I only just became interested in rockdust; I have been using it for a few months so much too early to make conclusions about anything.

I have read extensively about the subject lately and the issue of ph would seem to have most to do with whether the rock source has a lot of calcium. Granites/basalts don't, they have more potassium, etc. So I suppose one needs to be fairly sure of the rock type of the source, or possibly have the dust tested.

2-3 kg is what, about 4-6 pounds? About 4500 sq M per acre, to put in terms I'm familiar with, about 18,000 pounds per acre, a pretty whopping dosage. Best be sure the calcium content is very low. Hamaker recommended such huge amounts but other recommendations I have read are very much lower.

What about doing some tests on smaller areas at different rates?

Have you read "bread from stones", btw?

"the issue of ph would seem to have most to do with whether the rock source has a lot of calcium"

Calcium has absolutely positively no effect at all on soil pH except under the specific situation in which the pH is greater than 8.0 due to sodic soil conditions. That's a common misconception since most liming materials also happen to contain calcium but it is not the calcium that adjust the pH downward. Rather than type it all out again, see my posts concerning lime, pH and base cations in this thread if interested in an explanation.

(I'm really hoping this calcium = upward pH adjustment thing will die eventually)

Right, so I guess I'm okay using rockdust then?

Garg, apparently you are a chemist and/or soil scientist, and yet you didn't offer the OP the slightest help on their question.

I guess you aren't going to argue that "limestone" type rock powders do not strongly raise ph? Especially when used at about 9 or 10 tons per acre? So the correlation between Ca-containing rock and the raising of ph is exceedingly high, regardless of the precise process. I have never heard that finely powdered igneous rocks strongly raise ph. But on the other hand they are rarely used, whereas limestone is constantly used. So there is little data, and so the OP will be operating pretty much in the dark, so to speak.

Garg, calcium sulfate can raise a low ph and lower a high one, correct? How does that work?

Here is an interesting little dissertation on Ca sulfate, which states that it is of little use excepting on sandy low-S soils (which I have). It also says in passing that it is not the Ca in lime (Ca carbonate) that raises ph, as Garg stated.

My understanding of the effect of calcium was that it "bumped" the smaller Mag molecules off, which then bump the smaller K molecules, which bump the smaller sodium, which bump the smallest hydrogens out of the solution, which raises the soil ph.

Here is a link that might be useful: bumper crops

Oh dear, I thought this issue would be a lot simpler to resolve!

Oh dear, I thought this issue would be a lot simpler to resolve!

Which rock dust you apply will determine the answer to your question. If what you add is Rock Phosphate it may have no affect on your soils pH while Granite Dust or Greensand might.

Are there greensand deposits in the UK? And plus 200-mesh dust for anything other than limestone or rock phosphate is mighty tough to find, IME.

I have never used rockdust but I was curious about your question so I Googled it. It is milled volcanic rock quarried in Scotland.

It is advertised as 'helping with poor acidic soils' which are pretty rare in UK gardens anywhere, and very unlikely in SE England. But you say you don't want to be told you don't need it, so I will refrain.

Just going by what the manufacturers say, you shouldn't add lime as well as rockdust. Not because rockdust will raise the pH but because lime will 'lock up' the minerals in the rockdust.

The application rate recommended by the manufacturers (and they presumably will be trying to get you to use as much as possible) is 1/2 kg per sq metre, so your 2 - 3 kg psm looks like way too much.

I am just curious as to whether you have had your allotment for very long. Have you had disappointing yields in the past or are you trying to preempt hypothetical problems? If the latter maybe a year of growing in the conditions as they are will help form a picture of what your soil might or might not need.

Regarding your other post about lime and clubroot. Does your plot definitely have clubroot or are you just anticipating that it might be a problem? The allotment site where I have my plot does not have clubroot. It is not inevitable. If your land does not have it already and you grow your own brassicas from seed you may never experience it.

pnbrown - yes there is greensand in the UK.

and yet you didn't offer the OP the slightest help on their question.
That's not true. I don't know much about the specific "rock dust" being discussed.
However, it seemed to me that you were giving the O.P. the impression that if it contains calcium it will raise the pH. I nipped it in the bud, lest they come to a false conclusion linking calcium to alkaline reaction. (Which, the O.P. stated specifically and repeatedly is the one and only concern for the purpose of this particular post).

calcium sulfate can raise a low ph and lower a high one, correct?
Other than very isolated circumstances, which I have discussed in the past, no, that is not correct.

So then its value would be for soils low in Ca and S. Not uncommon in acidic sandy soils.

"acidic sandy soils" ... which are rare in SE England unless the OP gardens on the Surrey heathlands.

The link to the manufacturers does not seem to be working for me but here is another.

Parts of Scotland do have poor acidic soils where this product might be useful. But I really doubt it is of much value on a neutral soil in the Home Counties. However, without knowing where the OP is located exactly and without a soil test on his/her allotment it's all conjecture.

Here is a link that might be useful: Rock Dust

Agreed, Flora, I was musing upon it's use in some of my plots, which are low in Ca and S. And acidic. Probably not by coincidence.

If garg is correct then gypsum will not raise ph, since it doesn't have carbonate. Gypsum is often thought to lower high ph, maybe because of the S content, but that effect would be slight, I think.

So then its value would be for soils low in Ca and S. Not uncommon in acidic sandy soils.
Absolutely.

Gypsum is often thought to lower high pH, maybe because of the S content, but that effect would be slight, I think.
It will not lower the pH except under specific circumstances and deals more with the remediation of high sodium soils and the high pH that is sometimes secondary to that condition (but not always) rather than reactions specific to the sulfate in gypsum itself. Sulfate itself does not lower the pH in soil. Since I have a "linking to myself" thing going, see my response near the bottom of the thread regarding sulfate salts and their affect on pH here if interested.