Granulated gypsum for clay soil: what it does and what it will not
Granulated gypsum for clay soil is a mined clay breaker for ground where water sits on the surface after rain: its calcium pulls dispersed clay into crumbs that drain, with no change to soil pH. It is calcium sulphate screened to 2 to 5 mm, spread from a spreader or by hand and dug in or left for the rain to carry down.
It only does that on clay that disperses. The RHS puts it plainly: "Some, but not all, clay soils respond to extra calcium." So do the crumb test first. Drop a dry crumb of your soil into a glass of rainwater: a milky halo means the clay disperses and gypsum is the fix. If the water stays clear, your clay is heavy rather than dispersed, and compost will do more for its structure. Then treat a marked patch at 400 g/m² before doing the whole bed.
In plain numbers that is about 16% sulphur, 21% calcium and just under 2% magnesium. The 40% SO₃ figure is the one RB209, the AHDB fertiliser guide, uses for quarried gypsum, so sulphur rates translate straight across.
What it does
- Helps dispersed clay drain. Crumbs instead of an airless pan, so rain soaks in rather than sitting on top.
- Feeds sulphur roots can use straight away. Plants take sulphur up as sulphate, which is what gypsum is.
- Adds calcium without liming. No pH change, so it suits soil already where you want it, including ericaceous beds.
- Stops the surface crusting over. Keeps the top centimetre crumbly after heavy rain.
- Adds magnesium as well. 3% MgO, which most agricultural gypsums do not carry.
At a glance: mined calcium sulphate, 2 to 5 mm granules. Crumb test first. Autumn on clay, spring and autumn on lawns. Never mix into stored manure or slurry. Bagged and dispatched from Stockport; the 1.5 kg online size comes as two 750 g packs, with larger bags up to 30 kg.
Granulated, micronised or liquid?
Granulated gypsum (this product)
- 2 to 5 mm granule with none of the dust of a powder
- Best for whole lawns, beds and allotments
- Dissolves progressively with rainfall rather than all at once
- Mixes straight in with your other dry feeds in the spreader
Micronised gypsum powder
- Dissolves faster, because powder has far more surface touching the soil
- Better for potting mixes and precise dosing by the gram
- Dusty in wind and awkward to spread evenly over a large area
Liquid gypsum
- Liquid Gypsum goes on through a watering can or sprayer and is fastest to act
- Useful for a patch, or an established lawn you would rather not walk a spreader over
- Paired with a wetting agent for better ground penetration: see the Liquid Gypsum and Wetting Agent pair
The science
How calcium unsticks clay: flocculation
Clay particles are microscopic flat plates with a negative charge, and something has to balance it. Sodium holds the plates apart, so they slide over each other like wet playing cards: the greasy mud that dries into a pan. Calcium carries twice the charge in a smaller package and lets the plates pull together into crumbs. The gaps between crumbs are the drainage, the air and the room for roots.
The water matters too. Soil stops draining once the water in it becomes too pure (Quirk and Schofield, 1955), and clean rain on sodium-heavy clay is the worst case. Gypsum dissolves at about 2.5 grams per litre, roughly 200 times more readily than ground limestone, so it keeps calcium in that water while the slower swap does the work.
Rain soaked in twice as fast
On soils prone to crusting, gypsum at 5 tonnes per hectare under simulated heavy rain roughly doubled the rate water soaked in and cut soil washed off the surface by 30 to 50% (Miller, 1987). The trial used phosphogypsum, which dissolves faster than mined rock, so expect the same effect over a longer period.
Crumbs survived the frosts
Freeze and thaw is what smashes soil crumbs over a British winter. Calcium sulphate at 0.2% by weight kept crumbs measurably more stable across six soils. Lime, tested alongside, made almost no difference (Lehrsch, Sojka and Jolley, 1993).
No change to pH, and the sulphur reached the plants
Chaganti and colleagues (2019) found no pH change at either rate tested, while soil sulphur rose three to eightfold and leaf sulphur 15 to 97%. Gypsum is not lime and will not raise pH.
What it will not do
Fix compaction
- Gypsum changes soil chemistry, not soil physics. On ordinary soil it made no measurable difference to how hard the ground was. Compaction needs aeration, deep roots or a fork.
Reliably fix blossom end rot
- It is a watering problem far more than a calcium one. UC Cooperative Extension: "Adding calcium to the soil rarely alleviates the problem." Steady watering does more.
Correct bitter pit in apples
- Torres and colleagues (2024) found the calcium shortage in a bitter-pitted apple "is minimally corrected through improved Ca fertilization."
"Balance" your soil
- Fixed calcium to magnesium ratios have not survived testing. Yields were unaffected from 0.25:1 to 31:1 (Kopittke and Menzies, 2007).
References
- Quirk, J.P. & Schofield, R.K. (1955) The effect of electrolyte concentration on soil permeability. Journal of Soil Science 6:163–178.
- Miller, W.P. (1987) Infiltration and soil loss of three gypsum-amended Ultisols under simulated rainfall. Soil Science Society of America Journal 51(5):1314–1320.
- Lehrsch, G.A., Sojka, R.E. & Jolley, P.M. (1993) Freezing effects on aggregate stability of soils amended with lime and gypsum. CATENA Supplement 24:115–127.
- Chaganti, V.N., Culman, S.W., Dick, W.A. & Kost, D. (2019) Agronomy Journal 111(3):1109–1117. DOI 10.2134/agronj2018.10.0683.
- Chen, L., Kost, D., Tian, Y. et al. (2014) Effects of gypsum on trace metals in soils and earthworms. Journal of Environmental Quality 43:263–272.
- Kopittke, P.M. & Menzies, N.W. (2007) A review of the use of the basic cation saturation ratio and the "ideal" soil. Soil Science Society of America Journal 71:259–265.
- Torres, E., Kalcsits, L. & Nieto, L.G. (2024) Is calcium deficiency the real cause of bitter pit? A review. Frontiers in Plant Science 15:1383645.
- AHDB, Nutrient Management Guide (RB209), Sections 3–7, 2020–2023 editions.
How much gypsum to add to clay soil: rates per square metre
Autumn is the main time on clay: after rough digging, or after aeration on turf, so winter rain does the dissolving. New beds can also go in early spring, before planting. Lawns take it in spring and autumn. For sulphur nutrition, apply at or soon after planting, or in early spring for established crops. Otherwise any time the ground is neither frozen nor waterlogged.
Two very different jobs, two very different rates
Gypsum is used at one rate to feed a crop sulphur and calcium, and at a rate ten to thirty times higher to change the structure of a soil.
| Purpose | Product rate | Delivers |
|---|---|---|
| Sulphur nutrition (most vegetables, potatoes, peas) | 6–7 g/m² | 25 kg SO₃/ha |
| Sulphur nutrition (vegetable brassicas) | 12.5–18.8 g/m² | 50–80 kg SO₃/ha |
| Lawn maintenance | 50–100 g/m² | 0.5–1 t/ha |
| General soil conditioning | 200 g/m² | 2 t/ha |
| Heavy clay improvement | 400–600 g/m² | 4–6 t/ha |
| Digging into a new lawn before seeding or turfing | 500 g/m² | 5 t/ha |
200 g/m² for structure is also 800 kg SO₃/ha, far more sulphur than any crop needs in a season. Fine as a one-off, because the surplus washes through over winter. Not fine every year on the same ground.
Directions by application
New beds and borders
Broadcast over the cleared bed and fork into the top 5cm. Water in, or apply before rain. On very heavy or previously waterlogged ground go to 400 g/m². Leave a month before re-working the soil, with an untreated strip alongside to compare.
Heavy clay improvement
Incorporate to 10cm where you can; where you cannot dig, spread on the surface and let winter rain carry it down. Expect softer soil within twelve months and the full effect over two to three seasons. Add generous compost too: organic matter improves clay whatever its type. Keep within the maximum rate below, because heavy rates can wash magnesium and potassium out of the soil.
Gypsum for lawns: established lawns
Use a spreader set to the rate, ideally just before rain. If rain does not turn up within a couple of days, water it in. No need to rake in; the granules work down through the grass. Fine, closely mown turf is best treated between late winter and early summer.
Compacted, waterlogged or slow-draining turf
Hollow-tine, sweep up the cores, spread and brush the granules into the holes so the calcium gets down to the roots. The aeration relieves the compaction; the calcium helps the new air spaces stay open.
New lawns, before seeding or turfing
Rotavate or fork into the top 10–15cm, level, firm, water thoroughly and leave it a week before seeding.
Vegetable plots and allotments
As a sensible default, 20 g/m² raked in at planting covers the sulphur needs of almost everything on a plot. Crop by crop rates are on the next tab.
Containers and potting mixes
Roughly 60g in a 60-litre bag of compost, or 600g to 1.2kg per cubic metre. Peat-free composts are often short of calcium. Mix through while the compost is dry. For raised beds, use the bed rates above.
Other uses
| Use | Rate | Timing and frequency |
|---|---|---|
| Salt damage (de-icing spray, pet spots) | 122–244 g/m² on well-drained ground | As soon as the soil has dried enough to walk on. Single application, repeat the following season if needed. Water heavily afterwards. |
| Ground flooded with seawater | a single dose of 500 g/m² | Once it has dried out. |
| Compost heaps (open, turned heaps only) | around 450g per barrow load, or 20 kg per tonne of dry matter | As you build the heap. Each new heap. |
| Clearing a muddy pond | 146–336 g/m² of pond surface | Autumn works best. Once; repeat if runoff re-muddies the water. Start at the low end and wait a few days before adding more. Mix it into a slurry with pond water and spread it about rather than tipping dry granules in one spot. |
Applying it, step by step
- Weigh it. A rounded tablespoon holds roughly 20g of granules and a standard 250ml cup roughly 300g.
- Test first on clay. Crumb test, then a marked patch at 400 g/m² beside an untreated strip, compared after a winter.
- Spread evenly. Halve your spreader setting and do two overlapping passes; by hand, two passes at right angles.
- Incorporate where you can. Fork into the top 5–10cm on bare ground. Rates above 250 g/m² work considerably better incorporated. On turf, leave it on top.
- Let rain do the work. Roughly 25mm of rainfall dissolves about 62 g/m² of gypsum, so a heavy autumn works through a 200 g/m² dressing over a season. Sulphur and calcium are available within weeks.
Never mix gypsum into stored manure or slurry. Under anaerobic conditions the sulphate can be reduced to hydrogen sulphide, a dangerous gas. Spreading gypsum on ground after manure, or adding it to an open, turned compost heap, is fine. Granules are low-dust, not dust-free: wear gloves and avoid breathing dust when spreading.
Use it with
| Combination | Verdict |
|---|---|
| Garden Lime | Compatible and complementary: lime lifts pH, gypsum opens structure without touching pH. Apply together or in rotation. |
| Dry granular fertilisers | Yes. The granule mixes evenly with other granular feeds in the spreader; powdered gypsum segregates. |
| Compost | Yes, on the same ground. The combination beats either alone. |
| Other sulphur sources | Count the total. If you already apply Polyhalite, Kieserite, Epsom salt or an ammonium sulphate feed, reduce the gypsum. |
| Ericaceous plantings | Yes. No effect on pH, so blueberries, rhododendrons, azaleas and camellias stay on acid soil. |
- Liquid Gypsum and Wetting Agent pair: for lawns and beds where water sits after rain. The wetting agent gives the gypsum better ground penetration and better results.
- Polyhalite Fertiliser | 4-in-1 Mineral (K, Ca, Mg, S): adds potassium alongside calcium, magnesium and sulphur, from the same spreader.
- Granulated Rock Dust | Volcanic Basalt | Slow-Release: basalt, for the trace minerals gypsum does not carry.
Worth reading: how to feed a lawn the organic way and what organic dry amendments actually do.
Gypsum rates by crop: vegetables, fruit, ornamentals and pots
Sulphur rates follow RB209, converted to grams per square metre at 40% SO₃. RB209 applies them "where sulphur deficiency has been recognised or is expected": sandy soils in any rainfall, loam and light silty soils in a wet region, and clay only where more than 375mm of rain falls over winter. Apply at or soon after planting unless stated.
Vegetables
| Crop | Rate | Timing and notes |
|---|---|---|
| Brassicas: cabbage, sprouts, cauliflower, calabrese, kale, broccoli | 12.5–18.8 g/m² | The hungriest group for sulphur, which builds their flavour compounds. |
| Onions, leeks, garlic, shallots; peas and beans; carrots, parsnips, beetroot, swede, turnip; lettuce and leafy salads; courgette, squash, pumpkin, cucumber | 6–7 g/m² | At or soon after planting. |
| Potatoes | 6–7 g/m² | In the seedbed. Adds calcium without raising pH on acid ground. |
| Sweetcorn, radish, celery, asparagus, herbs | 6–7 g/m² | At or soon after planting; early spring for established asparagus. |
| Tomatoes, peppers, chillies, aubergine | 6–7 g/m² for nutrition; 56–112 g/m² pre-plant where soil calcium tests low | Worked into the bed before planting. Steady watering matters far more than anything you add. |
Fruit
RB209 notes that "fruit crops are not generally thought to respond to sulphur" and gives 15–25 kg SO₃/ha only where deficiency is recognised or expected: roughly 4–6 g/m², applied in spring.
| Crop | Rate | Timing and notes |
|---|---|---|
| Young fruit trees | 0.5–1 kg per tree, watered in well | At planting or in the first dormant season. |
| Strawberries, raspberries, currants | 4–6 g/m² | Spring. The RB209 fruit figure. |
| Blueberries and other ericaceous fruit | 4–6 g/m², or as part of a container mix at 1 g/litre | Spring. Calcium without undoing your acidification. |
Ornamentals
| Planting | Rate | Timing and notes |
|---|---|---|
| Herbaceous borders | 200 g/m² as a soil conditioner | Autumn or early spring, forked into the top 5cm before replanting. |
| Rhododendrons, azaleas, camellias, heathers | 4–6 g/m² | Spring. Start low. |
Do not exceed 1 kg per square metre in any single application or in any one year. In practice, garden rates should sit at or below 600 g/m². Going higher mainly washes magnesium and potassium out of the soil. If you are applying every year, get a soil test every few years to check where you have got to.