Residual value of zinc fertiliser for production of wheat

Zinc deficiency is common on the sandy acidic soils in south Western Australia for grain production of wheat grown with diammonium phosphate containing low levels of zinc contamination. The effectiveness of zinc fertiliser (zinc oxide or zinc contamination of single superphosphate that were widely used for crops in south Western Australia) was measured in 1996 for grain production of wheat, for zinc applied once only to plots, either in 1996 (current zinc) or in a previous year (previous zinc) (1983, 1984, 1986, 1990, 1992). Relative to current zinc applied as zinc oxide with diammonium phosphate, the effectiveness of previous zinc for dry matter, zinc uptake (zinc concentration x yield) and grain production of wheat decreased relative to the effectiveness of current zinc, the decrease being larger with increasing time since application. Thirteen years after application, the decrease in the effectiveness was about half for dry matter and grain production where wheat was grown with diammonium phosphate. Both currently and previously applied zinc fertiliser increased wheat dry matter, zinc content of the dry matter and grain yields. Zinc applied as a zinc contaminant in single superphosphate in 1983 produced wheat grain yields on the maximum grain yield plateau (about 2.4 t/ha) achieved for the 5 amounts of zinc oxide applied in the current year (1996). The critical concentration of zinc in the youngest emerged leaf and grain for diagnosing zinc deficiency was 12 mg zinc/kg. However, when relating the zinc concentrations in the youngest emerged leaf to the grain yield (prognosis), a zinc concentration of 14 mg zinc/kg was determined.