The information we will provide the farmer will answer the following questions: 1) How much K was removed by soybeans? 2) Did yields improve with added K? 3) Was the yield or the K removed different between tillage practices? 4) By how many ppm, did the soil K change following soybean harvest? 5) Did the soil analysis results differ between the methods of soil storage? 6) If so, what would the difference mean to a farmer’s fertilizer recommended rate? And 7) Does the K and P removed/bushel, or observed change in soil test levels differ between sites?

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Research Conducted
This trial estimated the removal of K from soil by soybeans following harvest at Hettinger, Minot, and Williston. At Carrington, (CREC) K and P removed by soybeans estimated from a trial that either received 50 lbs K or did not, across plots that had received the same N treatments applied in over three years on conventional till (CT), and no-till (NT). Soil samples from the top six inches of each plot before planting, mid-season, and at harvest, were divided into two groups, one group refrigerated and the other air-dried. The samples were analyzed by wet analysis at AgSource Laboratories, Ellsworth (IA).

Why the research is important to ND soybean farmers
There are some concerns about potassium (K) and P mining (removal without supplementation) in soybean fields, and as a result, yields of subsequent crops could be impacted if production continues without K fertilization. Also, reliability soil tests for K, and predictability of yield response when soil samples are stored by refrigeration or air-dried. This is important because only when a good soil available nutrient estimate is made from analysis that K fertilizer recommendations can be effective.

Final findings of the research
Application of 50 lbs K at CREC did not improve yields, protein or oil concentration. Aboveground biomass was significantly increased by K application under NT but not under CT. Test weight was significantly greater under NT. Yields were low due to drought conditions in 2018. At CREC yield average was 34 bu/ac, and a bushel removed 1.2 lbs K and 0.54 lbs P on average, which is relatively low compared to 0.65 lbs P/bu for a 69-bushel crop in a previous trial at CREC. Available soil K was greater at mid-season than before planting and after harvest (Figure 1), and was not necessarily less following harvest at the other sites. Soil analysis of refrigerated samples showed higher K values than air-dried samples. Available soil P after harvest was not less than before planting. More research is needed to assess the need for P fertilization of soybeans to improve yields. Potassium fertilization is likely to enhance yield for soils that the North Dakota state recommendations have clearly defined. Response depends strongly on type of clay.