Jane McGary janemcgary@earthlink.net
Sat, 05 Feb 2005 15:10:39 PST
Something to know about horticultural pumice is that it comes in two 
versions, washed and unwashed. The washed kind is what one finds in little 
bags in garden centers; it has had the fines (dust) washed out. It's used 
primarily as an alternative to Perlite and has the same disadvantage: it 
"floats" to the top of the pot unless mixed with something of the same 
general dimensions and 
density (i.e., fine bark). I find it horrible as an 
ingredient for bulb potting soil, and it is less likely to give the plants 
a nutrient boost, which the unwashed type certainly does. However, the 
washed pumice is a decent soil amendment in the garden and is useful in 
growing demanding alpines. I use only the unwashed type now, which is 
harder to find. I mix it with forest loam and very sharp coarse sand. I 
don't know if unwashed pumice is available except near the source. I think 
the place I get it from buys it from a nearby plant that uses it to 
manufacture building blocks.

Jane McGary
Northwestern Oregon

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