If your soil test finds high potassium, should you add additional potassium fertilizer?
This is a question that Adrian Johnston of the Potash & Phosphate Institute tackled in a recent paper, Potassium Responses Ñ Managing High Potassium Testing Soils. The following is a summary of his paper.
The answer is generally no, but there are situations when adding potassium fertilizer, or potash, can still be beneficial. My own research has determined that producers with high potassium levels, which can occur after repeated applications of manure, which contain high levels of potassium, may have problems with excessive potassium-to-calcium-and-magnesium ratios in forage.
Read Also
VIDEO: Green Lightning and Nytro Ag win sustainability innovation award
Nytro Ag Corp and Green Lightning recieved an innovation award at Ag in Motion 2025 for the Green Lightning Nitrogen Machine, which converts atmospheric nitrogen into a plant-usable form.
This can cause grass tetany or milk fever in animals that eat the feed. Adding potassium in these circumstances would be detrimental.
Plants use large amounts of potassium during the growing season, roughly 1.5 to three pounds per bushel of yield. However, only a small amount of this potassium is taken off the field in the form of grain.
The remainder is cycled back to the soil from the straw.
If the entire crop is removed from the field, such as with silage or hay, large amounts of potassium can be removed from the field. That can reach as high as 60 lb. per ton for alfalfa.
Generally, crops respond to potassium fertilizer with the greatest yield response when potassium levels are low.
Marginally adequate soil that tests between 250 and 350 lb. per acre should be monitored, especially if in the crop rotation whole plants are to be harvested, such as green feed, silage and forage.
Soil with high potassium levels also responds to additional potash, but not as much as soil with low levels.
Research by Western Co-operative Fertilizers Ltd. has found that on average barley showed no response to added potash on high potassium soil 60 percent of the time, a two to five bu. per acre yield response 25 percent of the time and a five to 10 bu. per acre increase 15 percent of the time.
Barley seems to be the most responsive in this situation, partly because the fertilizer may be adding critical potassium early in the growing season when the soil is cold, which is when chemical reactions that make potassium available in the soil are slowed down.
Potassium also varies across the field, especially in rolling landscapes, so sometimes the composite soil sample will produce a soil test value that is adequate in potassium but there may be areas within the field that are deficient and may benefit from potassium fertilizer.
When considering applying potash, producers must remember that it is made up of two nutrients that can be used by growing crops: potassium and chloride.
Soil that tests high in potassium can still be low in chloride and crops may still benefit from potash fertilizer.
While the odds are low that adding potash to soil with high potassium levels will result in a significant response, many producers apply starter potassium at 15 to 20 lb. per acre as insurance in case of a potential response.
Brent Flaten, P.Ag., CCA, is an agronomic crop enhancement specialist for Agricore United.
