Indian Standard Soil Classification System
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Classification Based on Grain Size
The range of particle sizes encountered in soils is very large: from boulders with dimension of over 300 mm down to clay particles that are less than 0.002 mm. Some clays contain particles less than 0.001 mm in size which behave as colloids, i.e. do not settle in water.

In the Indian Standard Soil Classification System (ISSCS), soils are classified into groups according to size, and the groups are further divided into coarse, medium and fine sub-groups.

The grain-size range is used as the basis for grouping soil particles into boulder, cobble, gravel, sand, silt or clay.

Very coarse soils
Boulder size
 
> 300 mm
Cobble size
  80 - 300 mm
Coarse soils

Gravel size (G)

Coarse
20 - 80 mm
Fine
4.75 - 20 mm
Sand size (S)
Coarse
2 - 4.75 mm
Medium
0.425 - 2 mm
Fine
0.075 - 0.425 mm
Fine soils
Silt size (M)
 
0.002 - 0.075 mm
Clay size (C)
< 0.002 mm

Gravel, sand, silt, and clay are represented by group symbols G, S, M, and C respectively.

Physical weathering produces very coarse and coarse soils. Chemical weathering produce generally fine soils.

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