STUDIES ON ALTERNATIVE MEANS OF LEGUME INOCULATION - APPRAISAL OF APPLICATION OF INOCULANT SUSPENDED IN IRRIGATION WATER (WATER-RUN INOCULATION)

Water-run inoculation is a novel means of inoculating crop legumes wit h species of Rhizobium or Bradyrhizobium. Inoculant suspended in irrig ation water is delivered into the seedbed. This procedure may be apt f or situations when a farmer has limited time to sow a large area and m ore conventional and time-consuming 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 verti cally only 5 cm to sowing depth. A peat inoculant of B. japonicum rema ined uniformly in suspension during flow of irrigation water over peri ods of 45 min and distances of 80 m from the point where the inoculant was introduced. With furrow irrigation on a poorly structured red bro wn earth, water-run inoculation applied at the normal (commercially re commended) rate did not initiate a satisfactory soybean symbiosis and was inferior to the more conventional methods, seed coat and seedbed i noculation. Rhizobial colonisation of seedling rhizospheres was limite d, nodulation was sparse, and low numbers of B. japonicum re-establish ed in the soil after harvest. Symbiosis was improved by higher rates o f inoculation and was particularly enhanced in an area where the irrig ation 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 effecti ve 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 leve ls of N2 fixation led to increased levels of N in the seed. We conclud e that water-run inoculation is not an appropriate means of legume ino culation in furrow-irrigated systems on poorly structured soils but it may be a practical option for inoculation of crop legumes grown under flood irrigation.