Class 7 Science
Mixture of rocks particles, sand and humus is known as soil. Soil supports the growth of plants by holding their roots and supplying water to them.
Formation of soil: Soil is formed by weathering. Weathering is the process of breaking down of rocks by the action of wind, water and climate. Formation of even one inch of soil takes many years.
Composition of Soil: Soil contains humus, water, clay, sand and gravel.
Humus: The decaying dead matters in soil are called humus. Humus makes the soil fertile and provides nutrients to the plants.
Soil Profile: Different vertical layers of soil are known as soil profile. The feel (texture), colour and chemical compositions of soil are different in each layer.
Layers of soil are referred as horizons. Horizons of soil are categorized into four types: A-horizon, B-horizon, C-horizon and Bedrock.
A horizon: Top layer of soil is called A-horizon. It is also called top-soil. A-horizon is generally dark in colour. It is rich in humus. A horizon is generally soft and porous. It retains more water.
Roots of small plants are entirely embedded in topsoil. Topsoil also provides shelter to many living organism, such as worm, insect, moles, rats, snakes, etc.
B-horizon: The next layer of the soil from top is known as B-horizon. This layer is harder and more compact than the top soil. This is generally known as the middle layer of soil. This is less rich in humus but contains most of the minerals present in the soil.
C-horizon: The third layer of soil is known as C-horizon. C-horizon is made of small lumps of rocks with cracks and crevices.
Bedrock: The layer below the C-horizon is known as bedrock. Bedrock is composed of rocks and is hard to dig with spades.
Types of Soil
Size of particles of soil is very important for the properties of soil. So, soils are categorized mainly in three types based on the proportion of size of particles. These are Sandy Soil, Clayey Soil and Loamy Soil.
Sandy Soil: Particles of sandy soil are larger in size. Because of being larger in size, sand particles cannot fit close together and hence there is enough space among them. These spaces are filled with air. Water drains quickly through sandy soil. So, sandy soil is called well aerated, light and dry.
Sandy soil is not fit for vegetation as it does not retain water. However, millets can be grown on sandy soil.
Clayey Sol: Particles of clayey soil are very small in size. This makes the particles of clayey soil packed tightly. There is very little space among the particles. Water does not drain quickly through clayey soil because of less space among particles. So, clayey soil is not well aerated and retains more water.
Clayey soil is used in making of toys. Clayey soil is good for growing paddy.
Loamy Soil: Particles of loamy soil are smaller than sand and larger than clay. Loamy soil is the mixture of sandy soil, clayey soil and silt. Silt is the deposit in river beds.
Loamy soil has right water holding capacity and is well aerated. This is considered as the best soil for the growth of plants.
Properties of Soil:
Percolation Rate of Water: Amount of water drained through water in unit time is known as percolation rate of water in soil. Percolation rate of water can be calculated using the formula given here.
Percolation rate (mL/min) = Amount of water in mL/Time taken to percolate
The percolation rate of water in sandy soil is fastest and in clayey soil is slowest.
Moisture present in soil: Amount of water present in soil is called moisture present in soil. The amount of moisture present in clayey soil is highest and that in sandy soil is lowest.
Absorption of water: Different soil absorbs different amount of water. Clayey soil absorbs the highest amount of water because of its higher water retention capacity. On the other hand, sandy soil absorbs the least amount of water because of its lower water retention capacity.
Soil and Crop
Different types of soil are found are found at different places. A particular soil type may not be suitable for certain types of crop. That’s why different types of crop are grown in different parts of the world.
Sandy soil: Sandy soil is not fit for any crop as it does not retain water. However, some thorns and bushes do grow on sandy soil. Millets can be grown on sandy soil.
Clayey Soil: Clayey soil is best suited for paddy, as it can retain water for a longer time. This is also suitable for wheat and grams.
Loamy Soil: Loamy soil is considered the best for almost all types of crops. It is suitable for lentils and other pulses.
Sandy Loam: Sandy loam soil is suitable for the growth of cotton plants as it can easily drain water and is well aerated.
Removal of topsoil by weathering agents; such as wind, water and ice is known as soil erosion. Soil erosion affects the fertility of soil, as humus present in the soil is eroded along with soil. It is important to prevent soil erosion.
Preventive measures for soil erosion:
- Deforestation should be prevented at all cost to prevent soil erosion.
- Shelter belt plantation of trees is good for preventing soil erosion.
- Terrace farming in hilly areas is effective in preventing soil erosion.