One of the major nutrients needed for crop growth is sulfur (S). In the past nitrogen, phosphorus and potassium were considered the major nutrients. Sulfur emissions from coal fired power plants supplied more sulfur than needed for good crop growth. But in my business as an independent crop consultant we have been recommending sulfur for many years now. I started consulting in 2006, and in those days we seldom saw the characteristic yellow striping of sulfur deficiency in plant leaves. Over the years we have seen sulfur deficiency symptoms gradually increase to the point that sulfur deficiency is a great concern.
There are several ways to test for sulfur in the soil, but many universities have questioned the usefulness of a sulfur test. Brookside Laboratories uses a Mehlich 3 extraction to measure soluble sulfur in the soil. Ten to 20 ppm is considered an ideal level. Midwest Laboratories procedures are a little different, but they also say that ideal levels are 10 to 20 ppm. It appears to me that experience plays a role in interpreting sulfur tests. I have found that you are certain to have sulfur-related issues when test levels fall below 10 ppm unless sulfur fertilizer is used.
According to the Sulphur Institute, “Sulphur is one of the 17 essential plant nutrients. It is essential for the growth and development of all crops, without exception. Like any essential nutrient, sulfur also has some key functions in plants:
  • Formation of chlorophyll that permits photosynthesis through which plants produce starch, sugars, oils, fats, vitamins and other compounds.
  • Protein production. Sulfur is a constituent of three S-containing amino acids (cysteine, cystine and methionine), which are the building blocks of protein. About 90% of plant S is present in these amino acids.
  • Synthesis of oils. This is why adequate sulphur is so crucial for oilseeds.
  • Activation of enzymes, which aid in biochemical reactions in the plant.
  • Increases crop yields and improves produce quality, both of which determine the market price a farmer would get for his produce.
  • With reference to crop quality, S improves protein and oil percentage in seeds, cereal quality for milling and baking, marketability of dry coconut kernel (copra), quality of tobacco, nutritive value of forages, etc.
  • It is associated with special metabolisms in plant and the structural characteristics of protoplasm.”
Dr. Shaun Casteel of Purdue University has recently released his research on sulfur use in soybeans. He got a good yield response using AMS (ammonium sulfate) at 100 pounds per acre. He is certain that it is not a nitrogen response, because the beans didn’t respond to urea applications.
Possible sulfur sources include:
  • Ammonium Sulfate – AMS supplies 20 pounds of sulfur per hundred.
  • Gypsum – Good Gypsum supplies 17 pounds of sulfur per hundred, but analysis can vary depend in on source.
  • Ammonium thiosulfate supplies about 3 pounds per gallon. It may be the most expensive but combines well with other liquid fertilizers.
  • Elemental sulfur supplies 90 or so pounds of sulfur per 100 pounds. Elemental sulfur is slow release, so if your soil test levels are low, you may not get good response the first year. It does persist a little better than other sources.
Remember that 60-bushel soybeans remove 21 pounds of sulfur per acre. While 80-bushel soybeans remove 28 pounds sulfur per acre. Sulfur is soluble so applying every year or every other year assures you have enough.

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About the Author: David Rahe

Rahe specializes in soil fertility, site specific fertilizer application, and soil management and is a Certified Professional Soil Scientist, Certified Professional Soil Classifier, and 4R Certified Crop Advisor. Rahe is a graduate of the University of Illinois with a bachelor’s in Agronomy. After a 25-year career with the Natural Resources Conservation Service in Illinois and Missouri, Dave began consulting in 2005. He is currently a partner with RPM Soils LLC and can be reached at