Wisconsin alfalfa survey raises concerns
Results of a 2010 survey of alfalfa stands in Wisconsin lends still more credence to the notion that growers in parts of the country need to be on the lookout for signs of sulfur deficiency in their crops.
For the survey, a team of University of Wisconsin (UW) researchers collected 39 plant samples from 35 alfalfa fields in 19 counties throughout the state. Analysis showed that 64% of the areas sampled were deficient in sulfur. Perhaps even more telling, 58% of the samples with a sulfur deficiency were from fields that were considered normal looking.
Carrie Laboski, a UW soil scientist who headed up the project, notes that in a similar Wisconsin survey conducted in 2000–01, sulfur was low in 38% of samples. "The take-home message from our survey isn’t that every farmer needs to start applying sulfur on his alfalfa stands," she says. "Rather, it’s that all growers need to be aware that sulfur deficiency can be an issue preventing them from achieving maximum yields."
Signs associated with low sulfur include a light green color, stunted and spindlier plants with delayed maturity and/or thin stands. For growers who suspect sulfur deficiency is holding back their alfalfa yields, Laboski recommends submitting tissue samples to a testing laboratory.
Samples can be gathered by snipping the top 6" of 30 to 40 plants in the late bud to first flower growth stage. Identifying a sulfur deficiency on the front end of the growing season allows you to make fertilizer applications earlier.
Test results will determine how much sulfur, if any, needs to be applied. For example, if the test shows less than 0.23% sulfur, most recommendations call for applying 25 lb. of sulfur in a sulfate form like gypsum (calcium sulfate), potassium sulfate or ammonium sulfate to correct the deficiency.
If a sample tests above 0.25%, applying sulfur isn’t necessary. With tests indicating sulfur of 0.23% to 0.24%, growers might want to limit sulfur application to a 10'x10' area to test the crop’s response to the fertilizer. "Depending on soil types and moisture conditions, it could take four weeks or so to see a response [greener, taller plants] to the application," Laboski says.