Soil inoculants are Microbial amendments that use beneficial microbes to enhance plant health. The microbes form a symbiotic relationship with crops that is beneficial to both the plant and microbes. Some soil inoculants promote plant growth by stimulating plant hormone production while others improve the plants uptake of nutrients. A small amount do both.
There are Microbial soil innoculants which have been shown to enhance a plants resistance to some common crop diseases. Resistance to powdery mildew and several root rots can be induced by the correct innoculants.
Nitrogen-fixing bacteria such as Rhizobium form symbiotic relationships within root nodules of beans and legumes. This symbiosis enhances the plants uptake of essentail nutrients in particular nitrogen, as well as increasing the available nitrogen in the soil. It's beneficial aspects is not limited to legumes as it increases available nitrogen for all plants.
Related innoculants / microbes enhance the availability of the other macronutrients such as phosphorus to the host plant.
Around 80% of Earths atmosphere is nitrogen gas (N2). Unfortunately as gas it is unusable by plants, animals, and most living organisms. Plants can die of a nitrogen deficiency while engulfed by atmospheric nitrogen [N2] they can't use. Living organisms use the liquid form [NH3] aka ammonia. Nitrogen-fixing bacteria absorb nitrogen in its gaseous form and convert it into nitrate or ammonia which are absorbable forms of nitrogen that the plants are able to use.
Cyanobacteria are another beneficial soil dwelling bacteria. Unlike teh Rhizobia class of bacterium they do not interact with plants and do not form nodules on plant roots. Instead, they are free roaming soil borne bacteria that improve the soils health and by default that of the plants grown in it. Some types of Cyanobacteria draw nitrogen from the atomosphere and soil and like Rhizobium convert it to nitrogen that plants can use.
Chemicals and Persistent tilling and working of the soil, although neccesary at times [The tilling, not the chemicals] has an effect on soil borne bacteria. In some cases they are almost non existent. The lack of a host crop, legumes, for extended periods will cause Rhizobium to go dormant, as time goes by their numbers dwindle and they eventually die off.
A legume inoculant is advisable when...
A. The soil has not hosted beans or legume for an extended period.
B. When the topsoil has been removed.
C. When chemicals such as synthetic fertilizers, fungicides and pesticides have been used.
Not all inoculants work on all legumes be certain you are applying the right inoculant for your crop. Many inoculants available to home gardeners are blends that contain different types of nitrogen fixing bacteria strains for various legumes,and it works well for most, but not all. Peanuts [a legume] are one example.
Some seeds come pre-coated with the correct strains, the labels will usually state "rhizocoated" which means it is already coated with the correct nitrogen fixing bacteria.
To apply inoculant moisten the seed and either sprinkle it on or roll the seed in the inoculant - its not rocket science just wanna get a healthy coat on the seed. Make sure you get a thick coat on the seeds too little could fail but you never have too much.
Once you have nitrogen fixing bacteria in your soil it should hang around for next years crops, but there is no guarantee. So regular seasonal inoculations are a good idea to ensure bountiful harvests. The amount of nitrogen recycled back to the soil following a legume crop can be minimal. Nearly all of the nitrogen fixed by the plant goes directly into the plant itself. Small amounts leak back into the soil for neighboring plants to use. Only when the legume plant is allowed to decompose back into the soil does significant amounts of usable nitrogen become available to neighboring plants.