When your only tool is a hammer, everything looks like a nail. So goes an old joke about someone who seems to be fixed on one issue or action. Applying enough fertilizer potassium (often referred to as potash, muriate of potash, or MOP) to forage crops seems to be my current hammer. It is too simplistic to think potassium application fixes every problem, but an increasing number of my farm forage inquiries end up involving a lot of discussion about this fertilizer nutrient. What I hope to do with this article is to help you better understand potassium and its importance in forage production.
First potassium is an essential nutrient for plant production. Potassium is involved in plant water relations – opening and closing of the leaf pores that regulate water flow through the plant. Potassium is also needed for plants to fully express their disease resistance. And potassium is well known for its role in the expression of winter-hardiness in perennial forage crops.
Potassium is removed in large quantities by forage crops, especially hay crops. Each ton of forage will remove about three to four times the potassium as phosphorus. Using a ‘balanced’ fertilizer like triple-19 (19-19-19) over a long period of time can cause a hayfield soil to have sky high soil test phosphorus numbers and soil test potassium values in the basement.
Soils differ in their ability to supply potassium to plants. For example, the Eden silty clay soils of Northern Kentucky typically have high levels of potassium but release added potassium rather slowly to crops. On the other hand, the Tilsit silt loam soils around Princeton are low in K but release added K readily. Frankly, the reasons behind the different K-supplying abilities of various soils are beyond my expertise and the space available in this column. The best way to determine K status of soils is through a soil test.
Timing matters. When potassium is applied to forages in the early spring, plants take up more potassium than needed, a process called luxury consumption. This surplus potassium is removed in the first hay cut, robbing the plant of the long term benefits of the added nutrients. Fall is the preferred time to apply potassium to avoid luxury consumption. When large amounts of K2O are needed, a split application may be needed, such as after the first cutting and again in early fall. With potassium and perennial forage crops, you are playing the long game for future returns.
Spring-applied potassium on grass pasture fields causes double trouble due to grass tetany or hypomagnesemia in cattle. High soil test potassium inhibits the uptake of magnesium by forage crops, and the resulting high potassium forage inhibits the uptake of magnesium in the rumen. Grass tetany is most frequently seen in mature cows in early lactation. These cows cannot mobilize magnesium from their bones fast enough to replenish that lost in milk, leading to tetany. Feeding a mineral high in magnesium is the best way to prevent grass tetany.
So how do you develop a potassium plan for forage crops? Here are the key takeaways:
Soil test and soil test often. Take the soil sample as long after the last application of potassium as possible, such as each year after the last cutting but before applying fall fertilizer.
Make sure your pH is in the right range. Do not add lime when your pH is too high. If your soil pH is low, apply lime according to UK guidelines. This will maximize the effectiveness of the fertilizers that you add, including potassium fertilizers.
Get your fertilizer custom blended to match soil test results. In fields with historic applications of Triple-19 we typically see high-very high soil test phosphorus and low-very low soil test potassium.
Add potassium according to soil test recommendations. If your soil test potassium is low, causing the recommended rate to be high, you’ll want to split that application.
~Dr. Jimmy Henning, originally in Farmer’s Pride. Subscribe to online or print editions https://thefarmerspride.com/