What are the pros and cons of different rock dusts?

Question

Rockdust is powdered rock that helps give plants extra minerals (such as silica for strong leaves and stems), trace minerals, calcium, iron etc. They can increase the soil pH somewhat, too. Rockdust is not fertilizer, as it does not directly contain the required amounts of nitrogen and phosphorus. However, it is said to contribute to soil bacteria that does make the soil more fertile, and whether or not it's fertilizer, it still helps plants to get bigger, stronger and healthier (it also helps to increase yields)—at least if all the reviews and articles I've read are correct.

The rock dusts of which I am familiar include Azomite, Agrowinn, basalt rock dust, glacial rock dust and diatomaceous earth (which usually isn't thought of as rockdust, but it kind of classifies). If you know any others, please add them to the list.

Here's what I know already:

• Azomite: It has a fair amount of silica (more than basalt rock dust, but probably less than diatomaceous earth; it's not the same kind of silica that is in sand, however, but I think silicon dioxide): about 65% silica. It has lots of positive reviews online, and it is probably the most popular rockdust with conventional gardeners. A post I saw on a forum (and its comments) indicates that it might be radioactive (gamma); the Azomite website has tested the alpha and beta (and claims it's not radioactive), but they have not tested for gamma radiation. Azomite comes from a unique source (volcanic ash) in Utah, unlike any other area in the world, they say. I believe I've heard that Azomite (if not rockdust at large) is good for helping to revive suffering trees.

• Agrowinn: This is another commercial rockdust. It has some claims over Azomite, but I don't really know much about it. I've seen it recommended that you reapply it regularly (which seems against the grain of other rock dusts—tell me if I'm wrong).

• Glacial rock dust: This is said to have more pollutants than volcanic rock dust (such as basalt), but a lot of people seem to like it.

• Basalt rock dust: Some sources say basalt rock dust is the optimal kind of rockdust, and I've never heard anyone deny that directly. It has less silica than Azomite and diatomaceous earth, for sure. It is higher in other minerals, however. It is said to be high in ferrous minerals. I've seen differing compositions listed for basalt rockdust. The silica content is probably (not sure) between 20 to 50% of its composition, which means it must have a lot more of something else if it has so much less silica. Basalt can be significantly different colors. I imagine the darker basalts probably have more iron or something than the lighter ones.

• Food grade diatomaceous earth: It's about 85% amorphous silica (the amount depends on what brand you get). It has some trace elements, aluminum and such. It's the only thing like a rockdust that I've actually used. I'm guessing that other than the silica, most other rockdusts are better for plants. However, the silica is not to be underestimated (however, other rockdusts have lots of silica, too; just not usually as much). I've personally used this more for other purposes than gardening (like eating it to cure food intolerances; yes, it actually worked for me there, although it had a negative affect on my short-term memory for a while, and made me constipated, probably due to heavy metal chelation). I've used it a little as an insecticide in an attempt to keep cutworms off my tomatoes. It helped some, but not all the time (perhaps it didn't when wet). Aphids didn't seem bothered by it, particularly. I'm speculating here, but rather than sprinkling it on the plant base, mixing a larger amount with the soil might be more helpful for cutworms. However, another solution (such as beneficial predatory insects) could probably be much better (for cutworms and aphids, that is).

• Granite rockdust: I've heard you can get this for free from a local rock quarry. The same might be true for certain other kinds of rockdust. If you get it from a quarry, make sure they haven't been using harmful chemicals and stuff in their work.

• Agricultural lime: This isn't thought of as rockdust, really. It's mostly to increase soil PH (or lower acidity). It has lots of calcium in it.

• Greensand: This isn't really thought of as rockdust, either. I think it applies a lot of a few certain minerals. I think this increases potassium in the soil. It contains glauconite (which by itself may lower soil PH, but I don't know if greensand does, particularly).

• Hornfels rockdust: I've only barely heard of this.

Anyway, I'm not looking for someone to say, "This rockdust is the best kind, or this is better than that."

What are some more attributes of various rock dusts and what are the pros and cons? I'm sure they all have pros and cons. That's what I want to know. I know I've listed a lot already, but I'd like to know more. I will probably pick the most thorough and informative answer (unless it's just a really good one that surprises me).

EDIT: This question only pertains to rockdust (not other remineralizers or soil amendments). If it's powdered rock of any kind, I'll count that (whether or not it's commonly referred to as rockdust).

Answer 1

First, I have to say that many of these are so similar, you really won't find a lot of pros and cons between them. They are very similar, and though some might do better than others in certain situations, I haven't really found a reason to differentiate when using as a soil helper, and they all work.

• Azomite: Can cost over twice what basalt or glacial rock dusts do. It comes from a specific volcanic ash deposit in Utah, so it won't be as sustainable as other dusts can be. It is easy to use in spreaders. See here for the mineral analysis.

• Agrowinn: Only one company sells it, and it is very expensive. It has far lower mineral count than azomite (except for Mg). Scroll to the bottom of this page for the analysis. It is still a good mineral source.

• Glacial Rock Dust: Much cheaper than the above minerals. You can use it on a larger application, as it's far more affordable. See the analysis here. It isn't necessarily ground as finely, so it can clog some spreaders.

• Basalt Rock Dust: Great mineral content for the price. I prefer it over glacial because it's less likely to contain heavy metals (which you'll find in alluvial and glacial dusts) and is more consistent in mineral analysis. See here for analysis. Here is a FAQ page that compares basalt and glacial rock dusts.

Those are all the dusts you listed. (EDIT: More have been added to the list since.) Any others will also be similar in the way these are.

You may like to know that while these have high mineral counts, they aren't all available to the plant. They have to break apart even more, which is most easily accomplished using an acid, such as humic acid. The cool thing is, humic acid is the acid produced when organic matter decomposes. So adding rock dust with compost (especially compost that's not quite finished) can increase availability by over 200%. You will also have better results adding the dust to soils high in organic matter, for the same reasons.

Answer 2

There is a distinct benefit to using greensand over DE or rock dust, that is the soil conditioning properties, as greensand adds a beautiful free-draining texture for organic potting soil applications. I must note that I use all 3 products in my typical mix, using 3mm or less DE, not dust or flour, primarily for drainage and silicon, azomite for the full range of minerals and i will water several times with 8% humic acid to make the minerals more available. I have heard that kelp meal is preferred for more sensitive plants. Drainage and nutrients are our biggest issues on our heavy clay soil and we also grow many vegetables in pots.

Answer 3

Steve Solomon recommends kelp over azomite as a better source of trace minerals. See http://www.motherearthnews.com/organic-gardening/homemade-organic-fertilizer-zmaz06jjzraw.aspx

Kelp Meal (dried seaweed) has become expensive, but one 50-pound sack will supply a 2,000-square-foot garden for several years. Kelp supplies more than just a complete range of trace minerals, it provides growth regulators and natural hormones that act like plant vitamins, increasing resistance to cold, frost and other stresses. Some rock dusts are highly mineralized and contain a broad and complete range of minor plant nutrients. These may be substituted for kelp meal, but I believe kelp is best. If your garden center doesn't carry kelp meal and can't order it, you can get it from Peaceful Valley Farm Supply of Grass Valley, Calif.

He has written books on the subject of increasing nutrient density by soil analysis, and amendment.