In the early 1900s, soil from a field that had previously grown a particular legume was often used as an inoculant for new fields. This was cumbersome, and had the added disadvantage that pathogens and nematodes might also be transmitted. Over the years, there has been a steady evolution in the carrier material used to transport inoculant cultures, while maintaining their viability and numbers. Carriers have included agar slants, compost, talc, bagasse , peat, and, more recently, liquid frozen or clay preparations.
Convenience is also a factor at peak planting times.
How these goals are best achieved can vary with the inoculant formulation used, with the cultural conditions that pertain to a particular crop, with environmental conditions, and even with the size of the host seed. This should ensure good nodulation under normal planting conditions. An alternative, also used in cases where the seed has been treated with a fungicide or bacteriocide that would harm the rhizobia is to dribble granular peat or liquid inoculants into the soil around or under the seed. Because of these different possible needs, and where the market warrants it, the larger inoculant companies manufacture several different products for each major legume to be inoculated. These products include sterile and non-sterile peats, and liquid or frozen formulations.