Volcanic rock

Dr Paul Chapman cyrtanthus@blueyonder.co.uk
Fri, 26 Jul 2002 03:25:03 PDT
Dear Mary Sue, Dave and all,

I'm scraping my distant memory cells, but my Ph.D. work had a lot to do with this.

Firstly, pumice is a volcanic rock; it was erupted at the surface of the earth and cooled quickly, as opposed to granite, which cooled slowly, well buried in the earth. As a result, the crystals of minerals in pumice are tiny, and generally need a microscope to be seen easily, because they didn't have time to grow to any size before the lava became solid, whereas crystals (that might be the same minerals) in granite are big enough to see easily.

Now comes the scientific bit - for the same total volume of crystal, there is much more surface area if there are lots of small crystals, rather than one big crystal. Believe it or not, the size of crystals in granite are about 1000 times bigger than those in pumice, which gives you 1000 times as much surface area. Most chemical reactions between solids and liquids take place on surfaces, so pumice will react about 1000 times as fast as granite - it will only take a few years to change, not a few thousand years.

When more or less any sort of igneous rock breaks down, the minerals don't simply dissolve in the water (except for the silica) - they change first to clays, which then eventually dissolve (if anyone wants a copy of my Ph.D. thesis....). So, that's the basic reason for clay building up in composts containing pumice - helped by the fact that, where there is peat as well, the water in the compost will become acid, which will speed up the reactions that break down the minerals.

On a slightly different topic, Dave mentioned that worms were a problem in his pots, and that Benomyl was a possible solution. I have exactly the same problem, and have been looking for a solution for ages. The only trouble is, I haven't seen any Benomyl containing fungicides around for ages, and thought they had been banned in the UK. Do you know the names of any that are still commercially available to amateurs?

Best wishes,


Dr Paul Chapman, Wallington, Surrey, UK
South London commuter belt suburbia - zone 9a - where we have a warm, sunny day today, and you can almost believe the zone number!

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