A recent report from Michigan State University reports no significant yield response from applying gypsum (calcium sulfate) on soybeans. It's a good example of why we never want to depend on one year of test results. The same study in 2014 showed a 6-bushel-per-acre positive yield bump in soybean.
Gypsum does have its uses as both a soil amendment and a fertilizer. There are measurements and formulas to determine how much to apply to remediate sodium and salinity. It's when used to supply calcium and sulfur that there are no clear measurements on which to base recommendations, and in the case of soil, it may take as much as six to eight years to notice a difference.
I first applied gypsum back in 2004 on 80 acres. I applied a ton of synthetic gypsum (Procal 40 from Soil Solutions) and have continued that practice every third year as both a soil amendment and a sulfur source. I began to notice significant changes in soil tilth in 2010 -- six years after the first application. I have never tried to measure a yield response.
It doesn't surprise me that this study contradicted itself in 2015. James DeDecker, author of the Michigan study, stated that most soils in Michigan don't lack calcium or structure that requires gypsum remediation. However, he points out that the courser-texture soils (sands) could respond since they have a smaller CEC value and are lower in organic matter and won't mineralize as much sulfur.
We have known for decades that sulfur is mostly needed on course soils. With reductions in atmospheric sulfur deposits (thanks to the Clean Air Act introduced in 1963) applications of supplemental sulfur are most commonplace for all crops including alfalfa, corn and soybeans.
DeDecker said in his study, "Inadequate levels of available sulfur are not only directly detrimental to soybeans, but can also inhibit uptake and utilization of nitrogen by the crop due to the synergistic roles of sulfur and nitrogen in protein synthesis." However, soybeans are not as responsive to supplemental sulfur as alfalfa and corn.
In 2014, the Michigan study applied 1/2 ton of bulk natural (powdered) gypsum and increased yields by 6.6 bushels per acre. In 2015, they used a different site and applied Gypsoil's synthetic gypsum and Calcium Products' pelletized SuperCal SO4 mined gypsum at three rates. Yields averaged 37.4 bushels per acre with no significant difference between treatments, and the untreated control averaged 38.2 bushels. Tissue tests found all treatments provided sufficient calcium, nitrogen and sulfur.
DeDecker believes the responses relate to soil quality differences between the two sites and differences in texture, CEC and organic matter. The field in 2014 had a CEC of 5.1 and organic matter of 1.9%. In 2015, CEC was 10.5 and organic matter was 2%. As a result, DeDecker concluded CEC and organic matter might be able to predict deficiencies and gypsum responses.
It will take years of additional calibration work at different rates, sites and soil types to develop gypsum application recommendations. The Michigan State work seems like a good start, and I hope we keep working toward this goal.
To read the full Michigan State University article "Soybean response to gypsum is dependent on soil quality," click on the following link: http://goo.gl/…
If you have a question, e-mail Dr. Daniel Davidson at firstname.lastname@example.org
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