Sulphur soil acidifier: 90% elemental sulphur to lower soil pH
Sulphur soil acidifier lowers soil pH so blueberries, rhododendrons, azaleas and camellias can take up iron and manganese properly. It is 90% elemental sulphur with 10% bentonite clay in a pastille that spreads dry with no dust, then breaks into fine sulphur once it is wet. Soil bacteria turn that sulphur into acid, a little at a time, and bring neutral or alkaline ground down towards pH 5.0 to 6.0. Those plants sit in the hydrangeas and ericaceous range. Growing notes are in the plant growing directory.
You will also see it sold as garden sulphur, yellow sulphur or sulphur chips. It is the same mineral, but plain chips have to weather down before much happens. When you use sulphur for soil, the dose is set by your soil type and the speed by soil temperature, so it works from spring to autumn, and a re-test tells you when to stop. Once you reach your target, the pH stays down for years.
What it's for
- Blueberries & cranberries: the most acid-hungry, pH 4.5–5.5
- Rhododendrons, azaleas & camellias: classic ericaceous shrubs, pH 4.5–6.0
- Acers, pieris & summer heathers: acid-loving structure and foliage. Winter heathers (Erica carnea, E. × darleyensis) tolerate lime and do not need this
- Neutral or alkaline beds: nudge them down for acid-lovers
- Raised beds: mix in before planting
At a glance: 90% sulphur, 10% bentonite. Test first. Not for pots or chalky soil. It only lowers pH. Rates by soil type are in How to Use. The wider picture is in how to increase or decrease your soil pH.
The pastilles are made by co-melting sulphur with the bentonite: nothing we blend, and nothing we add to.
Packed in Stockport: Dr Forest weighs and packs every bag.
The science: how sulphur acidifies soil
Soil bacteria do the acidifying. Sulphur-oxidising bacteria, mainly Thiobacillus and Acidithiobacillus, use the sulphur as an energy source and convert it to sulphate, releasing hydrogen ions as they go. It's those H⁺ ions that lower pH.
CO₂ + S⁰ + ½O₂ + 2H₂O → CH₂O + SO₄²⁻ + 2H⁺: two hydrogen ions for every sulphur atom oxidised (Havlin et al., 1999; Ohio State Extension). The sulphate left behind is also a plant nutrient.
How long sulphur takes to work, and why soil temperature decides
In warm, moist, well-worked soil a modest change shows within weeks; in cool or heavy soil it takes months, and autumn and winter applications act mainly from spring onward. A large shift, such as neutral ground down to blueberry pH, takes one to two years (Michigan State University Extension). The bacteria need air as well as moisture: in waterlogged soil, sulphur turns to hydrogen sulphide, which harms roots.
Why the bentonite is there
| Form | What reaches the soil | Handling |
|---|---|---|
| Milled powder | Fine particles spread thinly, with the most soil-exposed surface of any form | Dusty, drifts on the wind, unpleasant to spread |
| Sulphur-bentonite pastille (this product) | Fine sulphur, released as a cluster where each pastille sat | Spreads clean and dust-free at a measured rate |
| Plain granule | Stays intact for months; only the outer surface is exposed | Easy to spread |
| Coarse chips | Coarse throughout; weathers down over a season or more | Clean, but hard to spread evenly at a measured rate |
Soil type sets the dose
Clay and organic matter resist pH change, so they need far more sulphur than sand (RHS; Michigan State University Extension). Once acidified, soil pH typically stays low for 5+ years, but keep testing (Ohio State Extension, AGF-507).
It will not raise pH, and it will not hold on chalky soil with free lime: if a drop of vinegar fizzes on your soil, sulphur is the wrong tool (RHS). It is not for pots.
References
- Mullen R, Lentz E, Watson M (2016). Soil Acidification: How to Lower Soil pH. Ohio State University Extension, AGF-507.
- Royal Horticultural Society. Acidifying soil. RHS Advice.
- Germida JJ, Janzen HH (1993). Factors affecting the oxidation of elemental sulfur in soils. Fertilizer Research 35(1–2): 101–114. doi:10.1007/BF00750224. (This journal is now published as Nutrient Cycling in Agroecosystems.)
- Yang Z-H, Stöven K, Haneklaus S, Singh BR, Schnug E (2010). Elemental sulfur oxidation by Thiobacillus spp. and aerobic heterotrophic sulfur-oxidizing bacteria. Pedosphere 20(1): 71–79.
- Havlin JL, Beaton JD, Tisdale SL, Nelson WL (1999). Soil Fertility and Fertilizers: An Introduction to Nutrient Management, 6th ed. Upper Saddle River, NJ: Prentice Hall.
- Longstroth M (n.d.). Lowering the Soil pH with Sulfur. Michigan State University Extension.
- Chapman SJ (1996). Powdered elemental sulphur: oxidation rate, temperature dependence and modelling. Nutrient Cycling in Agroecosystems 47: 19–28. doi:10.1007/BF01985715
- Degryse F, Ajiboye B, Baird R, da Silva RC, McLaughlin MJ (2016). Oxidation of elemental sulfur in granular fertilizers depends on the soil-exposed surface area. Soil Science Society of America Journal 80(2): 294–305. doi:10.2136/sssaj2015.06.0237
- Hanson E, Hancock J (1996). Managing the Nutrition of Highbush Blueberry. Michigan State University Extension, Bulletin E-2011.
- University of Georgia CAES. Backyard blueberry leaves can tell the tale of soil deficiencies and pH problems.
- Cornell University. Berry Diagnostic Tool: blueberries: iron deficiency.
How to use sulphur to lower soil pH
Only acidify if your soil is neutral or alkaline: check with a pH kit. Drop a little vinegar on the soil first: if it fizzes, there's free lime or chalk and it can't realistically be acidified (RHS). Wear gloves, work in still air, and wear goggles and a dust mask on large areas (RHS). Err on the side of too little: you can always add more.
How much to use
Every rate below comes from a published university or horticultural guide, named next to the number. All rates are grams of pastilles per square metre, mixed into the top 15 cm. Always test your soil pH first, and only treat soil that is above the pH your plants need.
| Use | Rate | How to apply it | Source |
|---|---|---|---|
| Lower pH 1 unit, sandy soil | 55 g/m² | One dose | Ohio State, Purdue, University of California |
| Lower pH 1 unit, loam | 110 g/m² (2 doses) | Two doses of 55 g/m², 6 months apart | Purdue, University of California |
| Lower pH 1 unit, clay | 175 g/m² (3 doses) | Three doses of about 58 g/m², 6 months apart | University of California, Purdue |
| Most in one dose, beds | 73 g/m² | University of California | |
| Most in one year, bare beds before planting | 146 g/m² | University of California | |
| Most in one year, planted ground | 112 g/m² | University of Maine | |
| Lawns (only if pH above 7.5) | 15 g/m² (about one level tablespoon), at most twice a year | See Lawns below | University of Georgia |
| Established ericaceous plants | up to 25 g/m² a dose | At most twice a year, early spring and late autumn | University of Georgia, Cornell |
| Established blueberries | up to 85 g per plant per year | See Blueberries below | Oregon State, Cornell |
| Pots | Do not use | RHS |
Sources: Ohio State University; Purdue University Extension; University of California. For half a unit (for example pH 7.0 to 6.5), use half these amounts.
- Never more than 73 g/m² in one dose on beds, and no more than 146 g/m² a year on bare soil before planting, and at most 112 g/m² a year on planted ground (University of California; University of Maine). Retest before each further dose.
- About the RHS figure. The RHS gives 135 to 270 g/m² of sulphur powder, to take the top 15 cm from pH 7.0 to 7.5 down to 6.0 to 6.5 (RHS). That is higher than the university figures above, so treat it as the upper limit. The RHS itself advises several small additions over months.
What you're aiming for
Open ground & beds
Mix the pastilles into the top 15 cm and water in. Left on the surface they can take years to work (RHS). Never more than 73 g/m² in one dose or 146 g/m² a year on bare soil before planting, or at most 112 g/m² a year on planted ground (University of California; University of Maine). Wait at least 3 months, ideally 6, before retesting (University of California).
Pots & containers
The RHS lists sulphur as suitable for all soils but not potting media (RHS). For acid lovers in pots, use ericaceous compost and water with rainwater.
Established ericaceous plants
For rhododendrons, azaleas, camellias and heathers already in the ground, sprinkle over the root area out to the edge of the leaves (University of Georgia), lightly work in and water. Split the year's amount between early spring and late autumn (Cornell University). Retest before repeating.
Lawns
Most lawns are happy between pH 6.0 and 7.5 (Purdue University), so most lawns do not need sulphur. Only use it if a soil test shows pH above 7.5. Apply 15 g/m² (about one level tablespoon), at most twice a year, as heavier doses can scorch grass. Water in straight away with about 10 litres per square metre. Apply in spring or early autumn on mown grass, avoiding hot weather and dry, stressed grass (Kansas State University). Retest after the second dose.
Blueberries and other fruit
Blueberries need acid soil, ideally pH 4.5 to 5.5 (RHS). Most other fruit is happy at pH 6.0 to 7.0, so only use sulphur where a test shows the pH is above your crop's range.
- Before planting: use the soil-type rates above, mixed in to 15 to 20 cm, at least 6 months ahead (University of Georgia), ideally a year (Cornell University).
- Established blueberries: no more than 85 g per plant per year (Oregon State University; Cornell University), either once in early autumn or split between early spring and late autumn, and never more than 25 g/m² per dose over the root area (University of Georgia). Higher rates can burn or kill the plants (Cornell University).
- Retest every autumn (Oregon State University).
Brassicas and onions
Do not use Sulphur Pastilles to lower pH in brassica beds, which need pH 6.5 to 7.0. Where brassicas or onions are short of sulphur, use a sulphate fertiliser, which plants can take up straight away (AHDB RB209).
Which bag size do you need?
Sulphur is dosed by area, so the sum is simple: your area in m² × the rate for your soil = grams of product needed. A 6 m² border on loam needs 6 × 110 = 660 g to lower pH by one unit, given as two doses of 330 g, 6 months apart. Retest before each further dose.
Area each bag treats
| Bag | Sandy (55 g/m²) | Loam (110 g/m²) | Clay (175 g/m²) |
|---|---|---|---|
| 1.5kg | 27 m² | 13.6 m² | 8.6 m² |
| 4kg | 72 m² | 36 m² | 23 m² |
| 9kg | 163 m² | 82 m² | 51 m² |
| 15kg | 272 m² | 136 m² | 85 m² |
To lower pH by one unit in the top 15 cm. Loam is treated in two doses and clay in three, 6 months apart, and beds never get more than 73 g/m² in one dose or 146 g/m² a year on bare soil before planting, or at most 112 g/m² a year on planted ground (University of California; University of Maine). Retest before each further dose.
Typical projects
| What you are doing | On loam (110 g/m²) | Bag |
|---|---|---|
| Rhododendron border, 5 m × 1.5 m | 825 g | 1.5kg |
| New ericaceous bed, 10 m² | 1,100 g | 1.5kg |
| Acid border or small blueberry patch, 25 m² | 2,750 g | 4kg |
| Large planting, 100 m² | 11,000 g | 15kg |
On clay, use about 60% more. On sand, use half. Split each total into doses as above.
Step by step
- Test your soil. Take a pH reading and do the vinegar-fizz check for free lime.
- Work out the rate. Use your soil type and how far you need to drop (table above).
- Weigh it out. Use kitchen scales.
- Broadcast on the surface. Scatter evenly across the area. Do not bury the pastilles yet.
- Water, and give it a few days. The bentonite swells and breaks each pastille into fine sulphur: you will see them slump and spread.
- Now fork it in. Work the dispersed sulphur into the top 15 cm and keep the soil damp. The bacteria need moisture and air, never waterlogged.
- Re-test after at least 3 months. Top up cautiously if you're not there yet; it's easy to overshoot.
Once the pH is right, feed acid-lovers gently. Our Sulphate of Potash gives fruiting blueberries chloride-free potash, and Seaweed Plant Food is a mild, pH-safe biostimulant. If you want sulphur and calcium without lowering pH, that's a job for gypsum.