Advances in inoculant technology: a brief review

Legume seed inoculants by default must be compatible with current farming practice. It is rare that farming practice is changed to accommodate a technology that delivers a high quality inoculant. Production of inoculants, therefore, is an exercise in practical (applied) microbiology. The agricultural industry demands that an inoculant is easy to apply and that it works reliably under a range of conditions. Such demands call on a range of technologies and skills on the part of researchers, manufacturers and users. Inoculants are prepared from either submerged or semi-solid culture and applied either to the seed or directly to the soil in different forms. The form used is directed by the requirements of the legume host and its associated husbandry techniques. Agar, broth (concentrated and frozen), oil-dried, lyophilized and powdered carrier (mostly peat) formulations are used. Advantages and disadvantages of each are discussed briefly. Peat formulations dominate due to good survival of the root-nodule bacteria and their ease of use. There has not been an alternative (non-peat) formulation that has performed consistently better than the peat forms in field evaluation. Despite higher quality and longer shelf life of dried forms as inoculants per se they have not been superior to peat-based forms either as seed or soil-applied inoculant. The review looks briefly at aspects of production that may influence (improve) survival of root-nodule bacteria and concludes that there are windows of opportunity for new developments. Aspects that may influence quality of peat cultures and, in particular, those that may provide better survival of the inoculant on the seed include: growth phase at the time of mixing with a carrier, rate of drying and of re-hydration (both as inoculant and as inoculum on the seed), carrier characteristics (including additives and stickers) and inoculation methodology. Few significant advances have occurred in the 1990s. The challenge for the future is to develop a technology that provides long-term survival of root-nodule bacteria on the seed or in granules (for soil application) to the extent that 'on-farm' inoculation requirements can be replaced by a reliable pre-sale seed inoculation step.