Soil testing is the key component for determining the need for phosphorus (P) fertilization. Also, if fertilization is required, test results guide the rate of application recommended to optimize production. Through extensive laboratory, greenhouse, and field research studies, soil tests are calibrated against the expectation of response to applied P. That is, the research determines both the relative index of P availability to the crop being grown and an indication of the probability and magnitude of yield increase one should expect when P is applied – thus providing the interpretation of test results. This research also determines which test is best at predicting available P and crop response to P application.