How soil works
Plants make sugars from air, light, water and minerals. These sugars not only power the growth of the plant itself but are exuded through the roots into the surrounding soil. It is the primary food source for all underground biology. The job of the soil biology is to assist the plant to get access to moisture, mineral nutrients and protection from pathogens.
The numbers of these soil borne organisms is difficult to get ones head around. For example two handfuls of fertile soil contain more organisms than humans currently on the planet, lets say 7 billion. This is a very diverse collection too with fungi, bacteria, protozoa, nematodes being the most common.
Today we can get some idea of the diversity of soil life using DNA sampling techniques. One measure that we can now use tells us that the more bio-diverse the life in soil the more fertile and productive it is.
When you consider that 90% of all our food comes from the soil then its obvious that we have a vested interest in maintaining and even enhancing this healthy bio-diversity.