Question

Deep black soil is considered to be highly productive due to the high proportion of
(a) Silt and earthworm
(b) Clay and humus
(c) Gravel and Ca$^{2+}$
(d) Sand and Zn

Answer 1

Hint: The property of deep black soil to retain moisture for a long time has been an important factor for its high productivity. Its composition is essential for the growth of cotton.

Complete answer:
Clay content in deep black soil ranges from 40 to 60 per cent. Black soil is also abundant in humus. Slow absorption causes the black soil to retain the moisture for a longer time. This helps the crops, especially, the rain-fed ones, to survive the scarcity of water in dry seasons.
- The black soil retains moisture to a great extent. It swells and becomes sticky when wet in the rainy season. However, in the hot dry season, the moisture evaporates which leads to the shrinkage of soil. Broad and deep cracks appear which allows oxygenation of the soil in the deepest parts. This leads to an extraordinary increase in the fertility of the soil.
- As the soil is fertile, it is suitable for crops such as cotton, sugarcane, citrus fruits etc.
- The presence of magnetite, compounds of iron and aluminium, and humus are responsible for its black colouration.
- Based on origin, colour, composition, and location, the soils in India have been classified into following types: Alluvial soils, Red and yellow soils, laterite soils, Arid soils, Forest soils, Saline soils and Peaty soils.
So, the correct answer is ‘Clay and humus’.

Note: Humus is the thick black or brown substance that remains after most of the organic litter has been decomposed by organisms such as earthworms. They often help mix humus with minerals in the soil. The soil crumbles due to the presence of iron. This is beneficial for the Air and water to move easily through the loose soil and subsequently, oxygen is available to the roots of plants.