I find perlite is best mixed when mixing your own potting compost potting
compost to aid drainage & use vermiculite to slightly cover my seedlings
instead of compost.

--
Thanks Keith,Nottingham,England,UK.
"Chris Hogg" wrote in message
...
On Fri, 8 Apr 2005 01:32:20 +0100, "Miss Perspicacia Tick"
wrote:

What's the difference and when should one be used in preference to the
other? As I understand it, both perform the same function - that is to
improve drainage; should the former be used with cuttings and the latter
with more established plants (i.e. if you're repotting)? My father
appears
to use them interchangeably but I reckon that, if they could both be used
for identical tasks, then why would they both be sold? I was potting on
some
mystery plants (mystery because they were something new I'd bought and
I'd
lost the packet and brain fog means I forget my own name sometimes!) and
I
didn't know which to use, so I used vermiculite simply because we had
more
of it.

So could someone please explain the difference to me (Monty Don only
appears
to use vermiculite - I was hoping he'd use perlite for something so I
could
fathom it out for myself)?

Thanks

Vermiculite starts life as a relative of mica, which is flash heated
to a few hundred degrees C, when the water bound in the crystal
structure instantly turns to steam and expands the mica-like sheets to
give the result you see. The name comes from the latin 'vermis' - a
worm.

Perlite starts life as a volcanic glass, which is crushed and then
also flash heated but to a rather higher temperature than vermiculite.
The glass softens and water within the glass converts to steam and
foams up the glass. Not sure whether it's an 'open' or a 'closed' foam
structure though, i.e. whether the pores are interconnected and open
to the outside world, or whether they're just a mass of sealed
bubbles.

IME, vermiculite has an alkaline pH. I used to use it in potting mixes
for heathers but didn't understand why the foliage was getting
chlorotic (yellow), until I tested the pH of the vermiculite. I now
only use perlite.

Vermiculite can get quite soggy, but perlite doesn't to nearly the
same extent, so I suppose the argument that it retains moisture better
than perlite is probably true. But I also feel that vermiculite can
inhibit drainage, especially in potted plants, due to the flat platy
nature especially of the larger particles, and after it's been around
for a while. OTOH I am quite happy with the idea that vermiculite
retains nutrients better than perlite, due to what is known as its
'ion exchange capacity', whereas I doubt that perlite has an exchange
capacity of any consequence.


--
Chris

E-mail: christopher[dot]hogg[at]virgin[dot]net