Rock Dust – Good or Bad?

Here are two opposing articles to read on the subject of adding rock dust to your garden / agricultural land.  Take a read – what do you think?  Rock dust – good or bad?


Topping Soil With Rock Dust Could Suck Billions of Tons of CO2 From the Air and Increase Crop Nutrients

By Andy Corbley -Jul 14, 2020

Good News Network

“Spreading the dust of basalt rock over fallow fields could drain billions of metric tons of CO2 out of the atmosphere every year, says a new study published in Nature.

According to a team of primarily English scientists, mitigation of the worst effects of human-caused climate change will require both cutting carbon emissions, and the gradual removal of already existing greenhouse gases.

Soils normally absorb carbon from the atmosphere, but when mixed with the dust of basalt, which is rich in calcium and magnesium and also very abundant (as a mining and mineral by-product) you get a 2-fold benefit—crop production that is more nutrient, and an accelerated absorption of CO2.

The mixing of dust and soil increases the alkalinity, dissolving CO2 into non-organic carbon forms such as hydrogen carbonate ions: HCO3. These carbon-sequestering ions are removed via rainwater, and transferred to the ocean through runoff and drainage systems where they will act as carbon-prisons for 100,000 years.

“The logistical infrastructure to apply basaltic rock dust to managed croplands already exists owing to the common need to apply crushed limestone to reverse soil acidification resulting from intensive cropping,” write the authors in their study. “Thus, rapid deployment at large scale appears to be feasible…and has important ancillary benefits including mitigation of ocean acidification.”

If this can also cut the acid level in the ocean, which puts coral at risk, the idea now has a 3-fold benefit—a win-win-win.

“CO2 drawdown strategies that can scale up and are compatible with existing land uses are urgently required to combat climate change, alongside deep emissions cuts,” said Prof David Beerling, of the University of Sheffield, a lead author of the study. “ERW [dust spreading] is a straightforward, practical approach.”

Dust is even better than trees
Their modeling and analysis found that the emissions of serious CO2 producers Germany and Japan could be offset by treating half the world’s cropland with basalt dust, which would in theory be cheaper than other CO2 extraction strategies, with costs varying on local labor rates.

Tree-planting is mentioned in the study as a great way to extract CO2 from the atmosphere, but while mass-planting is often cheaper, the benefits are not as strong. Further they rely on the trees surviving a certain number of years for the benefits to fully take hold which is never a certainty.

Speaking with the Guardian, Prof. Breeling said of the basalt dust mixing: “If you could demonstrate to farmers in China and India, for example, that they are going to get crop yield increases and get paid $100 a ton for removing CO2, then it becomes really attractive.”

“Mining generates a continuous, but often discarded, finely powdered silicate by-product that is utilizable for dust mixing,” reads the study. They point out that it would require little to utilize existing silicate powders because of already existing mining infrastructure, potentially eliminating the CO2 generation from constructing tipper trucks, roads, or additional mines to produce the dust. Finally the authors mention that nations only need make an inventory of how much silicate byproduct they possess.

These numbers could then be plugged into their models and a more accurate and real-world assessment could be made on how far basalt dust mixing could go to reducing the effects of climate change.

And because it’s good for cropland, there are plenty of private sector incentives as well.”

Rock dust

(The article below has not been printed in full here – please follow the link to read the complete article)

Rock Dust – Can It Remineralize the Earth?

By: Robert Pavlis

Garden Myths

“Rock dust is a very popular soil additive especially with organic and permaculture groups. It is full of nutrients and 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.

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 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.

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.

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.

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

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 our soils are not losing fertility. They are not nutrient deficient. Therefore, rock dust, assuming it actually works, is a product that tries to solve a problem that doesn’t exist.

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.”


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