It's a mixture of decaying organic substances, such as dead leaves, used to fertilize soil
It is difficult to define humus precisely; it is a 
highly complex substance,... [DOC] [TLE] The importance of soil organic matter [PAR] Various types of human activity decrease soil organic matter contents and ... 
living and nutrient conditions of soil organisms, such as repetitive tillage or 
burning of .... useful species in the plant mixture and reduces the density of the 
plant cover, ....