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.
“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”.”
YES there are many different types of soil and they vary in their composition. From what I understand that through Albrecht’s research he found that soil with particular qualities and ratios grew plants with less diseases and less pest problems. Hence the need for mineral balancing.
Kinda surprised that someone so smart can miss the obvious. Yes soils all have different mineral balance but the ratios they work with have been found to be the optimum and bring the soil to an ideal for optimum growth.
Certainly I think there is much more need for citizen science. And actual good science….one sample is small and I can see a few issues with only one study already.
That if your soil is showing it is well balanced – probably you don’t really need rock dust. SO that partially states the obvious. Plus you disclude that Callihans work was confiscated. So you know that evidence thing. I think people who have previously tested their soils might use a general amendment to keep balancing without testing. So without reviewing their tests and data in the field you cannot truly say no evidence. You can say anecdotal evidence.
Where is the proof for this statement “soil with particular qualities and ratios grew plants with less diseases and less pest problems. Hence the need for mineral balancing.”?
Why would compost need to include manure to contain all minerals plants need? Does plant material need to be processed by digestive tract to free up the minerals and put them in a form plants can take up?
Compost does not need manure. Any plant material will be decomposed by microbes, and in the process they will release all of the minerals.
Excellent article, Robert! I couldn’t agree more, and thanks for posting a link to my video. You may already be familiar with Stephen Legaree’s videos on the topic as well. He sent crop samples to a lab for analysis and found no difference in nutrient density in crops grown in soil amended with rock dust. Yes, it’s citizen science, but the results are interesting. https://www.youtube.com/watch?v=B64NywRTPck
You might be interested in the recent paper linked below, which says:
“Benefits for crop protection against biotic threats from silicate
weathering arise from the production of soluble silicic acid, which
is readily taken up by plants, thereby improving stem strength
and increasing resistance to pests and diseases in major temperate (soybean and wheat, for example) and tropical (sugarcane, maize, rice and oil palm) crops. ”
http://www.life.illinois.edu/delucia/Beerling%20et%20al%20(2018)%20Farming%20with%20crops%20and%20rocks%20NaturePlants.pdf
But how fast is the process? “Enhanced weathering accelerates CO 2 reactions with minerals contained in globally abundant, Mg- and/or Ca-rich rocks,”. My limestone contains both Mg and Ca and it has been reacting with CO2 for millions of years – there is still lots here. If this weathering was working we would be seeing rocks disappear and we would have no CO2 problem.
? Limestone is already a carbonate, how can it react with CO2? You mean it is undergoing wheathering by dissolving in mildly acidic rainwater?
As for speed, the whole point of crushing/grounding the silicates is to accelerate weathering (hence “enhanced weathering”.
Ref 75 (http://planttuff.com/wp-content/uploads/2016/01/A_Review_of_Silicon_in_Soils_and_Plants_and_Its.99600.pdf) goes into more detail on Si-fertilisation and needs.
Most likely, for the majority of gardens, this is unecessary, but as you often say, it depends on the soil, which only tests can tell. I suspect many gardeners don’t need to lime their gardens (especially if they use a lot of compost instead of inorganic fertilisers), but depend on their practice and their soil, it might be beneficial to lime with ground silicates as opposed to simple carbonates.
Agree with this “As for speed, the whole point of crushing/grounding the silicates is to accelerate weathering (hence “enhanced weathering”.”
But how fast is ‘accelerated’? A quick look at the reference does not provide numbers for this.
I browsed a bit, but couldn’t find exact/compared rates depending on particle size (note that I’m a material chemist, not an agro chemist, so I’m not at all familiar with the field). In the last link, it points out to a few studies where yield and disease resistance where increase by silicates applications, and includes a rate of removal:
“Bazilevich et al., 1975; Reimers, 1990; Savant et al., 1997a).
Larger removal rates are estimated from high–Si-accumulator
crops such as wheat, sugarcane, and rice. Sugarcane removes
approximately 300 kg ha−1y−1, whereas rice is as much as 500 kg
Si ha−1y−1”
“Most notable are rice, sugar beet, sugarcane, and wheat
with an estimated shoot Si removal of 329, 1,408, 160, and
108 kg Si ha−1, respectively.”
I’ll keep reading, this is very interesting!
Is Greensand also of no value?
I did a little searching about green sand. It is a common recommendation by organic growers. As far as I can tell it decomposes very slowly, on the order of rock phosphate. It won’t add much short term value.
I wondered if using rock dust actually did anything for my garden. Now I know the answer. It amazes me how many times people tell you to add it to the garden on YouTube videos.
Thank you for clearing things up.
Kansasgardener.
Unfortunately, YouTube is becoming congested with rubbish. The same sensationalism that worms in magazines and web sites is getting lots of views on YouTube.
I’m disappointed! I have 50lb bag of azomite in my garage, waiting for my spring garden. I’ve used those sea minerals before but it killed everything I sprayed it on. I used azomite on some plants, it seemed to help but the plants weren’t doing too terribly before so it’s hard to tell if it had an effect. Do you think it would work as a dust to protect the plants from aphids and the like?
No. Add it to the soil – it will benefit your grandchildren.
Is there any practical use for this? Or do I dump it?
put it in the garden – it might help long term.
So my great , great grandchildren will benefit from my azomite applications? Splendid 😉
Maybe I’ll eat it then, get something out of it.
Funny – I just wrote that – the grandchildren part – as a reply to another comment.
One thing (the only thing?) in favour of the idea of rock dust (not the product) is that there are areas where fertile soil develops from loess or on the slopes of tropical volcanoes – but only as the plant community concentrates NPK in the soil. It’s an interesting question whether there are an infinite number of garden myths to be debunked
There is not a finite number of garden myths. Social media is creating them far faster than I can write about them.
Here is a nutrient test on a commercially available compost, including minerals. https://blackgoldcompost.net/PDF/CompostAnalysis_04042017.pdf
My feeling is that if you’re amending with compost, you have have all the mineral you need from. Extra mineral amendments aren’t needed.
That is correct. Since most plants use the same minerals to grow – all plants contain the minerals. Compost made from plants which includes things like manure will contain all the minerals plants need.
I wonder if my worms might use rock dust in their digestion and then pass it on to my plants.