Answer : 1. "A-horizon"
Explanation :

1. Humus is found in the topmost layer of soil (A-horizon).

2. Humus is a black or brown colored substance which makes the soil fertile.

3. It is an important source of nutrients for plants, and it helps the soil absorb water and air.

4. There are many ways to promote humus, including-

- Leaving crop residues in the field

- Using livestock manure

- making compost