what is soil

WHAT IS SOIL?

Soils are complex mixtures of minerals, water, air, organic matter, and countless organisms that are the decaying remains of once-living things. It forms at the surface of land – it is the “skin of the earth.” Soil is capable of supporting plant life and is vital to life on earth.

chances are that you haven't thought a lot about the soil under your feet, but you may be surprised at the complexity of soil. Soil varies in its composition and the structure of its particles, and these factors are closely examined by farmers, who need appropriate soil for planting crops, as well as engineers who may need to understand how soil is going to hold up under different demands. Soil is also vitally important to the sustainability of an ecosystem because it serves as the natural medium for the growth of vegetation. In this lesson, you will discover just what soil is and which factors are looked at when determining the structure and the types of soil.

So what exactly is soil? Soil can be defined as the organic and inorganic materials on the surface of the Earth that provides the medium for plant growth. Soil develops slowly over time and is composed of many different materials. Inorganic materials, or those materials that are not living, include weathered rocks and minerals. Weathering is the mechanical or chemical process by which rocks are broken down into smaller pieces. As rocks are broken down, they mix with organic materials, which are those materials that originate from living organisms. For example, plants and animals die and decompose, releasing nutrients back into the soil.

Soil Profile and Soil Horizons

Now, if you look down at the soil under your feet, you cannot tell very much about that soil. So when you study soil, it's helpful to grab a shovel and dig a hole that is big enough to reveal a vertical section of soil that ranges from the surface to the underlying rock, referred to as a soil profile. The soil profile is somewhat like the soil's fingerprint, and it will differ from other soil samples based on factors like its color, texture, structure and thickness, as well as its chemical composition.

Each layer of a soil profile is referred to as a soil horizon. These horizons are identified by letters. Horizon A is the upper layer, closest to the surface. You can think of this horizon as the topsoil. In fact, you can use this as a memory jogger to help remember the order of the horizons. The letter A is at the top of the alphabet and refers to the topsoil layer. As you move deeper into the layers of the soil profile, you have horizons B and C, giving us the three main horizons.

Types of Soil

There are three basic types of soil: sand, silt and clay. But most soils are composed of a combination of the different types. How they mix will determine the texture of the soil, or, in other words, how the soil looks and feels.

One type of soil is sand. Sand within soil is actually small particles of weathered rock. Sand is fairly coarse and loose so water is able to drain through it easily. While this is good for drainage, it is not good for growing plants because sandy soil will not hold water or nutrients.

Silt is another type of soil. Silt can be thought of as fine sand, and it will hold water better than sand. If you were to hold a handful of dry silt in your hand, it would feel almost like flour. If you were to add water to the silt in your hand, it would do a fair job of holding the water and feels slick and smooth.

Clay is very fine-grained soil. Its particles are even smaller than silt, so there is very little space between the fine grains for air or water to circulate. Therefore, clay does not drain well or provide space for plant roots to flourish. If you were a farmer, you would not want your field to be mostly clay. However, if you were a potter, you would think clay was the best type of soil. When moisture is added to clay, it can be molded into shapes, such as a pottery bowl or a building brick.

Now, we can consider loam as our fourth type of soil, even though it is really a combination of sand, silt and clay. Loam will vary depending on how much of each component is present, but generally if you are a gardener, this is the type of soil you want because it holds moisture, but also allows for good drainage. If you were to hold loam in your hand, you could mold it into a ball, but the ball would easily crumble when disturbed.

6 Major Types of Soil Found in India

The committee appointed by the Indian Council of Agricultural Research (ICAR), classified the Indian soil in the following main groups:

1. Alluvial Soils

2. Black Soils

3. Red Soils

4. Laterite Soils

5. Mountain Soils

6. Desert Soils

1. Alluvial Soil:

It is the most important type of soil found in India covering about 40 per cent of the total land area. It is very fertile and contributes the largest share of agricultural wealth. This soil supports nearly half of the Indian population.

The alluvial soil is found mostly in the Northern Plains, starting from Punjab in the west to West Bengal and Assam in the east. It is also found in the deltas of the Mahanadi, Godavari, Krishna and Kaveri rivers in the Peninsular India. The northern parts and the coastal areas of Gujarat also have some deposits of alluvial soil.

Many rivers originate from the Himalayan Mountains and bring a large amount of sediment with them. It is deposited in the river valleys and the flood plains. Thus, the parent material of the alluvial soils is always of transported origin.

The fine particles of sand, silt and clay are called alluvium. The alluvial soil can be divided into old alluvium, also called bangar, and new alluvium, called khadar. Remember, the new alluvium can be about ten thousand years old.

i. The new alluvium is deposited in the flood plains and deltas. These areas are flooded almost every year.

ii. The old alluvium is found on the higher side of the river valleys, i.e. about 25 metres above the flood level.

iii. The khadar soil is sandy and light in colour, while the bhangar soil is clayey and dark.

iv. The khadar soil is more fertile than the bangar soil.

v. The alluvial soils contain adequate potash, phosphoric acid and lime.

vi. They are generally deficient in organic and nitrogenous contents.

vii. The old alluvium often contains lime nodules, known as kankar.

The fertility of the alluvial soil varies from place to place. Due to its softness and fertility, alluvial soil is most suited to irrigation and can produce bumper crops of rice, wheat, maize, sugar cane, tobacco, cotton, jute, oilseeds, etc.

2. Black Soil:

The black soil is locally called regur, a word derived from Telugu word ‘reguda’. It is also called the Black Cotton Soil, as cotton is the most important crop grown in this soil.

The black soil is mostly found in the Deccan Trap, covering large areas of Maharashtra, Gujarat and western Madhya Pradesh. It is also found in some parts of Godavari and Krishna river valleys, covering parts of Karnataka, Andhra Pradesh and Tamil Nadu.

i. The black soil has been formed thousands of years ago, due to the solidification of volcanic lava.

ii. This soil is made up of extremely fine clayey material.

iii. The black soil is well-known for its capacity to hold moisture.

iv. This soil is rich in calcium carbonate, magnesium carbonate, potash and lime, but poor in phosphoric content.

v. During the rainy season, black soil becomes sticky and is difficult to till as the plough gets stuck in the mud.

vi. During the hot dry season, the surface of this soil develops cracks.

vii. These cracks help in the aeration of the soil.

viii. Actually the black soil should be tilled immediately after the first or the pre-monsoon showers.

Generally, in the upland regions, the black soil has low fertility, while in the valleys or lowlands; this soil is darker, deeper and very fertile. Due to high fertility and capacity to hold moisture, black soil is widely used for producing cotton, wheat, linseed, millets, tobacco and oilseeds. With proper irrigation facilities, this soil can also produce rice and sugar cane.

3. Red Soil:

The red soil occupies about 10 per cent area of India, mostly in the south-eastern part of the Peninsular India. This area encircles the entire black soil region. The red soil is found in Tamil Nadu, parts of Karnataka, southeast Maharashtra, eastern parts of Andhra Pradesh, Madhya Pradesh, Orissa and Jharkhand.

i. Most of the red soil has been formed due to weathering of igneous and metamorphic rocks.

ii. The red colour is due to the high percentage of iron contents.

iii. The texture of the red soil varies from sandy to clayey, and the majority being loamy.

iv. On the uplands, the red soil is thin, poor, and porous and has loose gravel.

v. In the lower areas, the soil is deep, rich, fine grained and fertile.

vi. This soil is rich in potash, but poor in lime, phosphate, nitrogen and humus.

With proper doses of fertilizers and irrigation the red soils can give excellent yields of cotton, wheat, rice, pulses, millets, tobacco, oilseeds, etc.

4. Laterite Soil:

The word ‘laterite’ has been derived from a Latin word meaning ‘brick’. The laterite soil is widely spread in India and is mainly found on the summits of the Western Ghats, Eastern Ghats, Rajmahal Hills, Vindhyas, Satpuras and Malwa plateau. It is well- developed in southern Maharashtra, and parts of Orissa, West Bengal, Karnataka, Andhra Pradesh, Kerala, Bihar, Assam and Meghalaya.

i. The laterite soil is formed under conditions of high temperature and heavy rainfall with alternate wet and dry periods.

ii. Such climatic conditions promote leaching of soil. Leaching is a process in which heavy rains wash away the fertile part of the soil.

iii. The laterite soil is red in colour and composed of little clay and much gravel of red sandstones.

iv. Laterite soil generally is poor in lime and deficient in nitrogen. The phosphate contents are generally high.

Due to intensive leaching, the laterite soil generally lacks fertility and is of low value for crop production. But when manured and timely irrigated, the soil is suitable for producing plantation crops like tea, coffee, rubber, coconut, arecanut, etc. It also provides valuable building materials.

5. Mountain Soil:

The mountain soil is generally found on the hill slopes covered with forests. In the Himalayan region such soil is mainly found in the valley basins, the depressions and the lesser steep slopes. The north-facing slopes generally support soil cover. Apart from the Himalayan region, this soil is also found in the Western and Eastern Ghats and in some parts of the Peninsular India.

i. The mountain soil is formed mainly due to the deposition of organic matter provided by the forests.

ii. This soil is rich in humus, but poor in potash, phosphorus and lime.

iii. It is heterogeneous in nature and varies from place to place.

iv. The mountain soil is sandy with gravels and is porous.

For getting high yields of crops, heavy doses of fertilizers have to be applied. In the Himalayan region wheat, maize, barley and temperate fruits are grown on this soil. This soil is especially suitable for producing plantation crops, such as tea, coffee, spices and tropical fruits in Karnataka, Tamil Nadu and Kerala.

6. Desert Soil:

The desert soil is found mostly in the arid and semi-arid regions, receiving less than 50 cm of annual rainfall. Such regions are mostly found in Rajasthan and the adjoining areas of Haryana and Punjab. The Rann of Kachchh in Gujarat is an extension of this region.

i. The sand in the desert areas is partly of local origin and partly being blown in from the Indus Valley.

ii. It includes even the wind-blown loess.

iii. The desert soil has sand (90 to 95 per cent) and clay (5 to 10 per cent).

iv. In some regions this soil has high percentage of soluble salts, but lacks in organic matter.

v. The nitrogen content is low, but the phosphate content is as high as in normal alluvial soil.

When water is made available through irrigation, the desert soil can produce a variety of crops, such as wheat, millets, barley, maize, pulses, cotton, etc. Shortage of water in the arid regions is the main limiting factor for agriculture.