Initial and residual effectiveness of molybdate fertilizer in two areas of south western Australia

A range of levels of sodium molybdate was applied to existing clover pastures at Boyup Brook and at Bakers Hill in south-western Australia. In the three subsequent years, further levels of molybdate were applied to some of the sites to assess the residual value of the initial application. The effect of the molybdate application on the molybdenum concentration in clover tops was measured. Plots of molybdenum concentration in tops versus molybdate applied curved upwards at first as each additional increment provided an increased response in concentration. However, at high concentrations of molybdate in clover tops the plots curved to the right at three out of four sites as each additional increment produced a decreased response. That is, overall, the curves were sigmoid. Increases in molybdate concentration were greatest on soils with least ability to retain molybdate as measured in the laboratory. Molybdate applied one year previously was about half as effective as currently applied molybdate. After two and three years it was about 20% as effective. A single application of rnolybdate remains effective for a long period on a soil of low ability to retain molybdate because the initial application is super-abundant - rather than because of large differences between soils in the rate of decline of effectiveness.