Soil is not inert: it is an almost unbelievably diverse, living community of microbes and animals. A single square yard (square meter) of woodland floor is typically home to 30 million nematodes (eelworms) and 250 different species of mites. Unlike plants, the microorganisms and animals of the soil community cannot make their own food and depend on organic matter from the world above, so more organic matter means more microbes and more soil animals.
To a very large extent, everything else in the garden depends on the health of this soil community. Healthy soil means healthy plants, which provide plenty of nectar for pollinators and lots of leaves for the herbivorous insects that are eaten by beetles, birds, predatory wasps, and spiders.
The soil community also contributes directly to the well-being of many of the larger and more conspicuous animals in the garden. Small soil animals like springtails are food for ground-dwelling beetles and spiders, while earthworms are a favorite food of frogs and toads, and even of larger animals such as raccoons and foxes. And don't forget the wildlife that inhabits the compost pile itself.
Decaying organic matter is a favorite haunt of many animals that would be rare or absent in a garden with no compost pile. Compost is a favored habitat for slow worms, one of the few animals in the garden that really like eating slugs.
A compost pile is a complete ecosystem, a world in miniature. Worms eat decaying vegetation and excrete organic compounds that enrich the mix, while their burrowing helps aerate the compost. As organic matter is passed through an earthworm's digestive system, it is finely ground and neutralized by calcium carbonate that is secreted by the worm's gizzard.
Millipedes, slugs, snails, and woodlice shred the plant materials, creating more surface area for fungi and bacteria to work on. Fly larvae (maggots) tunnel through the pile, eating everything in their path.
Fungi and actinomycetes (a group of organisms intermediate between bacteria and true fungi) get to work on the tougher plant residues that the bacteria leave behind. The microbes are food for organisms such as mites, nematodes, and springtails, which are in turn eaten by centipedes, ground beetles, rove beetles, spiders, and more exotic predators such as pseudoscorpions.
Finally, larger carnivores (for example, slow worms, shrews, and toads) move in, attracted by the warm, sheltered environment and the abundance of food.
Millipedes process and eat rotting plant material.
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