Calibration and assessment of soil tests for estimating fertilizer requirements. II. Fertilizer requirements and an evaluation of soil testing

Abstract

The estimation of the phosphorus and potassium fertilizer requirements of wheat and potatoes from soil test calibration equations is described. The procedure is based on the estimation of yield response to fertilizer at the site represented by the soil test and the application of economic principles to the function thus established. Soil testing may be justified on the basis of statistical tests of significance on these estimates. Since, however, testing can be expensive it should also be justified on economic grounds, such as by comparison of the profit obtained from the use of soil test estimates of fertilizer requirement with the profit from the use of some alternative estimate of fertilizer requirement. A preliminary evaluation may be made from a comparison of profits from soil test and average regional estimates of fertilizer requirement, using the calibration equation for the estimation of profits at different soil test levels. This method of evaluation separates the benefit due to testing from that due to the application of fertilizers. It is shown by this procedure that a soil test for the phosphorus fertilizing of wheat and for potassium fertilizing of potatoes in southern Ontario is worth while, given sufficient diversity of nutrient level within the region.