Residual effectiveness of copper fertilizer for wheat in Western Australia

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Abstract
In five field experiments the effect on wheat grain yield of a current application of fertilizer copper mixed with the superphosphate drilled with the seed was compared with the residual effect of fertilizer copper applied the same way 4 to 12 years earlier. In all cases the earlier application was superior, or equal, to the current application. Vegetative copper concentration, measured in two experiments, was increased much more by the original application than by the current application. All applied copper remained in the top 10 cm of soil 4 years after application in a form which could be extracted by 0.2 M ammonium oxalate at pH 3 when measured in one experiment. Reasons for the relative inefficiency of the current application are suggested. The residual effectiveness of fertilizer Cu in the field determined by this method measures the nett effects on Cu availability of processes acting in opposite directions. The processes enhancing Cu availability dominate or counter those processes such as soil 'fixing' reactions which, if present, are obscured.