Examination of the long-term data found small yield increases from potassium (K) fertilizer application regardless of source at three of the four locations. Yield increases were found for hard red spring wheat and corn at one location as well. Chloride was associated with a yield decrease of one bushel per acre at one of the four locations. The locations where yield was increased had low K soil test levels before application. This tells us that if K is needed, it should be applied regardless of source. The location in which yield was decreased by Cl had sufficient K in the soil as indicated by the soil test.
Trials where high rates of Cl was applied were done to look at a worse case scenario. Yield was decreased similarly when both potassium- and calcium-chloride was applied as a chloride source. At the Becker and Waseca sites, the yield reduction averaged four bushels per acre while there was an eighteen bushel per acre reduction at Morris, which was much drier during the 2020 growing season. Four varieties were planted at all three locations and all were affected the same.
The impact of Cl on soybean is not new and has been going on as long as soybean has been grown in the state. Most northern soybean varieties are considered “chloride includers,” which are susceptible to yield reduction from over-application of Cl. For growers in Minnesota, there is some concern that if too much Cl is applied directly ahead of the soybean crop, it could hurt soybean yield. While the exact tolerance is not known, the effect of Cl will be worse on poorly drained soils in years with low rainfall because Cl will not move out of the root zone.
In most cases, the yield reductions have been small. In our study, the three-year average yield at Waseca for soybean was 68 bushels per acre for the control and the average yield reduction was only one bushel per acre. However, in Morris, the high rate of Cl resulted in a yield decrease from 68 bushels to 50 bushels per acre, which would be a red flag to most growers. The fact is that a small reduction in yield will not be noticeable in the grand scheme of things, yet it is still a reduction in profitability while also paying for an unnecessary input.
Soybean fertilizer recommendations
Potassium chloride is the most economical source of K we have to apply, so managing this issue comes down to applying K fertilizer only where it is needed. If K needs to be applied ahead of the soybean crop, keep the rate low. Our soybean fertilizer recommendations currently suggest no more than 100 pounds of potassium chloride be applied ahead of the soybean crop but the actual tolerable rate can vary from one field to another. However, we have encountered yield reductions in sandy soils, so no field is immune to these issues.