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Geology/use of volcanic rock as mulch

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Question
I want to pile landscaping volcanic rock (which I can get free)on my beds, hoping that the rains would leach sufficient minerals into the soil in a reasonable amount of time.
What would you say would be the rate of actual leaching of minerals thru rainfall or watering from above?
Or is that unrealistic, and I should just buy powdered rock?

Answer
Jean:

"Volcanic" rock encompasses a lot of different mineralogies.  Scoria, the vesicle filled rock commonly sold as "volcanic" is found in two types, one red one black.  They are both pyroclastic meaning they are thrown from the volcano and cool as a kind of froth, hence the air pockets from the gas it contains.

They both contain different minerals hence the difference in color.  Pumice on the other hand is glassy.

Your idea to depend on leaching of minerals is a bit optimistic.  Leaching of minerals depends a lot on acids in the soil, humic acids, that form from decaying plant debris that form in the upper dark layer of the soil, the humus.

While you might get a bit of leaching I don't think it would be enough to feed your plants.  You also have to consider what the leaching does to the soil pH, or acidity.  Most pyroclastic rocks are acidic, meaning they have a high percentage of quartz and will with time turn the soil more acid than basic.  If the plants you are growing prefer more basic soil, high pH, then you will have to add a buffering agent to lower the pH.  

You might consult your local soil extension agent,

http://www.nrcs.usda.gov/wps/portal/nrcs/site/soils/home/

Look them up in the phone book for the local office and give them a call, they will be familiar with your local soils and can address your concerns based on local conditions and soils.

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Keith Patton

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I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.

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I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

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American Association of Petroleum Geologists
American Association of Photogrammetrists and Remote Sensing

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Bachelor and Master of Science
Registered Geologist in State of Texas

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