granite grit

Rodger Whitlock
Wed, 09 Nov 2005 12:05:54 PST
On  8 Nov 05 at 19:45, Linda Foulis wrote:

> < soil scientist enquired if my pot's top dressing was granite
> gravel.  It is (grower and developer grades of chicken grit)
> and he mentioned the fact that there was no wonder I didn't
> need to feed micronutrients because all 15 the plants need are
> made available in the right quantities from the top dressing.>
> At the risk of sounding like a complete moron, I really have
> to question the above written by John Lonsdale, last evening. 
> And my apologies before hand when someone proves to me why
> this can be so. How can micronutrients be derived from granite
> grit?

The stuff dissolves, though very slowly. You can't put 
granite in your coffee, stir it, and expect it to 
disappear like sugar, but yes, it dissolves ever so 
slightly. Simple as that. Remember, this is all about 
*micronutrients* and it doesn't take much of them to do the 

The other side of the coin can be experienced at any salt-water 
beach, where the particles of sand contain surprising amounts 
of sodium and chloride that have gradually penetrated the 
crystal lattices of the minerals making up the sand. The same 
sodium and chlorides will be released when moved to fresh water 
-- which is why salt-water beach sand is not usable as a soil 

>  Especially in a growing season, or even 10 growing
> seasons for that matter.  I can maybe see it if using a
> sandstone grit, if there is such a thing. All the granite I've
> ever come across has been an extremely hard rock and not
> inclined to erosion or break down in which micronutrients
> would be released, at least not in my life time.

You need to (a) visit the San Gabriel Mountains above Pasadena
in southern California, where the weathered granite is so
imcompetent you can kick a boulder into shards; and (b) go
look at the sand on a beach near you with a magnifying glass.
A lot of that sand is broken down granite.

Also: take a look at exposures of granite in your area -- not 
fresh blasted faces in roadcuts, but ones exposed naturally for 
millenia. You'll notice moss and such growing on them. The 
stuff is far from inert.
> Scientifically I am a moron as defined in any dictionary,

I'm not going to touch that line with a ten-foot pole!

> however this really stuck in my mind and I would like to learn
> how it is possible, especially in the case of granite.  The
> bag of granite grit #2 that I just picked up has been well
> washed and there is very little granite dust which could be
> the source of micronutrients, but even then what would it take
> to break down the granite dust to release those
> micronutrients?

Granite dust is a recognized source of nutrients, esp. 

Remember that granite is solidified magma from deep down in the 
earth and can be expected to contain some of every element. 

Rodger Whitlock
Victoria, British Columbia, Canada
Maritime Zone 8, a cool Mediterranean climate

on beautiful Vancouver Island

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