Perlite/Vermiculite?

[kitten]

Hi all, what is this stuff and what's the difference between them?  I'm sure i've read somewhere about using it in a thin layer on top of seeds rather than compost, but not sure what it's for, and which one i should be using!

[kitten]

[quizzical1]

I might be shot down in flames here, but I believe that both perlite and vermiculite can be mixed with compost to improve drainage, whereas vermiculite can can also be used to cover sowing in seed trays etc., and also aids water retention.
I've also heard that of the two, vermiculite is the safer option, because although they are both mineral products, vermiculite tends not to be as dusty as perlite. The dust from both of them could be inhaled, and won't dissolve in your lungs, so caution with both is advised.

QUOTE   Typical analysis of perlite
70-75% silicon dioxide: SiO2
12-15% aluminium oxide: Al2O3
3-4% sodium oxide: Na2O
3-5% potassium oxide: K2O
0.5-2% iron oxide: Fe2O3
0.2-0.7% magnesium oxide: MgO
0.5-1.5% calcium oxide: CaO
3-5% loss on ignition (chemical / combined water)

QUOTE   Materials such as clay, vermiculite, hydrogel and shredded bark will make soil hold more water

There are plusses for both though.

See http://www.perlite.info/hbk/0031401.htm