Growing medium

Jane McGary
Thu, 12 Jul 2012 15:56:52 PDT
Gastil wrote
>Having heard good things about pumice on this list,
>I went to our OSH and purchased a small bag of Black Gold brand pumice.
>It was about 10 times heavier than I expected pumice to be, but they 
>assured me that was just because it was wet.
>At home, I noticed it had a lot of dust, which I did not think I 
>wanted in my growing medium
>(not having read the note from Jane yet about the dust containing 
>useful minerals).
>When I washed the pumice, I noticed it sank. I expected pumice to float.

If the Oregon white pumice is bagged during damp weather, it will be 
wet. When I buy bags of it (4 cubic feet per bag, which is a BIG bag) 
in winter, the bag weighs 80 to 90 pounds; in summer it weighs about 
60 pounds. (Must remember to stock up in August!) And yes, it will 
sink, even when it's dry.

You can buy both washed and unwashed pumice here. I prefer the 
unwashed product because of the available nutrients in the fines. 
This pumice also raises the pH of a mostly acidic soil mix slightly.

A geologist friend of mine once told me that mixing peat with pumice 
will eventually cause the pumice to break down because of a chemical 
reaction. I have not noticed this, but I don't usually keep the seed 
mix (the only one with peat) more than three years.

I would never put vermiculite in planting soil, but I have used it to 
root cuttings and it's also excellent for packing bulbs for shipping, 
especially if they have to be kept very slightly moist.

It perhaps should be mentioned that many of the ingredients being 
discussed in this thread need to be handled with care not to breathe 
in much of the dust. I worry sometimes about developing silicosis 
from using so much sand, but a recent chest x-ray (for bronchitis) 
gave me the "all clear." Vermiculite is said to be hazardous in this 
respect. Milled sphagnum moss (not the same as sphagnum peat) has 
been implicated in transmitting a pathogen, but I don't recall the 
details; I use the former occasionally when growing plants from seed 
that are very prone to damping off.

Like Peter Taggart, I used to use forest humus as the organic 
component of my bulb soil -- when I had a forest. (Still have it, 
please buy it!) I never bothered to sterilize it, though I did sieve 
it and pick out the visible bugs. My feeling is that if you're 
growing your container plants outdoors, especially in the country, 
there isn't much point in sterilizing the soil. Soils for seeds 
should be more sterile, and for cuttings, even more.

Jane McGary
Portland, Oregon, USA

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