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Mark,
I don't know where that sentence after the part you quoted
came from, but that must be your comment, as it's not mine.
I was basing my comments about air space on basic particle
packing technology from my materials science background.
If you fill a room with basketballs or ping-pong balls, the
free space between them is the same. I'm not willing to do the math again,
but it's about 1/3 open space if I recall correctly, and that's as good as
it gets. If we're trying to make a dense body, you then take smaller
particles to fill in those free volumes, then smaller ones
to fill in the ones they leave behind and so on.
As we are obviously NOT trying to make a "dense body" out of
our potting media, a uniformly-sized, spherical particle is the idea from a free
air flow perspective, but has other problems that I addressed in my media
comparison article.
As to enchasing capillarity, I think the opposite of what you
stated is true: If we assume a constant absorption capability by the media
particles, capillarity occurs only at particle-to-particle contact points and
their immediate vicinities. Looking back at my particle size comment
above, you can see that in that perfect, uniform sphere scenario, each particle
is in contact with other particles at only eight points. If I mix in other
sizes, then the number of contact points, hence routes for capillarity flow to
occur, is increased. That is why my work with Semi-Hydroponic culture
focused quite a bit on finding the right combination of shape to maximize free
air space, and high moisture absorption of the medium to
compensate.
I'm not sure what you're going after with the "capturing
evaporation" comment, but it seems to me that getting humidity out of those free
air spaces and back into the particles - if that happens to a major degree at
all - is a surface area phenomenon, so again, smaller and/or a mix of particle
sizes would have a greater surface are than my "perfect, uniformly-sized
spheres, so would capture more. I actually doubt that much of that is
going on though, as the driving force to distribute those water molecules into
the surrounding atmosphere is a lot greater than the affinity they might have
with the media particles.
In the case of evaporation, the better the capillarity, the
wetter more of the surface area of the medium will be, and there will be more
evaporation. However, if the free air volume between particles is reduced,
then the "evaporation sites" lower in the pot do not communicate as well with
the surface, so the rate of evaporation will be decreased - it's a balancing
act.
Ray Barkalow - First Rays Orchids - www.firstrays.com Plants, Supplies, Artwork, Books and Lots of Free Info!
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