A balance of phosphorus and potassium helps keep alfalfa stands healthy, according to recent research at the Throckmorton-Purdue Agricultural Center.

“Alfalfa removes more potassium than any other field crop with the exception of corn silage, so growers need to pay close attention to potassium levels,” says Jeff Volenec, Purdue extension crop physiologist. “An imbalance between potassium and phosphorus fertilizer can actually be more damaging when you apply phosphorus and do not apply potassium. The stand thins out more quickly, which results in added weed encroachment.”

Keith Johnson, Purdue extension forage expert, notes that soil tests done in spring do not accurately reflect the amount of potassium available to plants. If a soil test was not done last fall, Johnson advises doing so after first harvest. A list of certified commercial laboratories is available at www.agry.purdue.edu/ext/soiltest.html.

“It’s important to have a soil test because serious consequences, such as milk fever in dairy cows and grass tetany in beef and dairy herds, can occur from applying too much potassium,” Johnson explains.

Potassium and phosphorus applications should be split – apply half after the first harvest and half after the last harvest so the alfalfa doesn’t take up more potassium than it needs. Anything more than 2% potassium in the forage on a dry-weight basis is a waste of money and potentially dangerous to grazing livestock, Volenec adds.

For highly productive Indiana fields, apply a total of 250 lbs/acre of potassium and 25 lbs/acre of phosphorus. “Growers may notice a slight reduction in forage quality, because high-yielding alfalfa is slightly more stemmy,” he says. “But it’s important to understand the yield advantages far outweigh this slight reduction in quality. Quality is a low-tier issue with well-managed alfalfa. It’s really about high yield and stand persistence. Quality will follow hand in hand if your fields yield well and have good persistence.”

At the very least, growers should replace potassium and phosphorus removed from fields during the summer. “For example, say a grower harvests 5 tons/acre of hay from a field this summer, which is 10,000 lbs, with 2% potassium in the tissue,” Volenec explains. “This results in the removal of 200 lbs of potassium. At a minimum, this grower should apply 200 lbs/acre of potassium, then have the soil tested in the fall and make the necessary adjustments.”

Volenec and Johnson recommend a soil test be taken every three to four years to make sure the plants have adequate nutrients available.

“With good balanced nutrition, we’ve had stands persist for six to seven years easily with really good yields,” Volenec says.

Detailed information about fertilizing alfalfa with these macronutrients is available at www.ces.purdue.edu/extmedia/AY/AY-331-W.pdf.