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.
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.
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”.
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
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.
Lab testing of Azomite added to soil showed no change in nutrient levels after two weeks.
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.
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.
That is for soil based medium, but what about soilless mixes? Would rock dust be a beneficial add to a mix comprised of only coir, vermiculite and perlite?
Does it decompose fast enough to provide any benefit? Peat is acidic, so maybe it releases some nutrients, but the manufacturers of the product need to demonstrate this.
Soilless mixes are usually used in containers and they should be fertilized with real fertilizer.
I grow trees, long term, in soilless mixes that I have been adding kelp meal and rock dust to. There is no peat used. If there is no benefit to it I’ll stop using it, but I figure it’s one less application of trace elements I need to apply in a liquid form.
Thoughts?
I don’t think rock dust does anything. If it did, why has no one published proof?
Hi
Thanks for the interesting article and interesting comments section! As a PhD candidate in research geology, I feel compelled to add that volcanic ash is very, very different than any type of crushed rock, as significant physical and chemical fractionation occur during the formation of both rock and ash. I would say that comparing ash to crushed rock was like comparing apples to oranges, but rock and ash are much more different than that!
The break down of rock into soil, referred to in geology as weathering, is a painstakingly slow process. Adding, for example, potassium rich mica to a soil could eventually increase the available K content, but probably not at timescales relevant to a garden, or even to humanity. That being said, weathering rates depend on many things, mainly rock composition (different minerals weather at drastically different rates) and climate, but also microbial processes. Some rock dust could change soil mineral contents, whereas other dust would have no effect at all, or even negative effects. This seems to be reflected by the contradictory and inconclusive scientific literature on the subject. Like most things, it’s complicated!
If you are going to apply rock dust, wear a mask and wash your hands afterwards. Asbestos often forms together with mica minerals and as a metamorphic mineral in basalt. Silica dust also poses a serious health risk in crushed rock. Depending on where you source your rock dust, there are lots of health risks to watch out for. I would strongly advise against going local, unless your supplier has done geochemical analyses with your intended use in mind. When I crush rocks, I wear a mask and work under a ventilator. What is inert at large grain sizes can be nasty for your health when crushed finely enough.
Finally, mine tailings are also “rock dust”. Some rocks are toxic and some aren’t, but isn’t it ironic that something miners struggle to dispose of because of its toxicity, is in high demand in the gardening world? Be careful out there!
Thanks for your geological insight. Very interesting.
Volcanic ash is no doubt more quickly incorporated into a soil.
Basalt soils are known for high fertility, thanks to a much higher percentage of phosphorus compared to granite soils, for example.
So, if I live in an area with granite soil, would I benefit from adding a basalt component? The answer is probably yes, but I might be wait a long time. Having a super fine basalt rock powder would speed the process up, but this might introduce some health and safety issues as you describe.
I’m thinking that a practical solution might be to grind it up super fine (talcum powder size, not sand size) and immediate mix with water into a thick slurry, and apply as a liquid soil amendment for safety reasons, and to enhance incorporation into the soil. This then serves as a slow release fertiliser, which might take a decade or more to be gradually digested by worms and microbes.
The main problem with synthetic fertilisers is side effects such as nitrification (acidification). The best way to protect against that is to have large amounts of organic matter in your soil so that you don’t need to add lime.
Very interesting article. You just saved me a bunch of money! The guy I was watching on YouTube was also recommending humic acid, or Leonardite. Any thoughts on that?
https://www.gardenmyths.com/humic-substances-humic-acid-garden/
Wow – you are fast! I did a search after I posted that comment, forgetting that you have a whole website here and probably already covered it. But thanks for the link. 🙂
Thank you for making this! I have been trawling through so much B.S. trying to learn more about rock dust. Ever since I started taking college science classes I have been astounded by the sheer volume of nonsense, misinformation, and pure tripe that tries to pass itself off as scientific. Thank you so much for taking a logical unbiased look at rock dust !!!
Hi brilliant. I have been adding building sand river sand lava rock dust. So I will have to stop using these amendments.
I’ve been thinking about using rick dust as an ammendment for my acidic sandy soil (in a high-rainfall area where nutrients leach out quickly).
My thinking is that it could bring the sandy soil somewhat closer to clay by adding fine particals (better nutrient retention) that act as (very) slow release fertilizer.
My understanding of why are clay soils so much more fertile than sandy soil (apart from the organic matter content which can be fixed easily) is that they are made of very tiny mineral particles which release nutrients more than big sand grains. Is this not true? Is there another reason why clay is more fertile? And what would you say makes soil around volcanoes so fertile?
“If plants don’t have the nutrients they need, they will not complete their life cycle – instead they die.”
This is not really true. It actually takes quite a lot of not optimal (stunted, slow, etc.) growth before a plant show nutritional deficiency (chlorosis for example) and even more before it dies (a plant can live chlorotic for years).
Decomposition “The best information I have is a casual comment that it is about 100 years. At that rate the product is essentially useless.”
Do you assume that it just do nothing for 100 years and then it decomposes all at once, releasing the nutrients? Or that it does nothing for 100 years and then start acting as a slow release fertilizer? Both is obviously a nonsense. I would assume it decomposes slowly all the time but possibly too slowly to add any benefit.
“bring the sandy soil somewhat closer to clay” – nothing will do this except millions of years.
“why are clay soils so much more fertile than sandy soil (apart from the organic matter content which can be fixed easily) is that they are made of very tiny mineral particles which release nutrients more than big sand grains.” – that is not correct. Organic matter is not part of clay – they are different particles in soil. Clay is not nutritious because it releases nutrients. Clay is charged and holds onto nutrients that come from other sources like organic matter, microbes, fertilizer.
““If plants don’t have the nutrients they need, they will not complete their life cycle – instead they die.”” is true. Try growing plants without calcium – they die.
“Do you assume that it just do nothing for 100 years” – no. It is a slow process that start when it is added and continues for several hundred years. But the rate of adding nutrients is so slow that it does not help your plants.
So the difference between clay and sandy soil is simply about nutrient holding ability, rather than one being inherently more fertile than the other?
What do you recommend for sandy soil? I add a lot of organic matter (horse/farmyard manure seems to work best) and also shop bought topsoil and obviously I feed plants although I am always worried about overfeeding so maybe I don’t feed enough. In some places, I have more amendments than the original soil. But still, plants that are heavy feeders (roses) grow better for me in pots than in the ground.
Is the answer simply continue with adding organic matter and maybe feeding more?
Organic matter will help sand hold onto nutrients. Fertilizer will feed plants in the short term. I think are needed for sandy soil.
Robert, I was recently advised at my local garden center to try Rock Dust to help an ailing Aloe plant, but I was skeptical. I have an Environmental Engineering background and I thought, as you also point out many times, how would rock dust work since the minerals in rocks are not bioavailable for a long long time and cannot immediately help plants. Turns out I just need to cut back on water to help the Aloe, but in my search on the benefits of rock dust, I found your column and am fascinated by the Q&A that follows. Your most common response, “Show me the scientific study.” reminds me so much of the “Show me the money” line in the movie Jerry Maguire. Thanks for keeping it real and scientific.
Hi,
Leaving out things like limestone, I agree minerals may be present in rock dust but are not readily available for plants because of grain size, soil characteristics, the molecular formation, etc. I am curious however about its physical properties. I have very fine, ground rock dust from well drilling that resembles clay. I am thinking about spreading it on a coarse sand and gravel soil around our cottage to improve water and mineral retention. What do you think?
Excellent Blog. Tha you all!!!
What do you make of the research coming out of Sheffield University here in the UK in 2020? Does it change your stance at all?
Thanks
If you add some links – I could look at the information.
Posted to the guy that raised it below:
This is recent:
https://www.theguardian.com/environment/2020/jul/08/spreading-rock-dust-on-fields-could-remove-vast-amounts-of-co2-from-air
“The chemical reactions that degrade the rock particles lock the greenhouse gas into carbonates within month…,”
https://www.nature.com/articles/s41586-020-2448-9
Looks like it does something after all
lol!
The first link is an unreliable source of information. – not a study.
The second link is not a study either. It is a modeling exercise “Here we use an integrated performance modelling approach to make an initial techno-economic assessment for 2050”, and does not even mention an increase in nutrients in the soil.
This links to a study showing 20% yield increase for sorghum:
“Here we report that amending a UK clay‐loam agricultural soil with a high loading (10 kg/m2) of relatively coarse‐grained crushed basalt significantly increased the yield (21 ± 9.4%, SE ) of the important C4 cereal Sorghum bicolor under controlled environmental conditions, without accumulation of potentially toxic trace elements in the seeds.”
https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.15089