Microorganisms or microbes are organisms that are too small to be seen by the naked eye; they can only be seen under the microscope. Several types of microorganisms occur in soil. Soil fertility and physical and chemical composition of the soil determine the distribution of the microorganisms in the soil. Microorganisms play many important roles in soil, such as degrading complex chemical substances, preserving organic materials, and nitrogen fixation. Their presence in soil is necessary for proper growth of plants.
The most common type of microorganism present in soil, bacteria occurs in greatest density in the upper layer of the soil. Micrococcus, chromobacterium, flavobacterium, bacillus, arthrobacter and pseudomonas are some of the more common species of bacteria found in soil. Bacteria perform a wide range of chemical transformations in soil, including disease suppression, nutrient transformation and degradation of organic matter. Their presence in soil is extremely beneficial to the plant life. Azobacter in particular is very efficient in converting atmospheric nitrogen to ammonia.
Several micro fungi are present in the soil, but their role in soil has not been as thoroughly studied as that of bacteria. The advantage of fungi is that they can act in places where bacteria are not able. For example, in conditions of drought, fungi, needing less moisture to survive than bacteria, continue to work on the soil well after drought has checked bacterial decay. The one limitation of fungi is that, except perhaps mycorrhizae, fungi are incapable of nitrogen fixation.
Actinomycetes, a type of microorganism with characteristics very similar to bacteria and fungi, are abundantly present in soil containing decomposed organic materials. Actinomycetes differ from fungi in the composition of their cell walls. Water logging in soil is not conducive to the presence of actinomycetes, but dry and semi-arid soils are able to sustain a sizable population. Actinomycetes are responsible for the characteristic musty or earthy smell of soil. They play an important role in soil with their ability to degrade many complex chemical substances. Actinomycetes are aplenty in soil that lacks in nutrients because they cannot sustain the competition from other microbes in a nutrient-rich soil.