9.1 Soil teeming with life

One day during the rainy season Paheli and Boojho observed an earthworm coming out of the soil. Paheli wondered whether there were other organisms also in the soil. Let us find out.

Activity 9.1

Collect some soil samples and observe them carefully. You can use a hand lens. Examine each sample carefully and fill in Table 9.1.

•  Discuss your observations with your friends.
  
•  Are the soil samples collected by your friends similar to the ones collected by you?
  

Boojho and Paheli have used soil in many ways. They enjoy playing with it. It is a great fun indeed.

Make a list of the uses of soil.

9.2 Soil Profile

Soil is composed of distinct layers. Perform the following activity to find out how these layers are arranged.

Activity 9.2

•  Do you see layers of particles of different sizes in the glass tumbler?
  
•  Draw a diagram showing these layers.
  
•  Are there some dead rotting leaves or animal remains floating on water?
  

The rotting dead matter in the soil is called humus.

You probably know that the soil is formed by the breaking down of rocks by the action of wind, water and climate. This process is called weathering. The nature of any soil depends upon the rocks from which it has been formed and the type of vegetation that grows in it.

We usually see the top surface of the soil, not the layers below it. If we look at the sides of a recently dug ditch, we can see the inner layers of the soil, too. Such a view enables us to observe the soil profile at that place. Soil profile can also be seen while digging a well or laying the foundation of a building. It can also be seen at the sides of a road on a hill or at a steep river bank.

The uppermost horizon is generally dark in colour as it is rich in humus and minerals. The humus makes the soil fertile and provides nutrients to growing plants. This layer is generally soft, porous and can retain more water. It is called the topsoil or the A-horizon. This provides shelter for many living organisms such as worms, rodents, moles and beetles. The roots of small plants are embedded entirely in the topsoil.

The third layer is the C-horizon, which is made up of small lumps of rocks with cracks and crevices. Below this layer is the bedrock, which is hard and difficult to dig with a spade.

9.3 Soil types

As you know, weathering of rocks produces small particles of various materials. These include sand and clay. The relative amount of sand and clay depends upon the rock from which the particles were formed, that is the parent rock. The mixture of rock particles and humus is called the soil. Living organisms, such as bacteria, plant roots and earthworm are also important parts of any soil.

The soil is classified on the basis of the proportion of particles of various sizes. If soil contains greater proportion of big particles it is called sandy soil. If the proportion of fine particles is relatively higher, then it is called clayey soil. If the amount of large and fine particles is about the same, then the soil is called loamy. Thus, the soil can be classified as sandy, clayey and loamy.

The size of the particles in a soil has an influence on its properties. Sand particles are quite large. They cannot fit close together, so there are large spaces between them. These spaces are filled with air. We say that the sand is well aerated. Water can drain quickly through the spaces between the sand particles. So, sandy soils tend to be light, well aerated and rather dry. Clay particles, being much smaller, pack tightly together, leaving little space for air. Unlike sandy soil, water can be held in the tiny gaps between the particles of clay. So clayey soils have less air. But they are heavy as they hold more water than the sandy soils.

The best topsoil for growing plants is loam. Loamy soil is a mixture of sand, clay and another type of soil particle known as silt. Silt occurs as a deposit in riverbeds. The size of the silt particles is between those of sand and clay. The loamy soil also has humus in it. It has the right water holding capacity for the growth of plants.

Activity 9.3

Collect samples of clayey, loamy and sandy soils. Take a fistful of soil from one of the samples. Remove any pebbles, rocks or grass blades from it. Now, add water drop-by-drop and knead the soil [Fig. 9.4 (a)]. Add just enough water so that a ball [Fig. 9.4 (b)] can be made from it, but at the same time it should not be sticky. Try to make a ball
[Fig. 9.4 (c)] from this soil. On a flat surface, roll this ball into a cylinder [Fig. 9.4 (d)]. Try to make a ring from this cylinder [Fig. 9.4 (e)]. Repeat this activity with other samples also. Does the extent to which a soil can be shaped indicate its type?

Can you suggest which type of soil would be the best for making pots, toys and statues?

9.4 Properties of Soil

You have listed some uses of soil. Let us perform some activities to find the characteristics of the soil.

Percolation rate of water in soil

Boojho and Paheli marked two different squares of 50 cm × 50 cm each, one on the floor of their house and the other on the kutcha (unpaved) road. They filled two bottles of the same size with water. They emptied the water from the bottles, one each, at the same time in the two squares. They observed that the water on the floor flowed down and was not absorbed. On the kutcha road, on the other hand, the water was absorbed.

Activity 9.4

For this activity divide yourself into three teams. Name the teams A, B and C. You will be finding out how fast the water passes down the soil. You will need a hollow cylinder or a pipe. Ensure that each team uses pipes of the same diameter. Some suggestions for obtaining such a pipe are given below:

1. If possible, get a small tin can and cut off its bottom.

2. If PVC pipe (approx. diameter 5 cm) is available, cut it into 20 cm long pieces and use them.

At the place where you collect the soil, place the pipe about 2 cm deep in the ground. Pour 200 mL water in the pipe slowly. For measuring 200 mL water you can use any empty 200 mL bottle. Note the time when you start pouring water. When all the water has percolated leaving the pipe empty, note the time again. Be careful not to let the water spill over or run down on the outside of the pipe while pouring. Calculate the rate of percolation by using the following formula:

 

For example, suppose that for a certain sample, it took 20 minutes for 200 mL to percolate. So,

 

Calculate the rate of percolation in your soil sample. Compare your findings with others and arrange the soil samples in the increasing order of the rate of percolation.

Have you ever passed through a farmland during a hot summer day? Perhaps you noticed that the air above the land is shimmering. Why is it so? Try out this activity and find the answer.

Activity 9.5

Take a boiling tube. Put two spoonfulls of a soil sample in it. Heat it on a flame
(Fig. 9.6) and observe it. Let us find out what happens upon heating.

Do you see water drops any where? If yes, where did you find them?

On a hot summer day, the vapour coming out of the soil reflect the sunlight and the air above the soil seems to shimmer.

After heating the soil, take it out of the tube. Compare it with the soil which has not been heated. Note the difference between the two.

9.6 Absorption of Water by Soil

Do all the soils absorb water to the same extent? Let us find out.

Activity 9.6

Take a plastic funnel. Take a filter paper (or a piece of newspaper sheet), fold and place it as shown in the figure (Fig. 9.7). Weigh 50g of dry, powdered soil and pour it into the funnel. Measure a certain amount of water in a measuring cylinder and pour it drop by drop on the soil. You can use a dropper for this purpose. Do not let all the water fall at one spot.

Pour water all over the soil. Keep pouring water till it starts dripping. Subtract the amount of water left in the measuring cylinder from the amount you started with. This is the amount of water retained by the soil. Record your results in your notebook in the following manner:

Weight of soil = 50g

Initial volume of water in the measuring cylinder = U mL

Final volume of water in the measuring cylinder = V mL

Volume of water absorbed by the soil = (U – V) mL

Weight of water absorbed by the soil = (U – V) g

(1 mL of water has weight equal to 1 g)

percentage of water absorbed

=

•  Which soil would have the highest percolation rate?
  
•  Which soil would have the lowest percolation rate?
  
•  Boojho heard from his neighbourer that 8–10 days after the rain, the level of water in a pond or well rises. Which type of soil will allow water to reach a well faster and in greater amount?
  
•  Which type of soil retains the highest amount of water and which retains the least?
  
•  Can you suggest any method to let more rain water percolate and reach the water underground?
  

9.7 Soil and Crops

Different types of soils are found in different parts of India. In some parts there is clayey soil, in some parts there is loamy soil while in some other parts there is sandy soil.

Clayey and loamy soils are both suitable for growing cereals like wheat, and gram. Such soils are good at retaining water. For paddy, soils rich in clay and organic matter and
having a good capacity to retain water are ideal. For lentils (masoor) and other pulses, loamy soils, which drain water easily, are required. For cotton, sandy-loam or loam, which drain water easily and can hold plenty of air, are more suitable.

Crops such as wheat are grown in the fine clayey soils, because they are rich in humus and are very fertile. Find from your teachers, parents and farmers the type of soils and crops grown in your area. Enter the data in the following Table 9.2:

Which kind of soil would be most suitable for planting rice? Soil with a higher or lower rate of percolation?