Alfalfa growers seeing light-green, stunted and spindly patches – or entire anemic-looking alfalfa fields – may want to apply sulfur this spring.

It could increase yields by up to 1 ton per acre per cutting, according to Iowa State University (ISU) research.

Or it may not, says Carrie Laboski, the University of Wisconsin soil scientist who recently analyzed alfalfa plant samples from around her state.

“Out of 39 samples that came in, 64% of those were low in sulfur (S), and that was from 19 counties,” she says. “It’s a limited survey, but sulfur deficiency is showing up quite a bit.”

At the same time, a third of the fields tested didn’t need any S, so Laboski recommends that growers monitor fields, send in plant samples for tissue analyses and possibly even soil-test problem areas.

They may want to check for potassium (K) deficiency while they’re at it, she recommends.

Laboski studied 31 normal-appearing and eight abnormal-looking samples from around the state in 2010. Of samples showing no symptoms, 58% were low in S and 45%, low in K. At least seven of the eight samples exhibiting a light green color and stunting lacked S and six needed K.

S concentrations from all tissue samples ranged from 0.11 to 0.34%, while K concentrations were 1.5-3.16%.

Alfalfa with tissue S levels at 0.25% are considered low and, at less than 0.23%, deficient. Plants with K concentrations of less than 2.25% are deficient, Laboski says.

Growers have hesitated to apply K at recommended rates because they’re not sure it would benefit them economically, she says. “But if we keep drawing down soil-test K levels in these hayfields, we’re going to have production problems in our other crops. And maybe even with alfalfa later.

“I would say sulfur is a bigger immediate issue on more fields, but we don’t know that for certain, and people still need to do some plant sampling to verify. If plant samples are coming back with less than 0.23% sulfur, there’s probably a good chance that applying sulfur will be profitable,” she adds.

Iowa State work backs that up, says John Sawyer, an ISU Extension field agronomist who, with colleague Brian Lang, studied problem silt loam and loam soils several years ago in the northeastern part of that state.

“The eroded, higher-landscape areas would show more deficiency and the lower draws would not,” Sawyer says.

In 2005 research, S was applied at 40 lbs/acre after first cut to three sites that had sported poorly looking, light-colored alfalfa. The nutrient doubled yields compared to those of alfalfa in a similar unfertilized site, Sawyer says.

Second- and third-cutting dry-matter yields averaged 2.76 tons/acre after an ammonium sulfate application and 2.49 tons/acre after calcium sulfate was applied during the 2005 trial. The unfertilized site averaged 1.18 tons/acre. One-ton first-cutting yield increases from S applied the previous year were also measured the next year.

Since that research, alfalfa growers in that region are managing for S deficiencies, Sawyer says. “It’s an ongoing thing. The tissue test is very useful for alfalfa.”

“Our major alfalfa-growing regions probably suffer a lot of the same problems” that Wisconsin is enduring, says Dan Kaiser, University of Minnesota Extension nutrient management specialist. “Alfalfa tends to be a high sulfur-demanding crop. As organic matter decreases, the potential sulfur that can mineralize also decreases, increasing the odds there will be a deficiency.”

U.S. sulfur deposition, shown in orange in the 1985 map, has been drastically reduced due in part to acid rain cleanup. Hay growers should now tissue test for sulfur, soil fertility experts say. Maps: National Atmospheric Deposition Program

S deficiency is on Penn State forage management specialist Marvin Hall’s mind, too. “Sulfur levels are dipping,” he says. “It’s something we need to be looking into.”

A large-scale national fertility study confirms that S deficiency is increasing, particularly in the western Corn Belt and central Great Plains.

The Fertility of North American Soils, 2010, published by the International Plant Nutrient Institute (IPNI), formerly the Potash and Phosphate Institute, reports soil-test results. Soil fertility specialists generally feel, however, that plant tissue testing provides more accurate S results.

The IPNI summary reports K removal “far in excess” of additions in the western Corn Belt and much of the Great Plains. Northeastern areas also had significant K declines. Soils in states east of the Mississippi River, as well as eastern Canadian provinces, “likely require annual K application to avoid yield losses,” the study adds.

“Growers may not be seeing much of a production issue in their hay crops,” says Laboski about the lack of K. “But a few consultants told me that they’ve seen more problems in first-year corn. Alfalfa actually has a greater need for potassium than corn, so I’m not exactly sure what’s happening. It may be related to root depth and type of root system.

“But the real issue is that if you start drawing down soil-test K levels too low, you’re going to have problems with production of alfalfa and your whole crop rotation.”

Laboski suggests that growers check S levels using plant tissue analysis and, while they’re at it, soil test to check K.

“To take plant samples, take the top 6” from one stem off each of 30-40 plants at bud-to-first-flower stage before any cutting. Although I think that growers, if they suspect S deficiency, would want to sample earlier in the growing season so that they have more time to apply fertilizer and realize a yield benefit,” she says.

Air-dry plant samples, then loosely pack them in a paper bag, seal the bag and mail it to an accredited lab.

S levels have been decreasing in part because less sulfate-S is being deposited through acid rain due to air-quality cleanup efforts. The high cost of fertilizer is likely keeping growers from adequately fertilizing with K, Laboski surmises.

After determining if and where S is deficient, she suggests applying 15-25 lbs/acre in a sulfate form – as potassium sulfate, ammonium sulfate, calcium sulfate or sul-po-mag, a combination of sulfur, potassium and magnesium.

“If they also need potassium, even though potassium sulfate can be a little more expensive, that might be a way to go.”

Elemental S is another option, but it isn’t immediately available to the crop so its benefits are limited, she says.­­­