Research Projects

Research Projects

Role of Polysulphate application and sulfur mineralization in maize and soybean nutrition

Prof. Mike Castellano and Prof. John Sawyer (Iowa)
Start: 2017 –  End: 2019

Prediction of sulfur (S) deficiency in corn and soybean, and crop response to S fertilizer, remains a major challenge. Historically, Iowa crops did not respond to S fertilizer application. However, as atmospheric S deposition and manure-based fertilization have decreased over time, crops have increasingly responded to S fertilizer application. As a result, farmers are demanding accurate plant and soil tests for decisions about S fertilization. Yet, reliable soil and plant S tests do not exist.

This proposal will evaluate plant and soil tests for sulfur fertilization, corn response to S fertilizer application, as well as response in the residual year post-fertilization. Four fertilizer S sources will be investigated: calcium sulfate, elemental sulfur, ammonium sulfate and polyhalite (polysulfate). Research will be focused on corn in northern to northeast Iowa due to high historical response to S fertilizer. For reasons that are not clear, corn responds to S fertilizer with greater frequency than soybean.

Our approach will take advantage of one Iowa State University research farm located at Kanawha, IA and 14 commercial producer fields. At present, we have identified one S-deficient field at the research farm and two potential S-deficient producer fields. We will identify additional S-deficient producer fields in the future. The two year study (2017-2018) will create 7 producer field sites in Year 1 and 7 new producer field sites in Year 2 (total of 14 producer field sites). Field sites established in Year 1 will not receive S fertilizer treatments again in Year 2, however, crop response will be measured in Year 2 due to the possibility for delayed or residual response to applied fertilizers.

Our work will focus on S-deficient fields and use a moderate rate of S (10-15 kg S ha-1) to maximize the likelihood of identifying an effect of S source on crop yield. We expect that differences in corn response to S source will result from differences in plant availability. We will measure soil extractable sulfate-S at four times from pre-plant to the corn silking (R1) growth stage. Small plants (V6 stage) and ear leaves at R1 will be sampled for S concentration. This sampling approach may help to determine the role of soil organic matter mineralization on soil-test S. The role of soil S mineralization is important because the lack of a reliable soil S test has been attributed to the variable contribution of soil S mineralization to crop S uptake. Variability in S mineralization within fields across years is affected by weather (i.e., temperature and moisture). We will conduct two years of replicated field trials with the following four