Potassium (K) is an essential element in plants and is considered one of the three macronutrients, along with nitrogen and phosphorus. Its relative amount is analyzed and reported in almost all routine soil samples. Therefore, it's obvious that it is important. Unfortunately, with recent price increases, it has gone from being the least expensive to the most expensive of the three macronutrients. In this article, I'll look at where potassium deficiencies are most likely to occur, the consequences of these deficiencies and management strategies to address these situations.
Plants take up potassium in the ionic form (K+). Since soils have a net negative electrical charge due to the clays and organic matter, the positively charged potassium is held magnetically to the clays and organic matter particles in the soil. The more clay and organic matter there is in the soil, the more strongly potassium is held. If there is little clay and/or organic matter in the soil, potassium can leach out of the root zone in heavy rains. Therefore, one likely place for potassium deficiency to occur is in very sandy soils with low organic matter contents.
Another likely spot for potassium deficiency to occur is in hay fields. Hay and silage remove large amounts of potassium because they remove entire plants. With higher hay and silage yields, more potassium is removed. Very little potassium is removed in grazing situations, and grain crops and cotton do not remove as much potassium as hay or silage. If harvesting forages and crop residues for biofuel becomes commonplace, potassium deficiencies will likely increase due to whole plant removal.
Potassium deficiencies are expressed in various ways due to the functions the element serves in the plant. One beneficial function of potassium is that it helps reduce the severity of some plant diseases. This is especially true in perennial forages, such as alfalfa and bermudagrass. If these species are deficient in potassium, the stand will probably not last as long as it would if potassium levels were adequate. Another beneficial function of potassium in plants is that it increases winter hardiness in perennial forages. Having adequate potassium will not make a cold-sensitive forage species thrive in Minnesota, but it can make a difference in how well it handles an abnormally cold winter within its adapted range. This, coupled with the disease resistance benefits of adequate potassium, helps explain why perennial forages grown in soils with optimum levels of potassium have longer-lived stands than those grown in potassium-deficient soils.
Since potassium fertilizers have increased in price, what are some management strategies to employ? First, collect soil samples from all fields you intend to fertilize. This will let you know if you actually have low levels of soil potassium. Do not apply potassium (or any other nutrient) without a soil test just because you think it may be lacking. This could be rather expensive if you don't need it.
Second, if you have low/medium soil potassium and you are cutting hay from your place, consider grazing the fields and buying your hay from somewhere else. This allows you to run more cows. Also, when you buy hay and feed it, you are adding nutrients, including potassium, onto your land from someone else's field. The value of nutrients you receive when buying hay and feeding it offsets a large portion of the cost of the hay. If you cut hay from your fields to feed to your cows, try to feed the hay back in that same field to recycle the nutrients rather than export them.
Third, if you are selling high yields of a premium hay crop (alfalfa or horse-quality bermudagrass), apply the rate of potassium recommended by soil test. While it may be expensive, you will more than make up the cost in increased yields and longer-lived stands.