Nature of Soil Separates: Physical, Mineralogical and Chemical

After reading this article you will learn about the physical, mineralogical and chemical nature of soil separates.

Physical Nature of Soil Separates:

Soil separates consist of stones, gravels, sands, silts and clays. Their nature is described briefly.

(i) Stone, Gravel and Sands (Coarse Fragments):

Stones and gravels range in size from 2 mm upward and may be almost rounded, irregularly angular or even flat. The distinction between stone and gravel is based on size and stone (> 3 inches) is generally larger than gravel (up to 3 inches).

Sand (2-0.05 mm average particle diameter) may be rounded or quite irregular depending on the amount of abrasion that they have received. When sands are not coated with clay or silt particles, such particles do not show any sticky, plastic property or any colloidal property.

Due to presence of sand particles in a soil mass, the passage of percolating water is rapid and thereby facilitates drainage and aeration. The water-holding capacity (WHC) of a soil is low due to presence of good amount of sand separates.

(ii) Clay and Silt (Finer Fractions):

The physical properties of soil are primarily dependent on total surface area. In addition to surface area, the chemical properties of the soil particles and organic matter content also exert influence. Surface area in square centimeters per gram or per cubic centimetre of soil is called “specific surface” of the soil.

Clays consist chiefly of secondary products of chemical weathering; have ultramicroscopic size, possessing large surface area than that of other fractions like silt and sand. Clay particles generally are mica like in shape and highly plastic when moist. When clay is wetted it tends to be sticky and is easily molded.

On drying it absorbs considerable heat energy and on wetting again, it evolves the same amount of heat. This phenomenon is known as “heat of wetting”. Clay particles exhibit properties of swelling, plasticity, cohesion and adhesion etc.

Silts are intermediate in size and show properties somewhat intermediary between sands and clays and are composed of original mineral fragments. Unsatisfactory physical properties will develop in presence of silt in higher amounts in soils and this poor physical condition can be slightly improved by supplementing adequate amount of sand, clay and organic matter to the soil.

Mineralogical Nature of Soil Separates:

(i) Sands and Silts:

These are the coarsest particles among other soil separates. They are fragments of rocks as well as minerals. Quartz (SiO₂) commonly dominates the finer grades of sand as well as silt separate. Besides this, there are presence of various other minerals like feldspars, micas, gibbsite, hematite and limonite.

(ii) Clays:

Coarser clay particles are composed of minerals like quartz and the hydrous oxides of iron and aluminium and other aluminosilicate minerals. Three main mineral types- Kaolinite, illite and montmorillonite are mostly present.

Chemical Nature of Soil Separates:

(i) Sands and Silts:

Since quartz SiO₂ is dominant of these two fractions, they are chemically inactive. Sometimes these fractions especially sands, contain different insoluble nutrient elements and hence cannot supply nutrients to plants. Silts (K-bearing minerals— micas) have been known to release potassium in soils and supply K to the plants.

(ii) Clays:

This fraction in soils is very active. Montmorillonite and Kaolinite are aluminium silicates. They carry sodium, iron and magnesium. Illite is hydrous mica, potassium aluminium silicates. It contains high potassium.