Michigan State University- Department of Crop and Soil Sciences
Sulfur (S) is an essential plant nutrient for all plants including forages. Because it is required in smaller amounts than the major elements and in much greater quantities than micronutrients, it is classified as a secondary element.
Most of the sulfur found in the soil is in the organic form and thus concentrated in the plow layer. The form of sulfur available to plants is sulfate (S04 2-), which is formed during the decomposition of soil organic matter by sulfur oxidizing bacteria that are present in all Michigan soils. Since sulfate is an anion, it is not attracted to the cation exchange sites on soils, making it subject to leaching. Because sulfur is subject to leaching, there can be considerable sulfur in the subsoil. Most of the sulfur in the subsoil is tied up with iron and aluminum and unavailable for plant uptake. It should be pointed out that the leaching of sulfur is considerably less than soil nitrates. Sulfur can also be removed from the soil by crop removal. Average crop removal estimated in Wisconsin from grain crops is less than 10 lbs./acre and 20 lbs./acre for alfalfa and corn silage. However, these removal rates can vary from site to site depending on the availability of soil sulfates. Many soil-testing laboratories have methods for evaluating SO4 –2 levels in the soil. The threshold levels for S04 –2 in the soil reported in the Great Lakes area is between 12 and 14 lbs./acre.
Sulfur in Plants
Sulfur is needed in building protein and is an essential component of amino acids. Plants and especially legume forage plants require adequate supplies of S for nitrogen metabolism. Inadequate S in forage plants can result in lower protein, off color, and lower yield. Critical plant tissue levels of sulfur in alfalfa found in Wisconsin research was estimated to be 0.20 % S.
Other Sources of Sulfur
Sulfur is found in livestock manure. The sulfur content varies with the kind, age and feeding programs. One ton of dairy cow manure contains an average of one pound of sulfur. Sulfur is also found in municipal sludge and wastewater. Sulfur is also present in the atmosphere, predominantly as sulfur dioxide (S02). Plants can absorb S02 directly from the air. Research in Wisconsin has shown that as much as 44 percent of the sulfur in alfalfa grown in mid-summer came from atmospheric sources. There are a large number of sulfur fertilizer carriers. A few familiar sources include Ammonium sulfate, Gypsum, and Potassium-magnesium sulfate.
Forage Crop Responses to Sulfur Fertilization
Christenson summarized sulfur fertilization studies conducted in Michigan from 1957 through 1985. He concluded that sulfur was generally not recommended for application to field crops because of rare and inconsistent yield responses. In a survey of 176 fields, only one case showed corn ear leaf sulfur concentration below the critical level. A two- year study with alfalfa-grass hay in Barry County showed no response to increasing sulfur rates. Another 3-year study with alfalfa showed a slight response to sulfur in one out of three years. Studies by Rand, Keeney and Walsh in Wisconsin showed a significant yield response in alfalfa in six of eight locations. With the exception of one site, all sites having significant yield responses to S had less than 0.20 % S in plant tissue. The alfalfa tissue from the check treatment at the two non-responsive sites contained 0.20 and 0.23 % S, respectively. These studies were done on well-drained soils with organic matter contents ranging from 1.1 to 2.8 percent and Sulfate S soil levels from 4 to 7 PPM.
Should we be fertilizing with sulfur for forage crops in Michigan?
Based upon the research conducted through 1984, it would appear there would be little or no response to sulfur fertilization on forage crops in most situations with the exception of course textured soils with low organic matter. However, this research was conducted in the years where there were significant amounts of sulfur deposited sulfur dioxide as a by-product of industrial air emissions. We know the emissions of sulfur dioxide are less today as a result of cleaner airs standards. As we move into higher forage crop yields, we are also removing more sulfur. If one suspects a potential sulfur deficiency, a quick check of the plant tissue should reveal whether there is sufficient concentration of S. Plant tissue level of 0.20 % or above is considered sufficient in alfalfa plant parts 14 inches to bud stage and above 0.19% in the ear leaf at silking time for corn.
Sulfur recommendations are not usually made for forage crops in Michigan, as the above research has not demonstrated a strong need. The limited research does show low levels of sulfur in some soils. In addition, most of the research conducted in Michigan was done prior to 1984. Since then, we have seen significant increases in our corn silage and alfalfa yields. In addition, we now have gone through more than two decades of less emissions of sulfur dioxide because of increased environmental regulations. Perhaps, now is the time to take another look at sulfur research on forage crops in Michigan. In the meantime, plant tissue tests should be done when a crop is suspect of being sulfur deficient. Using test strips on across fields, which are growing high yielding forage crops, may advisable to evaluate supplemental sulfur application only when plant tissue tests indicate concentrations below the critical levels.
- Christenson, D. R. 1986. Summary of Sulfur Studies from 1957-1985. Michigan State University Department of Crop and Soil Science File # 32.332.
- Hoeft, R. G., D. R. Keeney, and L. M. Walsh. 1972. Nitrogen and Sulfur in Precipitation and Sulfur Dioxide in the Atmosphere in Wisconsin. J. Environ. Quality, Vol. 1, no. 2.
- Killorn, Randy. 1983. Sulfur-An Essential Nutrient. Iowa State University Extension Publication # Pm1126. Ames, Iowa.
- Metson, A. J. 1973. Sulfur In Forage Crops. The Sulphur Institute. Technical Bulletin Number 20.
- Rand, R. E., D. R. Keeney, and L. M. Walsh. 1969. Availiability of and Crop Response to Sulfur in Wisconsin. University of Wisconsin. Research Report # 52.
- Reid. R. L., and G. A. Jung. 1974. Effects of Elements Other than Nitrogen on the Nutritive Value of Forage. “Forage Fertilization”. American Society of Agronomy. P. 420-424.
- Rhykerd, C. L., and C. J. Overdahl. 1972. Nutrition and Fertilizer Use. “Alfalfa Science and Technology”. American Society of Agronomy. Number 15. P.455-457.
- Robertson, L. S., M. L. Vitosh, and D. D. Warncke. 1976. Essential Secondary Elements: Sulfur. Michigan State University Extension Bulletin E-997.