Northeastern and Midwestern hay growers should watch for signs of sulfur deficiency in their alfalfa fields this spring, in part because of the lack of acid rain.

So says Dan Undersander, University of Wisconsin Extension forage specialist.

“The world has changed, and where we didn't have to apply sulfur in the past, now an awful lot of people do,” he says. Recent environmental efforts in more-industrialized areas have resulted in less acid rain, which at least had the side benefit of adding sulfur to soil.

Western hay growers, on the other hand, may get needed sulfate from irrigation water. Yet any grower with yellowing alfalfa plants may want to consider sulfur deficiency as a possible culprit.

A lack of sulfur can stunt alfalfa plants as well as make them turn color. But yellowing is also a symptom of several other problems, including aphanomyces, phytophthora root rot, low soil pH — even a lack of nitrogen. So Undersander recommends a plant tissue test or a soil test to confirm or rule out sulfur deficiency, because either test is a less-expensive option to testing for diseases.

Plant tissue tests, he says, are the most accurate. “Soil testing is an indicator of the potential of sulfur deficiency, but it is not precise because it does not take into account sulfur from multiple sources. I would recommend tissue testing.”

For accurate tissue-test results in any crop, sample at the correct stage of growth, says Undersander. He and other forage specialists recommend sampling alfalfa at mid-bud to early flower. Be sure to sample younger tissue, because sulfur moves from older to younger tissue.

“In the Midwest, we recommend sampling the top 6” of plants within a week prior to harvest. With corn and soybeans, you only have one shot in a year. But with alfalfa, because of multiple cuttings, you can do it potentially every time you cut. So one advantage to tissue-testing alfalfa is that you get more chances and a little wider range of sampling times.”

University of California experts similarly advise sampling right before harvest, when plants show regrowth shoots. They recommend clipping 40-60 stems for each sample, from at least 30 plants, just at their base near the soil. If comparing good growth to poor growth, collect 40-60 stems from each area. Take the middle third of the stems for sulfur analysis.

After that middle-third sample has dried down, the leaves are rubbed and separated from the stems and can be analyzed for sulfur while the mid-stem part can be checked for phosphorus and potassium. The top third of the plant can be analyzed for boron and molybdenum levels. Potassium is the only nutrient a tissue test does not measure.

Western forage specialists advocate taking bale cores and submitting them for tissue testing as a convenient way of gathering samples. (See “Tissue Tests Measure More” at tinyurl.com/tissuetest and “Look At Alfalfa Nutrient Needs Carefully This Spring” at tinyurl.com/nutrientneeds.)

Analyzing hay samples may be slightly less precise than analyzing younger tissue, but it will still indicate deficiencies if they exist, says Undersander.

“The key thing is to identify if you have sulfur issues and then consider what your crop needs. Once you bring sulfur back to a minimum level, and if you're not meeting that level from manure or other applications, then you should consider an annual fertilization,” he concludes.

Maps: National Atmospheric Deposition Program