Soil is one of the most important national resources of any country. The soil not only grows a variety of food and fodder crops required for men and animals but also produces raw materials for various agro-industries viz., sugar and starch factories, textile mills, canning and food processing units. It is a complex body showing great many variations in depth, colour, composition and behaviour. Every soil cosists of hard materials called mineral matter, soft and spongy organic matter, water, air and living organisms. Their proportion may vary, the soil has three dimensions namely, length, breadth and depth. Soils have primarily developed from different types of rocks. There are three principals kinds of rocks viz. igneous rocks, sedimentary rocks and metamorphic rocks.
A vertical section of the soil through all its layers and extending into the parent rock is known as soil profile. A soil profile is a historic record of all the soil forming processes and forms the unit of study in pedological investigation. It also helps in soil classification and forms the basis for practical studies of soils. A study of the soil profile is important from crop husbandry point of view, since it reveals the surface and the subsurface characteristics and qualities, namely depth, texture, structure, drainage conditions and soil-moisture relationships, which directly affect plant growth. It helps to classify the soils and to understand soil-moisture-plant relationships.
The profile is divided into four broad horizons called A, B,C and D. Depending upon further variations within these A2,A3,B1,B2,B3,C and D. The various organic layers lying above the surface mineral horizon also form a part of the soil profile. Both A and B horizons collectively are called as Solum. Solum plus parent material is sometimes referred to as regolith. The parent material is termed as bedrock of D horizon.
The surface of some soil in forested area has an organic horizon (o) above the mineral soils. In this layer the original forms of the plant and animal residues are observed. In the 02 horizon the original plant and animal forms cannot be distinguished. Such 0 horizon is absent in arable lands.A horizon- It consits of sub horizons A1, A2 and A3. A1 is the topmost mineral horizon containing a strong admixture of humified organic matter and dark colour. A2- It is lighter in colour having maximum eluviation of clay, iron and aluminum oxides having maximing leaching. A3 It is the transitional layer with properties more like A than B, sometimes absent. B horizon It may be incorporated at least in part in the plough layer or they may be considerably below the plough layer in the soils with deep A horizon having sub-horizons. B1 Transitional layer but more like B than A; sometimes absent. B2 Usually deeper coloured, maximum accumulation of clay minerals, iron and organic matter. B3 Transitional to C, more like B than C. C horizon This horizon excludes the bedrock from which A and B horizons have been formed. This horizon is less affected by pedogenic processes. Its upper layers may in time become a part of the solum as weathering and erosion continues. Accumulation of CaCO3 and
CaSO4 accur in this zone.D horizon It is bed rock.
The formation of soil and the development of profiles are dependent on the genetic and environmental factors, which vary considerably within and between regions, the variation in horizonation are frequent and common. The soils developed in a recent flood plain may have AC profile without any A2 whereas those in the red and lateritic soil area may have A1, B2 and C.
When a virgin soil is put under cultivation, the upper horizons become the furrow slice. Cultivation destroys the original layered condition of this portion of the profile and the furrow slice becomes more or less homogenous.
Many times, serious erosion produces a truncated profile. As the surface soil is swept away, the plough-line is gradually lowered in order to maintain a sufficiently thick furrow slice. The furrow is almost entirely within the B zone and the C horizon is correspondingly near the surface.
In deep soils, the soil profile may be studied upto 1m and a quarter and in others upto the parent material. The layers (horizons) in the soil profile, which vary in thickness, may be distinguished from the morphological characteristics which include color texture, structure etc.
Soils are classified by such characteristics as the kind and number of horizons or layers that have developed in them. The horizons are distinguished by texture, kind of minerals present and presence of salts and alkalies.
groups of soils that are commonly recognised in India are as follows:
Laterites and lateritic soils
Forest and hill soils
Saline and alkaline soils
Peaty and marshy soils