Potassium can be a neglected nutrient in forages, especially hayfields. Potassium is needed for many essential plant processes including stomatal opening and closing (regulates water status of plant), winter hardiness, and resistance to plant disease and stress. Fall is a great time to sample pasture and hayfields and apply needed fertilizer such as potash (K2O).
Silage crops are heavy users of K2O, and the stover/stems contain ¾ of the potash. If these fields are not amended with additional K2O according to soil test, subsequent forage crops will be K deficient. Repeated removal of hay crops without K2O replacement results in low to very low soil K2O test levels. Hay crops on these soils will have a diminished response to N, and can even appear nitrogen deficient after N fertilization.
A ton of fescue or orchardgrass hay will remove 17 to 19 lbs. of phosphate (P2O5) per ton compared to 53 to 62 lbs. of K2O. Using 20 and 60 for P2O5 and K2O removal respectively, a three ton hay crop will remove 60 lbs. of P2O5 and 180 lbs. of K2O. Replacement of these nutrients using 19-19-19 would require 900 lbs. of product per acre. Commonly used rates of 200 to 300 lbs. of 19-19-19 per acre would undersupply the K2O needed by 120 to 140 lbs. per acre.
To prevent potash from being limited in your hayfields, get a current soil test and then work with your fertilizer dealer to prepare a blended fertilizer that will supply recommended nutrients. Hay fields that are very low in potash will requires high application rates over time.