ICSE Class 6 Biology Chapter 6 Soil

Soil

Soil is the naturally occurring, uppermost layer of the earth and is formed from rocks and decaying plant and animal matter. The word 'soil' is derived from the Latin word 'solum' which means 'earthly material'. The science which deals with the study of soil is called pedology.

Soil As A Natural Resource

Soil is one of the most important natural resource. We know that we depend on plants for all our food. Land plants need soil for support, water and minerals. Healthy soil has more nutrients, air and water which help plants to grow well. Thus, life indirectly depends on soil.

Other uses of soil are mentioned below:

Soil is a rich source of metals like iron, calcium, aluminium, magnesium and so on which are used in different industries.

Soil is a source of clay which is used for making bricks, pottery and porcelain.

Water that seeps through the soil is stored underground as subsoil water. This water can be made available by boring wells.

Soil Profile

A vertical section of soil from the surface down to the bedrock, showing different horizons, is called soil profile. Different parts of the world have different soil profile. Soil profile usually shows three layers one above the other (Fig. 6.2) and is affected by climate and other factors.

A-horizon

This is the uppermost layer and is also known as top soil. It is soft, porous and contains lots of humus (dead and decaying plant and animal matter). It has a good water-holding capacity. It is also the most fertile part of the soil. Living organisms like insects, earthworms, bacteria and fungi are present in this layer.

B-horizon

It is found below the A-horizon. It is known as subsoil and contains sand, silt and clay. It is harder and more compact than the top soil. This layer is rich in minerals and iron oxide. Roots of tall plants normally reach the subsoil.

C-horizon

It is the third layer and consists of lumps of parent rock material.

Bedrock

It lies below the C-horizon and contains unweathered parent rock. It is not a part of the soil profile. It provides a base to the other three horizons.

Composition Of Soil

Take some soil from your garden and put it in a beaker. Add some water and stir it. Allow it to settle down. Observe the various layers. Take soil from different sites like the roadside, a pond and a field and compare the different layers.

The lowermost layer consists of large-sized particles called gravel. Just above the gravel layer is a layer of sand particles followed by silt and clay particles.

The different components of the soil are discussed below:

Inorganic matter

The soil contains inorganic matter such as clay, silica, chalk, nitrates, phosphorus, magnesium, calcium, sodium, iron and so on, which provide rich nutrients to the plants.

Organic matter - humus

Humus is the decaying remains of dead plants and animals. The decay takes place due to the action of microorganisms. During decay process, nitrogen, phosphorus and sulphur are released into the soil. This makes the soil fertile.

Air and water

The space between the soil particles is occupied by air and water. Air makes the oxygen available to the roots for respiration. Water with minerals dissolved in it is loosely held by the soil particles and is absorbed by the root hair. The capacity of a soil to hold water depends on the type of soil particles it contains (Activity 2).

Living organisms

Besides being a home for millions of bacteria, certain fungi and some blue-green algae, the soil is also a home for many more living organisms such as millipedes, centipedes, ants and earthworms.

Bacteria and fungi present in the soil play a major role in recycling of nutrients. They help in breaking down of organic matter or nutrients present in the soil. These nutrients are absorbed by plants from the soil. Some plants are eaten by animals. The nutrients in the dead remains of plants and animals are converted into nitrates by the action of decomposing bacteria and fungi and are released in the soil. These nutrients are again used by plants. Earlier we have read that earthworms are called farmer's friends. This is because earthworms improve the quality of the soil in two ways. As an earthworm moves through the soil, it ingests soil particles. It digests bits of food in the soil. Matter that cannot digest is passed out of the body. This adds to the nutrients of the soil and makes the soil fertile. Also as earthworms move through the soil they leave tube-like passages in the soil. Air and water that are needed by plants get accumulated in these passages. Plants grow well in soil in which earthworms live.

1 kg of rich farm soil contains 2 trillion bacteria, 400 million fungi, 50 million algae and 30 million protozoa as well as thousands of different worms and insects.

Teacher's Note

When you dig in a garden or park, you can observe different layers of soil and notice how living creatures like earthworms and insects help keep the soil healthy and fertile.

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