This is important as it allows the organisms to survive unfavorable conditions of many kinds. There are many conditions in soil that affect the growth of bacteria such as temperature, moisture, aeration and food. The methods of determining the actual numbers present are quite inaccurate, since many organisms will not grow in the artificial media commonly used. It is almost impossible to break up the clumps or colonies in such a way as to determine the number of individuals actually present. It can be determined that the numbers of bacteria in soil are very large, possibly ranging as high as three or four billions to a gram of soil Good soils in general carry the largest numbers.
Soil bacteria is classified under two heads, autotrophic and heterotrophic.
The autotrophic get their energy from the oxidation of mineral constituents, such as ammonium, sulfur,iron and most of their carbon from carbon dioxide. In numbers, they are insignificant, but since they include the organisms that support nitrification and sulfur oxidation, they are tremendously important in the sustenance of higher plants. Most soil bacteria, however, are heterotrophic, that is, their energy and carbon come directly from the soil organic matter. The general-purpose decay and ammonifying bacteria, as well as fungi and actinomyces, belong to this group. Many conditions of the soil affect the growth of bacteria.