Question

what is humas? what is its defenation

Answer 1

Humus is the organic material in soil. 
Humus is dark, organic material that forms in soil when plant and animal matter decays.

Answer 2

Humus is defined as black organic matter formed in soil due to the decay of dead plant and animal matter. It is organic because of accumulation of carbon and is hence dark in colour. It has very important constituents to keep soil healthy.