have a water-retentive, granular structure that facilitates the
presence of air and moisture throughout the mass creating perfect
conditions for microbial digestion to proceed.

This excrement is also the food for a diverse group of nearly
microscopic soil animals called secondary decomposers. These are
incapable of eating anything that has not already been predigested
by the primary decomposers. The combination of microbes and the
digestive enzymes of the primary and secondary decomposers breaks
down resistant cellulose and to some degree, even lignins. The
result is a considerable amount of secondary decomposition excrement
having a much finer crumb structure than what was left by the
primary decomposers. It is closer to being humus but is still not
quite finished.

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.