What are the two main categories of Soil?

On the basis of the mode of occurrences and the natural agents involved, soils are commonly classified into two categories as

1. Sedentary soils

2. Transported soils

1. Sedentary soils

These are also known as the residual soil. They occur directly on top of the parent rock. These are the residues left as insitu after weathering followed by transportation and consist of the insoluble products of rock weathering, which have escaped distribution through transporting agencies and which still mantle the rock from which they have been derived.

As such, the chemical composition of residual soils is defined principally by the nature of the parent rocks. The residual soils are often rich in humus. Laterites form the best example of residual soil.

Under marshy condition, accumulation of organic matter like peat gives rise to an organic type of soil. This is known as ‘cumulose soil’ and is included in the category of sedentary soils.

2. Transported soils

These are also known as drifted soils and this category of soil includes all those soils that have been deposited at places far from the parent rocks after being transported by the geologic agents. On the basis of the transporting agencies involved, these soils are classified as follows

(a) Colluvial soils

Under the influence of gravity material are removed from the mountains and get accumulated at the base of the steep slopes. The soils thus formed are stony and are never stratified.

(b) Alluvial Soils

These soils are generally confined to river basins and coastal plains and are mainly deposited by rivers. These are very fertile and supports vegetation.

(c) Glacial Soils

These soils are transported and deposited by glaciers. Boulder clay or till forms good soil at times.

(d) Aeolian soils

The wind-borne sediments, composed chiefly of silt and clay-fractions form scanty but fertile soil at times. The loess deposits form good examples of aeolian soils.

(e) Lacustrine soils

Materials transported by rivers and glaciers, collected in the lake basins, form soil in due course when the lakes dry up. They are stratified and are rich in organic matter.

Apart from the above important types of soil, there are sandy, coarse-grained soils formed from the sediments deposited in the coastal regions or on continental-shelf area, which constitute the marine soils.

Similarly, soil derived from the pyroclastic materials are known as Volcanic soils.