Californian humus is formed by low-temperature decomposition of wood chips over 3-5 years. Most composting processes work at higher temperatures which destroy the soil microorganisms. Soil humus is the component of soil that has been broken down by microbes to form soil organic matter (SOM). Leaves fall and form litter on the surface of the soil. These are broken down by fungi, which have evolved for the purpose of breaking it down to soil humus. Humus can be thought of as an ecosystem which contains many thousands of different types of microorganisms growing together and working in symbiosis with plants to build topsoil. The application of chemicals and tillage has killed-off microbes and destroyed soil biodiversity. In conventional (industrialized) agriculture - noting that this has only been in existence for about 60 years! - soil humus is lacking, and so plants have no microbes growing with them and no immunity. This leads to problems of disease, pests etc. Better results (yields) are obtained using organic methods to rebuild soil humus.
The Soil Food Web is a whole community of microorganisms that live in soil, of which there are four main types: bacteria, fungi, protozoa and nematodes, whose collective numbers total billions in a single teaspoonful of soil. The soil food web shows how they all relate to one another. In nature, plants grow and then eventually die, leaving dead material lying on the surface of the soil. At the vanguard are bacteria and fungi, which eat the plant material, and hence all the nutrients that were in the plants now become incorporated into the bodies of the bacteria and fungi. To make these nutrients available to plants again, requires the action of the soil predators - like sharks in the ocean or wolves in the pasture setting – nematodes and protozoa. These eat the bacteria and fungi continually, and excrete the nutrients in forms that can once more be used by plants. A perennial process is thus sustained: plants grow and die, bacteria and fungi consume the plant material, and are themselves eaten by predators which convert them to (and excrete them as) available nutrients that are taken up again, by newly growing plants.
e.g. In the case of nitrogen, plant mass is eaten by bacteria and the N becomes part of the bacterial biomass. The bacteria are eaten by protozoa, which excrete the N in plant-available (NH4+) form. This can reduce the necessary input of N-fertilizer by 50% because the microbes help convert the plant material to useful fertilizer. About half way through the video is some wonderful footage, recorded at 400x magnification, of a bacteria-feeding nematode actually eating a bacterium, as part of its daily diet of some 50,000 bacteria. As it eats each bacterium, it excretes plant-available N. California humus is used to form a “tea”, by extracting the humus into water, which can be applied to crops to inoculate the soil food web, eventually meaning that very low inputs of artificial fertilizers are required to grow crops, and an improved resistance to disease is achieved.