I'm going to start a new grow. Can I use the soil from the last grow? Or is it necessary to use new soil for some reason?
Use new soil. It's dirt cheap -- literally -- and the soil you've used before should be depleted and could have salt build-ups. Give those girls every potential advantage.
Reused soil produces superior pot, but you have to know how to ammend the soil properly. You can use the same soil for years, and it just keeps getting better and better. See here... http://forum.grasscity.com/general-indoor-growing/122198-soil-growers-only-topic.html -mu