Potassium can make a case for the 'Don't get no respect' award among the fertilizer nutrients. Nitrogen (N) gets most of the attention because of its showy results; nothing perks up a hay or pasture field faster. Calcium (Ca) and phosphorus (P) get lots of attention as 'bone makers' for our signature thoroughbreds. Even P gets the 'no such thing as bad press' award with its problematic relationship with water quality.
Lowly potassium just does not get any respect. Yet, after N, no nutrient is needed in greater amounts for hay or pasture. Essential for proper water relations, disease resistance and even winterhardiness, potassium is crucial for healthy plants. But when it comes to getting a little love in the fertilizer buggy, K is usually out of luck.
Don't fall into the trap of underapplying K to your hay fields. With hay, almost 100% of the minerals present in a hay crop are removed from the field. Hay crops remove three to four times as much K as P (K2O vs P2O5 forms). Kentucky soils cannot replace the K as fast as it is removed, and continuous haymaking without adequate nutrient replacement will severely reduce the available K in the soil. A soil test is the only way to know how much fertilizer is needed.
In the fall of 2017, I did a farm visit to a small farm whose owner was not pleased with the production from his hayfield. A quick look at the field revealed a whole lot of broomsedge and little of the cool season grass he was hoping to see. His UK County ANR agent had him pull a soil test, and the results were good for every measure except K, which was in the 'Very Low' category by UK's standards.
The very low K numbers were due to fertilizing only with 200 pounds of triple 19, which delivers 38 lb each of N, P2O5 and K2O per acre, and even this was not done every year. The field was cut for hay every year for at least a decade. So this field was grossly underfertilized for K every year, understandably resulting in low potash fertility.
The farmer agreed to let us conduct a replicated trial on this field, starting in the spring of 2018. Fertilizer treatments included all combinations of 180 lb of N, 45 lb of P2O5, and either 180 or 360 lb of K2O. These were compared to a 38-38-38 (N-P2O5-K2O) and 0-0-0 treatment, randomized and replicated four times.
The results were frankly dramatic. By the end of the first year, the proportion of broomsedge in the plots went from 89% to 14% by adding N, P and K. Annual yields for the plots received N, P and K were over 2 tons per acre greater than the unfertilized plots. Potash addition alone was responsible for half of the yield increase.
Fertilization had an unexpected benefit. Plots receiving P and K but no N had a marked increase in percentage red clover (visual basis). This clover was all volunteer, since no red clover had been overseeded on this field in over a decade.
There are many takeaways from this trial. First, proliferation of broomsedge is not only a low pH issue, it can also be due to low K levels. Second, low K will prevent N fertilizer from having its full effect. Third, getting P and K fertility up on unimproved forage fields can greatly stimulate clover even when it has not been seeded recently.
Yes, the levels of fertilizer used were high, but not unreasonable. Yes these fertilizer rates would be expensive, but not more expensive than killing a field and starting over. And the effect was seen in the first year.
For more information contact the Pulaski County Extension Office.