Lessons From a Statewide Nutrient Management Benchmarking Survey.

Large fluctuations in crop and fertilizer prices during the last five years caused many farmers to change their normal fertilizer management practices to better control the rising costs in crop production. The long-term effects of these changes on corn and soybean nutrient status are not known. A survey was conducted to characterize benchmark distributions of crop nutrient status that can be used to monitor changes over time and identify site-specific factors responsible for these changes.  In 2011, late-season digital aerial imagery of the crop canopy was used to select "Target Stress" (higher canopy reflectance and smaller plant biomass) and "Nonstress" areas to collect two soil and plant tissue samples within each of 505 corn and 376 soybean fields across Iowa.  The soil and tissue samples were analyzed for four essential nutrients and four micronutrients. For corn, more than 50% of samples had below optimal levels for S, Mn, B and for K, S, Cu, B for soybean. A categorical analysis showed that the soil test interpretations did not match those of plant tissue tests. In corn, the "Target stress" areas tended to test in a lower soil test category for Zn and Mn and in a lower tissue test category for N, P, S, and Mn. Fields with history of manure applications tended to have a higher nutrient status. Samples collected in areas with higher soil organic matter (SOM) and lower soil pH had a higher nutrient status than those collected in areas with lower SOM and higher soil pH. The results of this survey can be used to design on-farm yield response studies of nutrients that often test in below-optimal ranges to verify and refine currently used soil and plant tissue test interpretations.