Yellowing plants in second-year Nebraska alfalfa fields this spring may be short on nitrogen, says Bruce Anderson, University of Nebraska Extension forage specialist.
“What’s happening is the alfalfa is not forming nodules to make nitrogen because the soil is acid or the seed wasn’t inoculated,” says Anderson.
He’s received several calls about alfalfa that isn’t doing well, and says some of the fields have several things in common. The crop was planted last year, and if it was a spring seeding, didn’t grow vigorously in 2010. In most cases, alfalfa hadn’t been grown there for many years, no soil test was taken and the yellowing is variable across the field.
“If the alfalfa is yellow only in low areas, the problem can be phytophthora root rot,” says Anderson. “But what I see more often is a nitrogen deficiency.”
Alfalfa needs a soil pH above 6.2 and adequate inoculum to form nodules, he explains. Surface soils are becoming more and more acid because tillage and nitrogen fertilizer increase soil acidity, and lime is needed to neutralize it. Even if soil pH is acceptable, the correct type of Rhizobium bacteria also are needed. Fields that have not grown alfalfa for several years often need more bacteria added with the seed.
“Remember, soil testing, lime and inoculants pay when needed,” says Anderson.