Sulfur Fertilizers

Elemental sulfur is the most concentrated form of sulfur. However, it must be oxidized to the sulfate form before plants can use it. Elemental sulfur must be finely ground to particle sizes of 80-100 mesh to be oxidized and effective during the same growing season. This powdered material is difficult to apply as a fertilizer, since it is not soluble in water or liquid fertilizers. Its dust clings to skin and can make the applicator somewhat miserable for a couple of weeks. It is more often used as a soil amendment by using the strong acidifying effect to replace sodium with calcium on high pH alkali spots.

Prilled elemental sulfur is not effective unless finely ground sulfur is formed into granules or flakes using additives that bind the small particles together and disintegrate rapidly after soil application. Binding agents include sodium bentonite, sodium sulfate, calcium sulfate (gypsum), and calcium lignosulfate or combinations of these. These flaked materials will contain about 90 percent sulfur. Finely ground or molten sulfur can be added to anhydrous ammonia or to dry fertilizer during manufacturing. Also, finely ground sulfur can be added to suspension fertilizers.

Ammonium thiosulfate solution (ATS) (12-0-0+26S) is the most commonly used source of sulfur in fluid fertilizer. It weighs 11.5 pounds/gallon. After application to the soil, thiosulfate is decomposed to form approximately equal amounts of sulfate sulfur and elemental sulfur. Thiosulfate is an effective source of sulfur; however, since it also contains ammonium nitrogen, it has a high salt effect on seed and should not be placed with the seed when used as a starter.

Calcium sulfate (gypsum) contains 15-18 percent sulfur. It is found in ordinary superphosphate and is a byproduct of manufacturing concentrated superphosphate. Gypsum is an effective sulfur source, but not popular as a sulfur fertilizer because of the low sulfur content.

Other sulfur sources include fertilizers listed as nitrogen, phosphorus, or potassium fertilizers. Nitrogen-sulfur materials include ammonium sulfate (21-0-0 +24S), ammonium nitrate-sulfate (30-0-0 + 15S), ammonium phosphate-sulfate(13-39-0 +7S), and ammonium phosphate-nitrate (27-12-0+4.5S). Potassium-sulfur fertilizers include potassium sulfate (0-0-50+18S) and potassium magnesium-sulfate (0-0-22+22S). Sulfur is also a carrier for some of the micronutrients such as ZnSO4 and FeSO4.