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. Among the most important of these are the supply of oxygen and moisture, the soil temperature, the presence of organic matter, and the H-ion concentration of the soil solution.
All soil bacteria require for their growth a certain amount of oxygen.
Some can continue their activities with much less free oxygen than can others. Bacteria require moisture for their growth, optimum water for higher plants seemingly being satisfactory for their best development. Soil bacteria, like other plants, continue life and growth under a considerable range of temperature. The presence of a certain amount of organic matter is essential to the growth of most bacteria. Most bacterial thrive in a low acidity and low lime soil.