Building up Soil P and K in Low Testing Soybean, Rice, Fescue and Cotton Fields.

University of Missouri soil test laboratory recommendations for P and K fertilizer are based on three components: target level, crop removal, and build-up.  Target level is the amount of extractable nutrient found in a soil at which point applying more fertilizer containing the nutrient will probably not increase crop yields.  Crop removal is how much the nutrient is reduced in the soil annually from harvested forage, grain, or fiber.  Build-up is the additional fertilizer needed above crop removal to increase low- and medium-testing soil P and K to the target fertility levels for crop production. The objective of a long-term study is to evaluate the effects of P and K build-up periods on yields of tall fescue hay, cotton, and soybean and rice in rotation and to validate the build-up equations used in the MU fertilizer recommendation program. We concluded that a 4-year build-up program can be used by farmers without sacrificing hay yields in the first three years. No significant fescue hay yield increase was observed when using a 1-year build-up program compared to the 4-year build-up program. In rice experiments, the nitrogen only untreated check produced 806 to 1210 kg ha^-1 less rice than buildup programs and 672 to 874 kg ha^-1 less soybean.  Initially, we were concerned that using rice target thresholds would decrease soybean yields and that shorter buildup time would produce higher yields than long buildups but that does not appear to be happening. In cotton experiments, 1-year buildup programs produced 220 kg ha^-1 more than untreated checks. However, 1-year buildup programs on a fine sand soil failed to reach target K soil levels, suggesting that either insufficient K was applied or K⁺ may have leeched below the 15 cm soil depth.