EVALUATION OF SOIL TEST INFORMATION IN AGRICULTURAL DECISION-MAKING

The value of soil test information in planning application levels of f ertilizer is determined by using agricultural field plot data to estim ate the corn production function, to construct plausible prior distrib utions of nitrate levels and to determine the sampling distribution of soil test results. These quantities are used to determine the posteri or distribution of soil nitrate concentrations given the results of a soil test Optimal decisions concerning application levels of fertilize r are made with respect to this posterior distribution. Average reduct ions in fertilizer application rates range from 15% to 41%, depending on the form of prior information that is available. These reductions a re achieved by increasing the variability of application rates over ti me. Disregarding the uncertainty that remains after soil testing signi ficantly overstates the expected benefits of soil testing.