Studies on alternative means of legume inoculation: appraisal of application of inoculant suspended in irrigation water (water-run inoculation)

Abstract

Water-run inoculation is a novel means of inoculating crop legumes with species of Rhizobium or Bradyrhizobiunz. Inoculant suspended in irrigation water is delivered into the seedbed. This procedure may be apt for situations when a farmer has limited time to sow a large area and more conventional and timeconsuming means of inoculation may create a bottleneck during sowing. Field experiments with water-run inoculation of irrigated soybeans were conducted at 2 sites using furrow or flood irrigation. With furrow irrigation immediately after sowing, rhizobia-laden water had to infiltrate the soil laterally a distance of about 18 cm to reach the seed sown in single rows on hills (parallel ridges). With flood irrigation before sowing, water needed to percolate vertically only 5 cm to sowing depth. A peat inoculant of B. japonicum remained uniformly in suspension during flow of irrigation water over periods of 45 min and distances of 80 m from the point where the inoculant was introduced. With furrow irrigation on a poorly structured red brown earth, water-run inoculation applied at the normal (commercially recommended) rate did not initiate a satisfactory soybean symbiosis and was inferior to the more conventional methods, seed coat and seedbed inoculation. Rhizobial colonisation of seedling rhizospheres was limited, nodulation was sparse, and low numbers of B. japonicum re-established in the soil after harvest. Symbiosis was improved by higher rates of inoculation and was particularly enhanced in an area where the irrigation water ponded for 3-4 h allowing more time for the rhizobia-laden water to percolate the soil. With flood irrigation on a grey clay, an approximately normal rate of water-run inoculation induced an effective symbiosis especially when compared with lower rates of inoculation. Substantial populations of rhizobia developed in soybean rhizospheres, plant growth and nitrogen (N) content were enhanced, and higher levels of N2 fixation led to increased levels of N in the seed. We conclude that water-run inoculation is not an appropriate means of legume inoculation in furrow-irrigated systems on poorly structured soils but it may be a practical option for inoculation of crop legumes grown under flood irrigation.