Soil Separates: Classification and Characteristics

After reading this article you will learn about classification and characteristics of soil separates.

Classification of Soil Separates:

There are a number of systems of naming soil separates. They are:

(a) The American system developed by U.S.D.A.

(b) The English system, and

(c) The International system.

The international system which is commonly followed in India is given in the Table 2.1

Characteristics of Soil Separates:

(i) Physical nature of soil separates:

Sands are large-sized particles and have the large size of the pore spaces. Hence, they facilitate percolation and encourage aeration. Their water-holding capacity is low. They do not possess plasticity. Clay particles are smallest in size and possess fine pore spaces.

With decreasing particle size, there is decrease in aeration and percolation rate. The water-holding capacity of clay is very high. Properties such as plasticity, swelling, cohesion etc., are very high. Silt particles are intermediate in size. Silts also show properties somewhat intermediary between sands and clays.

(ii) Mineralogical characteristics:

Quartz commonly dominates the finer grades of sand as well as the silt separate. In addition, variable quantities of other primary minerals usually occur, such as feldspars and micas. Hematite and limonite minerals may be present. These impart various shades of red and yellow if present in sufficient quantity.

Coarse clay fractions are composed of minerals such as quartz and the hydrous oxides of iron (hematite and limonite) and aluminium. Another is the complex aluminosilicates. Three main mineral types—Kaolinite, illite and montmorillonite—are at present recognised.

(iii) Chemical makeup:

Since sand and silt are dominantly quartz (SiO₂), these two fractions are generally inactive chemically. On the other hand, clay particles are very active. Clays consist principally of secondary products of weathering. Chemically, Kaolinite and montmorillonite are aluminium silicates. They also carry in addition sodium, iron, magnesium.

Textural Classes:

According to the proportion of sand, silt and clay, a soil is given a name to indicate its textural composition. On this basis, soils are classified into 3 fundamental and broad textural classes: Sands, loams and clay. On the basis of these, additional class names have been devised.

There are two main systems of naming soils according to their texture. In U.S. Department of Agricultural Classification System, textural classes are: Sand, loamy sand, sandy loam, loam, silt loam, silt clay loam, sandy clay, silty clay and clay. The classification is from coarse to fine texture (Table 2.3).

In the American system as developed by the U.S. Bureau of Soils, ten textural classes are proposed. Textural classes based on the relative percentage of sand, silt and clay in a soil are given in Table 2.4.

Determination of Textural Class:

In the field, texture is commonly determined by ‘feel’ method. The soil is rubbed between thumb and fingers, preferably in the wet condition—sand feels gritty and its particles can be easily seen with the naked eye. The silt when dry feels like flour or talcum powder and is slightly plastic when wet.

Clayey material feels very plastic and exhibit stickiness when wet and are hard under dry conditions. A more accurate and fundamental method is ‘laboratory’ method which is based on mechanical analysis. The texture of a soil horizon is an almost permanent character. The size of particles in mineral soil is not subject to readily change. Thus, a sandy soil remains sandy and a clay soil remain a clay. Texture cannot be altered and thus is considered as basic property of a soil.