potting media

Michael Mace michaelcmace@gmail.com
Fri, 09 Aug 2013 11:25:46 PDT
I agree with Lisa, this is a very interesting discussion. Thanks to everyone
who has been contributing to it.

I wanted to comment on one thing...

>> I believe pumice & turface are added for moisture, aeration, CEC and to
reduce soil compaction; so why add the sand?

This reminded me of something I saw from an experienced grower years ago,
which has made more and more sense to me as I've experimented with various
potting mixes. This was the idea that in creating a potting mix, you're
looking to incorporate a variety of particle sizes. If the particles in your
mix are all small, air can't get in and things rot. If the particles are all
big, the mix dries out too quickly. There seems to be a magical balance of
drainage and moistness that happens when you get the right mix of sizes.

So the answer to the sand question might be that it's giving you some
smaller particles to complement the relatively large particles of pumice.
It also seems to help if you have a good balance between things that act
like sponges (materials like peat and compost) and things that do not absorb
water (sand, gravel, etc).  For example, if you look at UC Davis's
recommended potting mix, it is equal parts water-absorbing small particles
(peat),water-absorbing large particles (redwood compost), water-repelling
small particles (sand), and water-repelling large particles (pumice). Most
of the other successful potting mixes I've heard about seem to have that
kind of rhythm to them, even though the ingredients and proportions vary.

This helps to explain why some of us (including me) have been snake-bit when
using potting ingredients that had a lot of fines in them. I used perlite in
a mix because I thought it would give me large water-repelling particles.
But the perlite was full of fines (small particles), so my mix ended up too
dense and things rotted. Lesson: the condition of the materials you use is
just as important as the materials themselves.

The balance you hit among your potting ingredients interacts with your other
growing conditions: how humid is the air, how much sun do your pots get,
what's the average air temperature, are you growing in plastic or clay pots,
how big are the pots, etc. 

Because of the complex interactions between all of these things, I think
it's impossible to define the ideal growing mix for all bulb-growers in all
areas. You need to experiment and see what works in your conditions.

But the discussion of potting mixes is still very helpful to me because it
gives me ideas on things to try, and potential explanations when something
doesn't work.

San Jose, CA

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