Legume plants (alfalfa and clovers) require potassium for growth and winter survival. Alfalfa and clovers need about 50 to 55 pounds of potassium per ton of forage produced, grasses require slightly less. Plants require potassium for protein synthesis and for the opening and closing of stomata, which regulate air uptake and water loss by the plant.

Recycling potassium

The first step is to recycle as much potassium as possible. Potassium occurs mostly in the urine. It is very soluble and can readily be leached from manure with any juices that are lost. In fact, in a study we did, cow pies cattle left outside in October lost all their potassium by December, even though the cow pies were frozen most of the time. The high solubility means that potassium is lost from a manure pile as juices are lost or whenever rain leaches potassium into the soil below the pile. Similarly, on pasture over summer, if cattle urinate in the shade of a tree, the potassium is moved from the pasture to the tree. Make sure that all liquid from manure is collected and spread across the fields/pastures to recycle as much potassium as possible. Every pound lost is one that must be replaced at 60 cents per pound.

Good management can minimize the fertilizer that needs to be applied. The first step is to start with a recent soil test (less than three years old). First check to ensure that soil pH is adequate for alfalfa (pH 6.8 or higher). If soil pH is low, the stand will not yield well or persist regardless of additional fertilizer applied.

Potassium soil test categories are usually very low (VL), low (L), optimum (O), high (H), very high (VH) and excessively high (EH). When the soil test is optimum, the recommended fertilizer application rate is equal to the amount removed in the harvested crop. This is considered a maintenance application resulting in little change in soil fertility levels. You can calculate whether or not you have applied potassium to match removal since the soil test knowing your alfalfa yields since the soil test. You would take your yield (in tons) times 50 to 55 pounds of potassium and then subtract the amount you applied. The difference is the gain (or loss) is soil potassium level.

Some farmers did not apply the necessary fertilizer last fall to replace last year’s removal. Those fields will be below optimum. More importantly going into winter with low soil potash will reduce winter hardiness, increasing the risk of injured to kill stands over winter.

Potassium is extremely water soluble and moves quickly into the soil so an application of potash at any time will rapidly show response in the growing crop. Consider applying potassium to fields of alfalfa this spring that did not receive potassium last year, especially if a soil test indicates low potassium. The nutrient, if lacking, will greatly decrease yield. Similarly, on pastures I often see lack of potassium causing the legume to die out and then grass yield declines because there is little nitrogen being fixed.

Improving fertilizer efficiency

One method of improving potash fertilizer efficiency is splitting annual application to reduce luxury consumption of potassium by the plants. While the second application is extra effort, the split application will also cause lowered levels of potassium in the hay which many dairymen desire, especially for dry cows.

Remember to take credit for potassium from manure (this year the potassium in the manure is worth more than the nitrogen). Dairy manure averages about 5% potassium (10 pounds of potassium per ton or 21 pounds per 1,000 gallons). However the potassium is mainly in the urine, so if that is not captured most potassium is lost. You should test your manure to know the potassium content to allow taking full credit for that applied.

Thus while potassium fertilizer costs have increased dramatically in the last couple years, it is still a necessary nutrient for plant growth and must be supplied, especially to alfalfa and clovers for good growth and winter survival. Recycling as much potassium as possible and efficient nutrient management will reduce costs.

--Undersander is a University of Wisconsin Extension and Research forage agronomist.