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Granulated Volcanic Rock Dust UK | Basalt | Slow Release

Name: Granulated Volcanic Rock Dust UK | Basalt | Slow Release
Brand: Dr Forest
Price: 11.00 GBP
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Looking for the micronised version for foliar spray and root drench? → Micronised Rock Dust

Overview The Science How to Use FAQ

Granulated volcanic rock minerals — ancient diabase basalt for lawns, top-dressing & long-term soil remineralisation

Ancient Diabase Basalt 48% Silica 60+ Trace Elements Contains Zeolites OF&G Organic Slow-Release Granular

The most fertile soils on Earth — the deep, dark, mineral-rich soils of volcanic regions like Iceland, Java, the Azores, and the slopes of Mount Etna — share one thing in common. They sit on basalt. When volcanic rock weathers naturally over millennia, it releases a steady stream of minerals and trace elements that create conditions for extraordinary plant growth. Volcanic rock dust is the practice of accelerating that process deliberately — applying crushed basalt to garden soil to remineralise it in a single season rather than waiting thousands of years.

This product is ancient diabase basalt formed in volcanic eruptions millions of years ago, sourced from pristine geological deposits and crushed into a granular form for controlled, sustained mineral release. It contains 48% silica, 8.3% calcium, 5.1% potassium, 4.1% iron, plus magnesium, manganese, zinc, copper, boron, molybdenum, titanium, and dozens of other trace elements — along with unique mineral components including zeolites (which capture and slowly release nutrients, reducing leaching) and phonolites (which stimulate beneficial soil microbial activity).

The granular form is designed for sustained release over months and years — it goes down with a broadcast spreader and releases progressively over successive seasons, making it ideally suited to lawn renovation, raised-bed top-dressing and orchard floors where a dusty powder is not practical. It is less dusty and easier to handle than micronised basalt, and provides a long-term mineral reservoir in the soil that continues weathering and releasing trace elements with each watering and rainfall event. Completely natural, unprocessed, and OF&G certified organic.

48%Silica (SiO₂)

8.3%Calcium

5.1%Potassium

4.1%Iron

What volcanic rock dust granules are used for

Soil remineralisation for vegetable plots, beds and borders — decades of cropping and rainfall leaching strip trace minerals from garden soils; basalt granules restore the full mineral spectrum that creates genuinely fertile, productive soil
Silica supply for stem strength, pest resistance and disease defence — silica deposited in plant cell walls creates a physical barrier against piercing-sucking insects (aphids, whitefly, spider mites) and fungal penetration; basalt is the richest practical source of plant-available silica
Long-term trace element reservoir in soil and growing media — the granular form continues weathering and releasing minerals over months and years, building a sustained mineral reserve rather than a single-application spike
Improving flavour and sugar content in fruit, vegetables and herbs — trace minerals (iron, manganese, zinc, copper) are enzyme cofactors for sugar synthesis, organic acid production, and aromatic compound generation; mineral-rich soil produces measurably more flavourful crops
Potting soil and container media amendment — mix into growing media at the soil-build stage to provide the geological mineral base that bagged composts and peat-free mixes lack entirely
Compost heap enrichment — adding rock dust to an active compost heap supplies trace minerals to the finished compost and accelerates weathering (the microbial heat and acidity in compost breaks down the rock particles far faster than soil alone)
Lawn top dressing — broadcast across lawns for sustained trace mineral supply; less dusty and easier to spread than micronised powder; settles between grass blades and weathers into the root zone over successive seasons
Tree, shrub and hedging planting — mix into backfill soil to provide a long-term trace mineral reservoir in the root zone; the granules continue releasing minerals as the tree establishes over its first years

Granulated vs micronised rock dust — which to use

Granulated Basalt (this product)

Coarser granular form — designed for sustained, long-term mineral release
Less dusty and easier to handle, store, and spread than powder
Ideal for top dressing, lawn application, and mixing with dry fertiliser blends
Continues weathering and releasing minerals over months to years
The better choice for soil building, long-term amendment, and compost enrichment
Best for applications where you want a mineral reservoir that persists in the soil

Dr Forest Micronised Volcanic Rock Dust

Finely micronised powder — much faster mineral release due to greater surface area
Minerals become plant-available within weeks rather than months
Can be mixed into water for liquid application (forms a suspension)
The better choice for rapid remineralisation and immediate trace element supply
More dusty to handle — wear a mask when mixing dry
Use both: granules for the long-term reservoir, micronised for immediate availability

The science of volcanic rock dust: why basalt creates the world's most fertile soils

Basalt — the most mineral-rich common rock on Earth

Basalt is a dense igneous rock formed from cooled volcanic lava. It is the single most abundant rock type on Earth's surface, forming the ocean floors and the vast lava plateaus of volcanic regions. Its agricultural significance is that it contains the broadest spectrum of mineral elements of any common geological material — over 60 elements including every trace mineral that plants require. This is not a coincidence: basalt is the original source of most of the minerals found in fertile soils. When basalt weathers naturally over geological time, the minerals it releases form the foundation of soil fertility.

The observation that volcanic soils are exceptionally fertile is ancient — civilisations from Java to Sicily have farmed volcanic land for thousands of years. Modern soil science has confirmed why: freshly weathered basalt delivers silica, calcium, magnesium, potassium, iron, manganese, zinc, copper, boron, molybdenum, and dozens of other elements in forms that become progressively plant-available through chemical weathering and microbial activity. Applying crushed basalt to depleted garden soil replicates this natural process on a compressed timescale.

Mineral analysis — what this basalt contains

Silicon (SiO₂): 48% — the dominant component; provides plant-available silica for stem strength and pest resistance
Calcium (CaO): 8.3% — cell wall construction, soil aggregate stability, and enzyme cofactor
Potassium (K₂O): 5.1% — enzyme activation, sugar transport, and water balance
Iron (Fe₂O₃): 4.1% — chlorophyll synthesis, electron transport, and enzyme function
Magnesium (MgO): 1% — central atom in chlorophyll; essential for photosynthesis
Manganese: 0.2% — photosynthesis, enzyme activation, and nitrogen metabolism
Zinc: 760 mg/kg — protein synthesis, hormone production, and internode elongation
Copper: 6.5 mg/kg — lignin synthesis, pollen viability, and reproductive development
Boron: 0.1 mg/kg — cell wall formation, pollen tube growth, and sugar transport
Molybdenum: 9 mg/kg — nitrogen fixation in legumes and nitrate reduction in all plants
Also contains: titanium, zeolites, phonolites, sulphur, phosphorus, and dozens of ultra-trace elements

Zeolites and phonolites — the unique components

Zeolites are naturally occurring aluminosilicate minerals with a cage-like crystalline structure
The cages trap and slowly release nutrient ions — acting as a natural slow-release mechanism within the rock itself
This reduces nutrient leaching: minerals are captured by the zeolite structure and released gradually in response to plant root activity
Phonolites are a group of volcanic minerals that preferentially stimulate beneficial soil microbial activity
Research has shown that phonolite-containing rock dusts increase bacterial and fungal populations in treated soils
The combination of zeolites (nutrient retention) and phonolites (biological stimulation) is what makes this diabase basalt particularly effective as a soil amendment

Six mechanisms of action

Trace Element Remineralisation

UK garden soils have been progressively depleted of trace minerals by decades of cropping, NPK-only fertilisation, and natural leaching from British rainfall. Basalt rock dust restores the full spectrum — over 60 elements — in a single application. The granular form weathers gradually through contact with soil moisture and microbial acids, releasing minerals in a steady, sustained supply that mirrors natural geological weathering. This addresses the "hidden hunger" that limits crop quality even when NPK levels are adequate.

Silica for Plant Defence

Silicon is the second most abundant element in basalt (48% as SiO₂) and one of the most underappreciated nutrients in gardening. When absorbed by plant roots, silicon is deposited in cell walls as a physical barrier — a layer of opaline silica that piercing-sucking insects (aphids, whitefly, spider mites) and fungal pathogens cannot easily penetrate. Research has consistently shown that silicon-supplemented plants suffer less pest damage and lower rates of fungal infection. Basalt is the most economical and broadly available source of plant-available silicon for organic gardeners.

Soil Microbial Stimulation

Basalt rock dust provides mineral surfaces and trace element substrates that soil micro-organisms colonise and utilise. The phonolite fraction in particular has been shown to stimulate beneficial bacterial and fungal populations. As microbes weather the rock particles — dissolving minerals through the organic acids they produce — they simultaneously multiply, increasing the biological activity of the soil. This creates a positive feedback loop: the biology weathers the rock, releasing minerals; the minerals feed the biology, which weathers more rock.

Flavour & Sugar Enhancement

The trace minerals in basalt (iron, manganese, zinc, copper) are enzyme cofactors for the metabolic pathways that produce sugars, organic acids, and aromatic volatile compounds in crops. These are the compounds that determine flavour, sweetness, and aroma — the qualities that distinguish exceptional home-grown produce from bland supermarket equivalents. Mineral-rich volcanic soils consistently produce crops with higher Brix readings (sugar content) and more complex flavour profiles. Applying basalt to garden soil replicates the mineral conditions that make volcanic regions famous for food quality.

Zeolite Nutrient Retention

The zeolite minerals within this diabase basalt act as microscopic nutrient reservoirs. Their cage-like crystalline structure captures positively charged nutrient ions (potassium, calcium, magnesium, ammonium) and holds them against leaching. When plant roots release hydrogen ions during normal nutrient uptake, the zeolites exchange their captured nutrients in return — a natural, self-regulating slow-release mechanism embedded within the rock itself. This is particularly valuable in sandy soils and container media where nutrient leaching is the primary cause of poor performance.

Soil Structure Improvement

The physical presence of rock dust granules in soil improves aeration and drainage by creating stable mineral particles that resist compaction. Over time, as the granules weather, the released calcium and magnesium contribute to soil aggregate formation — the same flocculation process that makes volcanic soils so naturally well-structured. The silica released during weathering also contributes to aggregate stability. The net effect over successive seasons of application is a progressively better-structured, better-drained, and more workable soil.

Scientific References

Gillman, G.P. (1980). The effect of crushed basalt scoria on the cation exchange properties of a highly weathered soil. Soil Science Society of America Journal, 44(3), 465–468.
Epstein, E. (1999). Silicon. Annual Review of Plant Physiology and Plant Molecular Biology, 50, 641–664.
Beerling, D.J. et al. (2018). Farming with crops and rocks to address global climate, food and soil security. Nature Plants, 4, 138–147.
Leonardos, O.H. et al. (1987). The use of ground rocks in laterite systems: an improvement to the use of conventional soluble fertilizers? Chemical Geology, 60, 361–370.
Marschner, H. (2012). Mineral Nutrition of Higher Plants (3rd ed.). Academic Press. [Silicon and trace element nutrition]
Manning, D.A.C. (2010). Mineral sources of potassium for plant nutrition. Agronomy for Sustainable Development, 30(2), 281–294.

How to use volcanic rock dust granules: application rates & 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 — basalt 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.

Application rates

Outdoor beds, borders and vegetable plots

Rate: 100–300g per m² | Frequency: 1–2 times per year (spring and autumn)

Scatter evenly over the soil surface and fork or rake lightly into the top few centimetres. Water in well. Use the higher rate (300g/m²) for first-time applications on soils that have never received rock dust, and the lower rate (100g/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

Rate: 5–15g per litre of growing medium | Frequency: Once when building the soil mix

Mix thoroughly into potting compost, peat-free media, or coir 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 (pure coir, peat-free compost) and the lower rate for loam-based or compost-rich mixes that already contain some minerals.

Lawn top dressing

Rate: 100–200g per m² | Frequency: 1–2 times per year (spring and autumn)

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, more disease-resistant grass.

Compost heap enrichment

Rate: A few handfuls per barrowload of material | Frequency: Each time you add material

Sprinkle rock dust granules between layers of compost material. The heat, moisture, and microbial acidity within an active compost heap weathers the rock particles far faster than they would weather in open soil — dramatically accelerating mineral release. The finished compost will be enriched with the full basalt mineral spectrum, creating a genuinely mineral-complete amendment.

Tree, shrub and hedge planting

Rate: 2–3 handfuls (50–100g) mixed into backfill | Frequency: Once at 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 valuable for long-lived plantings where you want sustained mineral supply without repeat applications.

Raised beds — initial construction

Rate: 200–300g per m² mixed into the bed fill | Frequency: Once when building, then 100g/m² annually

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 100g/m² each spring as part of your annual bed preparation.

Step-by-step application

Measure the correct amount. For beds and lawns: 100–300g per m². For soil mixes: 5–15g per litre. A generous handful is approximately 50–75g.
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. Basalt 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.

Rock dust works best in biologically active soil

Minerals in rock dust are released through weathering — a process that is dramatically accelerated by soil micro-organisms. Bacteria and fungi produce organic acids that dissolve mineral surfaces far faster than water alone. This means rock dust is most effective in soil with active biology: soil that has been enriched with compost, treated with Grow-Kashi, or regularly fed with organic matter. In biologically inert growing media (fresh coir, sterile potting mix), mineral release from rock dust will be slower. For the fastest results, combine rock dust with a biological inoculant.

Works well combined with…

For comprehensive soil remineralisation, use the granulated rock dust alongside Dr Forest Micronised Volcanic Rock Dust — the micronised for immediate mineral availability, the granules for the long-term reservoir. Add to Dr Forest Mineral Mix for a complete soil conditioning programme that also includes clay minerals, gypsum, and sea-shell meal. Combine with Grow-Kashi to maximise the biological weathering that releases minerals from the rock. Mix into compost heaps alongside Scottish Seaweed Meal for the richest, most mineral-complete finished compost possible.

Frequently asked questions about volcanic rock dust

Is volcanic rock dust a fertiliser?

Not in the conventional NPK sense. Rock dust contains very low levels of nitrogen and phosphorus. What it provides is the full spectrum of trace elements (over 60), significant amounts of silica, calcium, potassium, and iron, and unique mineral components (zeolites, phonolites) that no NPK fertiliser contains. It is a soil remineraliser and conditioner — use it alongside a fertiliser, not instead of one. The fertiliser provides the macronutrients; the rock dust provides the mineral foundation.

What is the difference between the granulated and micronised rock dust?

Same basalt source, different particle size. The micronised powder has a vastly larger surface area and releases minerals within weeks — it is the fast-acting option. The granulated form (this product) releases minerals over months to years — it is the slow-release reservoir. The micronised is better for immediate remineralisation and liquid application. The granulated is better for top dressing, lawn use, and long-term soil building. Ideally, use both: the micronised for the quick hit, the granules for the sustained reserve.

Will it improve the flavour of my vegetables?

Yes. The trace minerals in basalt are enzyme cofactors for sugar synthesis, organic acid production, and aromatic compound generation. These are the metabolic pathways that determine flavour, sweetness, and aroma in crops. Soils rich in trace minerals produce crops with higher sugar content, more complex flavour, and higher vitamin and antioxidant levels. This is the fundamental reason volcanic soils produce the world's best-quality crops — and rock dust brings those same minerals to your garden.

What does silica actually do for plants?

Silicon absorbed by plant roots is deposited in cell walls as a layer of opaline silica — essentially a glass-like barrier. This physical reinforcement makes stems and branches stronger and more rigid, reducing lodging in tall plants and supporting heavier fruit loads. More importantly, the silica barrier makes it physically harder for piercing-sucking insects (aphids, whitefly, thrips, spider mites) to penetrate leaf and stem tissue, and harder for fungal pathogens to establish infection. Research consistently shows that silicon-supplemented plants suffer less pest and disease damage than unsupplemented controls.

Can I use this on lawns?

Yes — and the granular form is particularly well suited to lawn use. Broadcast 100–200g per m² and water in well. The granules settle between grass blades without smothering the turf (unlike powder, which can coat leaf surfaces). They weather into the root zone over successive months, providing sustained trace mineral supply and silica for stronger, more disease-resistant grass. Apply in spring and/or autumn.

How quickly will I see results?

Rock dust is a long-term soil investment, not a quick-fix product. The granular form releases minerals over months to years as it weathers. Visible effects — improved plant vigour, stronger stems, better pest resistance, enhanced flavour — develop progressively over the first growing season and accumulate with successive applications. If you need faster mineral availability, use the Dr Forest Micronised Volcanic Rock Dust alongside these granules. The effects of rock dust are cumulative and permanent — the minerals you add today remain in the soil indefinitely.

What are zeolites and phonolites?

Zeolites are naturally occurring aluminosilicate minerals with a cage-like crystalline structure that captures and slowly releases nutrient ions — reducing leaching and acting as a natural slow-release mechanism within the rock. Phonolites are a group of volcanic minerals that have been shown to stimulate beneficial soil microbial activity, increasing bacterial and fungal populations in treated soils. Both are naturally present in this diabase basalt and contribute to its effectiveness as a soil amendment beyond the simple mineral content.

How should I store it?

Anywhere dry. This is inert geological material — it does not degrade, spoil, clump, absorb moisture, or lose effectiveness regardless of how it is stored. It has an indefinite shelf life. A sealed bag in a shed, garage, or greenhouse is fine. It is one of the very few garden products that genuinely cannot go wrong in storage.