Now comes the final stage in humus formation.  Numerous species of earthworms eat their way through the soil, taking in a mixture of earth, microbes, and the excrement of soil animals.  All of these substances are mixed together, ground-up, and chemically recombined in the worm’s highly active and acidic gut.  Organic substances chemically unite with soil to form clay/humus complexes that are quite resistant to further decomposition and have an extraordinarily high ability to hold and release the very nutrients and water that feed plants.  Earthworm casts (excrement) are mechanically very stable and help create a durable soil structure that remains open and friable, something gardeners and farmers call good tilth or good crumb.  Earthworms are so vitally important to soil fertility and additionally useful as agents of compost making that an entire section of this book will consider them in great detail.

Let’s underline a composting lesson to be drawn from the forest floor.  In nature, humus formation goes on in the presence of air and moisture.  The agents of its formation are soil animals ranging in complexity from microorganisms through insects working together in a complex ecology.  These same organisms work our compost piles and help us change crude vegetation into humus or something close to humus.  So, when we make compost we need to make sure that there is sufficient air and moisture.

Decomposition is actually a process of repeated digestions as organic matter passes and repasses through the intestinal tracts of soil animals numerous times or is attacked by the digestive enzymes secreted by microorganisms.  At each stage the vegetation and decomposition products of that vegetation are thoroughly mixed with animal digestive enzymes.  Soil biologists have observed that where soil conditions are hostile to soil animals, such as in compacted fine clay soils that exclude air, organic matter is decomposed exclusively by microorganisms.  Under those conditions virtually no decomposition-resistant humus/clay complexes form; almost everything is consumed by the bacterial community as fuel.  And the non-productive soil is virtually devoid of organic matter.

Sir Albert Howard has been called the ‘father of modern composting.’  His first composting book (1931) The Waste Products of Agriculture, stressed the vital importance of animal digestive enzymes from fresh cow manure in making compost.  When he experimented with making compost without manure the results were less than ideal.  Most gardeners cannot obtain fresh manure but fortunately soil animals will supply similar digestive enzymes.  Later on when we review Howard’s Indore composting method we will see how brilliantly Sir Albert understood natural decomposition and mimicked it in a composting method that resulted in a very superior product.

At this point I suggest another definition for humus.  Humus is the excrement of soil animals, primarily earthworms, but including that of some other species that, like earthworms, are capable of combining partially decomposed organic matter and the excrement of other soil animals with clay to create stable soil crumbs resistant to further decomposition or consumption.