Soil formation is a cyclic process.
On the basis of geological origin soil can be divided into mainly two groups.
1. Soils of organic origin and
2. Soils formed by physical and chemical weathering of rocks
Organic soils are formed by the decomposition of plants and animals residue over time.
While soils by physical weathering formed when large rocks fall from the high lands under the action of gravity, impact, air and water break down it into smaller pieces.
Some of these pieces are large and some are very fine. These pieces are called soils. These pieces fall under the category of gravel and sand respectively.
Important thing to note here is that these soils retain the minerals that were present in the parent rock.
On the other hand, in chemical weathering rock decomposes by oxidation, hydration, carbonation and leaching by organic acids and forms finer soils such as silts and clays.
Silts are defined as coarser than clays.
On the basis of gain size we can arrange these soils by Gravels having the largest grain size, after that Sand has relatively smaller grains, and then come silts and lastly the clays have smallest grain size.
If weathered material or formed soils remain at the place where they were produced, they are called residual soils.
And if this weathered material is transported and deposited elsewhere by the action of wind, water, ice or any other agency then it is called transported soil.
Transported soils are further classified according to their transporting agency.
Lets see few important examples.
Alluvial soils : they are deposited by the running waters like rivers
Lacustrine soils: they are deposited by suspension in still and fresh water lakes.
Marine soils : are deposited from suspension in sea water
Aeolian soils : these are transported by wind.
Glacial deposit: these soils are transported by ice glaciars.