Soil is the uppermost layer of the Earth’s crust, capable of supporting life. You know that the earliest civilizations almost always arose near river banks. This happened because the river provided a continuous supply of water and the soil along the river banks was very fertile. Thus, human settlements in different parts of the world followed the distribution pattern of fertile soil and water on the Earth.
Soil provides water and minerals to plants for their growth. Without soil, there would be no vegetation and hence no animal life. Where do we look for soil? You would be tempted to say that soil covers the entire land surface of earth. However, this is not true. Large parts of Arctic and Antarctic regions are covered with ice and snow. Solid rocks present in mountains contain no soil. Ice, snow and rocks together constitute about one-fifth of the total land area.
Composition of Soil:
Composition of soil varies from place to place. It depends on the type of rock which it is formed. However, all soils contain the following components:
- Rocks particles: Tiny particles of soil derived form the parent rock.
- Humus: Humus consists of decaying remains of plants and animals.
- Living organisms: Soil contains a large number of micro-organisms like bacteria, algae, fungi and protozoa. Many small animals like insects, earthworms, spiders, scorpions, centipedes and millipedes are found in the soil. Burrowing animals like rats, moles, rabbits and guinea pigs make tunnels in the soil.
- Water: Water is present in pores between soil particles and helps the plants in their growth.
- Air: Air is also present in the soil pores and is essential for the survival of living organisms present in the soil.
A vertical section of the showing different layers is called soil profile. The different layers of the soil are called horizons. Let us find out more about the soil layers.
The three layers of soil are:
The uppermost layer of the soil is called topsoil or A-horizon. It is the darkest of all layers. The dark color is due to the presence of humus which makes the soil fertile. This layer contains roots of many plants which absorb water and nutrients from it. Soil is soft and porous with great capacity to hold water. The topsoil is home to many organisms like bacteria, algae, fungi, insects and worms.
The layer of soil below the A-Horizon is called subsoil or B-Horizon. Subsoil is harder and more compact than the topsoil. It does not contain humus and is, therefore, lighter in color than the topsoil. B-Horizon is rich in soluble minerals and iron oxides. Roots of certain tall trees may reach up to this layer.
The lowest layer of soil is called the C-Horizon. This layer consists of small pieces of the parent rock. Weathering of these pieces of rock results in soil formation. The rock pieces contain many cracks and crevices. Although no organic matter (humus) is present in this layer, it contains some minerals. Beneath the C-Horizon is present the solid parent rock called the bedrock. It is not possible to dig beyond the C-Horizon with a spade.
Formation of Soil:
Soil is formed by the braking down of large rocks over a long period of time. The process of breaking down of rocks into small epics by the action of agents of nature like wind, rain, running steams, glaciers, alternate high and low temperatures and soil organisms is called weathering. Let us study the agents of nature that lead to weathering of rocks.
After rains, water gets trapped in cracks of crevices of rocks. In winter, the trapped water freezes. You know that water expands on freezing. This expansion of water are thus broken into smaller pieces. Continuous movement of rain water or river-water over the smaller pieces of rock breaks them down further to form soil.
Large fluctuations in the day and night temperatures also lead to weathering. The rocks get heated up during the day and expand. At night, the rocks cool down and contract. This repeated contraction and expansion of the rocks weakens them and they break up into smaller pieces. These pieces may be broken down further to form fine soil particles by the combined action of various agents of nature.
Roots of trees:
Roots of trees penetrate deep inside the rocks and weaken them. Root exert pressure no the rocks causing cracks in them. The rocks crumble leading to the formation of soil.
Wind blowing across rock surfaces rubs against them and breaks away tiny particles of rock. These particles are carried to great distances by wind broken down from soil.
Classification of Soils:
Soil is mixture of particles of varying sizes. The texture of the soil depends upon the relative amounts of the particles of different sizes present in the soil. If the soil contains greater amount of small-sized particles, it is soft to touch but if it contains more of bigger-sized particles, it has a coarse texture.
Depending upon their size, soil particles can be classified are gravel, sand, silt and clay with the gravel having largest particle size. These particles are present in varying amounts in soil. Depending upon the amount of the different types of particles present, soil can be classified as under.
Sandy soil contains about 60% of sand particles with small amounts of silt and clay. It is very porous and its water-holding capacity is very low as all the water easily runs down through the large pores. This type of soil is, therefore, not good for the growth of plants. Sandy soil is found mainly in the deserts.
Clayey soil contains more than 50% of clay particles. Since the clay particles are very small in size, they are very tightly packed and do not allow water to drain out easily. Clayey soil, therefore, gets easily water-logged. It is poorly aerated because the tightly bound clay particles leave little space in between to trap air. Clayey soil is very sticky and thus tilling the soil is very difficult. However, clayey soil is rich in a mineral which makes it suitable for plant growth. Due to the great binding capacity of its particles, clayey soil is used for making toys and pots.
Loamy soil contains 30-50% of silt, 5-20% of clay and the rest is sand. It also contains humus. It has good holding capacity and sufficient space between its particles to trap air. Therefore, loamy soil is best suited for plant growth.
Types of Soil Found in India:
Different geographical regions in India have different kinds of soil. Soil differs in terms of its color, texture, particle size and water content. Six types of soil can be recognized in India.
1. Red Soil
This soil is red in color due to the presence of iron oxide in it. The topsoil contains quartz and clay particles in addition to iron oxide. It is, however, poor in humus, but can be made fertile by adding fertilizers and manures. This soil is also called red latosol and is found in interior regions of Kerala, Tamilnadu, Andhra Pradesh, Odisha, Southern Karnataka and Eastern Madhya Pradesh.
2. Black Soil:
Black soil is rich in iron, magnesium and is particularly suited for cotton and sugarcane cultivation. It is derived from rocks called basaltic rocks. Black soil is found in many parts of Maharashtra, Madhya Pradesh, Andhra Pradesh and Gujarat. It is commonly called regur.
3. Alluvial Soil:
Alluvial soil is formed by deposition of silt transported by the river from mountains. It contains gravel, sand and clay and is rich in humus. Greater humus content makes the soil fertile and suitable for cultivating wheat, rice and sugarcane. It is loamy in texture and is commonly called khaddar. It is mainly found in the plains of Haryana, Uttar Pradesh, Bihar, coastal of Odisha and Andhra Pradesh.
4. Desert Soil
Desert soil is sandy and porous with a coarse texture. Its water-holding capacity is low, hence, only thorny bushes and cacti can grow in it. However, desert soil can be used for growing crops if it is irrigated well. It is found in Rajasthan and parts of Gujarat. The Indira Gandhi Canal in Rajasthan has made cultivation of crops possible by providing a continuous supply of water.
5. Mountain Soil:
Mountain soil has the highest humus content of all soils found in India. Hence, it is very fertile, but varies in content from place to place. Mountain soil are found in the Himalayan region and north-east India
6. Laterite Soil:
Laterite soil is reddish, brownish or yellowish in color, as it contains a high percentage of iron and aluminum oxides. This soil is found in regions with heavy rainfall. It is sued for growing plantation crops like tea, coffee and coconut. It is mainly fund in the Western Ghats and parts of Tamilnadu, Andhra Pradesh, Odisha and Assam
Soil: A Natural Resource
Soil is a valuable resource. The existence of mankind is greatly influenced by this resource. It is useful to us in the following ways:
Source of food, clothing and shelter:
Soil supports the plant kingdom which proves food to man and animals. Plants provide fibers like cotton and jute which are used to make clothes and others articles. Plants also provide medicines, rubber, gum, resins, and wood for making furniture and for constructing houses.
Soil as a source of minerals:
Soil is the home of our mineral wealth. Minerals like iron, copper, silver and gold are dug out from the Earth’s crust and sued in the industry for several propose. Coal and petroleum are also obtained by digging out soil.
Soil as a source of building material:
Soil is used as a raw material for making bricks and mortar, pottery and porcelain.
Soil as a source of water:
Rainwater seeps through (percolates) soil particles and collects above the rock bed to from subsoil water. This is pumped out by means of hand pumps and other means and sued for drinking and irrigation.
Soil reduces the effect of flood:
Soil absorbs rain water and reduces the damage caused by flood. Soil also provides water to plants during dry season and keeps them alive.