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Granulated volcanic rock dust: the slow-release, easy-spread soil remineraliser
Zeolite-Bearing Volcanic Rock 48% Silica Slow-Release Granules Trace Elements Spreader-Friendly Packed in Stockport
Granulated volcanic rock dust is crushed volcanic rock pressed into granules, spread on soil to put back the trace minerals that cropping and rain wash out. The rock carries silica, calcium, potassium, iron, magnesium and dozens of trace elements locked in its mineral structure, and soil moisture, root acids and microbes release them slowly over seasons rather than weeks. This is the same rock as our micronised powder, just in a coarser form that goes through a lawn spreader and does not blow about.
Pick the granules for lawns, beds and borders, raised-bed fills and planting holes, where you want a mineral reserve that sits in the root zone for years. Pick the micronised volcanic rock dust when you want minerals moving faster, or something you can drench. Plenty of growers use both. One mined mineral, nothing added, bagged in Stockport in 1.5 kg to 15 kg sizes.
48%Silica (SiO₂)
8.3%Calcium (CaO)
5.1%Potassium (K₂O)
4.1%Iron (Fe₂O₃)
What volcanic rock dust granules are used for
- Remineralising tired beds and vegetable plots — 100–300 g per m² forked into the top few centimetres, once or twice a year, rebuilds the trace element supply that decades of picking and rain have taken out.
- Lawn top dressing — the granules run through a spreader, drop between the blades and weather into the root zone. No white dust on the turf, no cloud when the wind picks up.
- Raised beds and container mixes — bagged compost and peat-free media carry almost no rock minerals. Mixing granules in at build time gives the mix a geological base it would otherwise never have.
- Tree, shrub, rose and hedge planting — a couple of handfuls in the backfill puts a mineral reserve where the roots will be for the next twenty years.
- Compost heaps — a heap is warm, damp and acidic, which is exactly the environment that weathers rock fastest. Layered granules come out the other end as mineral-enriched compost.
- Silica supply — the rock is nearly half silica by weight. Plants grown without silicon are structurally weaker and more vulnerable to abiotic stress (Epstein, 1999).
Granulated or micronised: which rock dust do you need?
Granulated volcanic rock dust (this product)
- Coarse granules with a smaller surface area, so minerals release over months and years rather than weeks.
- Goes through a hand or lawn spreader; spreads evenly by hand too.
- Far less dusty to handle, store and apply.
- The choice for lawns, top dressing, raised-bed fills, planting holes and compost heaps.
- Cannot be suspended in water for a drench.
Micronised volcanic rock dust (the sister product)
- Fine powder with a very large surface area, so the same rock weathers far faster.
- Can be stirred into water and applied as a suspension to the root zone.
- Dusty. Wear a mask when mixing it dry.
- The choice when you want minerals moving this season, or you are treating pots and seed trays.
- Both together: micronised for the near term, granules for the reserve.
Packed in Stockport
Weighed and bagged in small batches at the Dr Forest unit in Stockport. No coatings and no animal by-products, so it suits vegan and no-dig gardens. Rock dust is not a fertiliser and will not replace one; it sits underneath your feeding as the mineral base. New to the idea? Our guide to volcanic rock dust, silicon and trace minerals covers what it can and cannot do.
The Science
The science of volcanic rock dust: why the slow route is the point
What this rock actually is
This is a zeolite-bearing volcanic silicate that cooled below the surface rather than erupting, which left its zeolite minerals intact. Volcanic rock of this kind is the parent material of many of the world's most productive farmed soils. Geologists class it as mafic, rich in magnesium and iron, with minerals that dissolve far more readily than the quartz and potassium feldspar in granite. That distinction matters. In a review of 48 crop trials with silicate rock powders, the consistent positive results came from mafic volcanic rocks; granites were patchy at best (Swoboda et al., 2022).
Our granules are this volcanic rock, crushed and pressed into a spreadable pellet. The mineral content is identical to the micronised version; only the particle size changes, and particle size sets the release speed. The more surface a rock exposes to soil water, roots and microbes, the faster it gives up its minerals (Harley and Gilkes, 2000). A granule exposes far less surface per gram than a powder, so it weathers slowly and lasts.
Composition and nutrient content
| Constituent | Content | Role in the garden |
|---|---|---|
| Silica (SiO₂) | 48% | Source of plant-available silicon: stem strength, stress tolerance |
| Calcium (CaO) | 8.3% | Cell walls; helps soil aggregates hold together |
| Potassium (K₂O) | 5.1% | Water regulation and sugar transport, released slowly |
| Iron (Fe₂O₃) | 4.1% | Chlorophyll synthesis |
| Magnesium (MgO) | 1% | The central atom in chlorophyll |
| Manganese | 0.2% | Photosynthesis and enzyme activation |
| Trace elements | Broad range | Zinc, copper, boron, molybdenum, titanium and others, in small amounts |
Read that table the right way round. The potassium and calcium are real, but they sit in mineral lattices and come out slowly, so rock dust will not feed a hungry tomato this month. What it does is put the full range of elements back into the soil in a form that does not leach. The rock also carries a zeolite fraction; zeolites hold positively charged nutrient ions in their crystal structure and give them up as roots pull on them.
Rock dust is a deposit, not a withdrawal. You are putting minerals into the soil bank and letting the soil decide when to spend them.
Mechanisms of action
01
Weathering releases cations and raises exchange capacity
Finely ground silicate rock added to seven highly weathered Queensland soils lifted pH, increased cation exchange capacity and raised exchangeable calcium, magnesium and potassium within three months. Only a fraction of the applied cations had converted to exchangeable form, so most of the reserve was still in the rock, and the gains survived leaching equivalent to a full tropical wet season's rain (Gillman et al., 2002, Applied Geochemistry). A 24-month incubation found the effects still strengthening at month 24 (Anda et al., 2015, Catena). Slow, and cumulative. That is the point of the granule.
02
Roots and microbes speed the rock up
In the laboratory, silicate minerals dissolve slowly. In soil, root exudates and microbial organic acids pull ions off mineral surfaces far faster than water alone, which is why field weathering rates repeatedly outrun laboratory predictions (Harley and Gilkes, 2000; Swoboda et al., 2022). The practical consequence: rock dust does most in a biologically active soil with compost or a microbial input alongside it, and least in a sterile, dry one.
03
Silicon builds stronger, more stress-tolerant plants
Silicon is not on the essential nutrient list, but plants grown without it are structurally weaker, more vulnerable to metal toxicity and abnormal in growth and reproduction compared with plants that have it (Epstein, 1999, Annual Review of Plant Physiology and Plant Molecular Biology). Volcanic silicate rock is the practical source. In a UK controlled-environment trial, relatively coarse crushed silicate rock on a clay-loam soil raised shoot silicon in sorghum by 26% and yield by 21%, with no potassium or phosphorus fertiliser added, though at a heavy 10 kg per m² (Kelland et al., 2020, Global Change Biology).
04
A UK field result in a temperate climate
Most rock dust trials come from the tropics; the 2022 review called the temperate evidence inconclusive. The best British data so far is a field trial on a sandy clay-loam in north-east England. Crushed silicate rock incorporated at seeding raised spring oat yields by 20.5% under direct drilling and 9.3% after ploughing, lifted soil pH by 0.2 to 0.3 units after 256 days and increased potassium and calcium in the crop, with no build-up of arsenic, cadmium, chromium or nickel in the grain (Skov et al., 2024, PLOS ONE). The rate was 18.86 tonnes per hectare, roughly 1.9 kg per m², far above garden rates, so read it as evidence that the mechanism works in a British field and do not expect the same yield lift from 200 g.
05
Potassium from rock, without chloride or salt
Silicate rocks are a legitimate potassium source, and the reactive ones release it far faster than granite: nepheline dissolves around 100 times more quickly than potassium feldspar (Manning, 2010, Agronomy for Sustainable Development). The potassium arrives without the chloride load of muriate of potash and without raising the soil's salt index, which is why rock dust can go on year after year.
Scientific references
- Swoboda, P., Döring, T.F. and Hamer, M. (2022). Remineralizing soils? The agricultural usage of silicate rock powders: A review. Science of the Total Environment, 807, 150976.
- Skov, K., Wardman, J., Healey, M. et al. (2024). Initial agronomic benefits of enhanced weathering using basalt: A study of spring oat in a temperate climate. PLOS ONE, 19(3), e0295031.
- Kelland, M.E., Wade, P.W., Lewis, A.L. et al. (2020). Increased yield and CO₂ sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil. Global Change Biology, 26(6), 3658–3676.
- Gillman, G.P., Burkett, D.C. and Coventry, R.J. (2002). Amending highly weathered soils with finely ground basalt rock. Applied Geochemistry, 17(8), 987–1001.
- Anda, M., Shamshuddin, J. and Fauziah, C.I. (2015). Improving chemical properties of a highly weathered soil using finely ground basalt rocks. Catena, 124, 147–161.
- Harley, A.D. and Gilkes, R.J. (2000). Factors influencing the release of plant nutrient elements from silicate rock powders: a geochemical overview. Nutrient Cycling in Agroecosystems, 56(1), 11–36.
- Manning, D.A.C. (2010). Mineral sources of potassium for plant nutrition. A review. Agronomy for Sustainable Development, 30(2), 281–294.
- Epstein, E. (1999). Silicon. Annual Review of Plant Physiology and Plant Molecular Biology, 50, 641–664.
Trial figures describe the published literature on crushed silicate rock, mostly at agricultural rates well above those on this page. They are not product-specific yield guarantees.
How to Use
How to use volcanic rock dust granules: application rates and guide
Granular: apply dry and water in
This is a dry granular product. Scatter on the soil surface, mix into growing media, or broadcast across beds and lawns. Water in well after application. The rock weathers through contact with soil moisture, so the more thoroughly it is incorporated into moist soil, the faster mineral release begins. The granules are far less dusty than micronised rock dust, but we still recommend wearing a mask if mixing large quantities in an enclosed space.
A mineral base, not a fertiliser
Rock dust carries almost no nitrogen or phosphorus and releases its potassium slowly. Keep feeding as normal and treat the granules as the layer underneath. Minerals in rock dust are released through weathering, a process that is greatly accelerated by soil micro-organisms. Bacteria and fungi produce organic acids that dissolve mineral surfaces far faster than water alone, so rock dust is most effective in soil with active biology: soil enriched with compost, treated with Grow-Kashi, or regularly fed with organic matter. In biologically inert growing media, mineral release will be slower. For the fastest results, combine rock dust with a biological inoculant.
Application rates
Outdoor beds, borders and vegetable plots
Scatter evenly over the soil surface and fork or rake lightly into the top few centimetres. Water in well. Use the higher rate (300 g/m²) for first-time applications on soils that have never received rock dust, and the lower rate (100 g/m²) for annual maintenance applications on previously treated soil. The minerals accumulate over successive seasons; each application builds on the previous one.
Potting soil and container media
Mix thoroughly into potting compost or peat-free media before planting. The granules provide a long-term mineral reservoir that continues releasing trace elements throughout the growing season and beyond. Use the higher rate for mineral-poor media (peat-free compost) and the lower rate for loam-based or compost-rich mixes that already contain some minerals.
Lawn top dressing
Broadcast evenly across the lawn. The granular form is easy to spread by hand or with a lawn spreader, and is far less messy than micronised powder on turf. Water in well after application. The granules settle between grass blades and weather into the root zone over successive months, providing sustained trace mineral supply and silica for stronger grass. Our guide to top dressing a lawn covers the method.
Compost heap enrichment
Sprinkle rock dust granules between layers of compost material. The heat, moisture and microbial acidity within an active compost heap weather the rock particles far faster than they would weather in open soil, so mineral release is much quicker. The finished compost will be enriched with the full rock-dust mineral spectrum.
Tree, shrub and hedge planting
Mix into the backfill soil when planting trees, shrubs, roses and hedging. The granules provide a mineral reservoir that persists in the root zone for years, continuing to release trace elements as the tree or shrub establishes. Particularly useful for long-lived plantings where you want sustained mineral supply without repeat applications.
Raised beds: initial construction
When constructing a new raised bed, mix rock dust granules into the soil or compost fill before planting. This provides the geological mineral base that raised bed media almost always lack. Top up with 100 g/m² each spring as part of your annual bed preparation.
Step-by-step application
- Measure the correct amount. For beds and lawns: 100–300 g per m². For soil mixes: 5–15 g per litre. A generous handful is approximately 50–75 g.
- Scatter or mix evenly. For beds and lawns, scatter as evenly as possible and fork or rake in lightly. For soil building, add to the growing medium and mix thoroughly.
- Water in well. The rock weathers through contact with moisture. Thorough watering after application begins the weathering process and starts mineral release. Rainfall will continue the process naturally.
- Apply 1–2 times per year. Rock dust is a long-term soil investment, not a regular feed. Spring and autumn applications build cumulative mineral reserves over successive seasons.
- Store dry. Keep in a sealed bag in a dry place. The granules are inert geological material with an indefinite shelf life; they will not degrade, clump or lose effectiveness in storage.
Works well combined with…
For a complete remineralisation, run the granules alongside Dr Forest Micronised Volcanic Rock Dust: the powder for minerals this season, the granules for the long-term reservoir. Grow-Kashi brings the microbial activity that weathers rock fastest. Mineral Mix extends the programme to clay minerals, gypsum and sea-shell meal, and a few handfuls of Scottish Seaweed Meal in the compost heap with the granules makes for a properly mineral-rich finished compost.
Storage and handling
Anywhere dry, indefinitely. Keep the bag sealed so the granules stay free-flowing for the spreader, and out of reach of children and pets as you would any garden product.
Questions & Answers
Frequently asked questions about volcanic rock dust
Not in the NPK sense. It carries very little nitrogen or phosphorus, and its potassium comes out slowly. What it supplies is silica, calcium, iron, magnesium and a wide spread of trace elements in a form that does not leach. Use it alongside a fertiliser: the fertiliser feeds the crop this season, the rock dust rebuilds the soil's mineral base underneath.
Same rock, different particle size. The micronised powder has a huge surface area and weathers within weeks; it is the fast-acting option and can be stirred into water. The granules release over months to years and act as the reserve, and they spread cleanly on lawns and beds. Many customers use the micronised rock dust for the first season and the granules for the long haul.
100–300 g per m² on beds, borders and vegetable plots, once or twice a year in spring and autumn. Use the top of that range the first time on soil that has never had rock dust, and 100 g/m² for annual maintenance. Lawns take 100–200 g per m². A generous handful is roughly 50–75 g.
Yes, and it is what the granular form is best at. Broadcast 100–200 g per m² by hand or through a lawn spreader and water in well. The granules fall between the grass blades without coating the leaf the way a powder does, then weather into the root zone over the following months. Apply in spring and/or autumn.
Slowly. Granules release minerals over months to years as they weather, so effects build over the first season and stack up with each annual application. Look for steadier growth and gradually better soil; there is no visible jump. If you want minerals moving this season, add the micronised powder alongside the granules.
The strongest evidence comes from tropical soils, and a 2022 review of 48 trials called the temperate evidence inconclusive. Since then a field trial in north-east England has shown crushed silicate rock raising spring oat yields by 9–20%, lifting soil pH and increasing potassium and calcium in the crop, at a rate of around 1.9 kg per m². Our garden rates are a fraction of that, so we sell it as a long-term soil investment.
Plants take up silicon from soil water and lay it down in cell walls. Plants grown without it are structurally weaker, more vulnerable to metal toxicity and abnormal in growth and reproduction. This rock is about 48% silica, and a UK trial found crushed silicate rock raised shoot silicon in sorghum by 26%.
The rock is alkaline (pH 11 in water) and does nudge pH upward over time; the English field trial recorded a rise of 0.2 to 0.3 units after 256 days at a heavy agricultural rate. At garden rates the shift is small and gradual. If you are deliberately keeping a bed acidic for blueberries or rhododendrons, apply lightly and check your pH each spring.
It is crushed rock with nothing added. Keep the bag out of reach as you would any garden product, and wear a mask if you are mixing a lot of it in an enclosed space, but there is nothing in it to trouble bees, birds or a dog that walks across a treated lawn.
Anywhere dry. It is inert geological material with an indefinite shelf life. It will not degrade, spoil or lose effectiveness however long it sits in the shed. Keep the bag sealed so the granules stay free-flowing.
Here. This is granulated diabase basalt in 1.5 kg, 4 kg, 9 kg and 15 kg bags, posted to any UK address. If you want a fine powder for drenches and sprays rather than granules for spreading, we also sell micronised rock dust from the same basalt source.
Long-term remineralising of beds, borders, raised beds and lawns. It adds 48% silica, 8.3% calcium, 5.1% potassium and 4.1% iron, plus magnesium, manganese and more than 60 trace elements, and it releases them slowly as the granules weather. The granules suit lawns because they settle between the grass blades without coating the leaves. Mix it into the fill when you build a raised bed. It isn't an NPK feed, so carry on feeding as normal.
For beds and borders, 100–300 g per m². Use 300 g the first time on soil that has never had rock dust, then 100 g per m² each year to maintain it. For lawns, 100–200 g per m² once or twice a year, in spring and autumn, watered in. For a new raised bed, mix 200–300 g per m² into the fill before planting, then top up with 100 g per m² each spring.
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