks.