A multi-site field evaluation of granular inoculants for legume nodulation

The most common method of inoculating legume crops in Australia is the application of peat slurry inoculant to seed. The recent introduction of granular (solid) formulations of inoculants into the Australian market has provided the potential to apply rhizobia with greater ease, but their efficacy has not been independently evaluated. Here, we compare the efficacy of a range of experimental and commercially-available granular inoculants on chickpea, faba bean, lentil, lupin and pea crops in comparison with un-inoculated treatments, and with conventional seed-applied peat slurry inoculants. Thirty-seven field experiments were established in Victoria, South Australia and southern New South Wales over five years. Peat slurry inoculants provided effective nodulation of all legumes. Granular inoculants varied markedly in their ability to improve grain legume nodulation. The size of response depended inversely on background nodulation from soil rhizobial populations. At sites with median background nodulation, peat granules and attapulgite clay granules placed with seed resulted in nodulation similar to peat-slurry-based inoculation, but treatments with bentonite clay granules did not increase nodule numbers much above those in un-inoculated treatments. The generally lower numbers of rhizobia g−1 in the bentonite granules, translated to lower rhizobia application rate to the soil. However, differences in number of rhizobia g−1 granule did not fully explain the nodulation differences between granules. Granule moisture content and granule particle size differed markedly between granule types but their influence on nodulation was not tested. Grain yields did not differ between attapulgite granules placed with seed, peat granules and peat slurry inoculants (all well-nodulated treatments), but were lower with bentonite granule inoculants. Yield differences within sites were related to nodulation and the differences between treatments attenuated as background nodulation increased. Overall, these studies demonstrate that certain granule types have the potential to be used in Australia with grain legumes, particularly in circumstances when seed-applied inoculants are problematic, such as where seed fungicides or insecticides need to be applied. However, granular inoculant formulations differ substantially in their potential to produce nodules on a range of grain legumes.