Exceptionally dry conditions this summer will result in low crop yield in much of Iowa, and weather forecasts call for normal or below-normal rainfall through the fall. These dry conditions and low yields will significantly reduce phosphorus (P) and potassium (K) removal with crop harvest, and may also affect soil test P, K and pH results, complicating test interpretations.
P and K removal with crop harvest
Removal of P and K with harvest influences the amount of available P and K in soils. Estimates of removal are used to decide fertilizer application to maintain soil-test P and K levels within the optimum interpretation category. The amount of P and K removed is calculated from the yield of harvested grain or biomass and its P and K concentration. It is easy to measure crop yield for an entire field and also for different field areas with grain yield monitors. Yield monitors are very useful to estimate yield (and P and K removal) for field areas with different yield levels due to drought or other reasons.
Taking grain samples for analysis is an option for estimating P and K concentrations, but the easiest and less costly approach is to use average concentrations per unit of yield for different crops and harvested plant parts, as listed in ISU Extension and Outreach publication "A General Guide for Crop Nutrient and Limestone recommendations in Iowa, PM 1688. Those values are long-term estimates, and grain analysis during the last few years indicate they represent the upper range of concentrations observed today. For example, values from PM 1688 for corn grain are 0.375 lb P2O5/bu. and 0.30 lb P2O/bu.; and for soybean grain values are 0.80 lb P2O5/bu. and 1.5 lb K2O/bu.
No matter the option used to estimate grain or biomass P and K concentrations, there is a great deal of uncertainty and variability concerning drought effects. Depending on moisture availability during different portions of the growing season, there could be relatively more or less grain dry matter production than nutrient uptake and translocation from vegetative plant parts to the grain, which would result in lower or higher concentrations, respectively.
When sampling grain or biomass for analysis, you should remember that there is large variability within fields, so take several samples to adequately represent the field or different field areas. Using averages from PM 1688 also is a reasonable approach because even with large variability in P and K concentrations, research has shown that yield level is the most important factor determining the amount of P and K removed. For example, say the grain P concentration ranges were 0.17 to 0.43 lb P2O5/bu. for corn and 0.43 to 1.1 lb. P2O5/bu. for soybean, and the K concentration ranges were 0.14 to 0.30 lb K2O/bu for corn and 0.43 to 1.8 lb K2O/bu for soybean. Therefore, concern about differences in grain P and K concentration due to dry conditions should be much less than getting good estimates of harvested yield.