Availability of previous and current applications of zinc fertilizer using single superphosphate for the grain production of wheat on soils of South Western Australia

Thirty field experiments on a range of soils in different rainfall zones of South Western Australia were used to examine the effectiveness, relative to freshly applied zinc (Zn) fertilizer of previously applied Zn fertilizer for grain yield of wheat. The soils had been fertilized with Zn at 0.2 to 1.2 kg Zn ha-1, 9 to 24 years previous. The effect of applied nitrogen (N) fertilizer on grain yield and Zn concentrations in the youngest emerged blade (YEB) was also examined. At all sites, the current application of Zn fertilizer to soils previously treated with Zn did not increase grain yield. The highest level of N fertilizer did not reduce grain yield where Zn had been applied previously or induce Zn deficiency in wheat plants. The lowest level of Zn (0.2 kg Zn ha-1, Experiment 17) applied 15 years earlier was still fully effective for maximum grain production. The application of currently applied Zn increased the Zn concentration in the YEB for 23 experiments. Application of N decreased Zn concentration in YEB in the 19 experiments, had no effect on the Zn concentration in 11 experiments, and increased Zn concentrations in two experiments. This was so for recent and previously applied Zn. In experiments where N decreased the Zn concentration in YEB, the concentration declined to 10 mg kg-1 in seven experiments. Zn concentration in the grain was increased by the current application of Zn in 25 experiments. It had no effect in five experiments (Experiments 11-13, 21-22). The application of N fertilizer decreased the Zn concentration of the grain for both previously and currently applied Zn in 20 experiments. Nitrogen decreased the concentration of Zn in the grain to 10 mg Zn kg-1 in seven experiments. Zinc extracted from the soil by DTPA was correlated with the amount of previously applied Zn fertilizer. DTPA-extractable Zn for the experimental sites were 0.3 mg kg-1, except for 2 experiments which were 0.2 mg/kg. The results show that where Zn fertilizer had been applied previously, applications of high levels of N fertilizer to cereal crops did not require further applications of Zn if superphosphate (400-600 mg Zn kg-1) was used in the cropping and pasture phase. This is because of contaminates of Zn in rock phosphate used to make superphosphate. However, the requirements for Zn for wheat grain need to be reconsidered if diammonium phosphate (DAP) is used for cropping and if superphosphate applications are less than 150 kg ha-1 during the legume crop or pasture species in rotation with the cereal.