Water retained by various soil ingredients

Diana Chapman rarebulbs@suddenlink.net
Wed, 24 Oct 2012 07:46:26 PDT
As Leo pointed out, there are other kinds of perlite.  I couldn't do 
without perlite, and use huge quantities of it, since I have hundreds of 
pots, some very large, and moving and carrying them is a daily task.  I 
started using the better quality perlite some time ago.  It has been 
specially heat treated, and comes in three grades, the medium one is 
about bean sized, and it has no dust or fines associated with it.  I was 
concerned with perlite dust being a health hazard.  It retains far less 
water, but it does have a tendency to float to the top of the medium 
more than the regular perlite. It can be found in 'grow shops', and 
costs about twice the amount of regular perlite.  I also use the regular 
perlite (which is misleadingly labeled 'medium') for my seed starting 
mixes, where I want more moisture retention and a nice even fine mix for 
the seedling roots.  It is very, very dusty, and you need to wear a mask 
when mixing it.  I must say also that I use finely ground peat, since it 
is sterile, evenly textured, and has anti-fungal properties.  Most 
bagged potting mixes contain ground wood products, which I believe are 
bad for bulbs and encourage fungal disease, plus are very hard to re-wet 
once they become dry.  Peat is hard to get wet too, but not if it is 
mixed liberally with pumice and perlite.  My preference would be to use 
horticulatural pumice instead of perlite, but it is only obtainable here 
in small bags, not suitable for my needs, plus it is expensive and 
heavy.  Of course I use it, but for my 'special' bulbs.

As Dave points out, there is a difference between water held between the 
particles and water absorbed by the particles, so I don't think it is 
that simple.  After all, if water is absorbed by particles, such as 
pumice or perlite, then slowly released, this is a very good thing for 
the roots, while with coarse sand or grit, there is only water between 
the particles, and once it is gone, there's nothing there for the 
roots.   I have rooted damaged bulbs in regular perlite, and the roots 
that grow in pure perlite are amazing, dense, widely branched, and 
clinging to the perlite particles.  This is the way I root cuttings also 
(for the garden).

Telos Rare Bulbs
> In message <fc9884635e19f6f68f3145bbc358c825.squirrel@http://www.possi.org/>,
> Leo A. Martin <leo@possi.org> writes
>> It is possible to buy large horticultural perlite. Particles are about the
>> size of dry beans. This retains much less water than the fine stuff. Used
>> by itself it is not good for anchoring roots so it needs something else
>> for more stabilization.
> One might distinguish between the water retained inside the medium and
> that held between the particles of it. Smaller particles can retain more
> water between themselves. The bigger gaps between bigger particles hold
> more air than water.
> A medium that took no water in at all would show only this effect, Hmm.
> cavity wall insulation consists of 2mm polystyrene balls...
> There are pictures of various mediums like pumice, usually with a size
> scale here:
> http://pacificbulbsociety.org/pbswiki/index.php/…

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