THIS IS WHAT A GEOLOGIST HAS TO SAY ABOUT IT...Hi Ness,
it's a bit of a con as it all depends on what your local
bedrock is in the first place. This dictates what, if any, minerals are
likely to be deficient. As, presumably, we are talking about your
allotment, the amount of local mineralisation is more than sufficient
for most biological requirements. The only ones needing occasional
replenishment are those taken up in relatively large quantities, i.e.
the CHNOPS - based minerals (carbon, hydrogen, nitrogen, oxygen,
phosphor and sulphur), all of which are obtained by decayed biotic
matter anyway. There is some very dodgy and selective geoscience on the
website, especially regarding the CO2 drawdown with respect to silicate
weathering, which from a mega-annular perspective is very complex.
Releasing more available silicate rock won't increase the weathering
rate, if that were the case then all soils/exposed rock would already be
fully depleted, which they patently are not.
That aside, rock dust is a by-product of all aggregate quarries
(I've got several chemical analyses here), but is re-used as a filler in asphalt by those quarries linked
to the industry. Sandstone and granite dust isn't generally used and the
former is particularly unsuitable for asphalt. Granite dust is already
indigenous to the local soils. The stuff they are selling as rockdust
appears to be from basic and ultrabasic rocks, such as basalt, dolerite,
peridote and their volcanic equivalents, locally called greenstone,
particularly rich in calcium, iron, magnesium, titanium, manganese,
potassium etc., being common around here and also indigenous to
Callington soils. Greenstone dust was produced at Lean quarry, Liskeard
and still used in the asphalt plant there, but is now sourced from
Blackhill quarry on the A30 near Lewannick. It is also produced at
Greystone quarry on the Launceston - Tavistock road (Lawhitton turning),
but in both cases is used in asphalt production and not usually sold. In
the case of Lean they augment the shortage of local dust with limestone
dust imported from Somerset, but Blackhill, Lean or Greystone might
give/sell you some of the local stuff if you flutter your eyelashes.
There are many other quarries producing crushed basalt/dolerite in
Cornwall (none in Devon that I know of), I'd have to check on the net to
see which ones are likely to be using it for asphalt, as the plants thin
out going west because the potential for adequate business decreases,
they survive (just) around here due to the proximity to Plymouth. You
stand more chance of a regular supply from a quarry using it just for
backfilling or mixing it into coarse aggregates.
The sales of Rockdust are primarily aimed at those gardeners
located on non-igneous bedrock, particularly limestone areas, as these
are likely to be the most deficient in minerals generally. This is borne
out by their participating sales outlets, all of which, in England and
Wales at least, are located on sedimentary bedrock. Unfortunately, it's
one of those businesses where you can convince people that they need
something they don't, largely through ignorance, in the same way that
shoppers down here are exhorted to buy limescale remover for kettles and
bogs.
HOPE THIS HELPS SOMEONE :)
