Class 7 Science Soil Notes

Texture of Soil

Rock break into smaller pieces of different sizes. This is why the size of soil particles also vary. The texture of the soil thus depends on the relative proportions of the various types of soil particles.

Depending on particle size, soil is of three types:

1. Sandy Soil            2. Clayey Soil           3. Loamy soil

Finally, depending upon the size of soil particles, soil can be classified into

Gravel                      5.0 mm (diameter)

Fine gravel               2.0 mm (diameter)

Coarse sand             0.20 mm (diameter)

Fine sand                 0.02 mm (diameter)

Silt                          0.002 mm (diameter)

Clay Less than          0.002 mm (diameter)

Soil Types

Different geographical regions have different types of soils with their own characteristic colour, texture and contents. Six types of soils are commonly found in India, as described below:

1. Red Soil or Red Latosol: This soil is red in colour due to the presence of iron oxide It contains quartz and clay particles but is deficient in nitrogen and humus. This soil can be enriched with nutrients by the addition of manures and fertilizers. This type of soil is found in the interior regions of Kerala and Tamil Nadu, Southern Karnataka, Andhra Pradesh, Orissa and Eastern Madhya Pradesh.

2. Black Soil or Regur: This rich black loamy soil is rich in minerals, humus and is porous in nature. It contains large proportions of iron and magnesium which are derived from basaltic rocks - the rocks formed from the lava of volcanic eruption This soil is ideally suited for growing sugarcane and cotton. In India, regur soil is found in Maharashtra, Andhra Pradesh , Madhya Pradesh and Gujarat.

3. Alluvial Soil or Khadar: It is the most important and widespread of all the soil type It covers approximately 40% of the land area . It is formed by the deposition of silt which has been brought down by the great Himalayan rivers - Sutlej, Ganga, Brahmaputra and their tributaries. In India it is found in the plains of Haryana, Punjab, Uttar Pradesh, Bihar, West Bengal, Coastal Orissa and Anthra Pradesh.

4. Desert Soil: This soil contains large proportions of sand. It is greyish brown in colour, porous in texture and is rich in soluble salt It does not have good water holding capacity but it can become fertile when watered. Sandy soil is mainly found in the Rann of Kutchh in Gujrat and in Rajasthan.

5. Mountain Soil: This is highly fertile soil and has the highest humus content among all the soils in India.

The proportion of its contents vary from place to place. It is found in the Himalayan region and North Eastern parts of India and some parts of the Western Ghats and other hilly ranges such as the Malabar range in Kerala..

6. Laterite Soil: It is red in colour and is found in regions of heavy rainfall with highly humid tropical climate It is clayey in texture and good for growing plantation crops like tea, coffee and coconut. In India, it is mainly found in the Westsern Ghats, parts of Tamil Nadu , Andhra Pradesh, Orissa and Assam.

Water Holding Capacity of Soil

The amount of water absorbed and retained by the soil is called its water holding capacity. It depends upon the amount of humus present in the soil and the texture of the soil. Since the texture of soil varies from place to place, its water holding capacity also varies.

Sandy Soil: Sandy soil has very poor water holding capacity because of the large spaces between the soil particles.

Clayey Soil: Clayey soil has very good water holding capacity, so in areas that have a lot of clayey soil, waterlogging is common. Waterlogging mean the collection of stagnant water above the soil whenever there is rain. The water logged soil cannot hold air for plant roots to breathe, hence plants cannot grow well in it.

Loamy Soil: It has good capacity to hold and absorb water. This soil is ideal for agriculture.

Soil Quality and Suitability

Quality of soil depends upon:

1. Fertility of Soil: The capacity of soil to sustain plant life is a fertility of soil. Soil fertility can be enhanced artificially by adding to the soil.

a) Chemical fertilizers

b) Biomanures like cow dung compost and wormicast to the soil.

2. Soil Particles: On the basis of the particle size, soil can be classified as gravel, fine gravel, coarse sand, fine sand, silt and clay.

3. Soil Texture: Relative proportion of different size of soil particles forms the texture of the soil. On the basis of texture, soil can be classified as sandy, clayey and loamy.

4. Water Content: Plants are propagated mainly in soil and get their water supply from soil. Chief source of water for soil is rainfall. As we have seen, the water holding capacity of different types of soils varie