Humus or soil organic matter is the chief source of minerals return to soil. It has a much water holding capacity. It can hold mineral ions by absorption and binds clay particles into aggregates. It makes soil porous thus increasing aeration of soil and also causes easy percolation of water in soil. Depending upon the nature of parent matter, vegetation and climatic condition it is of two types
(i) Mor humus : also called raw humus.
(ii) Mull humus : microbiologically rich.