Residual value of molybdenum trioxide for clover production on an acidic sandy podzol

The residual value (RV) of molybdenum (Mo) for clover (Trifolium subterraneum L.) production was measured for an acidic sand when the fertiliser was spread over the soil surface (topdressed). The RV of Mo, as molybdenum trioxide, the most widely used Mo fertiliser in Western Australia, was measured using yield of dry herbage (DM), Mo concentration of DM, and Mo content of DM (yield of DM multiplied by the Mo concentration of DM). The RV of Mo fertiliser was measured in 1993 for fertiliser Mo applied once only to plots not treated with Mo before, either in 1993 (current Mo) or 1-10 years previously (previous Mo). Relative to the nil-Mo treatment, additions of Mo fertiliser increased DM yield by about 1.20 t/ha (20% DM increase) in October. When 80 or 320 g Mo/ha was applied, all yields were on the maximum yield plateau except for Mo applied 10 years previously. The yield for the 80 g Mo/ha treatment applied 10 years earlier decreased by about 15% relative to the other current and previous Mo treatments. However, as measured using Mo concentration and Mo content in DM, there was a continuous decline in the effectiveness of previous Mo relative to current Mo the longer the previous Mo was in contact with the soil. The decrease in RV was about the same as measured using Mo concentration or content in tissue. Molybdenum applied 5 years previously was about one-third as effective as current Mo for Mo content or Mo concentration of DM.