Classification of soil is process grouping and pasting. Classification of soil in meaning of geomechanics must extend informations about engineering features of soil. Engineering features which are of primary significance are  compression, hardness, possibility of building.
Direct determination of  these features demands special equipment, time and qualified workers. The fundamental index of classification is size of granule, afterwards possible plasticity of soil.
Residual and transported soils are showing in boundless variations of granular size.
As a general base for classification, mostly inorganic soil, two main groups of class with granicnom value size of 0.075 mm or in some countries 0.0074 m90.(006 mm) is taken.
This limit of 0.075 mm (0.06 mm) is appeared from practical reason because human eye recognize in granular mass individual grains higher than this value but smaller is seen like amorphous mass.

Large grain soil vary with sizes of grains from stone (rock) fragments up to gravel and sand. Also, large grain soil can contents admixture of small fraction so, for the
determination  granulometric structure engineers always use sift experiment.

Small grain soil mostly is consisted of dust and clay. The most important feature of these grains is plasticity.

Organic soil always contents organic matter like humus. And humus is amorphous material appeared by decomposed  plants and animals products.