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    Dr Forest

    Sulphur Soil Acidifier

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    90% S + 10% Bentonite
    Lowers Soil pH
    Ericaceous & Blueberries
    Packed in Stockport

    Sulphur soil acidifier — lower your soil pH the natural way

    Sulphur soil acidifier is elemental sulphur (S) that soil bacteria oxidise into sulphuric acid, which lowers soil pH so acid-loving, ericaceous (lime-hating) plants can take up iron and manganese properly. Ours is a sulphur-bentonite pastille — 90% elemental sulphur with 10% bentonite clay. Spread it dry with no dust, then the moment it gets wet the bentonite swells and bursts the pastille apart into fine sulphur particles, giving the bacteria the surface area they need. It's the standard way to bring neutral or alkaline ground down towards pH 5.0–6.0 for blueberries, rhododendrons, azaleas, camellias and blue hydrangeas.

    You'll also see elemental sulphur sold as garden sulphur, yellow sulphur, sulphur chips or pastilles. Plain chips and granules have to weather down before much happens; the bentonite does that job for you in a few days. Because the acid is still made by soil biology it isn't instant — it works from spring to autumn when soils are warm, and rewards re-testing rather than guessing. Once you reach your target it holds the pH down for years.

    90%Elemental sulphur (S)
    pH 5–6Ericaceous target
    150–300g/m² per pH unit
    2×H⁺ ions per S atom

    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
    • Blue hydrangeas — keep the flowers blue (below about pH 5.5)
    • Neutral or alkaline beds — nudge them down for acid-lovers
    • Raised beds & containers — mix in before planting

    Sulphur vs the quick fixes

    Sulphur-bentonite pastilles

    Gentlest on plants and the cheapest per pH unit. Handles like a granule, then breaks down to fine sulphur once wet, so it acts far quicker than plain chips. Still needs warm, moist soil and a few weeks to a few months, but the effect lasts 5+ years.

    Iron or aluminium sulphate

    Act faster, but you need roughly 7–8× as much for the same drop, they can lock up phosphorus, and aluminium can build up in the soil. Better for a quick nudge than a lasting change (RHS).

    Packed in Stockport

    Weighed and packed by hand in small batches at our workshop in Stockport, Greater Manchester. The pastilles themselves are made by co-melting sulphur with the bentonite — nothing we blend, and nothing we add to.

    The science: how sulphur acidifies soil

    Elemental sulphur doesn't acidify soil on its own. Sulphur-oxidising bacteria — mainly Thiobacillus and Acidithiobacillus — use it as an energy source and convert it to sulphate, releasing hydrogen ions as they go. It's those H⁺ ions that lower pH.

    The oxidation reaction

    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.

    Warmth and moisture set the speed

    Relative sulphur oxidation rate versus soil temperature Oxidation barely starts below about 5 degrees C, rises steeply and plateaus between 30 and 40 degrees C, then falls. 0 50 100 Relative oxidation (%) Typical UK soil 8–18°C Plateau ~30–40°C 5 15 25 35 45 Soil temperature (°C)
    Temperature drives the rate. Germida and Janzen's review names soil temperature, water potential and aeration as the primary constraints on elemental-sulphur oxidation. Of the three, temperature is the one you cannot manage: oxidation barely starts below about 5 °C and runs fastest between 30 and 40 °C. In practice that means spring-to-autumn application in UK gardens, where soils sit well below the optimum all year. Sources: Germida & Janzen, 1993; Yang et al., 2010.

    Why the bentonite is there

    Relative oxidation speed by sulphur form Milled powder oxidises fastest, a dispersed sulphur-bentonite pastille is close behind, a plain granule is slower and coarse chips slowest. Fastest Close behind Slower Slowest Milled powder dusty, drifts Bentonite pastille once dispersed — Dr Forest Plain granule Coarse chips Oxidation speed
    Surface area is everything, and the bentonite buys it. Sulphur is oxidised on the surface of each particle, so finer means faster. Milled powder is quickest but drifts and is unpleasant to handle. A sulphur-bentonite pastille gets most of the way there without the dust: the clay imbibes soil moisture and the pastille disintegrates into finely divided sulphur. The released particles are still a little coarser than milled powder, so it is not quite as quick — but it is well ahead of a plain granule or chip, which has to weather down on its own. Sources: The Sulphur Institute; Incitec Pivot Fertilisers Agritopic — Sulphur; Lee, Boswell & Watkinson, 1988.

    How far, how fast

    Illustrative soil pH response over time in warm versus cool soil An illustration of pH falling from 7.0 towards the ericaceous target band, faster in warm soil than cool soil. Ericaceous target 5.0–6.0 7.0 6.5 6.0 5.5 5.0 Soil pH 0 2 4 6 Months after application Warm, moist soil Cool / heavy soil
    Illustrative, not a guarantee. There is a short lag of several days while the sulphur-oxidising bacteria establish, then oxidation runs at a roughly constant rate per unit of exposed particle surface — so the pace eases later because the particles are shrinking, not because the bacteria tire. Warm, moist, well-worked soil moves within weeks; cool or heavy soil takes months. A large shift, such as neutral ground down to blueberry pH, takes one to two years whatever you do. Always re-test. Pattern after Watkinson & Blair, 1993; timescale after Longstroth, MSU Extension.

    The mechanism, in short

    01

    Two protons per atom

    The acidity is biological: each sulphur atom oxidised releases 2 H⁺ (Havlin et al., 1999). No bacteria, no acid.

    02

    Temperature leads

    Temperature, moisture and aeration are the three primary constraints on oxidation. Almost nothing happens below about 5 °C, and the rate peaks between 30 and 40 °C (Germida & Janzen, 1993; Yang et al., 2010).

    03

    Bentonite makes the surface area

    The clay imbibes soil moisture and swells, bursting each pastille into finely divided sulphur that oxidises far faster than an intact granule (The Sulphur Institute). It has to wet where it lands to do that — see How to Use.

    04

    Buffering sets the dose

    Clay and organic matter resist pH change, so they need far more sulphur than sand (RHS; Michigan State University Extension).

    05

    Long residual

    Once acidified, soil pH typically stays low for 5+ years — but keep testing (Ohio State Extension, AGF-507).

    06

    Keep it aerobic

    In waterlogged soil, sulphur turns to hydrogen sulphide instead of sulphate, which harms roots. Damp, not saturated (Michigan State University Extension).

    1. Mullen R, Lentz E, Watson M (2016). Soil Acidification: How to Lower Soil pH. Ohio State University Extension, AGF-507.
    2. Royal Horticultural Society. Acidifying soil. RHS Advice.
    3. 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.)
    4. Watkinson JH, Blair GJ (1993). Modelling the oxidation of elemental sulfur in soils. Fertilizer Research 35: 115–126. doi:10.1007/BF00750225
    5. Lee A, Boswell CC, Watkinson JH (1988). Effect of particle size on the oxidation of elemental sulphur, thiobacilli numbers, soil sulphate, and its availability to pasture. New Zealand Journal of Agricultural Research 31(2): 179–186. doi:10.1080/00288233.1988.10417943
    6. 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.
    7. 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.
    8. Longstroth M (n.d.). Lowering the Soil pH with Sulfur. Michigan State University Extension.
    9. The Sulphur Institute. Elemental Sulphur (sulphur fertilizer types).
    10. Incitec Pivot Fertilisers. Agritopic: Sulphur.
    11. University of Georgia CAES. Backyard blueberry leaves can tell the tale of soil deficiencies and pH problems.
    12. Cornell University. Berry Diagnostic Tool — blueberries: iron deficiency.

    How to use sulphur to lower soil pH

    Test first, then go gently

    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 err on the side of too little — you can always add more.

    How much to use

    Sulphur needed by soil type to lower pH by about one unit About 150 grams of product per square metre on sand, 220 on loam, and 300 on clay. 0 100 200 300 g per m² 150 220 300 Sandy Loam Clay / organic
    Rule of thumb for the top 15 cm. About 150 g of product per m² on sandy soil, up to 300 g/m² on clay, with loam between at around 220 g/m². RHS puts that dose as enough to move the top 15 cm from pH 7.0–7.5 down to pH 6.0–6.5 — so depending where you start and what your soil is like, expect anywhere from half a point to one and a half. For a smaller shift, halve it. Source: RHS elemental-sulphur figures (135–270 g/m², top 15 cm), divided by 0.9 for the 90% pastille. RHS specifies sulphur powder; the loam value is our interpolation, as RHS publishes none.
    Soil type Drop ~0.5 pH Drop ~1.0 pH
    Sandy ~75 g/m² ~150 g/m²
    Loam ~110 g/m² ~220 g/m²
    Clay / high organic ~150 g/m² ~300 g/m²

    These are weights of product, which is what you actually scoop. Because the pastilles are 90% sulphur they deliver the RHS elemental-sulphur figures for the top 15 cm — 70, 100 and 135 g/m² for the smaller shift, 135, 200 and 270 g/m² for the larger. One honest caveat: RHS quotes those rates for sulphur powder. The bentonite is there to break these pastilles down to fine sulphur in the soil, which gets you close, though the particles it releases stay a little coarser than a mill would give. Test and repeat rather than over-dose — over-acidified soil has to be limed back.

    What you're aiming for

    Target soil pH ranges by plant Blueberries and blue hydrangeas 4.5 to 5.5; rhododendron, azalea, pieris and summer heather 4.5 to 6.0; camellia 5.0 to 6.5; veg, top fruit and lawns 6.0 to 7.0. Ericaceous zone 5.0–6.0 Neutral 7.0 Blueberry Blue hydrangea Rhododendron / azalea Pieris & summer heather Camellia Veg, top fruit & lawns 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Soil pH
    Where to aim. Most ericaceous plants are happy at pH 5.0–6.0; blueberries and blue hydrangeas want it lower, around 4.5–5.5. Blueberries are the least forgiving: above about pH 5.3 they begin to show iron chlorosis, the youngest leaves going yellow while the veins stay dark green. That is more than a point below neutral, which is why blueberry ground almost always needs correcting. Blue hydrangeas need available aluminium in the soil as well as a low pH — pH alone will not turn them blue. Sources: RHS; University of Georgia Extension; Cornell Berry Diagnostic Tool.

    Application rates

    Open ground & beds

    Rate: 150–300 g/m² per pH unit (top 15 cm)  |  When: spring–autumn, ideally before planting

    Broadcast evenly on the surface and water it. Leave it a few days for the pastilles to swell and break down, then fork into the top 15 cm. Burying them intact concentrates the sulphur into a small volume and slows everything down. Sandy soils take the lower rate, clay the higher. Re-test after 8–12 weeks and top up if needed.

    Containers & raised beds

    Rate: acidify the mix before planting  |  When: at potting

    For pots, it's safer to acidify peat-free ericaceous compost before planting than to dose a planted pot. Reckon on about 1.0 g per litre of mix on sandy blends, 1.5 g/L on loam and 2.0 g/L on compost-rich ones, for one pH point. Wet the mix so the pastilles break down, turn it through, then leave it a few weeks before testing. Avoid heavy doses around established roots.

    Established ericaceous plants

    Rate: light spring dressing  |  When: growing season

    For blueberries, rhododendrons and camellias already in the ground, scatter a light surface dressing in spring and water it in — no digging needed, since the pastilles break down where they land and the acid moves down with the rain. Re-test before repeating. Small and often beats one big dose.

    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 wanting a full pH point needs 6 × 220 = 1,320 g, so the 1.5kg bag. The chart and tables below do that arithmetic for every bag size.

    Area treated by each bag size on loam soil On loam at 220 grams of product per square metre, 500g treats 2.3 square metres, 1.5kg treats 6.8, 4kg treats 18, 9kg treats 41, 18kg treats 82 and 36kg treats 164. 500g 1.5kg 4kg 9kg 18kg 36kg 2.3 m² 6.8 m² 18 m² 41 m² 82 m² 164 m² 0 50 100 150 Area treated (m²) — loam, one full pH unit
    Loam at 220 g of product per m², one full pH point. Sandy soil goes roughly a third further; clay covers about a third less. The 4kg bag is the usual choice for a garden border. Derived from the rates above.

    Area each bag treats

    Bag Sandy Loam Clay / high organic
    500g 3.3 m² 2.3 m² 1.7 m²
    1.5kg 10 m² 6.8 m² 5.0 m²
    4kg 27 m² 18 m² 13 m²
    9kg 60 m² 41 m² 30 m²
    18kg 120 m² 82 m² 60 m²
    36kg 240 m² 164 m² 120 m²

    One full pH unit in the top 15 cm. For a half-point drop, every figure doubles — a 4kg bag covers 36 m² of loam instead of 18.

    Buying for a full year

    Most people need more than the initial dose. Even with the bentonite doing the grinding for you, oxidation is biological and the first season is variable, so the honest planning assumption is one full application in spring, a re-test at 8–12 weeks, and a half-rate top-up in late summer if you are not there yet. That is 1.5× the initial figure across the first year.

    Bag Sandy Loam Clay / high organic
    500g 2.2 m² 1.5 m² 1.1 m²
    1.5kg 6.7 m² 4.5 m² 3.3 m²
    4kg 18 m² 12 m² 8.9 m²
    9kg 40 m² 27 m² 20 m²
    18kg 80 m² 55 m² 40 m²
    36kg 160 m² 109 m² 80 m²

    Year one, including the likely top-up. After that the pH holds for years and you are only maintaining, so a bag lasts far longer.

    Typical projects

    What you are doing Year one, loam Bag
    One blueberry in a 45 cm pot (~40 L) ~80 g 500g does 6 pots
    Three blueberries, 2 m × 1 m bed 660 g 1.5kg
    Rhododendron border, 5 m × 1.5 m 2,475 g 4kg
    New ericaceous bed, 10 m² 3,300 g 4kg
    Acid border or small blueberry patch, 25 m² 8,250 g 9kg
    Large planting, 100 m² 33,000 g 36kg

    On clay, add about a third. On sand, take about a quarter off.

    Pots and raised beds — working by volume

    The rates above cover the top 15 cm, which is 150 litres of soil per square metre. Divide through and you get roughly 1.0 g per litre on sandy mixes, 1.5 g/L on loam and 2.0 g/L on compost-rich or clay mixes, for one pH point. A 40 L pot is therefore about 60–80 g. Wet the mix so the pastilles break down, turn it through before planting, and leave it a few weeks before testing — dosing a pot that already has a plant in it is much easier to overdo.

    Step by step

    1. Test your soil. Take a pH reading and do the vinegar-fizz check for free lime.
    2. Work out the rate. Use your soil type and how far you need to drop (table above).
    3. Weigh it out. A level tablespoon of granules is roughly 15–18 g — measure rather than guess.
    4. Broadcast on the surface. Scatter evenly across the area. Do not bury the pastilles yet.
    5. Water, and give it a few days. The bentonite swells and breaks each pastille into fine sulphur — you will see them slump and spread.
    6. 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.
    7. Re-test after 8–12 weeks. Top up cautiously if you're not there yet; it's easy to overshoot.
    Works well alongside

    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 — not this.

    Sulphur soil acidifier — questions

    Soil bacteria oxidise elemental sulphur into sulphuric acid, which lowers pH. That frees up iron and manganese for acid-loving plants and leaves behind sulphate, a plant nutrient. It only acidifies — it won't raise pH.
    It depends how far you are going. A modest correction — say half a point to a point on ordinary garden soil — takes weeks in warm, moist ground and months if it is cold or heavy; RHS puts it at weeks, or months over winter. A large shift is a different proposition: Michigan State reckon on one to two years to bring neutral soil down to blueberry pH, and up to five years of annual applications if the plants are already in the ground. Apply spring to autumn, re-test after 8–12 weeks, and judge it on the meter rather than the calendar.
    Roughly 150 g of product per m² on sand, up to 300 g/m² on clay, to drop about one pH point in the top 15 cm; halve that for a half-point. Those are product weights — at 90% sulphur they deliver the RHS elemental-sulphur rates of 135–270 g/m². Always test first, because clay and organic-rich soils buffer the change and need more.
    Yes, but give it time. Blueberries want pH 4.5–5.5, and on neutral or clay soil that is a big shift — Michigan State put it at one to two years, not one season, and advise applying and incorporating at least a year before planting. For bushes already in the ground, they cap the rate and spread it across several years rather than dosing heavily in one go. Correct the ground first, test, then plant.
    Yes, carefully. For pots, acidify the compost before planting. For plants already in the ground, use light spring dressings watered in, rather than one heavy dose around the roots.
    Sulphur is the gentlest and cheapest per pH unit and lasts for years, but it's slow. Iron and aluminium sulphates act faster, yet you need about 7–8× as much and they can tie up phosphorus, with aluminium building up over time (RHS).
    It's a natural mineral used widely in gardens, and the granules are low-dust. As with any soil product, wear gloves, keep it out of eyes, store it sealed and don't over-apply.
    Two naturally occurring minerals and nothing else: elemental sulphur with 10% bentonite, a natural swelling clay that is there to break the pastille down in the soil. Both are used by organic gardeners. We don't make certified-organic claims.
    Usually one of three things: the soil is too cold, it's waterlogged (which turns sulphur to hydrogen sulphide instead of acid), or there's free lime or chalk buffering it. Do the vinegar-fizz test — if it fizzes, sulphur won't hold (RHS; Michigan State University Extension).
    Weighed and packed by hand in small batches by Dr Forest in Stockport, Greater Manchester. The sulphur-bentonite pastilles are manufactured by co-melting the two minerals together, so what reaches you is exactly that and nothing else.
    To break the pastille apart. Sulphur is oxidised on the surface of each particle, so fine sulphur works and lumps don't — but fine sulphur is dusty, drifts on the wind and is unpleasant to spread. Ten per cent bentonite solves both problems at once: the pastille handles like a granule, then the clay imbibes soil moisture, swells and bursts it into finely divided sulphur exactly where you put it. The bentonite itself does no harm — it is an inert clay mineral, and if anything it slightly improves the soil's ability to hold nutrients.
    Same mineral, different form. "Garden sulphur", "yellow sulphur" and "sulphur chips" usually mean plain elemental sulphur, which has to weather down before it does much. Ours is a sulphur-bentonite pastille — 90% sulphur, 10% clay — spread dust-free, then breaking itself into fine sulphur the moment it gets wet. Powder-like speed with granule-like handling.
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