It is a very important component in the quality of topsoil, as it brings back to plant roots the nutrients they need, and adds to the friability of the soil (it's ability to hold moisture and air).
Humus is the end product of decomposition of dead leaves, wood, and animal matter by smaller and smaller organisms. Initially, larger creatures tear up the dead matter to obtain whatever food they can (ie, squirrels eating th meat of nuts, scavengers eating meat off carcasses). Then various insects, fungi and bacteria start to consume and break down the remaining tissues. The action of earthworms helps work this material below the surface, where it does the most good.
- Wile, Dr. Jay L. Exploring Creation With General Science. Anderson: Apologia Educational Ministries, Inc. 2000