Rock Dust – Can It Remineralize the Earth?

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Robert Pavlis

Rock dust is a very popular soil additive especially with organic and permaculture groups. It is full of nutrients and it is claimed that adding it to soil will replenish all of the nutrients that agriculture has taken out of our soil. This process of adding nutrients back to soil is known as mineralization.

This seems to make a lot of sense. We remove food from the land, and the food contains lots of minerals. At some point we need to put them back into the soil or else we will have soil that won’t grow anything. This seems logical but is it really true? Is our soil losing fertility? If it is deficient, can rock dust be used to solve the problem? How effective is rock dust and which type of rock works the best? Time to crush some myths about rock dust.

Azomite - a common brand of rock dust
Azomite – a common brand of rock dust

What is Rock Dust?

The simple definition is that rock dust, also known as rock powder and rock flour, is pulverized rock. It can be man-made or occur naturally. Cutting granite for commercial use produces granite dust. It is also a waste product from some mining operations. Glaciers naturally produce glacial rock dust. Rock dust is also found near ancient volcanoes and consists of basalt rock.

To be effective the rock needs to be ground into a very fine powder. That way it is more easily used by microorganisms and decomposed by environmental elements.

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Two common forms of rock, namely limestone and phosphate rock have been used for a long time to amend soil. Although these products are correctly called rock dust, they are usually not included when gardeners talk about “rock dust”, and I will exclude them from this post.

Is Rock Dust a Fertilizer?

Some commercial products call themselves a fertilizer and I even found one that was labeled like a fertilizer showing an NPK of 0-0-1, but by most legal definitions rock dust does not contain enough NPK to qualify as a fertilizer.

Claims Made for Rock Dust

Rock dust is claimed to add all kinds of minerals back to soil. These are the nutrients that plants need to grow. Because of this, rock dust products make all kinds of claims for growing bigger plants, producing higher yields, increasing disease resistance, etc. These are all valid claims if the soil is deficient of one or more nutrients and if rock dust adds the missing nutrient.

There are two clear questions we must answer to validate these claims and I’ll do that in the rest of this post.

Does rock dust add plant available nutrients to soil?

Is soil deficient of nutrients?

If the answer to either question is no, rock dust will not help plants grow.

Before answering these questions, let’s look at some other claims made for rock dust.

Helps restore the correct mineral balance in soil

To be true, this would mean that soil has some kind of “correct balance” to begin with and that this balance is important for plant growth.

It turns out that there are many different kinds of soil, and they vary widely in their mineral composition. There are plants that are adapted to and grow on just about any soil. There is no such thing as a “correct mineral balance”.

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When the correct balance is achieved organic matter is turned into humus

I have news for companies making this claim, microbes turn organic matter into humus in all kinds of situations. In leaf mold it is done without any soil. This is just nonsense from a marketing person reaching for straws.

Plants can complete their life cycle without the full range of minerals but will not produce at their full potential

If plants don’t have the nutrients they need, they will not complete their life cycle – instead they die.

Analysis reports show Lanthanum (La), Cerium (Ce) and Praseodymium (Pr) at 644 ppm

These are rare earth elements, which makes it sound as if you would want them in your soil – who does not want rare stuff? I have heard of the first two, but not praseodymium – I must have been away the day we did experiments with it!

The claims go on to say, “These elements act as cofactors for the methanol dehydrogenase of the bacterium Methylacidiphilum fumariolicum.” So what is this important bacterium?

Methylacidiphilum fumariolicum is an autotrophic bacteria, first described in 2007 growing in volcanic pools near Naples, Italy. It grows in mud at temperatures between 50 °C – 60°C (about 130 °F) and an acidic pH of 2–5.

I guess if you are gardening in hot acidic mud, you might need these rare earth elements to keep your autotrophic bacteria alive. For the rest of us, we don’t need these elements in our soil!

Basalt, an igneous rock, wasn’t processed or transformed by the environment, so the plant nutrients in it, are just as they were when they came out of the center of the Earth

This marketing person seems to be unaware of the fact that the minerals in rock can’t be used by plants until the environment, or life forms convert them into usable nutrients. “Transformed by the environment” is a good thing.

The other desirable quality of the best rock dust powders is that they are paramagnetic

That may be true, but there seems to be no published research to show that paramagnetic rock has any affect on plant growth. However, many pseudoscience groups do make such claims.

Mineral Content of Rock Dust

Rock dust does contain a lot of minerals. I have seen claims ranging from 60 up to 90 different minerals. Azomite is a common product and their analysis list of 74 minerals can be seen here.

I don’t dispute the claims, but there is no evidence that plants need all of these minerals. They use about 20 minerals – that’s it. The other 40 to 70 are not needed by plants.

How Much Should You Use?

I find that this question can tell you a lot about a product. If rock dust is good for gardens, how much should you use? What happens if you use too much?

One site had this recommendation;

3 tons/acre = 14 lb/100 sq. ft. = 1.25 lb/sq. yd.

or

7.5 tons/ha = 750 kg/1000 sq.m = 75 kg/100 sq.m = 750 grams/1 sq.m

But a rate even 8x higher can be used, although it would have to be incorporated into the soil.

You can add anywhere from 3 tons/acre to 24 tons/acre. If 3 was the right number, would 24 not be way too much? Would 24 not burn plants due to the high nutrient load? Only if the product actually added nutrients to soil.

Rate of Decomposition of Rock Dust

rock dust mine
rock dust mine

Earlier in this post, I posed the question, does rock dust add nutrients to soil. There is no doubt that adding rock dust adds the minerals, but I can also do that by laying a big bolder on top of the garden. The bolder will not help plants grow but it does add minerals to the garden. Unless the minerals in the rock decompose to release the nutrients in a form plants can use, there is little point in adding the rock dust.

For this reason I think that one of the most important questions we need to ask is, how quickly does rock dust decompose?

Some of my early reading on the matter indicated time frames of a hundred years. I have searched on many web sites selling rock dust and none have any claims or data to show decomposition happens even after 100 years or more. No one in the industry wants to put a number on this important property.

My recent visit to the Guelph Organic Conference allowed me to discuss rock dust with two suppliers. Neither one has been able to supply any details about decomposition. One never claimed to have such data, and the other only has it available in French – but they did not provide it.

Most studies that look at how quickly rock dust mineralizes are done in the lab. For feldspar, the estimated life of a 1 mm diameter grain is 921,000 years but field testing shows that this number may be as small as 100 years. A new Brazilian lab study using basalt dust indicates that nutrients become available in as little as 3 months. The soils used in these studies had a starting pH of 3.9 and 4.5. Release of minerals slows down dramatically as the pH increases.

The rate at which rock dust dissolves and releases its nutrients depends on the type of rock, the type of soil, the pH of soil, climate, and the mineral balance in the soil (ie presence of other minerals). There is almost zero dissolution in alkaline soil, and a much higher rate in very acidic soil. Low mineral, tropical soils dissolve faster than temperate soils. The studies that do exist have looked at mostly the release of potassium.

If you find some numbers on this please post them in the comments, or even better post them on our Facebook Group, called Garden Fundamentals.

Are Soils Nutrient Deficient?

This is also an important question to ask. Do we have a problem that needs to be fixed?

I had a closer look at this question in a previous post called Is Soil Fertility Decreasing? My conclusion was that growing food in our soil is not reducing its fertility. Therefore, rock dust, assuming it actually works, is a product that tries to solve a problem that doesn’t exist. There are certainly some soil around the world that nutrient deficient and rock dust may help there.

What Does Research Say?

Some papers report some improvements in plant growth with some soils but many show no change. There is limited field work done – it is almost all lab work. I did not find a single paper that measured the chemical characteristics of soil before and after adding rock dust to the field – maybe you can find one for me.

There is some evidence that rock dust may provide an important source of potassium in regions like Africa that tend to have soils which leach nutrients quickly and where fertilizer costs are very high.

Rock dust is used extensively in Brazil and now Embrapa, the Brazilian Agricultural Research Corporation, has come out and said, “there is not enough scientific information to recommend silicate agrominerals as a source of nutrients, especially potassium, or soil conditioners for agriculture.”

The science does not support the use of rock dust for most agricultural areas and even the suppliers of rock dust suggest it has no value in alkaline soil.

Update Nov 2023: A new meta study looking at silicate rock powders (SRPs) concludes that “Although the inherent inconsistency of SRP trials limits the degree to which they can be compared and interpreted, some major findings can be concluded”:

  • SRPs must be seriously considered as soil amendment for strongly weathered soils in the humid- and sub-humid tropics
  • Suggested rocks are those containing fast weathering minerals like feldspathoids, glauconites and basalts.
  • Results on soils in temperate regions remain inconclusive.

What about some citizen science results? This trial is interesting.

YouTube video

If this video does not play, try this link: https://www.youtube.com/watch?v=GxmSvZLqYHo

Summary for the Gardener

Most garden soil is not deficient of nutrients, so there is no point in adding more. If you do have a deficiency as shown by a soil test, add the nutrient that is needed.

For home gardeners, rock dust is a waste of money and natural resources.

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

100 thoughts on “Rock Dust – Can It Remineralize the Earth?”

  1. Robert Pavlis you may want to update your blog because you have been disproven.

    Here is a peer-reviewed study:

    “Basalt dust increased available phosphorus, potassium, calcium, and magnesium levels in the soil about twenty, ten, fifteen, and thirteen times higher than those without the basalt dust, respectively. Maize and bean plants grown in soils enriched with basalt dust showed macro and micronutrient accumulations, up to five times higher than plants without the use of basalt dust.”

    Potential of basalt dust to improve soil fertility and crop nutrition

    https://www.sciencedirect.com/science/article/pii/S2666154322001764

    Basalt dust can add calcium and potassium to the soil within 90 days. I don’t know why Pavlis claims it takes millions of years.

    Reply
    • 1) I never said it takes millions of years. The post reports on what others say.
      2) Thanks for posting the link.
      3) This is a pot culture test. The results are encouraging, but don’t mean very much until the work is redone in actual filed studies.
      4) I’ll updated the post to include the study.
      5) This one study does not “disprove” the statements in the post.
      6) The soils in this study had a pH of 3.9 and 4.5, and the post clearly says that minerals will be released faster in acidic soils. Most garden soils are not in this range.

      Reply
    • Did you read the paper? Which of these rocks are available to gardeners?

      Did you notice that positive results were only found in acidic soils in tropical climates? Or that almost all studies only looked at provide potassium to soils?

      Reply
      • Currently rockdust is available for smallscale farmers/gardeners, often marijuana growers. I am based in South Africa where a lot of diamonds are mined in a type of basalt called Kimberlite. Kimberlite is rich in olivine and olivine is the ideal stuff for enhanced weathering. Enhanced weathering releases the different nutrients in the rockdust and makes them available to plants.
        It is described that it works best in acidic soils. Many agricultural soils have to be treated to prevent acidification as a result of fertiliser applications. usually this is done with calcitic lime or dolomitic lime. Rockdust would do the same AND has more benefits.
        Also water promotes weathering, In South Africa most agriculture makes use of irrigation so also water necessary for weathering is available.
        Currently there are companies looking into commercialising rockdust so that it becomes available for large scale farming, thereby contributing to climate change mitigation as well as making agriculture more sustainable.

        Reply
  2. “I guess if you are gardening in hot acidic mud, you might need these rare earth elements to keep your autotrophic bacteria alive.” I can’t stop laughing. Love the blog and totally agree but this absolutely killed me.

    Reply
  3. Dear Robert, I love the fact that you always make me doubt and double check. Thanks for that.

    There are several studies that looked into rock dust and is not black and white. Some also looked at the change in soil chemical properties.

    What strikes me the most is the finding of some studies where rock dust was applied together with specific microbial inoculants, with compost, or preprocessed to enhance particles weathering. When the soil is biologically active, there are groups of microbes that help in turning the minerals in plant available forms. Likewise, the roots
    are capable to influence the pH in the rhizosphere and thus steer mineral release.

    These systems are inherently complex and what may work in one place may not work in another.

    This student in Austria collated a lot of info. It is not a peer reviewed paper itself, but some solid references are cited.

    https://unipub.uni-graz.at/obvugrhs/download/pdf/1510985?originalFilename=true

    hope you find this bit helpful

    Reply
    • Thanks for posting – interesting review.

      Here are some summaries.

      ” the scientific evidence about the true effectiveness is dispersed and contradictory, mainly due to the slow solubility of the rocks in the soil”

      “A significant prevalence of positive results was obtained when rocks were applied with compost, manure and specific bacteria or if they were acidulated in advance.”

      “Generally, silicate rocks contain a high percentage of elements that have little agronomic importance and large quantities have to be applied in comparison to chemical fertilizers” It seems like the main nutrient plants get from rock dust is potassium.

      “As a wide range of different materials is sold as ‘rock dust’ or ‘rock fertilizer’, no general conclusion can be madeAs a wide range of different materials is sold as ‘rock dust’ or ‘rock fertilizer’, no general conclusion can be made”

      “Trials suggest that the efficiency is best at places where it would be needed most, namely in tropical regions with weathered, highly nutrient depleted soils, where conventional fertilizers are rarely affordable and connected to declining use efficiencies.”

      Based on these comments, there is no reason gardeners should start using this product.

      Reply
  4. Great point. Even if there is a study one day proving some minerals are released to some degree over time , there will always be a more cost/time effective alternative for the minerals you are trying to add.

    Reply
  5. The issue is a little more complicated than just plant nutrients. See this podcast via the Leverhulme Centre for Climate Change Mitigation based at Sheffield University, UK. It has the potential to increase soil sequestration of carbon dioxide in the atmosphere – complex interactions with the soil bacteria, and also release of calcium and magnesium.
    https://open.spotify.com/episode/1vlecULewR7uediYMLvPNE?si=427rxVQjQwuH2Ty-upLU1A&dl_branch=1
    The secondary source of information that led me to this was a radio programme broadcast today on BBC Radio 4. For those who can access it (I’m guessing UK based only) here is the 15 minute programme:
    https://www.bbc.co.uk/sounds/play/m000z0k2

    Reply
  6. You have said that it takes hundreds if not thousands of years for the minerals in rocks to be released so the plants can use them but this is not true for all minerals.

    “Unless the minerals in the rock decompose to release the nutrients in a form plants can use, there is little point in adding the rock dust.”

    What about calcium? Calcium carbonate, in nature, commonly dissolves in water. It does not take hundreds or thousands of years. How do you explain all that calcium in drinking water?

    Hard water contains high concentrations of dissolved minerals from rocks, usually calcium does not take hundreds or thousands of years to get to us. Limestone which is usually composed of pure calcium carbonate has an enhanced solubility when it comes into contact with acid rain.

    Are you saying that the calcium present in rock dust will take hundreds or thousands of years to dissolve into the soil?

    Water (rainwater) mixes with carbon dioxide in the air or soil an acid solution called carbonic acid is former. This acid can flow through the cracks of rocks and it can cause it to dissolve. This is observable in hours, not hundreds of years.

    You can try this experiment take some vinegar and put it on some chalk the acid starts to dissolve the calcium carbonate. The vinegar dissolves the calcium carbonate and the chalk will be completely dissolved in a few days.

    “The best information I have is a casual comment that it is about 100 years. At that rate the product is essentially useless.”

    This statement is not true as shown with calcium. Some minerals present in rock dust can be dissolved with rainwater or water into the soil within days.

    Reply
    • Since when is calcium carbonate the same as rock dust???

      Even calcium carbonate dissolves slowly – have you ever seen a limestone rock in nature? Rain has a pH about 5.5 and has been dropping millions of years – the limestone rocks are still there – but a bit smaller.

      Reply
  7. Great article thanks, have you (or know of anyone who has) tried to grow any plants in straight rock dust? Perhaps throw in a few bacteria for good measure and bring it to life a bit and see what happens. I have read in ‘Organic Growing With Worms’ that said you can’t add too much rock dust so this would be the ultimate test of that. Also, I was thinking you could easily test to see if the plants absorb the minerals by applying it to soil that is deficient in one particular mineral that is in volcanic rock dust and then test to see if the plant (that grows in it) takes it up. Has anyone done that?

    Reply
    • Rock dust is just very fine sand. Bacteria would not survive since there is nothing for them to eat.
      Never seen anyone grow in rock dust.

      Reply

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