Determining net returns (profit) generated by phosphorus (P) and potassium (K) use comes from experimental data. The increase in yield will largely determine the profitability of P and K fertilization. Realistically, it is not possible to collect P and K response data on every farm. Consequently, actual responses to P and K use at a particular site are usually unknown. However, guidance in the rates of P and K to use can be determined from long-term university experiments on many soils in a given state over many years. These data provide a first approximation of realistic amounts to apply and the responses a farmer may expect.
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