Group index of soil usually varies by

The purpose of soil classification is to arrange various types of soils in to groups according to their engineering or agricultural properties.

Based on HRB system, the soil is divided into 7 primary groups, designated as A-1, A-2…A-7.

Triangular classification of U.S. public road administration is a commonly used system for textural classification.

Soil occurring in nature is composed of a different percentage of sand, silt, and clay size particles.

A triangle, having equal length of sides (i.e. Equilateral triangle) is used in textural classification chart.

Particle size classification used only as a designation of particle sizes, and do not signify the naturally occurring types which are mixtures of a particle of different sizes.

Indian standard classification (IS: 1948-1970) is based on the M.I.T. system.

For general engineering purposes, soil can be classified in terms of particle size, textural and unified soil.

Terms such as gravel, sand, silt, and clay are used to indicate grain sizes. These terms are used only as a designation of particle size.