What is vermiculite?

vermiculite Vermiculite is made from a naturally occurring, non-toxic, soft mica mineral.  It is heated to a high temperature to produce a lightweight, sterile, laminated structure, which ensures adequate soil aeration while at the same time having the ability to hold moisture and nutrients.

I have had good results using vermiculite, but this does not mean that is absolutely necessary.  Don't worry if you can't get hold of any, but keep an eye out, and ask at garden centres if you can.