SOIL CHAPTER 9 CLASS 7 SCIENCE NOTE- STUDY MATERIAL

SOIL CHAPTER 9 CLASS 7 SCIENCE NOTES

                                SOIL 

There are many natural resources on earth. Soil plays an important role among them. Soil provides shelter for many animals. We can see many organisms in the soil which dwell their life in it. It helps human beings also in many ways. Our crops, forests, water resources, buildings, and constructions are very much related to the soil.

You can collect soil samples from different places and observe the kind of insects and small organisms present in them. Different plants and herbs can be seen in it. Tabulate your result and make a comparative study of it.

 

Collect a sample of soil from your garden. Put that soil in a transparent glass tumbler of water. Stir it properly and allow it to settle down. Observe the distinct layers seen in the tumbler.

We can see that the heavy gravel particles are settling at the bottom of the tumbler. Just above that layer, we can observe sand particles. Clay particles are finer and smaller than sand and hence it will be settling on top of the sand. There will be a muddy water layer just above the clay layer. Water will become more clear as time passes and it will be visible as a separate layer. Very lighter and less dense things will be floating on the top layers. It may include dry leaves, animal remains, and twigs.

HUMUS

The rotting dead matter of plants and animals present in the soil is called humus. Normally the top layer of the soil will be rich in humus.

SOIL PROFILE

How soil is formed on earth? Have you ever thought of the formation of soil on earth?

The soil we see today has been formed through different processes that occurred in nature. The rocks that were there on earth undergone many changes due to climate change, rain, heat, and flowing water. 

Water that gets collected in the cracks of rocks will be freezing at night and it expands to make bigger cracks on it. It started breaking down into smaller pieces and might have carried by the wind or water. 

The process of breaking down rocks into the soil by the action of wind, water, and climate is called weathering.

When you are taking a ride in a hilly place, you can see the vertical cross-section of soil through which the roads are made. Have you noticed the difference in the colour and texture of each layer?

SOIL PROFILE

The vertical section through various layers of soil is called a soil profile.

These layers in a soil profile are called horizon. Each horizon will be of different colour, texture, depth, and composition.

We can discuss the features of these horizons in detail.

A-HORIZON or TOPSOIL

• It is the uppermost layer of the soil.
• It is dark in colour as it is rich in humus and minerals.
• It is soft and porous.
• It can retain more water.
• It is a shelter for many animals.
• It helps the roots of plant to spread easily through the soil.

B-HORIZON

• It is the next layer to the topsoil.
• It has a lesser amount of humus.
• It has more amount of minerals.
• It is generally harder and more compact than the topsoil.

C-HORIZON

• It is the third layer of the soil profile.
• It is made up of small lumps of rocks with cracks and crevices.

BEDROCK

• It is the last layer of the soil profile.
• It is the hardest layer.
• It contains a bed of rock which is difficult to dig with a spade.

TYPES OF SOIL

Soil formed from rocks in different places will be different in its composition.

Soil may contain different proportions of rock particles, clay, and humus.

The soil is classified into three types based on the proportion of particles present in it.

a. Sandy soil

b. Loamy soil

c. Clayey soil

The soil which contains a greater proportion of big particles is called sandy soil.

SANDY SOIL

If the proportion of fine particles present in the soil is higher, it is called clayey soil.

CLAYEY SOIL

If the proportion of large particles and fine particles are almost the same, it is called loamy soil.

LOAMY SOIL

DIFFERENCE BETWEEN SANDY, LOAMY AND CLAYEY SOIL

SANDY SOIL

• The particle size of the soil is large.
• There is a large space between the particles and it cannot fit closely together.
• Sand is well aerated.
• Water can drain quickly through the space between the particles.
• Less water holding capacity.
• Sand is light and dry.

CLAYEY SOIL

• The particle size is very small.
• The soil particles are tightly packed and there is very little space between them.
• Sand is less aerated.
• Water will not drain easily.
• Water holding capacity is more.
• It is heavier as it holds more water.
• It is the best soil for making pots, statues, and toys.

LOAMY SOIL

• Loamy soil is the best soil for the growth of the plants.
• It is a mixture of sand, clay, and silt.
• It has the right amount of humus.
• it has the right water holding capacity.

* SILT- It is the deposit of soil seen at the river bed. The particle size of silt is between sand and clay. It is very fertile as it carries all the minerals from the hills and mountains. it helps in the growth of plants.

PROPERTIES OF SOIL

The important properties that we are going to discuss are the percolation rate of the soil, moisture in the soil, and the water absorption capacity.

PERCOLATION RATE

Have you ever noticed how fast water seeps down through the soil? If you are collecting different samples of soil, the amount of water that seeps down will be different. Why it is so?

The time taken by the water to percolate through a particular sample of soil depends upon the particle size of the soil. We can observe that the time for percolation will be less in the case of sandy soil as it is loosely packed. Water goes down very fast through it.

So percolation rate can be calculated for a particular sample of soil by knowing the amount of water percolated and the time taken for percolation.

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

If a given sample of soil has taken 10 minutes to percolate 300 ml of water, the percolation rate will be 300/10=30ml/min.

MOISTURE IN SOIL

If you are heating a sample of soil from your garden in a test tube, you can observe tiny droplets of water getting condensed on the inner sides of the test tube. Where does this water come from?

It is due to the moisture or the water content present in the soil.

On a hot summer day, we can observe the air above the hot soil shimmering. The water vapour which is coming out of the soil reflects the sunlight and it appears to shimmer. Hence we can conclude that there is moisture present in the soil.

ABSORPTION OF WATER BY SOIL

Different types of soil will be absorbing water to a different extent.

The amount of water retained by clayey soil will be more than that of sandy soil.

An activity can be performed to observe this. 

Take a funnel and pour a known amount of soil into it. Note down the initial volume of water in a measuring jar and start pouring that into the soil. Make sure that you are pouring it all over the soil. 

Look at the tip of the funnel. When it starts dripping, stop pouring and note down the final volume of water. The difference will give you the volume of water absorbed by the soil.

The percentage of water absorbed by the soil can be found using the equation;

Percentage of water absorbed = U-V/W x100

where U is the initial volume of water in the measuring cylinder, V is the final volume of water in the measuring cylinder and W is the weight of the soil.

SOIL AND CROPS

Wheat and gram - clayey soil and loamy soil which is good at retaining water.

WHEAT

Paddy- clayey soil is rich in clay and organic matter and can retain more water.

PADDY

Lentils and pulses- loamy soil that drains water easily.

Cotton - sandy loam or loamy soil that can drain water easily and are well aerated.

COTTON FARM

In this chapter, you have learned about the formation of soil, soil profile, types of soil, and the properties of soil. You have already studied soil erosion and soil pollution in your geography lessons. The activities that are mentioned in the NCERT textbook can be performed at home to make the concepts more clear. This abstract of soil chapter 9 class 7 notes will definitely help you to enhance your knowledge of this chapter.