It’s about time to get serious about the use of sulfur in forage fertilizer programs. That’s because government efforts at reducing air pollution have resulted in removing much of this essential nutrient from precipitation. That’s good for air quality but not for the sulfur status of cropland. A generation ago there was so much sulfur in precipitation that acid rain was big news and some Adirondack and New England lakes were reported to be “dying”. We don’t hear much about acid rain now because the amount of sulfur in precipitation is a fraction of what it used to be. Look at the accompanying maps showing sulfate depositions in the U.S.; pay particular attention to the Northeast and upper Midwest. Between 1985 and 2008 sulfate depositions in much of the Northeast decreased by over half and are now under 10 lbs/acre, which is less than crop removal for many crops and particularly for alfalfa. Four tons of alfalfa per acre removes about 20 lbs of sulfur, much more than is now contained in annual precipitation. Corn harvested for silage removes about one pound of sulfur per ton of silage; using corn silage yields on your farm, you do the math.
This coming crop season you should be on the lookout for light green (chlorotic) areas in your alfalfa fields. These would most likely occur in patches rather than over the entire field. If you see these areas, get a tissue analysis done on both the normal-appearing and chlorotic areas. If the affected areas test less than 0.25% S, apply a sulfur-containing fertilizer such as calcium sulfate, potassium sulfate or sulfate of potash magnesia (sul-po-mag). Calcium sulfate is the cheapest source of sulfur at about $0.60 per pound of S. If you also need potassium, the sulfur in potassium sulfate winds up costing about $0.55 per pound. If sulfur is confirmed to be deficient in corn or grasses, ammonium sulfate is an excellent choice, supplying both nitrogen and sulfur. However, don’t apply ammonium sulfate to alfalfa since the alfalfa doesn’t need the N. Manure contains some sulfur, so you’re more likely to encounter sulfur deficiencies on fields that don’t get much (or any) manure. Some soil test labs include sulfur in their standard analysis. However, rely on tissue analysis to determine the sulfur status of your crops because there’s not a good correlation between soil and tissue status for sulfur. Forage analyses may be useful as an early-warning sign, but because forage analysis represents all the harvested portion of the alfalfa plant while tissue analysis uses only the top 6”, a low forage analysis sulfur level should be confirmed by a tissue analysis.
What’s at stake? Midwest university agronomists found yield increases of over 50% when they fertilized sulfur-deficient alfalfa (tissue analysis: 0.16% S) with 24 lbs of sulfur/acre, costing about $14.00. That’s a big return for a modest investment!
The Miner Institute Farm Report is written primarily for farmers and other agricultural professionals in the Northeastern U.S. and Eastern Canada. Most articles deal with dairy and crops topics, but also included are articles dealing with environmental issues and global agriculture as well as editorial commentary.
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