Soil fertility comprises three interrelated components: physical fertility, chemical fertility and biological fertility. Biological fertility, the organisms that live in the soil and interact with the other components, varies greatly depending upon conditions and it is highly complex and dynamic. It is the least well-understood fertility component. In addition to soil fertility, soil microorganisms play essential roles in the nutrient cycles that are fundamental to life on the planet. Fertile soils teem with soil microbes. There may be hundreds of millions to billions of microbes in a single gram of soil. The most numerous microbes in soil are the bacteria, followed in decreasing numerical order by the actinomycetes, the fungi, soil algae and soil protozoa. A better understanding of soil microbiology is essential if agricultural production is to meet the needs of a growing world population. In many regions, the healthy microbe population is still being threatened, and not promoted, by agricultural practices.