The primary source of potassium (K) fertilizer is KCl which contains 50% chloride (Cl) by weight. Soybean yield decreases have been found during recent years and decreased grain protein and increased oil have occurred for soybean within K trials conducted across Minnesota. Purchasing inputs that may reduce the yield of a crop is problematic and Minnesota soybean producers need better fertilizer guidelines to ensure greater profitability of nutrient inputs. Potassium research and improved guidelines are needed as soybean producers are increasingly focusing on inputs to further increase yield across the state, and little guidance is available across the Midwest on maximum rates of potash fertilizer which should be applied to reduce the risk of chloride (Cl) toxicity in soybean. Recent research in Minnesota has demonstrated the need for S to be applied to soybean. Research in the Midwest has indicated that the application of sulfur (S) may mitigate Cl impacts. Additional research would be beneficial to study whether S can help mitigate yield decreases in soybean due to high rates of Cl being applied to further increase yield potential of soybean across the state. The effect of K and Cl on grain quality also must be assessed studying protein, oil, and amino acid distribution as past research in Minnesota has shown that S can increase cysteine and methionine content while K application decreased these two essential amino acids. The proposed research will help better refine potassium and sulfur guidelines for soybean maximizing grain yield and quality parameters.

Evaluate the long-term impact of potassium rate and timing in a corn/wheat-soybean rotation on soybean grain yield and quality.
a) Quantify yield effects when K fertilizer is applied at different times and rates in a two-year rotation containing soybean.
b) Quantify changes in soybean protein and oil concentration based on long term K fertilization strategies.
c) Correlate K and Ca or Mg in plant tissue to determine impacts on soybean yield.

Determine if the application of Cl has negative effects of soybean grain yield and quality.
a) Quantify yield impacts for K fertilizer sources applied with and without Cl on soybean varieties that vary in IDC (potentially salt) tolerance.
b) Track rates of Cl buildup in poorly drained soils.
c) Quantify plant tissue Cl concentration and correlate tissue Cl concentration to the concentration of other essential nutrients.

Determine whether sulfur should be considered as an essential nutrient and applied to soybean in Minnesota.
a) Assess the impact of sulfur on soybean grain yield and seed quality.
b) Determine if the application of sulfur can reduce negative impacts of K or Cl on soybean seed quality or seed yield.
c) Further develop a set of sulfur guidelines for soybean production in Minnesota.

Update:

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Update:
Work is progressing on the project. I have all yield and soil test data compiled and some of the plant tissue data. All NIR seed quality analysis has been run and the data compiled and checked for needed re-runs. We are in the process of completing a portion of the tissue analysis in my lab. I requested an extension on the project to allow us to complete the tissue analysis before the final report is submitted as two of the three projects field work has been completed as of the end of the 2021 cropping season. Once the tissue results are available, I will submit the final report for the 2021 project.

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Update:
I do not have any additional data to report from the last reports due on May 31. We finished all lab work for the project completing first- and re-runs of samples collected during 2021. I have gone through the data but I have not summarized the final data for the final report submission. I should be able to complete the final report by the end of 2021 to complete this project.

Update:
All required lab work is not complete for the 2021 project. I have all data uploaded and am working on the final analysis. Final report is being prepared. I also recently updated the P and K removal database that I have been tracking P2O5 and K2O removed per bushel harvested of corn and soybean. That information will be included in the final report.

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Update:
I am uploading a file for the final report and also the final draft of the IDC publication that was part of this projects goals.

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Data indicates that soybean grain yield can be impacted by the source of K applied and that Cl may result in a risk for a decrease in yield. Corn and wheat yield do not appear to be strongly affected by Cl application from the initial data, but Cl is building in the soil at two of the four locations. When soil test K is in a responsive range, less than 200 ppm, K fertilizer should be applied, and the source of K does not matter for soybean, wheat, or corn. Application of K as little as 80 lbs of Cl per acre can reduce yield and the greatest risk is when K is applied directly ahead of the soybean crop and soil tests indicate a response to K should not occur. The application of K itself can reduce seed protein concentration which is an additional reason to reduce K application ahead of the soybean crop.

The over impact of chloride on soybean yield varies by location. The five year study showed a 1-2 bushel per acre reduction of yield at three of the four locations which reductions happening at Lamberton, Morris, and Crookston. Corn and wheat yield were not reduced by chloride indicating a greater tolerance and potential for applying high rates of potash ahead of tolerant crops instead of directly ahead of soybean. The bulk of all fertilizer applications occurred in the fall post harvest. One question that needs to be addressed is whether the impact of chloride is worse for fall or spring applied potash. That question cannot be addressed from this research.

Data from the second study does point to the negative impact of Cl on soybean grain yield. High enough rates of Cl can negatively impact both soybean grain yield and seed protein concentration. The plant tissue data from both studies shows that Cl will be taken up by the plant if it is in the soil where it can readily move with soil pore water. Sites do vary in the potential to carry Cl in the soil which also can affect the severity that Cl may have on soybean grain yield reduction. However, irrigated sites like Becker also present a risk as Cl in the irrigation water may compound the impact of Cl applied in fertilizer. While soil test K can be low in irrigated soils it would be better to not apply high rates of KCl ahead of the soybean crop. Other research is looking at optimal rates of K application for soybean and timing of application (fall versus spring). The effect of S on Cl is inconclusive currently.

Overall findings from this study are:
1) Crops will take up chloride from the soil regardless of whether the crop needs the nutrient.
2) Chloride in potash fertilizer is highly soluble and will be readily taken up by crops.
3) Soybean yield can be reduced by chloride but the reduction may be small and not easily detectable by growers.
4) Potash applications should be limited to no more than 100 lbs of actual product applied per acre directly ahead of the soybean crop regardless of whether the fertilizer is applied in the previous fall or in the spring.
5) The data shows no direct benefit of sulfur application to soybean for increasing seed yield or protein concentration. This work is ongoing.