SOIL | SPEED NOTES | SCIENCE | CBSE | CLASS 7

Chapter – 9 Soil Soil is the uppermost crust that covers the earth. It is a mixture of rock particles and humus. Soil is important for life onthe earth.

Formation of Soil: (i) Soil is formed by weathering ordisintegration of parent rocks. (ii) Physical weathering isdegradation of rocksby physical agents like water, ice,wind, sun, etc. (iii) Chemical weathering is chemical decomposition of rocks. (iv) Biological weathering is decomposition of parent rocksby bacteria andmicroorganisms.

Soil Profile: It isthe vertical section of soil fromthe ground surface to the parent rock. Soil profile is a section through different layers of the soil, Various layers are called horizons.

Constituents of Soil: Soilconsists of bothliving and non-living matter. These constituents make thesoil fertile.

Types of Soil: Soilis of different types: clayey, loamy and sandy.

(i) Sandy soil: It contains soil particles with a diameter of 0.2 to 2.0 mm. It comprises of around 60% sand along with some clay. It has very low water retention capacity. It is not richin humus. (ii)Clayey soil: It contains soil particles witha diameter of less than0.2 mm. It has an excellent water retention capacity and aircirculation is sufficient. (iii) Loamy soil: It contains a good mixture of sand, clayand humus. Ithas a good

water retention capacity andair circulation issufficient. Percolation rateof water isdifferent in different types of soil. It ishighest in the sandy soil andleast in theclayey soil. Different types of soils are used tocultivate different types of crops. Clayand loam are suitable for growing wheat, gram andpaddy. Cotton isgrown in sandy loam soil. Soil holds waterin it, whichis called soilmoisture.

The capacity of a soilto hold water is important forvarious crops. Clayey soil isused to makepots, toys andstatues.