Soil NCERT Science Class 7 Chapter 9 Summary

Soil
Upper layer of earth crust, is called soil. 
The mixture of rock particles and humus is called the soil. 

Humus
The rotting dead matter in the soil is called humus. 

Weathering
Formation of soil by break down of rocks by the action of winds, water and climate, is called weathering. 

Nature of soil depends on
The parent rock 
Vegetation grows on it. 
Wind, Climate, rainfall, temperature, light and humidity. 

Soil profile 
A vertical section through different layers of the soil is called soil profile. 
Horizon
Each layer of soil differing in texture, colour, depth, and chemical composition, is called horizon. 

A-Horizon 
It is dark in colour. 
It is rich in humus. 
Soil is fertile. 
This layer is soft and porous. 
This layer can retain more water. 
It is also called top soil. 

B-Horizon 
This layer has lesser amount of humus. 
This layer has more amount of minerals. 
This layer is harder. 
This layer is more compact. 

C-Horizon 
This layer is made up of small lumps of rocks with cracks and crevices. 

Bedrock 
The layer below C-Horizon is bedrock. 
It is hard layer. 

Types of soil 
Sandy soil 
Clayey soil 
Loamy soil 

Sandy soil 
The soil that contains greater propertion of big particles is called Sandy soil. 
The soil is loosely packed. 
Space between particles - more 
Water retaining capacity - less 
Air in soil - more 

Clayey soil
The soil that contains greater propertion of fine particles is called clayey soil. 
The soil is tightly packed. 
Space between particles - less
Water retaining capacity - more
Air in soil - less

Loamy soil
The soil that is mixture of clay, sand and silt, is called loamy soil. 
Size of particles - medium 
Space between particles - medium
Water retaining capacity - medium (good for crop) 
Air in soil - medium

Percolation rate 
Percentage of water absorbed 
Where U= Initial volume of water 
            V= Final volume of water 

Clayey soil and loamy soil are both suitable for growing crops. Because, such soil are good at retaining water. 






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