Agricultural microbiology is a branch of microbiology dealing with plant-associated microbes and plant and animal diseases. It deals with the microbiology of soil fertility such as microbial degradation of organic matter and soil nutrient transformations. Bacteria which are more dominant group of microorganisms in the soil and equal to one half of the microbial biomass in soil. Population is 100,000 to several hundred millions for gram of soil Autochthnous Zymogenous groups. Actinomycetes are intermediate group between bacteria and fungi, which are widely distributed in soil. Fungi are seen in surface layers of well-aerated and cultivated soils-dominant in acid soils. Common genera in soil are Aspergillus, Mucor, Penicillium Trichoderma, Alternaria, Rhizopus. Algae – found in most of the soils in number ranges from 100 to 10,000 per g. Protozoa are unicellular and population ranges from 10,000 to 100,000 per g of soil. Most of the soil forms are flagellates, amoebae or ciliates. Derive their nutrition by devouring soil bacteria. Found abundant in upper larger of the soil. They are regulating the biological equilibrium in soil.