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Most plants grow best where the soil is slightly acid to neutral (range of pH 6.5 to 7.2). Some plant species require acid soil conditions to thrive, including azaleas, gardenias, and blueberries. Certain materials can be applied as a soil amendment to decrease the soil pH or to acidify neutral to slightly alkaline soils. This can potentially improve plant growth for these species. For example, hydrangea flowers may not be naturally blue because the soil may not be sufficiently acid. Acidifying the soil can restore blue flowers.
Acidifying materials include elemental sulfur, iron sulfate, and aluminum sulfate. If soils contain free carbonates (“excess lime”), amending the soil for acid-loving plants may not be feasible. This Crop File is intended to provide interested individuals with information about materials and rates to adjust soil pH where appropriate.
Soil pH is a measure of the hydrogen ion (H+) concentration in the soil solution. “Soil solution” refers to the soil water which contains dissolved minerals and nutrients. The higher the hydrogen ion concentration in soil solution, the lower the soil pH.
Amendments applied to acidify the soil must undergo certain chemical or biological reactions. The hydrogen ions released during these reactions create acidity and cause soil pH to decrease.
Long-term use of certain nitrogen fertilizers may also cause a gradual build of soil acidity, so routine soil testing and monitoring is important.
Soil amendments should be applied before planting to allow time for soil reactions to be completed. Amendments can be surface applied to established turf or plantings. Amendments can be applied to holes or trenches to acidify a small part of the root zone for trees or shrubs.
Acidifying soil gradually over several years is advisable. Two or three smaller applications a year apart are better than a single large application.
Acidifying soils may create additional problems. When soil pH is adjusted, some minerals in the soil become more soluble, and thus more available to the plant. In the case of aluminum and manganese, this increased solubility may result in plant toxicity.
Annual soil sampling and analysis is important to monitor changes. Soil pH can change seasonally, so samples should be collected at about the same time each year.
Elemental sulfur probably the most economical product for large-scale treatment, but it has to be converted or “oxidized” by soil microorganisms to sulfuric acid to affect soil pH. “Thiobacillus“ are a bacteria species that are largely involved in sulfur oxidation, the conversion of elemental sulfur to sulfate.
The reaction of elemental sulfur when applied to the soil can be described as:
elemental sulfur + oxygen + water => Thiobacillus bacteria + time => sulfuric acid => hydrogen ions
2S0 + 3O2 + 2H2O => Thiobacillus =>
2H2SO4 => 4H+ + 2SO4-2
2H2SO4 => 4H+ + 2SO4-2
It is important to note that the process of sulfur oxidation is a slow biological process rather than a rapid chemical reaction. Do not expect quick changes in soil pH. It may take a few months or longer to change soil pH to the desired level because of the microbial activity rate.
Elemental sulfur should be incorporated into the soil to increase the speed of oxidation. The rate at which soil pH will decrease is related to both the Thiobacillus activity and the fineness of the sulfur materials. The bacteria are most active when the soil is moist and warm (above 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 late the following spring or early summer.
The Thiobacillus bacteria are aerobic, meaning they require oxygen. The amended soil or media must not be saturated or flooded, which creates an anaerobic condition. If saturated, oxygen is eliminated and the elemental sulfur is converted to hydrogen sulfide (H2S) by anaerobic bacteria. Hydrogen sulfide has a "rotten egg" smell and kills 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.
Prilled sulfur fertilizer products, like Tiger Sulfur 90® or Dispersul®, have shown good results. They are manufactured by forming finely ground elemental sulfur into pellets with bentonite clay for ease of application.
Aluminum sulfate changes the soil pH almost immediately because the aluminum produces the acidity as soon as it dissolves in the soil. The acidifying reaction is chemical rather than biological.
aluminum sulfate + water =>
gibbsite (solid) + hydrogen ions + sulfate
Al2(SO4)3 + 6H20 =>
2Al(OH)3 + 6H+ + SO4-2
Multiply the recommended elemental sulfur rate by “7" to calculate the aluminum sulfate application rate. Do not apply more than about 5 to 10 pounds per 100 square feet at one time. Aluminum sulfate reacts quickly and has the potential to induce aluminum toxicity to plant roots.
Make aluminum sulfate applications in three-week to fourweek intervals. Water frequently to avoid excessive levels of soluble salts, but do not saturate the soil to avoid saturation or flooding.
Repeated aluminum sulfate applications can result in a buildup of soluble soil aluminum to toxic levels. Treated soils should be sampled after every fourth or fifth application to monitor the change in soil pH and make adjustments to future application rates, as necessary.
Iron sulfate may also be referred to as “ferrous sulfate (FeSO4)” or “ferric sulfate (Fe2(SO4)3).” These amendments undergo a chemical reaction in the soil to produce hydrogen ions and to lower soil pH:
iron sulfate + oxygen + water => iron oxide (solid) + hydrogen ions + sulfate
4FeSO4 + O2 + 6H2O => 4FeOOH + 8H+ + 4SO4-2
Iron sulfate reacts much faster than elemental sulfur, usually within three to four weeks following application. Multiply the recommended elemental sulfur rate by “6" to calculate the iron sulfate application rate. Do not apply more than 6 to 8 pounds of iron sulfate per 100 square feet at one time. Make applications in one to two-month intervals. Water frequently between applications to avoid excessive levels of soluble salts, but do not saturate or waterlog.
A common misconception is that sulfate-containing fertilizers like gypsum (calcium sulfate), potassium sulfate, or Sul-PoMag will lower the soil pH. Sulfate ions (SO4=) have no effect on soil pH. Other fertilizer components (like ammonium or thiosulfate) may have some effect on pH, but the soil reactions are different.
Crop Files 2.03.106, 2.03.017, and 2.03.108 give suggested application rates of elemental sulfur to reduce the initial soil pH to the desired soil pH. Rates are given in “pounds per 100 square feet” for surface application to large areas. Rates are also calculated as “ounces per 10 cubic feet” and “ounces per 10 gallons” when treating beds or containers. Multiply the elemental sulfur rates by “7" if aluminum sulfate is used instead of elemental sulfur. Multiply by “6" if iron sulfate is used. These are suggested rates and should be monitored by routine soil analysis.
If the soil is calcareous (contains free carbonates) additional sulfur will be required to neutralize the carbonates. Increase the suggested rates by 10% to 15% for a “LO” excess lime test; about 30% to 50% for a “HI” excess lime test. Adjust iron or aluminum sulfate applications accordingly.
Additional sulfur may be needed to adjust the pH of soils or mixture with high organic matter content (4% OM or more). Increase the elemental sulfur rate by about five percent for each additional percent of organic matter over 4% OM. For example, if the mixture has 10% OM, increase the rate by 30% (10 - 4 = 6 and 6 x 5% = 30%). Adjust iron or aluminum sulfate applications accordingly.
Maintenance applications may be needed if the water used for irrigating contains medium to high levels of bicarbonate. Bicarbonate that accumulates in the soil from water application tends to increase the soil pH.
The rates below assume that soils are non‐calcareous; adjust sulfur rate for free carbonates and organic matter, as necessary. Multiply elemental sulfur rates by 6 if using iron sulfate or by 7 if using aluminum sulfate
¶ Table 1a. Desired ending pH = 6.5 (pounds per 100 square feet) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 0.8 | – | 1.6 | 2.4 | – | 3.2 | 4.0 | – | 4.8 |
| 8.0 | 0.8 | – | 1.5 | 2.3 | – | 3.1 | 3.8 | – | 4.6 |
| 7.5 | 0.6 | – | 1.3 | 1.9 | – | 2.5 | 3.2 | – | 3.8 |
| 7.0 | 0.4 | – | 0.8 | 1.2 | – | 1.6 | 2.0 | – | 2.4 |
| >6.5 | 0.1 | – | 0.2 | 0.2 | – | 0.3 | 0.4 | – | 0.5 |
| 6.0 | --- | --- | --- | ||||||
| 5.5 | --- | --- | --- | ||||||
| 5.0 | --- | --- | --- | ||||||
| 4.5 | --- | --- | --- | ||||||
¶ Table 1b. Desired ending pH = 6.5 (ounces per 10 cubic feet) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 2.6 | – | 5.1 | 7.7 | – | 10.2 | 12.8 | – | 15.4 |
| 8.0 | 2.6 | – | 4.8 | 7.4 | – | 9.9 | 12.2 | – | 14.7 |
| 7.5 | 1.9 | – | 4.2 | 6.1 | – | 8.0 | 10.2 | – | 12.2 |
| 7.0 | 1.3 | – | 2.6 | 3.8 | – | 5.2 | 6.4 | – | 7.7 |
| >6.5 | 0.3 | – | 0.6 | 0.6 | – | 1.0 | 1.3 | – | 1.6 |
| 6.0 | --- | --- | --- | ||||||
| 5.5 | --- | --- | --- | ||||||
| 5.0 | --- | --- | --- | ||||||
| 4.5 | --- | --- | --- | ||||||
¶ Table 1c. Desired ending pH = 6.5 (ounces per 10 gallons of soil or media) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 0.3 | – | 0.7 | 1.0 | – | 1.4 | 1.7 | – | 2.1 |
| 8.0 | 0.3 | – | 0.6 | 1.0 | – | 1.3 | 1.6 | – | 2.0 |
| 7.5 | 0.3 | – | 0.6 | 0.8 | – | 1.1 | 1.4 | – | 1.6 |
| 7.0 | 0.2 | – | 0.4 | 0.5 | – | 0.7 | 0.9 | – | 1.0 |
| >6.5 | 0.0 | – | 0.1 | 0.1 | – | 0.1 | 0.2 | – | 0.2 |
| 6.0 | --- | --- | --- | ||||||
| 5.5 | --- | --- | --- | ||||||
| 5.0 | --- | --- | --- | ||||||
| 4.5 | --- | --- | --- | ||||||
¶ Table 2a. Desired ending pH = 5.5 (pounds per 100 square feet) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 1.6 | – | 3.9 | 6.3 | – | 8.6 | 10.9 | – | 13.2 |
| 8.0 | 1.6 | – | 3.9 | 6.2 | – | 8.5 | 10.8 | – | 13.0 |
| 7.5 | 1.5 | – | 3.6 | 5.8 | – | 7.9 | 10.1 | – | 12.3 |
| 7.0 | 1.2 | – | 3.2 | 5.1 | – | 7.0 | 9 | – | 10.9 |
| 6.5 | 0.9 | – | 2.5 | 4.1 | – | 5.7 | 7.3 | – | 8.9 |
| 6.0 | 0.5 | – | 1.6 | 2.8 | – | 4.0 | 5.2 | – | 6.4 |
| >5.5 | 0.0 | – | 0.6 | 1.2 | – | 1.9 | 2.6 | – | 3.2 |
| 5.0 | --- | --- | --- | ||||||
| 4.5 | --- | --- | --- | ||||||
¶ Table 2b. Desired ending pH = 5.5 (ounces per 10 cubic feet) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 5.1 | – | 12.5 | 20.2 | – | 27.5 | 34.9 | – | 42.2 |
| 8.0 | 5.1 | – | 12.5 | 19.8 | – | 27.2 | 34.6 | – | 41.6 |
| 7.5 | 4.8 | – | 11.5 | 18.6 | – | 25.3 | 32.3 | – | 39.4 |
| 7.0 | 3.8 | – | 10.2 | 16.3 | – | 22.4 | 28.8 | – | 34.9 |
| 6.5 | 2.9 | – | 8.0 | 13.1 | – | 18.2 | 23.4 | – | 28.5 |
| 6.0 | 1.6 | – | 5.1 | 9.00 | – | 12.8 | 16.6 | – | 20.5 |
| >5.5 | 0.0 | – | 1.9 | 3.80 | – | 6.10 | 8.30 | – | 10.2 |
| 5.0 | --- | --- | --- | ||||||
| 4.5 | --- | --- | --- | ||||||
¶ Table 2c. Desired ending pH = 5.5 (ounces per 10 gallons of soil or media) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 0.7 | – | 1.7 | 2.7 | – | 3.7 | 4.7 | – | 5.6 |
| 8.0 | 0.7 | – | 1.7 | 2.7 | – | 3.6 | 4.6 | – | 5.6 |
| 7.5 | 0.6 | – | 1.5 | 2.5 | – | 3.4 | 4.3 | – | 5.3 |
| 7.0 | 0.5 | – | 1.4 | 2.2 | – | 3 | 3.9 | – | 4.7 |
| 6.5 | 0.4 | – | 1.1 | 1.8 | – | 2.4 | 3.1 | – | 3.8 |
| 6.0 | 0.2 | – | 0.7 | 1.2 | – | 1.7 | 2.2 | – | 2.7 |
| >5.5 | 0.0 | – | 0.3 | 0.5 | – | 0.8 | 1.1 | – | 1.4 |
| 5.0 | --- | --- | --- | ||||||
| 4.5 | --- | --- | --- | ||||||
¶ Table 3a. Desired ending pH = 4.5 (pounds per 100 square feet) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 2.4 | – | 6.3 | 10.1 | – | 14.0 | 17.8 | – | 21.7 |
| 8.0 | 2.4 | – | 6.2 | 10.0 | – | 13.9 | 17.7 | – | 21.5 |
| 7.5 | 2.3 | – | 6.0 | 9.7 | – | 13.4 | 17.0 | – | 20.7 |
| 7.0 | 2.0 | – | 5.5 | 9.0 | – | 12.4 | 15.9 | – | 19.4 |
| 6.5 | 1.7 | – | 4.8 | 8.0 | – | 11.1 | 14.3 | – | 17.4 |
| 6.0 | 1.3 | – | 4.0 | 6.7 | – | 9.4 | 12.1 | – | 14.8 |
| 5.5 | 0.7 | – | 2.9 | 5.1 | – | 7.3 | 9.5 | – | 11.7 |
| 5.0 | 0.1 | – | 1.7 | 3.2 | – | 4.8 | 6.4 | – | 7.9 |
| >4.5 | 0.0 | – | 0.2 | 1.1 | – | 1.9 | 2.7 | – | 3.6 |
¶ Table 3b. Desired ending pH = 4.5 (ounces per 10 cubic feet) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 7.7 | – | 20.2 | 32.3 | – | 44.8 | 57.0 | – | 69.4 |
| 8.0 | 7.7 | – | 19.8 | 32.0 | – | 44.5 | 56.6 | – | 68.8 |
| 7.5 | 7.4 | – | 19.2 | 31.0 | – | 42.9 | 54.4 | – | 66.2 |
| 7.0 | 6.4 | – | 17.6 | 28.8 | – | 39.7 | 50.9 | – | 62.1 |
| 6.5 | 5.4 | – | 15.4 | 25.6 | – | 35.5 | 45.8 | – | 55.7 |
| 6.0 | 4.2 | – | 12.8 | 21.4 | – | 30.1 | 38.7 | – | 47.4 |
| 5.5 | 2.2 | – | 9.3 | 16.3 | – | 23.4 | 30.4 | – | 37.4 |
| 5.0 | 0.3 | – | 5.4 | 10.2 | – | 15.4 | 20.5 | – | 25.3 |
| >4.5 | 0.0 | – | 0.6 | 3.5 | – | 6.1 | 8.6 | – | 11.5 |
¶ Table 3c. Desired ending pH = 4.5 (ounces per 10 gallons of soil or media) |
|||||||||
| General soil texture class ==> | Sandy | Medium | Clayey | ||||||
| Typical CEC, meq/100g ==> | 5 | – | 10 | 15 | – | 20 | 25 | – | 30+ |
| Initial soil pH: | ‐‐‐‐‐ Estimated elemental sulfur rate, lb S/100 sq ft to 6‐inch depth* ‐‐‐‐‐ | ||||||||
| 8.5 | 1.0 | – | 2.7 | 4.3 | – | 6 | 7.6 | – | 9.3 |
| 8.0 | 1.0 | – | 2.7 | 4.3 | – | 5.9 | 7.6 | – | 9.2 |
| 7.5 | 1.0 | – | 2.6 | 4.2 | – | 5.7 | 7.3 | – | 8.9 |
| 7.0 | 0.9 | – | 2.4 | 3.9 | – | 5.3 | 6.8 | – | 8.3 |
| 6.5 | 0.7 | – | 2.1 | 3.4 | – | 4.7 | 6.1 | – | 7.4 |
| 6.0 | 0.6 | – | 1.7 | 2.9 | – | 4 | 5.2 | – | 6.3 |
| 5.5 | 0.3 | – | 1.2 | 2.2 | – | 3.1 | 4.1 | – | 5 |
| 5.0 | 0.0 | – | 0.7 | 1.4 | – | 2.1 | 2.7 | – | 3.4 |
| >4.5 | 0.0 | – | 0.1 | 0.5 | – | 0.8 | 1.2 | – | 1.5 |
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