Development and evaluation of liquid inoculants.

Changes in the scale and planting equipment used on large soybean farm operations in North America have forced inoculant manufacturers to develop alternatives to peat based inoculants that are too cumbersome for large-scale field application and tend to plug precision air seeders used on large farms. A liquid inoculant formulation with good field performance characteristics that uses low cost materials and are easily attainable by small producers could overcome many problems associated with processing solid carriers. The field performance was evaluated of two prototype formulations with a network of 27 collaborators in sixteen countries including three in Vietnam to compare the liquid formulations to local inoculant products and to sterilised Canadian peat-based inoculant. LEGUME inoculation is an old practice in many agricultural systems. Because of excellent performance characteristics, ground, neutralised peat (humus) has become the standard carrier material for the manufacture of inoculant in North America, Europe and Australia. Large, easily accessible deposits of true peat are rare, especially in the tropics. This has led to research into alternative carrier materials such as lignite, charcoal, coir dust, and compost of various origins, sugarcane filter mud, bagasse, soils mixed with various organic amendments, clay, expanded mica (horticulture vermiculite) and liquid formulations with various additives. Most materials, however, have not demonstrated performance characteristics equivalent to peat based inoculant products. Changes in the scale and planting equipment used on large soybean farm operations in North America have forced inoculant manufacturers to develop alternatives to peat based inoculants. Their focus has been directed toward liquid inoculant formulations for several reasons. Peat based inoculants are too cumbersome for large-scale field application and tend to plug precision air seeders used on large farms. Liquid formulations have an in that they can be applied to seed as it passes through seed augers on the way to the planting machinery. Liquid formulations may have advantages for smaller scale agricultural systems in the tropics and their local inoculant manufacturer. First, most inoculant producers in the tropics do not have easy access to quality peat deposits. Even when they do, mining, drying, grinding and neutralising peat to a standard consistency is expensive and requires a considerable capital investment in equipment for small production operations. Other materials, such as composted wastes may be more available but are even more difficult to process to consistent characteristics. A liquid inoculant formulation with good field performance characteristics that uses low cost materials and are easily attainable by small producers could overcome many problems associated with processing solid carriers. We organised a project that aimed to develop and evaluate a liquid inoculant formulation that 1) was low in cost, 2) used readily available, non-perishable materials, 3) could be grown under normal fermentation conditions, 4) supported high cell numbers in storage, and 5) promoted nodulation better than local inoculant products and equivalent to the best quality peat carrier materials. We took a multifaceted approach to the problem including 1) identifying compounds that detoxified seed exudates and promoted cell survival upon rapid desiccation after application to seed, 2) strain selection for survival and superior BNF capacity, 3) preconditioning cells to withstand stress after 1 University of Hawaii NifTAL Project, Department of Tropical Plant and Soil Science, University of Hawaii at Manoa, 1000 Holomua Rd, Paia, HI 96779 USA niftal@hawaii.edu