Compared to the atmosphere, soil is a place where temperature fluctuations are small and slow. Consequently, soil animals are generally intolerant to sudden temperature changes and may not function well over a very wide range. That's why leaving bare earth exposed to the hot summer sun often retards plant growth and why many thoughtful gardeners either put down a thin mulch in summer or try to rapidly establish a cooling leaf canopy to shade raised beds. Except for a few microorganisms, soil animals breathe oxygen just like other living things and so are dependent on an adequate air supply. Where soil is airless due to compaction, poor drainage, or large proportions of very fine clay, soil animals are few in number.
The soil environment is generally quite moist; even when the soil seems a little dryish the relative humidity of the soil air usually approaches 100 percent. Soil animals consequently have not developed the ability to conserve their body moisture and are speedily killed by dry conditions. When faced with desiccation they retreat deeper into the soil if there is oxygen and pore spaces large enough to move about. So we see another reason why a thin mulch that preserves surface moisture can greatly increase the beneficial population of soil animals. Some single-cell animals and roundworms are capable of surviving stress by encysting themselves, forming a little "seed" that preserves their genetic material and enough food to reactivate it, coming back to life when conditions improve. These cysts may endure long periods of severe freezing and sometimes temperatures of over 150 degree F.
Inhabitants of leaf litter reside close to the surface and so must be able to experience exposure to dryer air and light for short times without damage. The larger litter livers are called primary decomposers. They spend most of their time chewing on the thick reserve of moist leaves contacting the forest floor. Primary decomposers are unable to digest the entire leaf. They extract only the easily assimilable substances from their food: proteins, sugars and other simple carbohydrates and fats. Cellulose and lignin are the two substances that make up the hard, permanent, and woody parts of plants; these materials cannot be digested by most soil animals. Interestingly, just like in a cow's rumen, there are a few larvae whose digestive tract contains cellulose-decomposing bacteria but these larvae have little overall effect.
After the primary consumers are finished the leaves have been mechanically disintegrated and thoroughly moistened, worked over, chewed to tiny pieces and converted into minuscule bits of moist excrement still containing active digestive enzymes. Many of the bacteria and fungi that were present on the leaf surfaces have passed through this initial digestion process alive or as spores waiting and ready to activate. In this sense, the excrement of the primary decomposers is not very different than manure from large vegetarian mammals like cows and sheep although it is in much smaller pieces.
Digestive wastes of primary decomposers are thoroughly inoculated with microorganisms that can consume cellulose and lignin. Even though it looks like humus, it has not yet fully decomposed. It does 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.
