Topsoil is not a static medium that can be harvested and mined like a mineral. The vitality and fertility of the topsoil relies on the complex dynamics of the humusphere, an ecosystem that includes the excrementation and incrementation activities of the plant matter that has its roots in the topsoil. As leaves, branches, twigs, stems, trees and roots of plants die each year, their delitrius is deposited on the surface of the soil. Through the action of worms, microbes, and other soil organisms, this organic matter is bioconverted back into humus in the topsoil. Once there, it supports a wide variety of animal life, the most abundant of which are the earthworms.
Earthworms are the custodians, caretakers and cultivators of the humusphere.
They are the dominant animal form, representing over 1,000 lbs. per acre in a native deciduous or prairie topsoil. These worms physically convert soil debris into humus by towing it deep into their burrows where it is acted upon by enzymes excreted by these worms. Once it is properly decomposed, vermicomposted in a manner of speaking, the worm returns to this larder in the root cellar and consumes it as food. Once digested, the worm cast is deposited back on the surface of the soil as stable humus. Benefits of this action of worms also include the mechanical modification of the soil through the action of tunneling and burrowing.