Class 7 - Science - Chapter 9 - Soil

Soil

 

Introduction

• Natural resources are the substances which come from natural environment and can exist without the action of mankind.

• Along with several natural resources such as rivers, mountains, plants, animals, minerals; soil is one of the most important natural resource.

Soil is important to us because-

• It holds the root of the plants for support of the plant and root absorbs nutrient and water from soil for the growth of plants.

• Many macro and microorganisms live in the soil.

• Soil is required for agriculture and in turn agriculture provides food, cloth and shelter for all living organisms.

• Soil produces refreshing fragrance after first rain on the earth.

Nature of soil

• When rocks break down by the action of wind, water or any other source, it forms small particles which together form soil and this process of formation of soil is called weathering of soil.
• The various small particles are sand and
• Nature of a soil depends upon from which rock it has been formed that is the parent rock. For example, rock which contains silica forms only sandy soil.
• Nature of a soil also depends upon the vegetation that has been grown upon it. For example, pedalfer a fertile and black soil forms where there is more of deciduous forest.

Soil horizon and soil profile

• Different layers of soil are called soil horizons and the cross sectional arrangement of these layers in soil is called soil profile.
• Each layers of soil have different texture and color, different depth and varying chemical composition.
• Layers of soil can be seen while digging a well or when the foundation of a building is laid.
• Humus is the rotting dead matter present in the soil which makes the soil fertile.
• Plants absorb their nutrients from the humus in the soil.

The different layers of soil are-

1. A- horizon- 
• This is the topmost horizon and also called as topsoil. 
• Contains large amount of humus and minerals. 
• Topsoil is soft and porous. 
• Capacity of water retention of this soil is more. 
• Hold the roots of the plants entirely.
•  House of many living organisms such as worms, rodents, moles and beetles.

     2.   B-horizon-

• Second layer of soil profile and also called as middle layer. 
• Contains larger amount of minerals than humus. 
• More compact and harder than topsoil.

    3.    C-horizon-

• Third layer of soil profile. 
• Small lumps of rocks are found with cracks and crevices.

   4.     Bedrock-

• The last layer of soil profile. 
• Very hard. 
• This layer is even difficult to dig with a spade.

Types of soil 

• Weathering of rocks form small particles of sand and clay.
• The amount of sand and clay in soil depend upon the parent rock from which the particles form.
• Soil can be also defined as the mixture of rock particles i.e. sand, clay and humus.
• Based on the amount of various sized particles, soil can be divided into three types-

1. Sandy soil- 

• When the soil contains large amount of big particles i.e. sand, the soil is called as sandy soil. 
• Large sand particles have space filled with air in between the particles because these cannot pack closely. 
• Water drain quickly thus sandy soils are light, mixed well with air, dry and have low water retention capacity.

      2. Clayey soil- 

• When the soil contains large amount of small particles i.e. clay, the soil is called as clayey soil. 
• Small clay particles have little space for air because these pack closely and water can be easily held in the very small spaces. 
• Clayey soils are heavy because of greater water holding capacity than sandy soils; these are wet and not well aerated.

      3. Loamy soil- 

• When the soil contains equal amount of big and small particles i.e. equal amount of sand and clay, the soil is called as loamy soil. 
• Along with sand and clay, another type of soil slit which is having size in between sand and clay form loamy soils. 
• It contains appropriate amount of water for the growth of plants. 
• Humus is also found in loamy soils. 
• Loamy soils are wet, have great water retention capacity and well aerated. 
• If loamy soil contains more sand then the soil is light and if contains more clay then the soil is heavy.

Comparison of different types of soil

Sandy soil	Clayey soil	Loamy soil
Contains sand	Contains clay	Contains both sand and clay.
Particles are large	Particles are small	Particles are both large and small
Soil is dry	Soil is wet	Soil is wet
Well aerated	Less aerated	Well aerated
Low water retention capacity	High water retention capacity	High water retention capacity
Soil is light	Soil is hard	Can be hard or light.

Properties of soil

Percolation rate of water in soil

• Percolation is the property of absorption of water by soil.
• If we pour a bottle of water on the floor and another bottle of water in the soil, the water on the floor will flow down but water in the soil will be absorbed.
• The percolation rate is the amount of water absorbed by any soil at any given time.
• Formula for calculating percolation rate is,

       Percolation rate (ml/min) = amount of water (ml) / percolation time (min).

• For example, If 200 ml of water is percolated through the soil sample in 40 min. Calculate the rate of percolation.

Solution – Rate of percolation is = amount of water (ml)/ percolation time (min)

                                                  = 200 ml/ 40 min

                                                  = 5 ml/ min

Ans- The rate of percolation is 5 ml/ min.

Moisture in soil

• Moisture is the amount of water present in soil.
• During summer, the water content in the soil evaporates and moves up which reflect the sunlight, thus the air above soil seems to shimmer.

Absorption of water by soil

• Different soil samples absorb water in different extent.
• The absorption of water by different soil can be find out by taking 50g of different soil samples.
• Pour the soil in different funnels placed in different beakers and pour water in the soil with the help of measuring cylinders.
• Continue pouring until it starts dripping.
• Now, check the amount of water left in the measuring cylinder and subtract it from the initial amount which was taken.
• The resulted amount of water is retained by the soil.
• Formula to calculate percentage of water absorbed is
• Percentage of water absorbed = {(U-V)/50} X 100
• 50g is the amount of water absorbed. 
• U is the initial volume of 
• V is the final volume of water (ml)

Soil and crops

• Crops in the soil are affected by the type of soil and also the climatic factors.
• Climatic factors such as rain, humidity, temperature, wind brings some changes in the structure of soil and hence affect the vegetation.
• For example

• Wheat and gram require clayey and loamy soil for growth.

• Rye grows in sandy loam.

• Paddy requires clayey and soil rich in organic matter which have a good water retention property.

• Loamy soils are required to cultivate lentils and other pulses.

• Sandy loam soils which can hold large amount of water are ideal for the cultivation of cotton, potato, and asparagus.

• Clayey soils are rich in humus and very fertile and hence ideal for the growth of rose, wheat.

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