Soil colour

Soil color is produced by the minerals present and by the organic matter content. Yellow or red soil indicates the presence of oxidized ferric iron oxides. Dark brown or black color in soil indicates that the soil has a high organic matter content. The presence of other minerals can also affect soil color.

Soil color is influenced by its mineral composition as well as water and organic contents. For example, soils high in calcium tend to be white, those high in iron reddish, and those high in humus dark brown to black. Soil needs only about 5% organic material to appear black when wet.

It is mainly known as black cotton soil because this soil is most suitable for the cotton crop. Along with cotton, the soil is suitable for the cultivation of crops like groundnut, wheat, tobacco, chillies, and jowar.

This soil is black in colour mainly because it is formed by weathering of lava rocks and are high iron, alumina and magnesia content and is best suitable for cotton crop production.

Black soil is formed by the decomposition of lava rocks and hence found mostly in volcanic regions. There are five main factors which controlled the operation of the soil processes. They are: Parent material, Climate, Organisms, Topography and Time.

The following are five important characteristics of Black soil: It is fine textured and clayey in nature. It has high amounts of lime, iron, magnesium and generally low quantities of phosphorus, nitrogen and organic matter. It is black in colour since it is formed from weathered lava rocks.

Under Reamed Piles in Black Cotton Soil It is the safest and economical option to construct the foundation in black cotton soil. In this type of foundations, the building or a house is anchored to the ground at the depth where the ground movement due to changes in moisture content is negligible.

Kharif in dependable rainfall areas (750mm-1250mm/yr) means considerable loss of water through runoff, considerable loss of nutrients, considerable soil erosion and loss of one crop. Black soil have low fertility and are poor in organic matter, nitrogen, available phosphorous and zinc.

The mat foundation is suitable for black cotton/expansive soils whose bearing capacity is less for suitability of spread footings and wall footings.

Of these, lime stabilization is one of the techniques which is in use for stabilizing black cotton soil from the past few decades. Use of lime reduces the high plasticity of black cotton and makes it workable. Also reaction between lime and soil makes the soil-lime mixture more strength resistant [1].

The following techniques can be used for improving bearing capacity of soil as per the site condition.

1. Increasing depth of foundation.
2. Draining the soil.
3. Compacting the soil.
4. Confining the soil.
5. Replacing the poor soil.
6. Using grouting material.
7. Stabilizing the soil with chemicals.

For main walls or for load bearing walls, the width of the trench should be dug 40 cm wider than width of foundation. Then fill the space on either side of the trench (i.e. 20 cm in each side) with coarse sand. This is done to separate the foundation masonry from direct contact with black cotton soil.

Black cotton soil is heavy clay soil, varying from clay to loam; it is generally light to dark grey in colour. Cotton grows in this kind of soil. The soil prevails generally in central and southern parts of India.

The under-reamed pile foundation is the best for black cotton soil. It has bulbs at its lower end. If this bulb is given below the critical depth of moisture movement in black cotton soil, the foundation will be secured to the ground, and it would not move with movement (shrinkage and swelling) of soil.