The yield level and P or K removal should be used to determine the application rate needed to maintain soil-test levels in the Optimum category.
Publication PM 1688 includes suggested average P and K concentrations in common harvested crop parts. As economic returns in Fig. 1 show, there is high response variation within the Optimum category for various reasons.
A removal-based rate will maximize yield, but often the small yield increase does not offset application costs, especially with current low prices. Therefore, if the producer’s economic condition is particularly bad or there is uncertain land tenure, a fraction of the estimated removal-based rate or only starter could be applied when prices are low. This may increase profits in the short-term, but higher nutrient rates will be needed in the future because soil-test values will decline.
The decline in soil-test values is much slower than many believe, and producers having high-testing fields or field areas could save money by withholding or reducing application rates until levels decrease to the optimum category. They should be aware that there is a good long-term relationship between P and K removal with harvest and soil-test values, but not necessarily consistent from year to year, especially for K.
Figure 2 shows average results across five long-term experiments in a corn-soybean rotation. Results from these and other long-term experiments show that, on average, and depending on the yield level, about ten years are needed for soil-test P or K values borderline between the High and Very High categories to decrease to the Optimum level, where maintenance based on removal is recommended.
(Figure 2:Relationship between cumulative P and K removal with grain harvest and soil-test values over time for plots not fertilized with P or K (Bray-1 for P and ammonium-acetate for K from dried soil samples)