In this ILSoyAdvisor webinar, Dr. Andrew Margenot, Associate Professor in Crop Sciences at the University of Illinois Urbana-Champaign, reviews data collected in an Illinois Soybean Association check-off funded research project. He highlights the current situation and challenges in soybean production in Illinois regarding phosphorus (P) management. Modern soybean production requires high P demand due to high grain P concentration, resulting in increasing yields over the past decades. Soybeans account for approximately 40% of P removal in a typical corn-soy rotation.

The Illinois Agronomy Handbook’s recommendations for soybean P management are outdated. The last update of P fertilizer recommendations is likely pre-1970, and soybeans are treated as residual feeders, with P application recommended at the start of the corn-soy rotation. There are also limited quantitative recommendations on P fertilizer placement and timing specific to soybean.

Monoammonium phosphate (MAP) and diammonium phosphate (DAP) comprise around 80% of the total P fertilizer used in Illinois but can result in nitrogen (N) losses, especially when fall-applied. Since the ‘extra’ N of MAP and DAP are generally unnecessary for soybean, avoiding MAP and DAP as P sources in front of soybean can provide appreciable positive impacts on water quality. N-free P sources such as triple superphosphate (TSP) with a formulation of 0-46-0 can avoid N losses from MAP or DAP.

Over two seasons in the central (Urbana) and southern (Ewing) Illinois sites, yield and P removal were similar regardless of the tested P sources, rates, and timing-placement at the two sites over the two-year period. TSP was found to be as effective as MAP and DAP for yield while avoiding N loss. A maintenance rate of 75% was considered permissible over a short timespan (2 years), although it was overestimated by an average of 31% due to lower-than-expected grain yield – pointing to the need for multiyear maintenance rates with a longer running average. The timing and placement of P fertilizer (fall vs. spring, broadcast vs. banding) did not significantly affect yield in the study.

Based on research findings, a few recommendations are offered. Depending on the availability of TSP, it is suggested to use TSP instead of DAP or MAP for soybean, especially for fall application, in both corn-soy and soy-soy rotations. Due to difficulties in accurately estimating maintenance rates in short timespans, longer-term management (3-6 years) is recommended to balance overall P input (P fertilizer applied) and output (grain P removal). Spring application of MAP and DAP is advised to minimize N losses. Broadcasting is more practical than banding, and the study did not observe any yield advantages of banding over broadcasting over two seasons in the central (Urbana) and southern (Ewing) Illinois sites. Soybean production systems can experience N leaching loss even without applying N as MAP or DAP, particularly in higher organic matter soils of central and northern Illinois. Thus, practices that reduce background N loss, such as cover cropping, are still beneficial for water quality.