Alfalfa growers in Minnesota typically apply potassium fertilizer twice each year, once during the growing season and once in the fall to promote overwintering.
However, recent research by University of Minnesota scientists indicates that a second fall potassium application has no effect on alfalfa’s winter hardiness.
Their data show no difference between spring alfalfa stands that received a fall K application and those that did not. This contrasts with older information and common knowledge prevalent today.
The researchers attribute their findings to two likely factors:
Modern alfalfa varieties have been bred for greater winter hardiness, so they don’t need as much K for overwintering as past varieties.
Previous research showing a benefit from fall K application was likely done on soils with much lower K levels than is available today.
The U-M researchers recommend testing soils and applying potash once during the growing season at the recommend rates.
The university also provides an online alfalfa calculator to help farmers pick optimal phosphorus and potassium application rates.
Potassium is thought to help plants such as alfalfa resist diseases and aid in overwintering in cold environments, according to Craig Sheaffer, a professor in the Department of Agronomy and Plant Genetics and lead researcher on the study.
Soil testing and K application is especially important for newly seeded alfalfa and younger stands.
Another reason to avoid fall K application is the greater potential for loss via leaching in sandy soils and surface runoff on other soil types.
“With the current economic situation, nobody wants to spend money on fertilizer and lose it to leaching and runoff,” says Dan Kaiser, Extension soil fertility specialist, who participated in the research.
Sheaffer and Kaiser do not recommend K application based solely on crop removal or fertilizing with a goal of maintaining soil K tests above current critical levels of 160-200 ppm. Doing so can result in over-fertilization.
Potassium can be taken up by plants in excess, so applying more potassium fertilizer than the crop needs will result in “luxury uptake” — nutrient removal from the soil without increasing yields.
“You can’t ‘bank’ potassium, so overapplying means a lower return on investment,” Kaiser says. And feeding lactating dairy cows forage with K concentrations greater than 2% can predispose some cows to milk fever, a disease that causes weakness and sometimes death.
Research was conducted at four locations in Minnesota from 2011 to 2014, which included some severely cold winters. The four sites were located near Becker, Rosemount, Lake City and St. Martin. The experiments looked at five rates of K fertilizer and compared yield, nutritive value, K tissue concentration and soil K test values.
The study was funded by Minnesota’s Agricultural Fertilizer Research and Education Council.