Rock dust: an honest look

Rock dust is crushed silicate rock. It holds most plant nutrients but gives them up over years, not weeks, and the trial record in temperate soils is mixed. We sell one ourselves, so here is what the published evidence supports, what it does not, and when a New Zealand gardener should spend the money elsewhere.

What is in basalt and andesite dust

Two open access papers publish whole rock analyses of quarry dusts used in trials.

The Tawau rock is one of the six and the closest to an andesite. One basalt was below the copper detection limit of 40 mg/kg. Neither paper reports boron. A total is what the rock holds, not what a plant can take up this season. Our listing for Ultra Paramagnetic Andesite Rock prints an analysis, which we have not repeated here because we could not check it against a laboratory report. [LAB RESULT NEEDED: laboratory report for the andesite with units, method and date, including boron, nickel, chromium, cadmium, lead and arsenic]

How slowly silicate rock weathers

Nutrients come out only as the silicate minerals dissolve. A geochemical review warned that slow dissolution may rule rock powders out unless soil and powder are well matched (Harley and Gilkes 2000). Four things set the pace.

• Particle size. Dissolution happens at the grain surface, and halving grain diameter doubles the surface per gram. Quarry dusts can be coarse: the oat trial below used basalt with a median grain of 1,147 micrometres (Skov et al. 2024).
• pH. Acid speeds silicate dissolution, and near neutral is the slow end of the range (Palandri and Kharaka 2004). Near neutral pH, even the faster silicates in basalt dissolve at least 30,000 times more slowly than calcite, which is why rock dust is not a substitute for lime (Skov et al. 2024). A living soil held at pH 6.2 to 7.2 is in that slow range.
• Biology. Roots and microbes release acids and organic compounds that attack mineral surfaces, and plant uptake and leaching keep the reaction going (Harley and Gilkes 2000). This speeds up a slow process. It does not make it fast.
• Temperature. From published activation energies for two pyroxenes (40.6 and 78.0 kJ per mole), the same grain dissolves 2.4 to 5.3 times more slowly at 10 °C than at 25 °C (Palandri and Kharaka 2004; our arithmetic). Cool soils are slow soils.

Rates also fall with time. Feldspar in fresh granite weathered more and more slowly over six years in laboratory columns, and field rates were orders of magnitude lower again (White and Brantley 2003). The best field figure we found is from Illinois: after four yearly dressings of 50 tonnes per hectare, about 16% of the calcium and magnesium in the basalt had been released (Beerling et al. 2024).

What the trials and reviews found

A 2022 review of 48 crop trials found promise for highly weathered tropical soils and called the benefits for temperate soils inconclusive (Swoboda et al. 2022).

Trials that found little or nothing. A three year outdoor experiment with a commercial volcanic rock dust, on three soil types with wheat and then forage, found no effect on plant growth, plant nutrient content or soil microbial activity. The authors called the dust probably inert (Ramezanian et al. 2013). In Western Australia, granite dust had no effect on wheat yield in the field, while NPK fertiliser lifted it about threefold, and as a potassium source the dust was 0.02 to 14% as effective as potassium chloride (Bolland and Baker 2000). An earlier glasshouse study there found granite powder helped wheat on one responsive soil by supplying potassium, while diorite powder did nothing (Hinsinger et al. 1995).

Trials that found a benefit. Sorghum yield rose 21% with basalt at 10 kg per square metre in a controlled environment (Kelland et al. 2020). Maize and soybean yields rose 12 to 16% over four years with 50 tonnes per hectare each year on an acid soil (Beerling et al. 2024). Spring oats yielded 9 to 21% more in one dry English season after 18.86 tonnes per hectare, which the authors put down mainly to a small pH rise from calcite in that basalt (Skov et al. 2024). Note the rates: 19 to 100 tonnes per hectare is 1.9 to 10 kg per square metre. A sprinkle does not reproduce these trials.

The paramagnetism claim

Paramagnetism is real physics: every material has a magnetic susceptibility, a measure of how strongly an applied field magnetises it. A geophysics reference table lists andesite at 170,000, basalt at 250 to 180,000, quartz slightly below zero and magnetite at 1,000,000 to 5,700,000 (volume susceptibility, in millionths of an SI unit) (Hunt et al. 1995). Magnetite is ferrimagnetic, not paramagnetic, so a high reading on a volcanic dust most likely reflects how much iron oxide it holds. That is our inference, not a test of our product.

The idea that this property drives plant growth comes from Philip Callahan's 1995 book (Callahan 1995) and articles in a farming magazine. One article describes student projects and in-house tests with sealed vials of rock dust buried in pots, and notes that some results were observational only and so not publishable in scientific journals (Dykstra 2000). We searched for a controlled, peer reviewed trial that separates a magnetic effect from the minerals in the rock and found none. A review of plants and magnetic fields describes the wider literature on weak fields as full of contradictory reports with few independent replications (Maffei 2014). Treat a paramagnetic reading as a description of the rock, not as evidence of a benefit.

The arithmetic: one bed, one dressing

Take a 10 square metre bed dressed at 2.5 kg per square metre, the crop rate on our listing: 25 kg of dust. Using the Tawau figures above:

• Zinc: 25 kg × 70.6 mg/kg = 1,765 mg, or 1.8 g.
• Copper: 25 kg × 27.2 mg/kg = 680 mg, or 0.7 g.

Now the same amounts as sulphates. Zinc sulphate monohydrate is 36.4% zinc (65.38 ÷ 179.46), so 1,765 mg of zinc is 4.8 g of zinc sulphate. Copper sulphate pentahydrate is 25.5% copper (63.55 ÷ 249.69), so 680 mg of copper is 2.7 g.

Those are totals locked in the rock. If zinc came out at the pace calcium and magnesium did in Illinois, about 16%, the release would be 1,765 mg × 0.16 = 282 mg, the zinc in 0.8 g of zinc sulphate. That step is an illustration, not a measurement. Either way, 25 kg of rock carries the zinc of under 5 g of sulphate and delivers a fraction of it in the first few years. The Illinois trial did measure more zinc and manganese in grain, but at 50 tonnes per hectare every year (Beerling et al. 2024). Trace elements are the weakest reason to buy rock dust.

What rock dust may legitimately offer

• A slow reserve of calcium, magnesium and potassium. In a three month incubation of seven highly weathered Queensland soils, finely ground basalt (under 150 micrometres) raised pH, cation exchange capacity and exchangeable calcium, magnesium and potassium, with most of the cations still held in reserve (Gillman et al. 2002). In Illinois, 50 tonnes of basalt per hectare a year released at least 23 kg of potassium per hectare a year (Beerling et al. 2024).
• Silicon. Sorghum shoots held 26% more silicon with basalt (Kelland et al. 2020). Silicon is beneficial rather than essential, and yield responses are mostly reported in accumulator crops such as rice and sugarcane on highly weathered soils (Haynes 2014). If silicon is the goal, laboratory rate data put wollastonite roughly 170 to 1,200 times faster than two pyroxenes common in basalt (Palandri and Kharaka 2004; our arithmetic), so Calsil Boost is the more direct tool.
• Carbon. Weathering silicates takes up carbon dioxide, but estimates run from 10.5 tonnes per hectare over four years in Illinois (Beerling et al. 2024) to about 10 kg per hectare a year on dry, lime-rich English soil (Buckingham et al. 2022).
• Peat mixes. We found no controlled trial in a peat based living soil. Growers add rock dust for weight, grit and a long term reserve. That is practice, not proof.

One caution: basalts can carry nickel and chromium (see the table), and modelling of 40 tonnes per hectare a year found soil copper and nickel limits could be passed within a decade (Dupla et al. 2023). Ask any supplier, including us, for a full analysis.

Is it worth the money in New Zealand?

More likely worth it: strongly weathered soils with low nutrient reserves. In New Zealand those are the Oxidic, Granular and Ultic soils, mainly in the northern North Island, which together cover under 5% of the country (Manaaki Whenua Landcare Research n.d.). Also long lived beds and reused living soil where you want a slow reserve, if the dust is fine and cheap enough to use by the kilogram per square metre. We found no published New Zealand vegetable trial.

Not worth it: fixing a shortage this season, supplying trace elements, raising pH, or paying for a paramagnetic number. Steve Solomon, whose gardening advice rests on soil tests and measured minerals, said of rock dust in 2017, "I've always had my doubts", and asked for proof by leaf analysis (Solomon 2017). That is his opinion, not a trial, but the test is a fair one.

What to do

1. Get a soil test before buying any mineral: see where to test your soil and the soil planner.
2. Correct pH with lime, such as Superfine AgLime, not rock dust.
3. Correct a tested trace element shortage with measured grams of a sulphate or chelate, such as zinc sulphate, never on a guess.
4. If you still want rock dust, treat it as a slow reserve. Choose a fine grade, work it into the root zone, and price it per square metre at the label rate first.
5. Test the claim yourself: leave one strip or pot without it and send leaves from both to a lab.

References

• Beerling, D.J., Epihov, D.Z., Kantola, I.B., Masters, M.D., Reershemius, T., Planavsky, N.J., Reinhard, C.T., Jordan, J.S., Thorne, S.J., Weber, J., Val Martin, M., Freckleton, R.P., Hartley, S.E., James, R.H., Pearce, C.R., DeLucia, E.H. and Banwart, S.A. (2024) 'Enhanced weathering in the US Corn Belt delivers carbon removal with agronomic benefits', Proceedings of the National Academy of Sciences, 121(9), e2319436121. https://doi.org/10.1073/pnas.2319436121
• Bolland, M.D.A. and Baker, M.J. (2000) 'Powdered granite is not an effective fertilizer for clover and wheat in sandy soils from Western Australia', Nutrient Cycling in Agroecosystems, 56(1), pp. 59 to 68. https://doi.org/10.1023/A:1009757525421
• Buckingham, F.L., Henderson, G.M., Holdship, P. and Renforth, P. (2022) 'Soil core study indicates limited CO2 removal by enhanced weathering in dry croplands in the UK', Applied Geochemistry, 147, 105482. https://doi.org/10.1016/j.apgeochem.2022.105482
• Callahan, P.S. (1995) Paramagnetism: Rediscovering Nature's Secret Force of Growth. Metairie, Louisiana: Acres U.S.A. Library record: https://openlibrary.org/books/OL823361M
• Dupla, X., Möller, B., Baveye, P.C. and Grand, S. (2023) 'Potential accumulation of toxic trace elements in soils during enhanced rock weathering', European Journal of Soil Science, 74(1), e13343. https://doi.org/10.1111/ejss.13343
• Dykstra, T.M. (2000) 'Paramagnetic effects on plant growth', Acres U.S.A., September. Reprint available at: https://www.biostim.com.au/paramagnetic-article.pdf (Accessed: 4 October 2026).
• Gillman, G.P., Burkett, D.C. and Coventry, R.J. (2002) 'Amending highly weathered soils with finely ground basalt rock', Applied Geochemistry, 17(8), pp. 987 to 1001. https://doi.org/10.1016/S0883-2927(02)00078-1
• 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), pp. 11 to 36. https://doi.org/10.1023/A:1009859309453
• Haynes, R.J. (2014) 'A contemporary overview of silicon availability in agricultural soils', Journal of Plant Nutrition and Soil Science, 177(6), pp. 831 to 844. https://doi.org/10.1002/jpln.201400202
• Hinsinger, P., Bolland, M.D.A. and Gilkes, R.J. (1995) 'Silicate rock powder: effect on selected chemical properties of a range of soils from Western Australia and on plant growth as assessed in a glasshouse experiment', Fertilizer Research, 45(1), pp. 69 to 79. https://doi.org/10.1007/BF00749883
• Hunt, C.P., Moskowitz, B.M. and Banerjee, S.K. (1995) 'Magnetic properties of rocks and minerals', in Ahrens, T.J. (ed.) Rock Physics and Phase Relations: A Handbook of Physical Constants. AGU Reference Shelf 3. Washington, DC: American Geophysical Union, pp. 189 to 204. https://doi.org/10.1029/RF003p0189
• Kelland, M.E., Wade, P.W., Lewis, A.L., Taylor, L.L., Sarkar, B., Andrews, M.G., Lomas, M.R., Cotton, T.E.A., Kemp, S.J., James, R.H., Pearce, C.R., Hartley, S.E., Hodson, M.E., Leake, J.R., Banwart, S.A. and Beerling, D.J. (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), pp. 3658 to 3676. https://doi.org/10.1111/gcb.15089
• Lewis, A.L., Sarkar, B., Wade, P., Kemp, S.J., Hodson, M.E., Taylor, L.L., Yeong, K.L., Davies, K., Nelson, P.N., Bird, M.I., Kantola, I.B., Masters, M.D., DeLucia, E., Leake, J.R., Banwart, S.A. and Beerling, D.J. (2021) 'Effects of mineralogy, chemistry and physical properties of basalts on carbon capture potential and plant-nutrient element release via enhanced weathering', Applied Geochemistry, 132, 105023. https://doi.org/10.1016/j.apgeochem.2021.105023
• Maffei, M.E. (2014) 'Magnetic field effects on plant growth, development, and evolution', Frontiers in Plant Science, 5, 445. https://doi.org/10.3389/fpls.2014.00445
• Manaaki Whenua Landcare Research (n.d.) 'Soil orders: Oxidic Soils, Granular Soils and Ultic Soils', New Zealand Soils Portal. Available at: https://soils.landcareresearch.co.nz/topics/soil-classification/nzsc/soil-orders (Accessed: 4 October 2026).
• Palandri, J.L. and Kharaka, Y.K. (2004) A compilation of rate parameters of water-mineral interaction kinetics for application to geochemical modeling. U.S. Geological Survey Open-File Report 2004-1068. https://doi.org/10.3133/ofr20041068
• Ramezanian, A., Dahlin, A.S., Campbell, C.D., Hillier, S., Mannerstedt-Fogelfors, B. and Öborn, I. (2013) 'Addition of a volcanic rockdust to soils has no observable effects on plant yield and nutrient status or on soil microbial activity', Plant and Soil, 367, pp. 419 to 436. https://doi.org/10.1007/s11104-012-1474-2
• Skov, K., Wardman, J., Healey, M., McBride, A., Bierowiec, T., Cooper, J., Edeh, I., George, D., Kelland, M.E., Mann, J., Manning, D., Murphy, M.J., Pape, R., Teh, Y.A., Turner, W., Wade, P. and Liu, X. (2024) 'Initial agronomic benefits of enhanced weathering using basalt: A study of spring oat in a temperate climate', PLOS ONE, 19(3), e0295031. https://doi.org/10.1371/journal.pone.0295031
• Solomon, S. (2017) Interviewed by Tad Hussey in 'Episode 6: Steve Solomon Covers Soil Remineralization & Nutrient Density', Cannabis Cultivation and Science Podcast, KIS Organics, 24 June. Transcript available at: https://www.kisorganics.com/blogs/podcast/cannabis-cultivation-and-science-podcast-episode-6 (Accessed: 4 October 2026).
• 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. https://doi.org/10.1016/j.scitotenv.2021.150976
• White, A.F. and Brantley, S.L. (2003) 'The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field?', Chemical Geology, 202(3 to 4), pp. 479 to 506. https://doi.org/10.1016/j.chemgeo.2003.03.001