Prediction of plant response to fertilisers by means of soil tests

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Abstract
Data from 82 rates-of-fertiliser trials on wheat, in which double superphosphate was the source of phosphate, were used to evaluate the predictability of grain yield responses from soil-test measurements. Yield data were reduced to single values for correlations with soil phosphorus tests by calculating (a) relative yields % and (b) the first principal components of the yield responses. Regression of these parameters on values from eight different soil tests, using a quadratic model in the square root scale, showed that the Olsen soil test modified to 16-h extraction and a soil: solution ratio of 2.2 ml : 100 ml accounted for more variation than other soil tests examined. The correlations with soil tests were closer for the relative yields than for the first principal components. Predictions of yield responses and their confidence limits were calculated for the rates of phosphatic fertiliser used in the trials from regressions of the absolute yield responses on this modified Olsen soil test, and the profitability of applying phosphatic fertiliser was assessed. Predicted relative yields were calculated by a similar regression. The profitability of phosphate applications for selected soil phosphate levels was determined.