⇦ Back to Fertilizer Lime Amendment Technology and Use Home
Sulfur-containing materials are generally used to supply fertilizer nutrients, but are also used as soil amendments to correct sodium problems and lower soil pH. Fertilizers containing sulfur in the sulfate form are quite soluble. Plant roots take up sulfur as the sulfate ion (SO4=), so sulfate fertilizers are immediately available when dissolved in the soil water.
Sulfur fertilizer sources, including sulfide, elemental sulfur, and thiosulfate, require microbes to convert the sulfur into a sulfate form before it becomes plant available. Thiobacillus bacteria are the primary organisms involved in this conversion process. They require free oxygen and water to complete the reaction. This conversion process usually creates acidity that can be useful for amending soil.
Table 1 lists some common sulfur fertilizers and basic properties.
Ammonium sulfate is very soluble, providing both nitrogen and sulfur. Sulfur is in the sulfate form - immediately available upon dissolution of the fertilizer granule.
Ammonium sulfate creates acidity by forming nitric and sulfuric acids (HNO3, H2SO4) during the nitrification process (ammonium converting to nitrate) and by dissociation of the sulfate ion - shown in equations 1, 2, and 3. The sulfate ion itself does not contribute directly to soil acidity, but may combine with the basic ions - calcium and magnesium - and carry them away during leaching.
1. (NH4)2SO4 + 4O2 ➔ Thiobacillus ➔ 2HNO3 + H2SO4 + 2H2O
2. 2HNO3 ⇌ 2H+ + 2NO3-
3. H2SO4 ⇌ 2H+ + SO4=
Ammonium thiosulfate (ATS) is a liquid fertilizer manufactured by reacting sulfur dioxide, elemental sulfur, and aqueous ammonia. A similar process is
used to produce thiosulfate fertilizers containing potassium, calcium, and magnesium.
ATS can be mixed with UAN solution, ammonium polyphosphate (10-34-0), and other liquid fertilizers. ATS has a density of about 11.1 lb/gal and a pH of 7.0 to 8.5. When mixed with 10-34-0, the solution pH will be between a pH of 5.8 to 7.0.
ATS is a “reducing agent,” creating acidity through oxidation of both the nitrogen and the thiosulfate ion. The thiosulfate molecule is converted tetrathionate, then to sulfate, then is is plant available. Equations 4, 5, and 6 describe the reactions that release hydrogen ions and create acidity.
4. (NH4)2S2O3 + H2O ➔ 2NH4+ + S2O3=
5. 2NH4+ + 3O2 ➔ 2NO3- + 8H+
6. S2O3= + O2 + 2H2O ➔ 2H2SO4 ➔ 4H+ + 2SO4=
The equations show a total of 12 hydrogen ions are generated from the transformation of one ammonium thiosulfate molecule: eight from nitrification and four from sulfur oxidation.
ATS must be placed at least two inches from the seed if used in a starter or pop-up fertilizer. It should not be placed in direct seed contact because it can inhibit germination especially when mixed with ammonium polyphosphate (10-34-0).
ATS is often mixed with nitrogen solution for application through sprinkler systems. It should not be applied directly to foliage unless diluted with irrigation water.
We suggest that no more than 3.5 gallons per acre (about 10 lb. S/ac) be applied in a single application. Heavier application rates of ATS have been observed to cause foliar injury appearing as lateral brown bands on corn leaves, particularly in sandy soils
ATS applied in irrigation water has been suggested to help offset the effect of sodium or to relieve iron deficiency chlorosis symptoms when banded in problem soils, but the effect has not been consistent.
Elemental sulfur (S) materials can range from 80% to 99% pure sulfur, but common agricultural fertilizers are 90% sulfur. Elemental sulfur (99% S) is often finely ground, mixed with a binding agent, then granulated to produce other fertilizers.
Elemental sulfur should be treated as a slow-release sulfur product because it may take weeks or months for it to be completely dissolved. It can lower soil pH quickly in very sandy soils because of their low buffering capacity.Pure elemental sulfur is insoluble, so the Thiobacillus bacteria must convert it to sulfate through chemical oxidation before it becomes plant available.
7. 2S + 3O2 + 2H2O ⇌ Thiobacillus ⇌ 2H2SO4 ⇌ 4H+ + 2SO4=
8. 2H+ + CaCO3 ➔ Ca++ + CO2 + H2O
This reaction creates acidity by forming sulfuric acid (H2SO4) which can then dissolve the carbonates found in calcareous soils. The carbonate is released into the atmosphere as carbon dioxide, while the calcium ion remains behind.
Elemental sulfur can be used to acidify soils planted with agricultural or horticultural crops. This may be too expensive for use on large areas of low to moderately-priced crops. Applying elemental sulfur has been successful at eliminating iron chlorosis problems in turf, but requires caution. As the sulfuric acid neutralizes calcium carbonate, it forms calcium sulfate which can affect salt-sensitive plants, like bluegrass.
This reaction can also be used to help treat sodic soils by providing a source of soluble calcium that can replace the sodium accumulated on the cation exchange sites.
Gypsum (calcium sulfate) is an effective sulfate source for plants and is also used in treatment of sodic soils. Microbial transformation is not necessary as it is immediately available for plant use when dissolved by water. It is a neutral salt, having no direct effect on soil pH.
Agricultural grade gypsum is typically a byproduct of wallboard manufacturing. It is available in a finely ground form and in granulated or pelletized form.
Potassium-magnesium sulfate is a combination of two very soluble salts. This product is typically called “Sul-Po-Mag™” or “K-Mag™”.
It is processed into a fertilizer grade material from an ore called “langbeinite”. The sulfur is in a sulfate form, requires no transformation, and does not affect soil pH
KMgS should be applied as close to planting as feasible because the sulfate could be leached by rainfall.
Ammonium polysulfide is produced by reacting aqua ammonia with hydrogen sulfide (H2S) using heat. It can be injected directly to the soil, metered into open-ditch irrigation systems, or can be mixed with certain other fertilizers, including anhydrous ammonia or ammonia solutions. Ammonium polysulfide is not completely compatible with liquid fertilizers that are highly acidic or have a high salt concentration.
¶ Table 1. Common Sulfur Fertilizer Materials |
||||||
| Name | Grade | Form | Chemical Formula | |||
| % N | % P2O5 | % K2O | % S | |||
| Ammonium sulfate | 21 | 0 | 0 | 24 | Dry | (NH4)2SO4 |
| Ammonium thiosulfate | 12 | 0 | 0 | 26 | Liquid | (NH4)2S2O3 |
| Elemental sulfur | 0 | 0 | 0 | 80 – 90 | Dry | S2 |
| Gypsum | 0 | 0 | 0 | 15 – 18 | Dry | CaSO4 |
| Potassium-magnesium sulfate | 0 | 0 | 22 | 22 | Dry | K2SO4 • 2MgSO4 |
| Ammonium polysulfide | 20 | 0 | 0 | 40 – 45 | Liquid | (NH4)2Sx |