Alfalfa removes large amounts of potassium (K) from the soil. Soil potassium levels impact plant health and can improve the ability of alfalfa to overwinter. If there is too little potassium, the stand will produce less tons per acre. Manure application can help replenish K in the soil, but even then additional potassium fertilization is often needed.
A study sponsored by the Agricultural Fertilizer Research and Education Council evaluated the effect of K fertilization on alfalfa stands across Minnesota with diverse climates and soil fertility. The alfalfa was harvested for four years beginning in the seeding year.
Craig Sheaffer, Jake Junger, and Dan Kaiser, plant and soil specialists at the University of Minnesota, explain in a recent article about the results of the study and what alfalfa producers need to know.
“The effect of K fertilization on forage yield varied by location and stand age,” the specialists say. Stands with a lower initial soil test K levels had the greatest response while those with higher levels had little to no response.
Yields were best for stands between 1 and 2 years old and were drastically lowered in 3- and 4-year-old stands. “There was no response to K fertilization in 4-year-old stands,” the specialists report.
Across all locations, K fertilization did not improve stand or stem density, but below crown and root mass was increased with the fertilizer.
According to the specialists, K fertilizer did not boost forage quality but instead reduced crude protein and digestibility in the forage. Additionally, if K fertilizer is applied at higher rates than the recommended levels, it can lead to luxury consumption and K accumulation; this can affect cattle health. High levels of forage K lead to parturient hypocalcemia, more commonly known as milk fever, in dairy cows.
Overall, potassium fertilization will improve forage yields when applied at the recommended levels based on the soil test. However, it will not necessarily prolong alfalfa stand life or sustain forage productivity as the stand ages.
Alfalfa varieties differ in yield potential but not in their response to potassium fertilization. There is no economic or biological advantage to applying fertilizer at rates above what is recommended, so knowing soil fertility levels and fertility appropriately can save money and help achieve maximum forage value.