Alex wrote:
How are they made, and what are they made of?

I'm looking into making them myself, but find it hard
to find some how-to materials online.

Looks like it's basically just compost and miscellaneous other plant
materials.

http://www.vegfamily.com/amy-olga/tip4.htm

http://www.pfaf.org/leaflets/vegorg.php

--
Jennifer