5. Effects of incorporation of molybdenum compounds in the seed pellet on inoculant survival, seedling nodulation and plant growth of lucerne and subterranean clover

Summary-Four molybdenum compounds were mixed with lime and applied as coatings to inoculated seed of lucerne (Hunter River) and subterranean clover (Mount Barker). The seed was sown immediately in molybdenum-deficient soil in the field or stored for periods up to 84 days before sowing. As storage time lengthened, the survival of both lucerne and clover rhizobia was adversely affected by sodium molybdate but not by molybdic acid, ammonium molybdate or molybdenum disulphide. This effect was reflected in poorer nodulation in the sodium molybdate treatments. Nitrogen fixation, using foliage nitrogen content as an index, was always higher in the molybdenum treatments than in the no-molybdenum controls. Both species appeared able to extract molybdenum from molybdenum disulphide. Otherwise, there were no treatment differences in plant growth, but there was a significant relationship between the proportion of seedlings nodulated by the inoculant strains and the amount of nitrogen fixation. It is concluded that seed-applied molybdenum would benefit pasture establishment in some circumstances and would not interfere with inoculant survival or seedling nodulation provided that sodium molybdate was not used for the purpose. It has long been known that symbiotic nitrogen fixation in legumes is seriously inhibited by molybdenum deficiency (Anderson and Thomas 1946) and that the molybdenum requirement for optimum fixation is appreciably greater than the requirement for optimum utilization of nitrate nitrogen (Anderson and Spencer 1950). The precise role of molybdenum in nitrogen fixation is still not understood, but it is known that one of the proteins (component I, molybdoferredoxin) comprising nitrogenase complex, the enzyme system responsible for the reduction of dinitrogen to ammonia, contains molybdenum (Smith