Question

Why is organic matter (humus) an important part of soil?
A. It helps to improve water infiltration
B. It can break down the organic population
C. It converts nitrogen in the air into nitrates used by plants
D. It is rich in nutrients, which is important for fertility

Answer 1

Hint: Soil profile shows 4 distinct layers called horizons
Horizon A, Horizon B, Horizon C and Horizon D
Humus lies in the horizon A which is organic matter undergoing decay by microbial action.

Complete answer: Humus is the most important part of the soil.
It consists of partially decayed organic matter.
It makes the soil porous, thereby increasing its air and water holding capacity.
Humus is rich in nutrients that promote plant growth.
Being black in color humus absorbs heat to warm up the soil.
So the correct answer is option is D. It is rich in nutrients, which is important for fertility.

Additional information: Horizon A is the topsoil. It is darker and a looser texture than underlying horizon B. Plant and animal matter collects at the surface of this horizon, forming the litter. Below the litter is the humus.
The horizon B has soil particles smaller and usually more compact than in the horizon C.
Minerals brought rainwater from the upper horizon accumulate in this horizon.
The horizon C consists of the weathered material derived from the intact parent rock that forms the horizon D.

Note: Humus is that part of soil being organic-rich in nutrients and makes the soil fertile.