By Craig Sheaffer, Professor, Department of Agronomy and Plant Genetics, University of Minnesota
Forage crops such as alfalfa remove large quantities of potassium (K) on an annual basis. When manure application is not an option, fertilizer must be purchased to supply K to alfalfa. Potassium can impact plant health, potentially affecting the ability of alfalfa to overwinter. Under-application of K can result in less tons produced per acre. While K fertilizer has historically been cheap compared to the other major macronutrients, supplying removal rates of K to alfalfa annually can result in a significant expense to alfalfa producers.
Research funded by AFREC evaluated forage yield and quality at three locations across Minnesota with diverse climate and soil fertility. Alfalfa stands were harvested for four years beginning in the seeding year.
What does the research say?
Eight modern alfalfa varieties that differed in fall dormancy reaction and high levels of disease resistance had a similar yield and forage quality response to potassium fertilization.
The effect of K fertilization on forage yield varied by location and stand age. Predictably, based on current Minnesota soil test recommendations, the greatest response occurred on soil with initial STK of 35 ppm. There was no response where the initial soil test was 160 ppm.
Forage yields were greatest for 1- and 2-year-old stands and decreased dramatically in 3- and 4-year-old stands. There was no response to K fertilization in 4-year-old stands, and, over all locations, K fertilization did not increase stand or stem density.
Below-ground crown and root mass were increased by K fertilization. Potassium fertilizer reduced forage nutritive value. Reductions in forage crude protein and digestibility are related to decreased leafiness and decreased maturity as forage yield increased.
Application of high rates of K fertilizer to achieve maximum yields will result in luxury consumption of potassium and accumulation in the herbage and roots. High levels of forage K due to luxury consumption may induce parturient hypocalcemia, or “milk fever,” in cows.
Diminishing economic returns on K fertilizer investment are likely if K fertilizer is applied at high rates relative to recommended levels. Despite an increase in K uptake with K fertilization, a net increase in STK was observed after accounting for K removal during harvest, which could be related to the release of K from soil minerals.
What do producers need to know?
Potassium fertilization is important for high forage yields when applied at recommended levels based on soil testing and University of Minnesota guidelines. However, it will not prolong alfalfa stand life or sustain forage productivity as stands age beyond the third production year.
There is no economic or biological advantage for applying fertilizer at rates above those recommended by the University of Minnesota guidelines to achieve high alfalfa yields. Potassium fertilizers should not be applied at levels above those recommended with a goal of accumulating K in the soil.
Potassium fertilization will not increase forage nutritive value but can lead to luxury consumption and K accumulations above those suitable for dry cows.
Intensively harvested alfalfa varieties do differ in forage yield but do not differ in forage yield response to K fertilization. Therefore, producers should select varieties that yield and persist well under their harvest systems and management regimes.