SOIL - THEIR TYPES AND PROPERTIES

Soil is one of the most important natural resources. It supports the growth of plants by holding the roots firmly and supplying water and nutrients. It is the home for many organisms. 

Soil is essential for agriculture. Agriculture provides food, clothing and shelter for all. Soil is thus an inseparable part of our life. The earthy fragrance of soil after the first rain is always refreshing.

Notes: Polythene bags and plastics pollute the soil. They also kill the organisms living in the soil. That is why there is a demand to ban the polythene bags and plastics. Other substances which pollute the soil are a number of waste products, chemicals and pesticides. Waste products and chemicals should be treated before they are released into the soil. The use of pesticides should be minimized.

Soil Profile:

Soil is formed by the breaking down of rocks by the action of wind, water and climate. This process is called weathering. The nature of any soil depends upon the rocks from which it has been formed and the type of vegetation that grows in it.

Soil is composed of distinct layers. A vertical section through different layers of the soil is called the soil profile. Each layer differs in feel (texture), color, depth and chemical composition. These layers are referred to as horizons. 

Soil profile is a section through different layers of the soil, Various layers are called horizons.

A-horizon: The uppermost horizon is generally dark in color as it is rich in humus and minerals. The rotting dead matter in the soil is called humus.The humus makes the soil fertile and provides nutrients to growing plants. This layer is generally soft, porous and can retain more water. It is called the topsoil or the A-horizons This provides shelter for many living organisms such as worms, rodents, moles and beetles. The roots of small plants are embedded entirely in the topsoil.

B-horizon: The next layer has a lesser amount of humus but more of minerals. This layer is generally harder and more compact and is called the B-horizon or the middle layer.

C-horizon: The third layer is the C-horizon, which is made up of small lumps of rocks with cracks and crevices. 

Below this layer is the bedrock, which is hard and difficult to dig with a spade.

Soil types:

Weathering of rocks produces small particles of various materials. These include sand and clay. The relative amount of sand and clay depends upon the rock from which the particles were formed, that is the parent rock. The mixture of rock particles and humus is called the soil.

Soil can be classified as sandy, clayey and loamy. 

1. Sandy soil:

Living organisms, such as bacteria, plant roots and earthworm are also important parts of any soil. The soil is classified on the basis of the proportion of particles of various sizes. If soil contains greater proportion of big particles it is called sandy soil.

Sand particles are quite large. They cannot fit closely together, so there are large spaces between them. These spaces are filled with air. We say that the sand is well aerated. Water can drain quickly through the spaces between the sand particles. So, sandy soils tend to be light, well aerated and rather dry.

2. Clayey soil:

If the proportion of fine particles is relatively higher, then it is called clayey soil. Clay particles, being much smaller, pack tightly together, leaving little space for air. Unlike sandy soil, water can be held in the tiny gaps between the particles of clay. So clay soils have little air. But they are heavy as they hold more water than the sandy soils.

Clayey soil is used to make pots, toys and statues.

3. Loamy:

If the amount of large and fine particles is about the same, then the soil is called loamy. The best topsoil for growing plants is loam. Loamy soil is a mixture of sand, clay and another type of soil particle known as silt.

Silt occurs as a deposit in river beds. The size of the silt particles is between those of sand and clay. The loamy soil also has humus in it. It has the right water holding capacity for the growth of plants.

The sizes of the particles in a soil have a very important influence on its properties.

Properties of Soil :

Percolation rate of water in soil:

Percolation rate of water is different in different types of soil. It is highest in the sandy soil and least in the clayey soil.

Rate of percolation can be calculated by using the following formula:

Percolation rate (mL/min) = Amount of water (mL) / Percolation time (min)

Moisture in soil:

Soil holds water in it, which is called soil moisture. The capacity of a soil to hold water is important for various crops.

While passing through a farmland during a hot summer day, Perhaps you might have noticed that the air above the land is shimmering. This is because, On a hot summer day, the water vapour coming out of the soil reflect the sunlight and the air above the soil seems to shimmer. 

Absorption of Water By Soil:

Soils absorb water, however different types of soil have different absorption capacity.

Percentage of water absorbed can be calculated as  ((U-V) /W) *100 where, 

U is Initial volume of water (in mL).
V is final or remaining volume of water (in mL).
W is weight of soil (in Gram).

Volume of water absorbed by the soil = (U – V) mL

Weight of water absorbed by the soil = (U – V) g

1 mL of water has weight equal to 1 g.

Soil And Crops:

Soil is affected by wind, rainfall, temperature, light and humidity. These are some important climatic factors which affect the soil profile and bring changes in the soil structure. The climatic factors, as well as the components of soil, determine the various types of vegetation and crops that might grow in any region.

• Clayey and loamy soils are both suitable for growing cereals like wheat, and gram. Such soils are good at retaining water.
• For paddy, soils rich in clay and organic matter and having a good capacity to retain water are ideal.
• For lentils (masoor) and other pulses, loamy soils, which drain water easily, are required.
• For cotton, sandyloam or loam, which drain water easily and can hold plenty of air, are more suitable.
• Crops such as wheat are grown in the fine clayey soils, because they are rich in humus and are very fertile.

Soil erosion:

The removal of land surface by water, wind or ice is known as erosion. Plant roots firmly bind the soil. In the absence of plants, soil becomes loose. So it can be moved by wind and flowing water. Erosion of soil is more severe in areas of little or no surface vegetation, such as desert or bare lands. So, cutting of trees and deforestation should be prevented and effort should be made to increase the green areas.

Do you know why horse dung is mixed with soil to prepare 'matkas' and 'surahis'?

This is because, burnt horse dung helps open up the pores in the soil. So that water could percolate out of the matkas and surahis, evaporate and cools the water inside. 

Generally such practices are used in  Sohagpuri (Sohagpur, a place in Madhya pradesh) surahis and matkas, which are famous in even far off places like Jabalpur, Nagpur, Allahabad, etc.