Michael Mace michaelcmace@gmail.com
Tue, 02 Jul 2013 22:48:41 PDT
What a fun conversation!

A couple of notes:

--In the US, beware of confusing Dry Stall with Stall Dry:
Stall Dry is a mixture of diatomaceous earth and clay, used like kitty
litter for horses. Not a good potting ingredient.
Dry Stall is pumice, with very angular particle sizes ranging from about 2-6
mm, and contains very few fines ("fines" being a sophisticated word for tiny
dust-like particles).

Jim, whatever you got in that bag, it doesn't sound like the Dry Stall you
can buy in California.

--Regarding the goodness or badness of fines, I think everything is
relative. If you have too much fines, your soil mix doesn't have enough air
in it and your bulbs rot. If you don't have enough fines, the soil dries too
quickly and your plants wither. 

I've caused very big problems for myself in the past when I failed to take
into account the amount of fines when mixing up a batch of potting soil. For
example, for several years I used a lot of perlite in my soil mix, thinking
that perlite increased drainage. But the perlite I was using had a lot of
fines in it, so it actually acted as a water-retaining ingredient. When I
mixed it with peat, the result was a soggy mass that air couldn't penetrate.
And then I wondered why some of my bulbs died.

The key to good planting mix, as many of the old-time growers say, seems to
be getting a good balance of moisture-retaining ingredients and
drainage-encouraging ingredients. Not too much fines, and not too little.
You want a soil mix that air can penetrate but that will keep the roots from
completely drying out. In other words, it should remain damp for a long
time, but should not stay sopping wet for long at all.

Gastil's beautiful work on the Growing Medium page on the wiki gives a ton
of useful information on this subject.

San Jose, CA

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