Volcanic rock dust: silicon and trace minerals | Dr Forest
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Volcanic rock dust: trace minerals, silicon and flavour
By Joe, Founder of Dr Forest · April 2026
I came to volcanic rock dust through tomatoes. Most of what I have learned about flavour comes back to soil minerals.
Fifteen years cooking in restaurant kitchens taught me that the same tomato variety can taste of a lot or a little depending on how it was grown. Working out why led me into soil chemistry. The short version is that flavour comes from a great deal of small chemistry the plant only does properly when it has a full set of minerals on hand: not just nitrogen, phosphorus and potassium, but the trace elements most fertilisers leave out.
That is how I came to basalt. Volcanic rock is essentially a mineral concentrate. It is the rock that produces the most fertile soils on earth: Iceland, Java, the slopes of Mount Etna. Crushed and applied to a depleted UK soil, it does most of what those volcanic regions do naturally, just considerably faster.
Here is the short answer up front, for anyone skimming. Volcanic rock dust is finely ground basalt, applied to soil to supply silicon and over 60 trace minerals. These are the elements NPK fertilisers leave out: the cofactors plants use to make flavour compounds, build stronger cell walls, and resist drought, fungal disease and insect feeding. It is also one of the few soil amendments that actively removes carbon dioxide from the atmosphere as it weathers in the soil.
What follows is the case for adding rock dust to a UK garden, allotment or smallholding: what is in it, why silicon matters more than most fertiliser literature lets on, what the latest peer-reviewed research from the US Corn Belt and UK farms actually shows, and how to apply both micronised and granular forms.
What volcanic rock dust does
What it supplies
Silicon plus 60+ trace minerals
- Silicon at roughly 48%, the richest practical natural source
- Calcium, magnesium, iron, potassium and manganese
- Boron, copper, zinc, molybdenum, cobalt and nickel
- Selenium, vanadium, lithium and other rare trace elements
- Naturally occurring zeolite minerals for nutrient retention
What it does in soil
Slow remineralisation
- Refills depleted trace mineral reserves over months and years
- Raises soil pH gently, countering rainfall-driven acidification
- Stimulates soil microbes and mycorrhizal fungi
- Strengthens plant defence via silicon deposition in cell walls
- Removes atmospheric CO₂ as the rock weathers in the soil
Puts back the minerals decades of cropping took out. Pulls carbon out of the air while it does.
First, what is volcanic rock dust?
Volcanic rock dust, also called basalt rock dust, basalt powder or rock minerals, is finely crushed basalt. Basalt is a dark, dense igneous rock formed when volcanic lava cools at the earth's surface. It is the most abundant rock type on the planet's crust, and its mineral composition happens to be remarkably well suited to plant nutrition.
A typical basalt analysis looks more like a multivitamin than a fertiliser. Roughly 48% silicon, 8% calcium, 5% potassium, 4% iron, 1% magnesium, plus manganese, zinc, copper, boron, molybdenum, titanium, cobalt, nickel, selenium, vanadium, lithium, strontium and dozens more in trace quantities. This is not a manufactured ratio. It is what nature settled on over geological time, and it is the same ratio that makes the soils around active and dormant volcanoes the most fertile in the world.
Hidden hungerWhy your soil needs remineralising
Every harvest removes minerals from the soil. Every winter rain leaches the soluble ones downwards through the profile. Every year of microbial activity consumes organic matter and the trace elements bound up inside it. Over decades, this one-way extraction depletes the trace minerals plants depend on for flavour, pest resistance, nutritional density and shelf life.
Most fertiliser programmes replace nitrogen, phosphorus and potassium and ignore everything else. Soil scientists call the result hidden hunger: plants that grow visibly fine but are subtly missing the cofactors they need to do their best chemistry. Hidden hunger is one of the reasons the same tomato variety, grown in the same season under the same regime, can taste of more in one plot and less in another. The plots that test for higher mineral density also tend to test for higher Brix, more complex aromatic profiles, and longer storage life.
UK soils are particularly affected. Centuries of intensive cropping plus a wet climate that leaches minerals out faster than they can be replaced means many UK garden and allotment soils are noticeably depleted in zinc, boron, manganese, molybdenum and silicon. Volcanic rock dust supplies all of them in a single application.
Trace mineralsThe elements NPK ignores
If I had to pick one thing rock dust does that most fertilisers miss, it is supplying the cofactors plants use to make their own flavour and defence chemistry.
Plants do not produce flavour directly. They produce sugars, organic acids, phenolics, terpenes, glucosinolates and dozens of aromatic volatiles. Most of those production lines run on enzymes that need a specific metal cofactor to work. Iron, copper, manganese, zinc, molybdenum, nickel, cobalt, vanadium and selenium all act as enzyme cofactors at one point or another in plant biochemistry. Take any of them out and the chemistry runs at half pace.
A few specifics worth knowing.
Boron is involved in sugar transport into developing fruit and in fruit cell wall integrity. Borderline boron deficiency regularly turns up as bland, soft-shouldered fruit on otherwise healthy plants.
Cobalt is required by the rhizobial bacteria that fix nitrogen in legumes, and it sits at the heart of vitamin B12 chemistry. Cobalt-deficient soils produce legumes that fix nitrogen poorly and have lower protein density.
Selenium is a trace nutrient that plants take up readily even though they do not strictly require it. UK soils are notably low in selenium compared with much of mainland Europe, and selenium-rich crops are a recognised nutritional benefit. Basalt happens to supply it.
Nickel is the cofactor for urease, the enzyme plants use to mobilise nitrogen from urea-form sources. Nickel was only confirmed as essential to plant nutrition in 1987. Most fertiliser programmes still do not include it.
Vanadium has been studied for its role in nitrogen fixation in some non-legume systems and as a low-level biostimulant for fruit set and seedling vigour.
You will not find these elements on a typical fertiliser label. They are present in volcanic rock dust because basalt is what it is, and because the rock has been in the ground for a very long time accumulating exactly the trace mineral profile a plant needs.
The minerals a plant uses to make flavour are not on the bag of any NPK fertiliser. They are in the rock.On trace mineral feeding
The fourth macronutrient
Silicon is the second most abundant element in the earth's crust. It is also routinely ignored in plant nutrition. This is a real oversight, particularly for UK gardeners dealing with fungal disease, drought stress or pest pressure, all of which silicon helps a plant push back against.
Plants take silicon up as monosilicic acid in soil water and deposit it inside their cells as amorphous silica, also known as phytoliths. The silica accumulates in cell walls, leaf surfaces and the cuticle, and the deposition has several measurable effects.
From root to leaf surface
Slow release into flavour compounds
Silicon is a structural element and a defence trigger. The two roles compound.
The point about secondary metabolites is the one most underappreciated. A 2020 review in the Journal of Experimental Botany pulled together two decades of research showing that adequate silicon does more than deposit as physical armour. It activates the phenylpropanoid pathway, the enzyme chain plants use to produce phenolics, flavonoids and lignin. Those compounds protect the plant. They also colour, flavour and preserve the harvest.
If you have ever wondered why the same tomato variety from a mineral-rich allotment tastes deeper and stores longer than one from a peat-bag in a polytunnel, silicon is part of the answer. Volcanic rock dust is the most practical natural source on a UK plot.
Enhanced weatheringThe carbon side of the same reaction
This is the part that has moved volcanic rock dust from a niche soil amendment into mainstream peer-reviewed climate science. When basalt weathers in soil, the silicate minerals react with water and dissolved CO₂ to form bicarbonate ions. The bicarbonates are carried by groundwater into rivers and eventually the ocean, where the carbon stays locked away on geological timescales: effectively permanent removal from the atmosphere. The process is called enhanced weathering, and it is now recognised by the IPCC, by Nature, and by PNAS as a scalable carbon dioxide removal strategy alongside reforestation and direct air capture.
What is striking about the agronomic studies of the last five years is that the carbon and the agronomic benefits stack rather than trade off.
The point worth taking from this is not the climate one alone. The same chemistry that pulls carbon out of the air is the chemistry that releases minerals to the plant. Carbon removal and crop yield rise from the same reaction, just observed from different ends.
Two formsMicronised or granular?
There are two practical forms of basalt rock dust. Each answers a different question.
Micronised and granular, factor by factor
| Factor | Micronised | Granular |
|---|---|---|
| Particle size | Powder grade, very fine | 1–4 mm granules |
| Speed of release | Weeks to months | Months to years |
| Best use | Potting mix, deficiency response, foliar drench | Topdressing, lawns, long-term soil building |
| Potting mix rate | 5–10 g per litre | Not ideal for potting mix |
| Outdoor bed rate | 100–200 g per m² | 100–300 g per m² |
| Lawn rate | Possible but dusty | 100–200 g per m² |
| Frequency | Once or twice a year | Once a year, can leave longer |
| Handling | Dusty, wear a mask in still air | Easy to spread, no dust |
| Compost addition | Excellent | Good |
Apply, water in, leave to it. Basalt weathers via contact with soil moisture, so the more thoroughly it is incorporated into moist soil, the faster mineral release begins. Adding rock dust to an active compost heap is particularly effective: the microbial activity and heat in a working heap accelerate weathering, and by the time the compost is finished, the basalt has already begun releasing.
Decision guide
Which form for which job?
It depends on what you are growing in and what timeframe you are working on.
Beds reward granular
You are feeding this season's crop and the soil that will grow next year's. Granular basalt's slow release matches the timescales of a perennial bed or a long-running allotment plot. A single application persists through several seasons.
Pots respond best to micronised
A small soil volume gets through nutrients faster and silicon gets used quickly. The faster weathering of the powder form lets the plant access minerals within the same season.
Lawns take granular, topdressed
Apply across a lawn in spring or autumn, water in, and let it work. Granular won't dust onto the wind, won't smother the grass, and releases steadily as the lawn grows through it.
Either form, used generously
Microbial activity in an active heap accelerates weathering of the rock surfaces. By the time the compost is finished, the basalt has already begun releasing minerals into the surrounding organic matter.
Long-term growers run both
Granular sets the slow background, the mineral reserve that quietly releases over years. Micronised is the lever you reach for when a plant looks deficient or you are starting a new bed and want minerals in solution this season. Most growers I know who care about flavour use both, applied at different times for different purposes.
Volcanic rock dust, micronised and granular
Diabase basalt with naturally occurring zeolite minerals for nutrient retention. Suitable for use in organic gardens. Made in Stockport, Greater Manchester. Plant-based with no animal by-products and no slaughterhouse waste, in line with the rest of the Dr Forest range.
A long view
Most fertiliser is bought to feed this season's crop. Rock dust is bought to feed the next decade's. You apply it once or twice a year, water it in, and forget about it. The plants grown on a slowly remineralising soil will taste of more, store longer, and resist drought and disease better than the ones grown on a soil that only ever sees nitrogen, phosphorus and potassium. That is most of the story. The carbon you remove from the atmosphere while you do it is a useful side effect.
Frequently asked questions
What is volcanic rock dust?
Volcanic rock dust is finely ground basalt, a dense, dark igneous rock formed from cooled volcanic lava. It contains over 60 minerals and trace elements including silicon at roughly 48%, plus calcium, iron, magnesium, potassium, manganese, zinc, copper, boron, molybdenum, cobalt, nickel and selenium. Applied to soil it remineralises depleted reserves and supplies plant-available silicon over months to years.
Does volcanic rock dust really make food taste better?
It can, particularly if your soil is low in trace minerals to begin with, and most UK garden soils are. Plants make flavour compounds, aroma volatiles and antioxidant phenolics using enzymes that depend on trace metal cofactors like manganese, zinc, copper, molybdenum, cobalt and nickel. A peer-reviewed review in the Journal of Experimental Botany in 2020 also showed that adequate silicon activates the phenylpropanoid pathway, which is responsible for many of the phenolic compounds that carry flavour and storage life. Volcanic rock dust supplies all of these in a single application. The effect builds over seasons rather than appearing in the first week.
Why is silicon important for plants?
Silicon strengthens cell walls by depositing as amorphous silica, creating a physical barrier against fungal hyphae and insect feeding. It reduces transpirational water loss, improving drought tolerance. It also activates secondary metabolite production, the phenolics and phytoalexins plants use for chemical defence, which double as flavour, aroma and antioxidant compounds in the harvest. Basalt at roughly 48% silicon is the richest practical natural source.
What is the difference between micronised and granular rock dust?
Micronised is ground to a powder for fast mineral release, useful for potting mix, foliar drench and deficiency response in the same season. Granular releases minerals slowly over months to years and is better suited to topdressing, lawns and long-term soil building. Both are the same diabase basalt mineral, just at different particle sizes.
Does volcanic rock dust help with carbon sequestration?
Yes. Basalt weathering reacts silicate minerals with water and dissolved CO₂ to form stable bicarbonates that are washed into groundwater and eventually the ocean, where the carbon stays locked away on geological timescales. A 2024 PNAS field trial measured 10.5 tonnes of CO₂ removed per hectare over four years from basalt-amended farmland, alongside higher crop yields. A 2025 paper in Nature projected that enhanced weathering across US agricultural land could remove 0.16 to 0.30 gigatonnes of CO₂ per year by 2050.
How do I apply rock dust in the garden?
Micronised: 5 to 10 grams per litre of potting mix, or 100 to 200 grams per square metre to outdoor beds, worked into the top few centimetres. Granular: 100 to 300 grams per square metre as a topdressing, or 100 to 200 grams per square metre on lawns. Both forms can be added generously to a compost heap during heap building. Apply once or twice per year, with spring and autumn the ideal windows. Water in after application.
Is volcanic rock dust suitable for organic gardening?
Yes. Volcanic rock dust is a natural mineral with no chemical processing, just quarried, crushed and ground. It is suitable for use in organic gardens and allotments. Dr Forest's volcanic rock dust is plant-based and contains no slaughterhouse by-products or animal-derived ingredients of any kind.
Can I use rock dust with other fertilisers?
Yes, rock dust works best alongside a complete feeding programme. It supplies trace minerals and silicon but contains very little nitrogen, phosphorus or potassium, so it complements rather than replaces a regular fertiliser. Dr Forest incorporates basalt rock dust into the Premium Tomato Fertiliser, the Fruit and Vegetable Fertiliser, and the wider premium fertiliser range, alongside polyhalite, seaweed, humic acid and alfalfa meal.
Sources cited
The working, shown
- Beerling, D.J. et al. (2024). Enhanced weathering in the US Corn Belt delivers carbon removal with agronomic benefits. PNAS, 121(9), e2319436121.
- Beerling, D.J. et al. (2025). Transforming US agriculture for carbon removal with enhanced weathering. Nature, 638, 425–434.
- Kelland, M.E. et al. (2020). Increased yield and CO₂ sequestration potential with Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil. Global Change Biology, 26, 3658–3676.
- Skov, K. et al. (2024). Initial agronomic benefits of enhanced weathering using basalt: a study of spring oat in a temperate climate. PLOS ONE.
- Ahanger, M.A. et al. (2020). Integration of silicon and secondary metabolites in plants: a significant association in stress tolerance. Journal of Experimental Botany, 71(21), 6758–6774.
- Vienne, A. et al. (2022). Enhanced weathering using basalt rock powder: carbon sequestration, co-benefits and risks in a mesocosm study with Solanum tuberosum. Frontiers in Climate, 4, 869456.