⇦ Back to Fertilizer Lime Amendment Technology and Use Home
Elemental sulfur can be applied as a soil amendment to acidify a soil and lower soil pH. Application rates depend not only on the soil pH, but also on soil texture and other characteristics. Differences in soil characteristics can result in differences regarding speed of reaction, amount of pH decrease per unit of sulfur applied, etc. Routine soil testing is important to monitor the success of the acidification process.
Elemental sulfur has to be converted or “oxidized” by soil microorganisms to sulfuric acid to reduce soil pH. It is important to note that the process of sulfur oxidation is a biological process (slow) and not a chemical reaction (rapid).
The basic acidification reaction can be described as:
elemental sulfur + oxygen + water => Thiobacillus bacteria + time => sulfuric acid => hydrogen ions
2S0 + 3O2 + 2H2O => Thiobacillus => 2H2SO4 => 4H+ + 2SO4-2
The hydrogen ions (H+) released during these reactions create acidity, causing soil pH to decrease. The rate at which soil pH will decrease is related to the microbial activity, application rate, and the fineness of the sulfur materials.
“Thiobacillus” refers to a species of bacteria that are largely involved in sulfur oxidation (conversion of elemental sulfur to sulfate). These bacteria are aerobic microbes, requiring oxygen to survive.
Thiobacillus and other bacteria are active when the soil is moist and warm (above 50° to 55°F). Microorganisms are not active in the winter, so fall or winter applications of elemental sulfur will have little effect on the soil pH until the following spring when microbes resume activity. Full oxidation of the elemental sulfur may take several months even with soil temperatures of 70° to 85°F.
The soil should be irrigated to maintain soil moisture, but do not overirrigate. If elemental sulfur is applied to saturated or flooded soils, the anaerobic bacteria that survive convert the elemental sulfur to hydrogen sulfide (H2S, “rotten egg” smell). Hydrogen sulfide can kill plant roots.
“Flowers of sulfur” are finely-ground particles of elemental sulfur (99% S). The fine particles have a large surface area and react more rapidly than coarser particles. These powdery products can be difficult to handle and apply on a field basis.
Prilled or pelleted fertilizers are manufactured by combining finely ground elemental sulfur with bentonite clay for ease of application. The clay expands as it absorbs moisture and the pellet breaks apart to disperse the fine sulfur particles. These fertilizer products typically have a 90% sulfur content (e.g., Tiger Sulfur™ 90® or Dispersul™).
Gypsum is the common term for calcium sulfate (CaSO4 • 2H20). It will not acidify soils. Gypsum dissolves in the soil and dissociates into calcium and sulfate ions (Ca+2 and SO4-2), but does not produce the hydrogen ion (H+) necessary for acidification.
Gypsum is applied to soils with excess sodium to improve soil structure and permeability. These soils typically have a pH of 8.0 or greater. The elevated pH is due to formation of sodium carbonate and sodium bicarbonate salts (Na2CO3, NaHCO3).
The pH may decline once the sodium ions are displaced by the calcium ions supplied in the gypsum. The change of soil pH is due to eliminating the sodium salts, not an acidifying process. There would be no significant change of soil pH if gypsum is applied to soils that are not impacted by excess sodium. The pH may increase if the soil sodium accumulation is an ongoing problem.
Sulfate ions (SO4-2) have no effect on soil pH, but a common misconception is that sulfate-sulfur fertilizers can be used for acidification. These products include gypsum (CaSO4), potassium sulfate(K2SO4), and potassium-magnesium sulfate (K2SO4•2MgSO4; K-Mag™ or Sul-Po-Mag™).
There might be a very slight drop in soil pH if these fertilizers are applied at high rates. This is due to a slight increase in soil salinity from the fertilizer that can affect the internal operation of the pH electrode, but this is not true acidification.
Other materials can acidify soils; like ammonium (NH4+) or thiosulfate (S2O3-2) fertilizers, ferric sulfate, or peat. The soil reactions are different than those for elemental sulfur and the required rates are significantly higher.
Sulfuric acid can be used effectively for reducing soil pH as it does not rely on microbial oxidation. It is generally much more expensive, is unpleasant to handle, and is very corrosive.
The recommended rates in Tables 1 through 4 are calculated to provide a base application rate. The rates are intended to adjust the soil acidity to the desired end-point for a 12-inch soil depth in a non-calcareous soil. The rates increase as soil clay content increases because of increased reserve acidity potential. This rate also assumes no free carbonates and soil
organic matter below 4%.
The routine soil test cation exchange capacity (CEC) by summation calculation can be used as a rough estimate of the soil texture. Note that the calculated CEC may overestimate the actual clay content if the soil contains high excess lime (I.e., reactive, free carbonates).
Elemental sulfur should be incorporated into the soil to increase the speed of oxidation. Soil pH adjustment does not happen quickly, so give the material time to react. It may take a few months or longer to change the soil pH to the desired level due to microbial activity required to convert the elemental sulfur. The soil pH adjustment should be monitored with routine soil sampling and analysis. This will assure that the applied sulfur is having the desired effect.
Soluble salt levels may increase due to formation of the sulfate ion during sulfur oxidation. Soil pH and soluble salts should be sampled routinely to monitor the progress of acidification.
Avoid “over acidification” with higher application rates. Make the first application at one-half or one-third of the total requirement to lower the pH gradually, perhaps over two to four years. Check the soil pH routinely, then apply the remaining requirement over time as needed.
Excess carbonates, high organic matter, or irrigation water can affect the elemental sulfur application rate.
Carbonates: Calcareous soils contain reactive carbonates; an excess lime rating of “LO” or “HI”. Additional sulfur over and above the base requirement will be required to neutralize the carbonates.
When elemental sulfur is fully converted to sulfuric acid, it will react with about three times its applied weight of soil carbonates. Thus, to neutralize a soil that contains 2% carbonate, could require about six tons of sulfur per acre to neutralize the carbonates alone. Then additional sulfur would still be needed to make the intended.
Organic matter: Soils with an organic matter content of 4% or more require sulfur application over the base recommended base rate. Suggested rates are 500 lb. S/ac. for each additional 1% organic matter per each additional pH unit to be reduced.
Irrigation water: Some irrigation waters contain significant bicarbonate (HCO3-) concentrations which tend to neutralize a portion of the soil acidity. Maintenance sulfur applications may be needed to offset the effect of the bicarbonate applied through irrigation.
The following set of formulas can be used to calculate the estimated elemental sulfur application rates for acidifying a soil to a final target pH value.
Factor_1 = (CEC * 19.7) - 19.8
Factor_2 = (CEC * -321) + 277
Factor_3 = (CEC * 53.2) - 991
Factor_A = CEC * -18
Factor_B = (CEC * 302) + 5
Factor_C = (Factor_1 * pH_end * pH_end) + (Factor_2 * pH_end) + Factor_3
SRec_base = (Factor_A * soil_pH * soil_pH) + (Factor_B * soil_pH) + Factor_C
SRec_adj = (OM * 500) + (XSL * 650), IF OM% > 4
SRec_final = (SRec_base + SRec_adj) * (Depth / 6)
pH_end = final soil pH target
CEC = cation exchange capacity, mEq/100g
soil_pH = result from soil test
OM = organic matter percentage from soil test if ≥ 4%; use zero if < 4% OM
XSL = excess lime rating from soil test; where “NO” = 0, “LO” = 1, and “HI” = 3; enter actual carbonate percentage if available from lab analysis
Depth = actual soil sample depth, inches
SRec_base = base elemental sulfur rate for non-calcareous soil, as lb. S/ac. to 6-inch depth
SRec_adj = sulfur rate adjustment for organic matter and excess lime, as lb. S/ac.
SRec_final = final, depth-adjusted sulfur rate, as lb. S/ac.
Note: These rates assume treated soils are non-calcareous. Adjust sulfur rate for free carbonates and organic matter, as necessary. Suggest adding 1 ton of elemental sulfur for each 1% of soil carbonate and an additional 500 lb. elemental sulfur for each 1% of soil organic matter over 4%. Split apply rates over two to four years. Reaction speed and final effect will depend on site-specific conditions. Split high rates into two or three applications at least one year apart. Monitor progress of pH adjustment carefully with soil analysis to avoid developing excess acidity or excess salinity.
¶ Table 1. Target pH = 7.5 |
|||||||
| General soil texture class ==> | coarse (sandy) | medium (loamy) | fine (clayey) | ||||
| Typical CEC*, meq/100g ==> | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| Initial soil pH: | ----- Elemental sulfur rate, lb S/ac to 6-inch soil depth ----- | ||||||
| 8.5 | 115 | 220 | 320 | 420 | 520 | 630 | 830 |
| 8.0 | 100 | 190 | 280 | 370 | 460 | 550 | 730 |
¶ Table 2. Target pH = 7.0 |
|||||||
| General soil texture class ==> | coarse (sandy) | medium (loamy) | fine (clayey) | ||||
| Typical CEC*, meq/100g ==> | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| Initial soil pH: | ----- Elemental sulfur rate, lb S/ac to 6-inch soil depth ----- | ||||||
| 8.5 | 210 | 400 | 590 | 780 | 970 | 1160 | 1540 |
| 8.0 | 200 | 370 | 550 | 730 | 910 | 1080 | 1440 |
| 7.5 | 140 | 260 | 380 | 500 | 620 | 740 | 980 |
¶ Table 3. Target pH = 6.5 |
|||||||
| General soil texture class ==> | coarse (sandy) | medium (loamy) | fine (clayey) | ||||
| Typical CEC*, meq/100g ==> | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| Initial soil pH: | ----- Elemental sulfur rate, lb S/ac to 6-inch soil depth ----- | ||||||
| 8.5 | 345 | 670 | 1000 | 1330 | 1650 | 1980 | 2640 |
| 8.0 | 330 | 640 | 960 | 1270 | 1590 | 1900 | 2530 |
| 7.5 | 270 | 530 | 780 | 1040 | 1300 | 1560 | 2070 |
| 7.0 | 160 | 320 | 470 | 630 | 780 | 940 | 1250 |
¶ Table 4. Target pH = 6.0 |
|||||||
| General soil texture class ==> | coarse (sandy) | medium (loamy) | fine (clayey) | ||||
| Typical CEC*, meq/100g ==> | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| Initial soil pH: | ----- Elemental sulfur rate, lb S/ac to 6-inch soil depth ----- | ||||||
| 8.5 | 515 | 1030 | 1540 | 2060 | 2570 | 3090 | 4120 |
| 8.0 | 500 | 1000 | 1500 | 2010 | 2510 | 3010 | 4010 |
| 7.5 | 440 | 880 | 1330 | 1770 | 2220 | 2660 | 3550 |
| 7.0 | 340 | 680 | 1020 | 1360 | 1700 | 2050 | 2730 |
| 6.5 | 190 | 380 | 570 | 770 | 960 | 1160 | 1550 |
¶ Table 5. Target pH = 5.5 |
|||||||
| General soil texture class ==> | coarse (sandy) | medium (loamy) | fine (clayey) | ||||
| Typical CEC*, meq/100g ==> | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| Initial soil pH: | ----- Elemental sulfur rate, lb S/ac to 6-inch soil depth ----- | ||||||
| 8.5 | 725 | 1480 | 2230 | 2980 | 3730 | 4480 | 5980 |
| 8.0 | 710 | 1450 | 2190 | 2930 | 3660 | 4400 | 5880 |
| 7.5 | 650 | 1330 | 2010 | 2690 | 3370 | 4050 | 5420 |
| 7.0 | 550 | 1120 | 1700 | 2280 | 2860 | 3440 | 4590 |
| 6.5 | 400 | 830 | 1260 | 1690 | 2120 | 2550 | 3410 |
| 6.0 | 200 | 440 | 680 | 920 | 1150 | 1390 | 1870 |
¶ Table 6. Target pH = 5.0 |
|||||||
| General soil texture class ==> | coarse (sandy) | medium (loamy) | fine (clayey) | ||||
| Typical CEC*, meq/100g ==> | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| Initial soil pH: | ----- Elemental sulfur rate, lb S/ac to 6-inch soil depth ----- | ||||||
| 8.5 | 980 | 2010 | 3050 | 4090 | 5120 | 6160 | 8230 |
| 8.0 | 960 | 1990 | 3010 | 4030 | 5060 | 6080 | 8130 |
| 7.5 | 900 | 1870 | 2830 | 3800 | 4770 | 5730 | 7660 |
| 7.0 | 800 | 1660 | 2520 | 3390 | 4250 | 5120 | 6840 |
| 6.5 | 650 | 1360 | 2080 | 2800 | 3510 | 4230 | 5660 |
| 6.0 | 450 | 980 | 1500 | 2020 | 2550 | 3070 | 4120 |
| 5.5 | 210 | 500 | 780 | 1070 | 1360 | 1640 | 2210 |
¶ Table 7. Target pH = 4.5 |
|||||||
| General soil texture class ==> | coarse (sandy) | medium (loamy) | fine (clayey) | ||||
| Typical CEC*, meq/100g ==> | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| Initial soil pH: | ----- Elemental sulfur rate, lb S/ac to 6-inch soil depth ----- | ||||||
| 8.5 | 1270 | 2640 | 4010 | 5380 | 6750 | 8120 | 10860 |
| 8.0 | 1250 | 2610 | 3970 | 5330 | 6690 | 8040 | 10760 |
| 7.5 | 1190 | 2490 | 3790 | 5090 | 6400 | 7700 | 10300 |
| 7.0 | 1090 | 2290 | 3480 | 4680 | 5880 | 7080 | 9470 |
| 6.5 | 940 | 1990 | 3040 | 4090 | 5140 | 6190 | 8290 |
| 6.0 | 740 | 1600 | 2460 | 3320 | 4180 | 5030 | 6750 |
| 5.5 | 500 | 1120 | 1740 | 2360 | 2990 | 3610 | 4850 |
| 5.5 | 220 | 560 | 890 | 1230 | 1570 | 1910 | 2580 |
*CEC =soil test Cation Exchange Capacity by summation
Mullen R., et.al. 2007. Soil Acidification: How to Lower Soil pH. Ohio State Univ. Ext., FactSheet AGF-507-07.
Mitchell, C. and J. Adams. Lowering Soil pH in Soil Acidity and Liming - Part 2, Internet Inservice Training. hubcap.clemson.edu/~blpprt/lowerpH.html
British Columbia Min. of Agric. and Food. 1991. Acidifiying soil. Soil Fact Sheet 638.100-1. www.agf.gov.bc.ca/resmgmt/publist/600Series/638100-1.pdf
Minnesota Coop Ext. Svc. Soil Acidification. http://www.extension.umn.edu/distribution/cropsystems/components/5886_6-9.pdf
Santa Barbara Co. Coop. Ext. Acidifying the Soil in Blueberry Production Guide. cesantabarbara.ucdavis.edu/files/75423.pdf
Zhi-Hui, et. al. 2010. Elemental Sulfur Oxidation by Thiobacillus spp. and Aerobic Heterotrophic Sulfur-Oxidizing Bacteria. 20:71-79.