What is Soil ?

Any part of earth's surface that supports vegetation also bears a covering of soil. Soil is thus usually defined as "any part of earth's crust in which plants root". Muddy bottoms of ponds, porous rock surfaces, ravines or glacial deposits, bottoms of lakes, peats etc., all are thus soils. But this is a limited definition of soil, as we know that soil is actually formed as a result of long-term process of complex interactions leading to the production of a mineral matrix in close association with interstitial organic matter-living as well as dead. In soil formation, modification of parent mineral matter takes fairly a long time. Such a modification is actually the result of interactions between climatic, topographic and biological effects.

Soil is thus not merely a group of mineral particles. It has also a biological system of living organisms as well as some other components. It is thus preferred to call it a soil complex, which has the following five categories of components. (i) mineral matter. It is a matrix of mineral particles derived by varying degrees of breakdown of the parent material-rock. (ii) soil organic matter (Humus). It is an organic component derived from long and short-term addition of material from organisms growing above and below ground i.e., plants, animals, and microorganisms. (iii) soil water (soil solution). It refers to all water contained in soil together with its dissolved solids, liquids and gases. Soil water is held by capillary and absorptive forces both between and at the surface of soil particles. Soil water in reality is a dilute solution of many organic and inorganic compounds, which is the source of plant mineral nutrients. (iv) soil atmosphere. It occupies the pore space between soil particles, which, at any time, is not water-filled. Its composition differs from the aboveground atmosphere in the sense that it is normally lower in oxygen and higher in carbon dioxide content. and (v) biological system. To the above, there may also be added, the biological system, as each soil has a distinctive flora and fauna of bacteria, fungi, algae, protozoa, rotifers, nematodes oligochaetes, molluscs and arthropods,

A perusal of the above shows that soil is not merely mineral matter, but a complex of several other types of component. Thus, biologically the soil may be said as "the weathered superficial layer of the earth's crust in which the living organisms grow and also release the products of their activities, death and decay." According to Dokuchayev (1889), a famous Russian pedologist "the soil is a result of the actions and reciprocal influences of parent rocks, climate, topography, plants, animals and age of the land." It can be represented by the following formula: S = (g.e.b) At

where, s-soil; g-geology; e-environment; b-biological influences; t-time.