Understanding soil inoculants
Soil biology is important for keeping agricultural systems healthy and productive. Living soil is complex. It includes creatures that cannot be seen with the naked eye, such as bacteria, fungi, actinomycetes, protozoa and nematodes, as well as creatures such as insects and earthworms. This community of organisms is bound together in a food web that affects the soil's chemical and physical properties. We care about these properties because they also affect plant growth and health.
Practices such as adding manures or composts to soil, planting cover crops and rotating crops are all aimed at rebuilding and maintaining soil organic matter, recycling and retaining nutrients, and decreasing soil diseases. These practices are usually associated with increased microbial biomass and increased soil organism diversity.
A healthy soil can contain billions of bacteria, fungi and other microorganisms in one teaspoon. Depending on soil conditions, the populations of these different microorganisms rise and fall. Some microbial populations increase quickly when fresh cover crops or other plant residues are added to the soil. For example, some microbes are able to use the readily available sources of carbon from fresh plant residues like humans use carbohydrates. These microbes decrease as the carbon sources are used up, causing other microbes that break down the less available sources of carbon like cellulose and lignin to increase. The point is that there are many native microorganisms in the soil that respond quickly when conditions are favorable for their growth.
As we continue to recognize that soil biology plays an important part in crop production, interest in soil inoculants continues to grow. Inoculants are used for a variety of reasons. In some cases, we add soil organisms that have a known beneficial effect. For example, some bacteria, like rhizobia, form a symbiotic relationship with certain host plants, like legumes. A symbiotic relationship is one that is mutually beneficial. In return for the plant feeding it carbon from photosynthesis and giving it a home, the bacteria can "fix" atmospheric nitrogen into a form that the plant can use. Some fungi, like mycorrhizae, can also form a symbiotic relationship with plants, scavenging phosphorus and other nutrients for the plant to use. Some bacteria and fungi do not form a symbiotic relationship with plants, but, when added to soil, can promote plant growth, suppress plant pathogens or both.
The easiest way to think about soil inoculants is to divide them according to their mode of action: biofertilizers or plant growth promoters, biopesticides and plant resistance stimulants.