Define humus and state its function.

(N/A) Humus is the dark-coloured,partially decayed organic matter found in the top layer of soil.
Its functions are as follows:
$1$. It makes the soil porous,which increases the soil's water-holding capacity and air content.
$2$. It is rich in nutrients,which promotes the growth of plants.
$3$. It improves soil structure,allowing plant roots to penetrate the soil more easily.