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.
so, which rock dust do you reccommend? I’d like one that isn’t too dusty, but works fast…maybe 75 years instead of 100. thx!
None – they don’t work!
Genuine rock dust with over 70 minerals and trace elements with a particle size of less than 8 microns, in the presence of microorganisms, will show results in less than 3 – 4 weeks
Prove it. Post a link to a study that shows this.
http://www.rock-dust.co.za/
See opening page “Tomato Trials” These trials have been replicated many times by potential clients with similar results. Bottom line, genuine rock dust works.
That is an unpublished report on a company site selling rock dust – proves nothing.
Provide the link for this study in a scientific journal.
If you had taken the time you may have noticed that my name appears on that website. I am the owner of that business and I am the one that did that trial. Perhaps you should do similar trials and see for yourself that you are mistaken in your opinion regarding rock dust
So you the owner of a business selling rock dust, and find that it works!
1) That tells us nothing about the product.
2) Until such work is published in a scientific journal, we do not have any facts to verify that the product works.
Why not hire a university to test it on real field garden soil, and have them measure nutrients before and after. It is actually an easy test – but manufacturers of rock dust don’t want to do it. I wonder why?
Maybe rock dust works for tomatoes but it does little to other vegetation, positively or negatively. I say that because of this: my father had a marble /granite factory on the outskirts of Verulam (KZN) and would produce so much sludge from cutting / polishing said marble / granite that he built dams out of the dried sludge to store the water. The vegetation around said dams didn’t profit from it and neither was it detrimental.
However I also chicken farmed free range on the same property – broilers and egg layers. Above water sludge did drain into the outer rain-catchment channels which the egg layers would drink out of. They produced eggs the size of goose eggs, all with a double yolk. Was it because of the “rock dust”? I have no idea.
Tomatoes are very similar to other plants. Most plants have similar root systems and absorb nutrients the same way.
Rock dust does not directly help plants.
Earthworm have to injest this rock salt to digest their food which is decaying matter. Without this dust their digestive system does not function. The product coming out of the digestive system is one of the best fertiliser, I hope you can see the connection on how rare earth from America has helped Amazon.
Thank you.
Earthworms do ingest bits of rock, but they don’t need mined rock dust.
I just had a bore drilled. The top section was a white and red clay substance. The lower layer was blue rock, basalt perhaps and quartz.
We put the blue stuff under our fruit trees ‘just in case’
My alpacas hurled themselves into it all with such joy, they love a dust bath.
I’m wondering if the dust is dangerous to us? There is so much of it.
I’m not convinced Rock dust isn’t a good thing. NASA has reported that 22,000 tons of Rock Dust is blown west from the Sahara desert each year and settles in the Amazon basin, fertilizing the Rain forest primarily with phosphorous. I’m a believer, and we have to start thinking of the future not just instantaneous results. So be it if the rock dust will benefit the plants 50 years from now. 50 years is nothing comparatively. Especially with the depletion of nutrients in our soils with 50 years of using primarily chemical salts as fertilizer. https://www.nasa.gov/content/goddard/nasa-satellite-reveals-how-much-saharan-dust-feeds-amazon-s-plants
The reference does not actually say that the dust fertilizes the Amazon. It only says the dust is deposited. At some point the P will become available, but when?
If you are making the point that using rock dust for future generations, I have no problem with that and if manufacturers of the product made this point I would have no issue with the product.
Our soils are not being depleted. https://www.gardenmyths.com/soil-fertility-decreasing/
Very good point Robert Pavlis!
I think most of this is generally correct. I would add that it is soil dependent and here in southern Florida where the soil is very sandy with almost no water holding capacity, the rock dust helps hold water and increases CEC.
My goodness you certainly opened a can of worms. If it helps Dr. Francis Gouwin used to write a garden coulomb in The Bay Weekly here in Maryland. He agreed with you. Unfortunately he past about a year and a half ago. I miss him greatly. When I sent my first soil samples in to Waypoint local in Virginia for testing the phosphate levels where 550 ppm. way to high I never added anything with phosphate since. My next test 4 years later came back 350 ppm. still high but much better. I’ll send in more samples this winter and see. I was very low in sulfur and a little low in magnesium I have added some flowers of sulfur and a little Epsom Salts every year and its now in the low but normal range . This is in the vegetable garden about 1000 sq. ft. I use an inch or 2 of leaf grow on top, every other year with two 40 lb. bags of pelletized agricultural lime. and the same amount of bloom (a local sewage process soil conditioner) every other year with no lime. my soil ph has been between a steady at 6.2 to 6.7 the cheep (14.00 dollars) have been within .2 of the Waypoint tests every time .Oh yea I only top dress the gardens with compost because I noticed the tomatoes would have a growth spurt the last 2 weeks in August witch I attributed to the compost finishing out under the heavy woven ground cloth by late summer (reusable if you rotate with your plants) the results speak for themselves. Both Dr. Gouin and I have experimented with adding Bloom in on top of container plants planted in our garden soil in pots. very good results at liest 1/3 more production. and in my wife’s butterfly gardens almost double the blossoms on 1/3 larger plants my test where not in perfectly identical controlled conditions but the Dr.’s where. You should look up The Bay weekly and his articles in the archives of the garden section. He was the best and not a bull shitter. .Thanks again. Michael Morgan mislandmike@hotmail.com .
it’s interesting how many of these comments ignore, or don’t know of, the basic facts of chemistry and plant nutrition – but I guess that’s your mission
I have been reading The Intelligent Gardener by Steve Solomon. He claims that he was so unhealthy that his teeth fell out while doing organic farming in Oregon and consuming the food he produced. He advocates that each soil situation must be tested and not only the mineral content of the soil is important but also the overall mineral balance is crucial.
I am very dubious of his claims and believe using lots of well made compost and wood chip mulch will produce excellent (not perfect) food in most situations.
From what I have read so far he doesn’t consider the benefit of getting as much carbon as possible into the top soil. The only place the carbon can come from is the atmosphere. There is huge potential for reducing the CO2 concentration in the atmosphere by carbon fixation in the soil, a process that is millions of years old, but which can be greatly accelerated by human intervention.
Thank you so much for you insightful and thought provoking comment. No need to site references, I’ll just go with what a total stranger says anonymously on the internet instead of the article that seems well balanced and has scientific and anecdotal references.
You are a true king among men.
Phil Callahan has done extensive research on this top. He is a top rated scientist with a background in biology, chemistry and physics. He has written a thorough book on this topic called Paramagnetism. A website you should check out is http://www.psicounsel.com/lowlev.htm on the low level of energies in agriculture. According to Phil Callahan all highly productive agriculture soils have a paramagnetic force in excess of 300 10 -6gauss. These soils produce healthy plants with a strong immune system. Soils below 100 10 -6 gauss are not produce, regardless of organic matter content. These soils produce weak plants that are subject to insect and microbe attacks. Weak plants in turn create weak animals and humans.
Sorry, but paramagnetism is not an accepted phenomena by the scientific community.
If he has done extensive research on rock dust, then post a link to a study that shows it work in the field.
This is a really poorly written article for someone who says their a gardener. Rock dust mainly contain silica, silica plays a huge role in cell growth and disease prevention in plants, in organic soils you can never have to much of something, the plants will use what it needs and not use what it doesn’t need. I’ve used mineral rock dust on old plants that have been established for years only to see some of the biggest blooms to date. My proof is in my growing.
re: “in organic soils you can never have to much of something” – that is not true. Too much phosphate becomes toxic to plants.
When you do some comparative trials with proper controls – let us know.
Effect of a rock dust amendment on disease severity of tomato bacterial wilt
https://link.springer.com/article/10.1007/s10482-012-9781-4
This is a second post with research showing certain benefits of rock dust (zeolite.) This isn’t a panacea, but it counters your claims that there are “zero” benefits to rock dust: https://www.sciencedaily.com/releases/2013/08/130822091034.htm
There is no link to the study.
The title of the study is “Enhancing the Growth of Plants on Coal Waste Using a Biological Fertilizer” – I don’t know too many gardeners who garden on coal waste!
The Science News summary is : “A simple mixture of organic waste, such as chicken manure, and zeolite, a porous volcanic rock, has been developed into a powerful fertilizer which can also reclaim desert or contaminated land.” Chicken manure alone will do this.
This is a greenhouse study – not a field study.
The original soil had a low pH of 5.1, the ideal pH for tomatoes is 6-6.8. Rock dust increased the pH to 6.8, but the conditions of the experimental setup reduced the pH of the control to 4.8, way below the ideal pH for tomatoes.
Ca and Mg levels did go up with rock dust, but iron and manganese went down – but the study failed to apply statistics to these numbers wo we really don’t know if this is true.
There was “no significant difference in tomato plant height, stem diameter and biomass”.
“The raised soil pH and Ca content were the key factors for the rock dust amendment controlling bacterial wilt under greenhouse conditions” – This could have also been achieved by liming the soil.
So we can conclude that in acidic soil, in a laboratory condition, rock dust will increase the pH of the soil, but even with this increase the plants did not grow any better. The same results can be achieved with lime.
There is no evidence in this study to show that rock dust increased micronutrients in the soil.
You’ve written: “Chicken manure alone will do this.” This misses the conclusion in the study:
“By comparing the dry weights of the plants grown in the extended control experiment, it is found that using organic waste without the zeolite component results in 45% of that of plants grown in the amended substrates. A similar result was found when substrates containing zeolitic tuff and no organic waste were investigated. In this instance, the plants had an average dry weights of some 46% of those in substrates amended with the biofertilizer. It was therefore found that the presence of both components of the biofertilizer is essential for it to sponsor nitrification to a degree that produces considerable plant enhancement.”
https://www.researchgate.net/publication/261279295_Enhansing_the_Growth_of_Plants_on_Coal_Waste_using_a_Biological_fertilizer
We do not garden on coal waste. Why not post a link done on normal agricultural field soil?
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But I digress.
I live in the high desert, so I probably have plenty of “minerals” in my soil. BUT, in my opinion, it appears that the only negative impact of rock dust is on the wallet. For me, it seems like a cheap test. So I’ll probably throw some on the garden and see what happens. But no way will I tell friends and family how great it is until I see a real scientific study.
Sorry for “necro posting” to this thread.
Hi! I also live in a high desert of Oregon. Would love to know how your experiment turns out!
Sorry but this is ridiculous, you are writing a post on something you have done no trials on yourself with huge amounts of speculation. Do some controlled trials and post your results and then write a post.
I’ve been using rock dust for plants and animals for years and have excellent results. It might not be for everyone but at least speak from an informed place instead of this sensationalism gardening crap.
Since you are so sure it works – provide a link to some a scientific study that shows nutrients increase after adding rock dust.