Trials to test delivering rhizobia through dry clay granules


By Associate Professor John Howieson and Ron Yates, Centre for Rhizobium Studies, Murdoch University

Large-scale trials later this year will build on the promising results achieved from a better method of inoculating rhizobia into soils, using dry clay granules as the carrier.

Nitrogen fixation with legumes and rhizobia is worth $2 billion annually to Australian agriculture. To protect and enhance this asset, rhizobial research in southern Australia has put great effort into selecting inoculant quality Rhizobium strains capable of high nitrogen fixation and with excellent survival in soils.

Most of this research has been funded by the GRDC and delivered through the National Rhizobium Program. This program has delivered rhizobial inoculant strains of very high quality to farmers for the rapidly growing suite of pulse and pasture legumes in Australian agriculture.

Traditional peat-based inoculants currently used by growers have practical limitations. To help overcome the problem, Bay Classic Pty Ltd are collaborating with researchers at the Centre for Rhizobium Studies to develop an alternative inoculant and delivery system – dry clay granules.

With peat-based inoculants, Rhizobium cells start dying immediately after the peat is removed from the protective packet (which has been held at 4°C). The death rate may be rapid for some strains and slower for others. Manufacturers are successfully managing this mortality by producing peat inoculants with very high counts, that still deliver sufficient numbers of rhizobia on seed to produce nodulation under most conditions.

However, to achieve this, the legume seed has to be sown immediately after inoculation into moist soil. Compounding the rapid death rates, particularly with pulses, is the toxicity of fungicide dressings applied to seed. This has substantially restricted the way farmers may inoculate and sow legumes with respect to timing of the operation. In contrast to peat inoculants, air-dried clay granules hold rhizobia in a state of “suspended animation”.

Cells do not die rapidly, so the granules can be stored in the shed under ambient conditions and then drilled with the seed or fertiliser. This gives farmers total control of legume inoculation and greater fl exibility with regards to the time of sowing.

About 25 experiments were conducted around Western Australia in 2003 with very promising results. Granules achieved similar nodulation patterns to conventional peat inoculant at most sites, and better nodulation where the legumes were sown into marginal moisture. It is planned to release the dry clay granules in pre-commercial quantities for “proof of concept” trials on farmers’ properties in 2004.

For more information:
Associate Professor John Howieson, 08 9360 2231,
Bay Classic Pty Ltd, 08 9756 6121,

GRDC RESEARCH CODE UMU00003, program 4