Effects of cutting back on potassium

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

With the continued high price of potash many farmers are considering cutting back on the amount that is applied. In making decisions about how much potassium to apply or how much one can cut back, it is very helpful to have soil test information indicating the available potassium (K) status of the various fields being farmed.

When the K soil test value is less than the critical value, applying no potassium will definitely result in yield loss. The critical varies with the CEC (cation exchange capacity) of the soil. For soils with CEC values of 6, 12 or 18 the critical value will be 90, 105 or 120 ppm K, respectively. The further the soil test value is below the critical value the greater the yield loss will be. This applies for all crops.

For farmers looking to cut back on potash, it is best to look on fields where the soil test is above the critical value where the normal recommendation is to apply an amount equal to crop removal. A 180 bu/a corn crop will remove about 50 lb K2O/acre and a 60 bu/a soybean crop will remove near 85 lbs K2O/acre. The risk of significant yield loss from applying no K or a reduced rate on these fields is generally low for the current growing season. However, it needs to be recognized that as K is removed from the “soil bank” a reduction in the available soil K level will occur. The decrease occurs more rapidly in sandy soils than in clayey soils. At some point in time this will need to be replaced.

In soils with CEC values of 6, 12 and 18, the available soil K test value may decrease approximately 1 ppm for about each 5-6, 9-10 and 13-14 lbs K2O/acre removed from the soil. Therefore, if 50 lbs K2O/a is removed and none is applied, the available soil K may decrease in the neighborhood of 9.1, 5.2 and 3.7 ppm per year. Applying 25 lbs K2O/a in the starter fertilizer will reduce this rate decrease. Prioritize potash use for those fields with soil test values below the critical K soil test value and look to cut back on fields that have soil K values above the critical value where yield is less likely to be impacted.

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