Is Sulfur Needed for Alfalfa Production ?
Sulfur needed to produce a ton of forage is about 6 lb, which is considerably less than nitrogen, potassium, and calcium; about the same as phosphorus and magnesium; but much greater than the micronutrients boron, copper, zinc, and manganese (Table 1). Phosphorus is generally the most limiting nutrient in North and South Dakota and northwestern Minnesota, but potassium is more frequently limiting in Wisconsin and southeastern Minnesota.
Sulfur deficiencies appear to be more common today than 15 to 20 years ago. Highly sandy, low organic matter soils are known to be deficient in sulfur for good crop production, including alfalfa. Therefore, sulfur fertilization has been a common practice on these soils. But we have found two sites that have 2 to 3% organic matter and loam to sandy loam texture that are deficient in sulfur.
The sites we have found deficient in sulfur for alfalfa have fairly characteristic symptoms, especially during late summer and fall. During the spring, the alfalfa may not show deficiency symptoms due to organic matter breakdown over the winter releasing adequate sulfur for the first crop, but the breakdown is inadequate for later crops. Alfalfa plants with sulfur deficiency generally are stunted (half to two -thirds the normal height), yellowish green to yellowish brown, and have about half to two-thirds the stem density of nor mal alfalfa. In addition, lower lying areas of the field generally were dark-green, taller, and actively growing. My first impression when I found the first deficient site was that the alfalfa may be inadequately nodulated causing a nitrogen deficiency. A test strip of nitrogen fertilizer however had no effect.
Soil test ( 6-inch similar to P and K) for sulfur deficiency is not as reliable as the P and K test s. However, any soil testing less than 20 lb S/acre in the fall is a candidate for possible S deficiency for alfalfa. The most reliable method to determine if this soil is truly S deficient when indicated b y a soil test is to apply a test strip with a sulfur-containing fertilizer like ammonium sulfate. If the site is deficient, the alfalfa color will turn dark green and the density of stems will increase considerably. Remember, alfalfa is a deep tap-rooted crop that may have sulfur available at deeper depths than what is sampled in the soil test and may not respond to sulfur fertilization.
Forage yields were increased by sulfur fertilization on the S-deficient site near Lake Park, Minnesota (Table 2). Forage yield was increased 80 to 88% by sulfur fertilization in 2005 and 41 to 50% in 2006. Forage yields in 2005 would have been greater with sulfur fertilization if more had been applied. We applied 20 lb S/acre, but this was used by the first two harvests and the third was basically deficient. Forage yield in 2006 was not limited by sulfur since a second 20 lb/acre treatment was applied following the second harvest, but it was limited by rainfall. Forage yield of the elemental sulfur treatment in 2005 was higher than the check, but not as great as the other sulfur sources. But in 2006 the yield of the elemental sulfur was similar to other sulfur sources. The additional year to break down the elemental sulfur allow adequate sulfur for the environmental conditions.
Forage quality; crude protein, acid detergent fiber, neutral detergent fiber, hemicellulose, cellulose, and relative feed value; was not significantly affected by sulfur fertilization in the first harvest of 2005.
I suggest producers check their alfalfa fields in the fall for areas that are yellowish green to brown with other areas of the field dark green. These may be just small areas on a hillside or could be the majority of the field like the Lake Park site. If such areas are found, try a test strip with some form of sulfur. You might be surprised with the response. For example, the test strip we tried on alfalfa resulted in the first documented case of sulfur deficiency in sugarbeet in the North Dakota/Minnesota sugarbeet growing area the next year.