Superphosphate on wheat: The cumulative effect of repeated applications on yield response

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Abstract
The cumulative effects of repeated applications of superphosphate on wheat yield responses, over a period of 26 years (10 cycles of a rotation system), were examined. Since cumulative effects are the sum of current and residual effects and current effects diminish with repeated applications of superphosphate, this approach enabled residual effects to be inferred. Residual effects were apparent at all rates of application (30, 60, 90, and 120 lb/acre), being greater at the three higher levels. The influence of rainfall in the growing period (April to October) on cumulative effects was also examined. April rainfall increased yield at all levels of application, whereas September rainfall increased yield only at 90 and 120 lb/acre of superphosphate. The results indicate that, for this particular experimental site, residual phosphorus has become more important in determining yield responses than currently applied phosphorus.

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