Most clover/trefoil/alfalfa innoculants are grown on a sterile peat solution. The peat will be irradiated to kill any microbes, etc, that might be there already. They then put colonies of rhizobia bacteria in the peat solution (usually they put the microbes on the peat, put the peat in a bag and ship it out quickly) and the microbes grow on the peat. It is then mixed with your seed so there are viable bacteria of the right species right in the rooting zone of your seedling clover. Once it is in the soil it is usually there for good (unless your pH is really low, or you have really low OM) and will innoculate your clover every time you put seed on that field.
