Sulfur Management In Rainfed Areas of North America.

Sulfur management in rainfed areas of North America has changed in recent years due to less atmospheric deposition of S, higher crop yields which increase total S requirements, and less tillage with subsequent slower S mineralization from organic matter during spring.  The objectives of this presentation are to (1) briefly review reasons for increased S deficiencies; (2) identify various soil, crop and climatic situations associated with S deficiencies; and (3) discuss S fertilizer sources that are being utilized to prevent S deficiency in agronomic crop production.  Atmospheric deposition in the eastern areas of North America have decreased from >27 kg SO₄^2- ha^-1 in 1985 in many areas to <15 kg SO₄^2- ha^-1 in 2008.  Corn, wheat, soybean and cotton yields have increased greatly during this period with a resulting increase in S requirements.  Tillage has decreased significantly during 1985 to 2011 with the result that less organic matter is mineralized in surface soils and more S is being retained in the soil.  In addition, soils under no-till or minimum tillage systems warm more slowly in the spring with a subsequent early season reduction in S availability to plants.   For these reasons, S responses have been seen on winter wheat more often than on summer annual crops such as soybeans. Sandy textured soils with low organic matter are more susceptible to S deficiencies due to lack of organic sources and greater leaching.  Sulfur fertilization of cotton, corn and winter cereals on Southeastern Coastal Plain soils is an increasingly standard practice while more areas are receiving S in the Corn Belt and Delta regions.  Sulfur fertilizer sources include ammonium sulfate (dry granular and liquid), ammonium thiosulfate, elemental S, and S supplied as both sulfate and elemental S in phosphate fertilizers.  These fertilizers are applied broadcast preplant, in starter-bands with N and P, and with N as top-dress or side-dress applications.