The uppermost layer of the earth’s crust is known as soil. It is a mixture of rock fragments and organic matter which has decomposed into constituent nutrients.Soil formation is influenced by the weathering and erosion processes that are defined by a region’s climate. Apart from this, the nature of the parent rock, topography, vegetation cover etc., also determine the type of soil that is formed.
Pedogenesis is the process of soil formation under the action of various forces of nature such as wind, flowing water etc.Soil can be classified into three types based on the texture of grains found in it.Sandy soils – if the size of soil grain is in the range of 2 to 0.05 mm.Silt – if the size of soil grain is in the range of 0.
05 to 0.002 mm. It is usually found on the river bedsClayey soils – if the size of soil grain is less than 0.002 mmSandy soils have enough gaps between their grains to drain water quickly. Hence, these soils tend to be dry, light in weight, and well aerated.Clayey soils are more tightly packed, because of the smaller grain size, leaving little gap for air. They do not allow water to easily pass through them. They tend to retain water in the pore spaces in between them.
Hence, these soils are heavy.Loamy soils are a mixture of sand, clay, and silt. This is considered to be the best kind of topsoil for growing plants. The soil pores hold the right amount of water. These soils also have a good humus content.Crops of different kinds grow their best in different varieties of soilspaddy grows well in clayey soils which have a good capacity to retain water and also contain good organic contentLoamy soils which are good at retaining water help in the growth of cereals such as wheat and gram.Pulses and other lentils need soils which drain water easily. Loamy soils are suitable for their growth.
Cotton needs soils which retain a lot of water and also have air spaces. Hence, loamy soils are suitable for them.Soil Profile and Soil HorizonThe different layers of the soil are arranged in a vertical manner, which is known as the soil profile.The layers are usually horizontal in orientation, parallel to the soil surface.Each layer differs the ones adjacent to it in terms of color, texture, chemical composition, and depth.Each layer of the soil is termed a horizon.Soil horizons are mainly identified based on their physical features such a color and texture.
The topmost horizon is usually dark in color. It contains humus, which is decomposed organic matter, and various minerals. This makes it the best layer for the growth and survival of plants. It is generally porous and can retain moisture to a good extent.
This is called the topsoil or the A-horizon.The layer next to it has lesser humus content but is rich in minerals. Unlike the A-horizon which is loose and soft, this layer is harder and more compact. This is called the middle layer or the B-horizon.The bottom layer is composed of the parent rock in the form of smaller lumps.
This is called the C-horizonO-horizonThis layer contains organic matter which is either undecomposed or partially decomposed. It includes leaves, twigs, mosses, and lichens.This layer lies on the top of the decomposed organic matter and the weathered rock material.A-horizonAlso known as the surface soil or the topsoil.It contains decomposed organic matter which is mixed with weathered rock minerals.Nutrients like iron, aluminium, clay, and organic matter are sometimes dissolved and carried out this layer.When the erosion is more pronounced, an eluviated layer appears at the subsurface or the base of the A-horizon.E-horizonIt is a light colored eluviated layer, that is eroded of its nutrients.It underwent significant leaching which took away the nutrients like iron, aluminium, clay etc., leaving behind resistant, undissolvable materials like quartz, sand, and silt.It is an older layer usually found in between the A-horizon and the B-horizon.B-horizonAlso known as the subsoil, this layer lies below the A-horizon and the E-horizon.It contains minerals which reflect the physical and chemical alteration of the parent rock.It is a zone of illuviation i.e., the nutrients that are leached out of the A-horizon and E-horizon get accumulated here.Hence it is rich in iron, aluminium oxides, clay, and organic compounds.C-horizonAlso known as the parent rock, it comprises of the parent rock material which eroded from the layers below and got accumulated in this horizon.It is in the form of large rocks which are sometimes unbroken.It also acts as a zone of accumulation of soluble inroganic compounds which percolate down to this horizon.R-horizonAlso known as bedrock, this forms the bottommost layer of the soil profile.It comprises of largely unbroken rock strata, as a continuous hard mass.In situ soils exhibit many features of the bedrock strata.