At any place where parent material is weathering over a period of time, there develops layers of soil one over the other in progressive state of maturity. Such a vertical section of soil is called soil profile. The soil profiles are characteristic of mature soil and are made up of succession of horizontal layers (horizons) , each of which varies in thickness, colour, texture, structure, acidity, porosity and composition. In general soils have four horizons – an organic or O- horizon and three mineral horizons (A,B,C horizons).
(a) Horizon O. The uppermost horizon of soil profile is called horizon-O or litter zone.
(b) Horizon A . Underlying the litter zone is the horizon A or top soil. This horizon is usually sandy.
(c) Horizon B. It is also known as subsoil and is formed with clayey soil. It contains a little humus also.
(d) Horizon C . It is at the bottom of soil profile and contains weathered rock of parent material. It is light coloured and it virtually lacks organic material. Below this zone hard rocks are found.
Types of Soil-
Soil – texture may classified into following groups:
1. Sandy Soils. This type of soil consists of loose and dry sand particles ( particle size 1.00 to 0.05 mm) alongwith 10% each of clay and silt (size 0.05 to 0.002 mm) . These soils have poor water holding capacity and low nutrient content.
2. Clayey Soils. Clay particles (size less than 0.002 mm ) have very small interspaces and inhibit the free circulation of air and water . They become water logged on getting water and are unsuitable for plant growth. Clay soils consist of 60% sand , 58% silt and 31% clay.
3. Loamy Soils. They contain sand, silt and clay in almost equal proportion. Loam is the best soil for vigorous plant growth.
4. Sandy Loam Soils. Such soils consist of sand (60-80%) silt (50%) and clay (20%) . They are also suitable for plant growth.
5. Clay Loam Soils . These soils contain predominantly clay particles (30%) alongwith sand (80%) and silt (15%) Such soils also favour plant growth.
