Yuhei Nakayama

Ph.D. Level Student
University of Illinois at Urbana-Champaign
Advised by Dr. Andrew Margenot

Comprehensive Evaluation of Phosphorus Best Management Practices For Soybean To Increase Nutrient Use Efficiency, Profitability, and Water quality

Though soybean yield has continued to increase over recent decades, Illinois’ recommendations on P management for soybean are outdated, with fertilizer often applied only at the start of the two-year corn-soybean rotation. Updating recommendations on the 4Rs (right source, rate, timing, and placement) of P fertilization for soybean will help expand toolbox for farmers to maintain yield while minimizing environmental impacts. Ammonium phosphates (MAP and DAP) are the common P fertilizer sources in the U.S. Midwest, and application of ammonium phosphates also supply additional nitrogen (N) that do not generally increase soybean yields but can potentially lead to N loss to the environment. In contrast, N-free triple superphosphate (TSP), once the dominant P fertilizer in Illinois, is equally effective in supplying P while minimizing N loss. The aim of this work was to comprehensively evaluate 4Rs of soybean P management to help verify and update the recommendations in Illinois while assessing the potential impacts of 4Rs on water quality. Specifically, we conducted field trials on the two dominant soil types in Illinois of Mollisols (Champaign Co.) and Alfisols (Franklin Co.) to evaluate treatment combinations of P sources (MAP, DAP, and TSP), rates (maintenance rate and 75% maintenance rate), timing (fall and spring), and placement (broadcast and banding). We measured soybean grain yield, P uptake and removal, and P use efficiency to evaluate soybean productivity, and N and P leaching using ion-exchangeable resin lysimeter to evaluate potential co-benefits of P management on water quality. Results from the first year suggested no response (p > 0.10) of soybean grain yield to treatments, but showed reduction of nitrate-N leaching with TSP compared to DAP and MAP on Mollisols in fall applications (p < 0.05).