Biological functions:

• Amino acids: cysteine, methionine. organic-N/organic-S in plants typically has a 30-40:1 molar ratio
• coenzymes and prosthetic groups
• glutathionine: antioxidant and precursor to phytochelatins
• alliins in allium (w/ allinase produce pungency in onions) and glycosinolates in brassica (produces isothiocynates) for chemical defenses and S storage.

No toxicity to sulfate beyond salinity, S uptake beyond needs stored in part as sulfate. Deficiency symptoms are yellowing of whole plant, moderately phloem mobile. Response to S fertilizer in many crops and regions. With the reduction of sulfur at the smokestacks, agriculture may be losing the only benefit of acid rain!

Sulfur inputs to soil:

• Precipitation and dry deposition (source: volcanoes, marine aerosols, coal burning)
• Fertilizers (both intentionally used to fertilize for S and incidentally contained in N-P-K fertilizers)

Sulfur outputs:

• Crop removal
• Leaching
• (Volatilization)

Soil Testing:

Water extract for sulfate; occasionally use CaCl₂ or Ca(H₂PO₄)₂ solutions in order to displace sulfate from anion exchange sites (if present). Captures existing sulfate levels, but does not account for sulfate losses due to leaching or accumulation from mineralization of organic-S.

Sulfur Fertilizers

• Potassium magnesium sulfate, K₂SO₄^.2MgSO₄: 22%S
• Sulfur-coated urea: 10-20% S
• Single Superphosphate (13.9%S as gypsum; only 1% S in triple superphosphate)
• Ammonium sulfate, (NH₄)₂SO₄, 24% S
• Elemental Sulfur, S (100%S): oxidizes to sulfuric acid, used also as acidifying agent
• Ammonium thiosulfate (ATS), (NH₄)₂S₂O₃, 26% S, 12% N
• Also Al₂(SO₄)₃, and FeSO₄, also used as acidifying agents

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This page was last modified by Phillip Barak, Univ. of Wisconsin, on 5 Jan 1999. All rights reserved.