worm castings are an excellent source of nitrogen, should be mixed into the soil, not left on top ... it may 'wash down' into the soil as you water, but pretty expensive stuff to not be using it correctly ... choice is up to you ... also, go with more perlite, about 20%, if you can ... no biggie on that, it does help drainage and aeration to the roots, though ... I don't like vermiculite, holds too much moisture, and like perlite, floats around when watered ... good luck :thumbsup: