Definition of Soil 

Soil Definition

Soil is an important natural resource that decides the diversity of life in an area. It is the top surface layer of the Earth’s crust. Rock particles make a large part of the soil. The other constituents which make up the soil are :

1. Mineral matter = 45%
2. Organic matter = 5%
3. Water = 25%
4. Air = 25%
5. Living organisms

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Formation of soil

The process of soil formation is so slow that the soil is regarded as a non-renewable resource. Over long periods of time, thousands and millions of years, the rocks at or near the surface of the Earth are broken down by various physical, chemical and some biological processes. This is called weathering and results in formation of fine particles of soil.

Physical weathering 

Weathering caused by climatic factors is called physical weathering. The various climatic factors are :

1. Sun : The rocks get heated up and expand under the influence of solar radiations. Since all the parts of the rocks do not expand and contract at the same time, cracks appear in the rocks and ultimately the large rocks break up into smaller pieces.
2. Water : Water influences soil formation in the following two ways :

• The water gets into the cracks developed by the unequal heating of the different parts of the rocks. On freezing, the water expands in rock crevices and breaks the rocks.
• Hard rock wears away over long periods of time by the flowing water. Fast flowing river always carries big and small particles of rock downstream. These rocks rub against other rocks and result in the wearing of rocks into smaller particles. The water takes these particles of rocks and deposits them down its path.

3. Wind : Strong winds continuously rub against rocks, erode them and thus help in soil formation.

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Biological weathering

Living organisms like lichens, mosses, etc. influence the formation of soil.

1. Lichens : They live on rocks and produce acids. The acids corrode the surface of rocks to form thin layer of soil.
2. Mosses : Mosses later grow on such surfaces and cause the rock to break up further.
3. Roots : The roots of trees enter the cracks and provide anchorage. In due course, roots grow bigger and force the cracks to widen.

Chemical weathering 

The rocks also undergo chemical weathering by processes like hydrolysis, hydration, oxidation and reduction.

Composition of the soil

The soil is composed of different types of particles of varying sizes. The various particles are:

• Gravels – Particle size greater than 2 mm in diameter.
• Sand particles – Particle size varies from 0.05 mm – 2 mm.
• Silt particles – Particle size ranging from 0.005 mm – 0.05 mm.
• Clay particles – Particle size less than 0.005 mm.

Humus and the micro organisms in the soil decide the quality of the soil. It causes the soil to become porous and allows water and air to penetrate deep underground. The mineral nutrients found in a particular soil depends on the rocks it was formed from.

The nutrient content of a soil, the amount of humus present in it and the depth of the soil are some of the factors that decide which plants will thrive on that soil.

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Types of soil

On the basis of particle size, soil is classified into the following types:

• Sandy soil

It consists of large proportion of sand particles and very small quantity of silt and clay. This soil cannot hold much water. It is found in deserts and is unfit for plant growth.

• Clayey soil

It contains large proportion of clay particles and small amounts of humus and silt. Being compact, it can hold water but cannot trap air. It is also not suitable for plant growth.

• Loamy soil

It contains relatively larger quantities of clay, silt, sand particles and humus. This is the best soil for plants as it is porous, holding good amount of water and air as well.

Soil (land) pollution

The contamination of soil with solid waste, chemicals (through industrial wastes or acid rain), fertilizers and pesticides, reducing its fertility is called soil pollution.

Sources of soil pollution

The main sources of soil pollution includes:

1. Solid wastes : The solid wastes coming from residences, cattle sheds, industries, etc. make the surroundings dirty and pollute the soil.
2. Chemicals : The industrial wastes consisting of a lot of chemicals are generally dumped in vacant sites and they pollute the soil. Some chemicals discharged into the air eventually come down as dry deposition or as acid rain and pollute the soil.
3. Excess of fertilizers and pesticides : Fertilizers and pesticides when used in excess in agricultural fields pollute the soil. From the soil, many non-biodegradable chemicals (e.g., DDT) even enter the food chain and show biological magnification.

Harmful effects of soil pollution

1. Soil pollution reduces the fertility of soil leading to reduction in crop yields.
2. Solid waste makes the surroundings dirty and also emit foul smell due to their decomposition.
3. Soil pollution may kill and reduce the diversity of organisms living in the soil including the earthworms, micro-organisms etc. which are instrumental in making the rich humus.

Soil erosion

Soil is a complex mixture of non-living materials and living organisms. It provides anchorage to plants and is also a source of nutrients and water to the plants. Majority of the plants, thus, grow in the soil. The top layer of soil is very fertile and is often carried by environmental agencies.

This removal of the top layer of the soil from one place to another by certain agents such as strong winds, fast flowing water, is called soil erosion.

Causes of soil erosion

The various causes of soil erosion are:

1. Strong winds : Strong winds carry away the loose top soil when it is uncovered.
2. Heavy rains : Heavy rains or frequent floods wash away unprotected top soil down into the streams, rivers, etc.
3. Improper farming and suspended cultivation : Farmers loosen the top soil of agricultural fields for cultivation or for removing the weeds. Sometimes, due to certain reasons, when these fields remain uncultivated for a long time it becomes prone to erosion.
4. Human activities : Large scale deforestation by humans lead to soil erosion.

Effects of soil erosion 

The various effects of soil erosion are loss of fertility, desertification, landslides, floods, etc,

Prevention of soil erosion 

Certain measures can effectively reduce soil erosion. These include intensive cropping, sowing grasses in uncultivated land, terrace farming along the slopes of hilly areas, making embankments along the river banks, etc.