Soil is a very complex living ecosystem, comprising of billions of organisms from thousands of species. In fact there are more organisms in a teaspoon of soil than there are people in the world! Meaning that in many cases there is a larger number, and a greater diversity, of organisms under our feet than there is above ground.
Microscopic organisms (or microorganisms), such as bacteria and fungi, have a number of extremely important functions. Including the release of nutrients from dead organic material (through decomposition); improvement of soil structure; carbon cycling and storage; nitrogen cycling; degradation of pollutants; and control of soil borne pathogens to name a few.
The number, type and activity of species of microorganism present in soil are affected by a combination of a number of different environmental conditions including temperature, moisture, aeration, pH, and food sources available to them.
Like any living thing, microorganisms can only live in conditions where they have: a food source; water; can breathe; and are warm enough to function.
Because they are too small to see, a simple and common way to study the microbial community is to measure their activity i.e. decomposition rate. The simplest method of determining decomposition rate is with the use of a litter bag experiment.
This consists of a mesh bag filled with a known mass of plant material (i.e. a tea bag!) being buried in the ground, and after a certain period of time the bag is retrieved and re-weighed. The material lost from the bag is a result of that material being decomposed by microorganisms in the soil.
Therefore we know that the greater the mass reduction in the bag, the higher the decomposition rate and the more active the microbial community.