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Dr. Peter H. Graham
439 Borlaug Hall
1991 Upper Buford Circle
St Paul, MN 55406

Are inoculants packaged in different ways? 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. A good inoculant carrier will absorb large quantities of the culture medium in which the rhizobia was grown; will maintain the added organisms viable for periods of six-twelve months; will contain a minimum of contaminant organisms; and can quickly and easily be used in the field. The goal in inoculation is: to supply the number of rhizobia necessary to ensure that the legume in question is well nodulated and able to satisfy most of its nitrogen needs from fixation; to ensure that a high proportion of the nodules are produced by the inoculant strain(s) and not by less effective soil or wind-borne rhizobia; to ensure that once the season is over and the nodules decompose, the inoculant organism(s) persist in the soil and dominate in the nodulation of that host in subsequent plantings. 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. Where the inoculant is seed applied, the Canadian mandated minimum for soybean is 105 rhizobia/seed, whereas the smaller alfalfa receives only 103 rhizobia/seed. This should ensure good nodulation under normal planting conditions. Where conditions are less than ideal -- for example, where rainfall after planting is uncertain, where soil is highly acid, or where competitive but ineffective rhizobia are already present in the soil -- higher than normal rates of inoculation will be necessary. Particularly if the legume to be inoculated is small-seeded (e.g. white clover), it may be physically impossible to ensure that adequate numbers of rhizobia adhere to the seed. 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. Inoculant application rates achieved using soil inoculation may be ten to fifty-fold those of seed inoculation, while even higher levels are possible with inoculants supplied through irrigation water (Ciafardini and Barbieri, 1987). A dry (12% liquid) granular clay inoculant formulation was released for use with peas in 1998, and can be broadcast or band applied, and also provides high numbers of rhizobia per hectare. 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. For soybean, researchers at the University of Guelph have undertaken comparative studies since 1988, with the following general conclusions (D.J.Hume and J.A.Omielan, pers. comm): In first year plantings of soybean, yield responses to inoculation ranged from 15-25%, with best results obtained using sterile peat carriers. At least part of the reason sterile peat carriers have performed so well appears due to the greater number of organisms they contain. In 1996, five sterile carriers tested averaged 5 x 109 rhizobia/gram, whereas the two non-sterile carriers tested averaged only 1.9 x 108 rhizobia/seed. Non-sterile seed-applied inoculants and granular preparations performed more or less equally over the period 1988 to 1996 While seed applied liquid inoculants performed a little worse than sterile carrier inoculants from 1990 to 1995, the seed applied and in-furrow liquid inoculants used in 1996 performed at least as well as the sterile seed applied inoculants. This improvement was attributed to better survival of rhizobia in the inoculant between manufacture and use. Inoculants performed more or less equally in conventional and no-till situations.