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Digital Aerial Imagery Guides a Statewide Nutrient Management Benchmarking Survey
P. M. Kyveryga, T. M. Blackmer
Iowa Soybean Association

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 corn and soybean production. It is difficult to predict the long-term effects of these changes on plant nutrient status. A survey was conducted to establish benchmark distributions of crop nutrient status that can be used to track changes over time and identify site-specific factors responsible for these changes.  In 2011, late-season digital aerial imagery (DAI) 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 samples were analyzed for four essential nutrients and several micronutrients. For corn, > 50% of samples were found below optimal levels for S, Mn, and B and for K, S, Cu, and B for soybean. A survey categorical analysis showed that the soil test interpretations often did not match plant tissue test interpretations. In corn, the "Target stress" areas tended to test in a lower soil test category for Zn and Mn, and in a lower plant tissue test category for N, P, S, and Mn. Fields with a history of Manure applications usually had a higher nutrient status. Samples collected in areas with higher soil organic matter (SOM) and lower soil pH usually had a higher nutrient status than those with lower SOM and higher soil pH. The low cost DAI of the crop canopy helped to collect soil and plant tissue samples to characterize statewide crop nutrient status. The results of this survey will be used to conduct on-farm experiments to measure yield responses to nutrients that often tested below optimal levels within smaller geographic areas across Iowa.

Keyword: Digital aerial imagery, soil and plant tissue testing, crop nutrient status, statewide survey, benchmark distribution.