For sure add in the BioChar, will help will sequestering mucho carbon. If you are still looking for good EWC, and can drop $80, get those Worm Gold Plus, its about $40 for a 20lb bag shipped. Stuff is real good man, real good.
Hey, just a quick post on the topic of air stones. I was brewing 5 gallons a time with two air stones until one stone broke where the tubing connects to the stone itself. Since that stone broke, I've run a few brews with just one stone.
Well, I just went out and picked up a new air stone...
http://www.boogiebrew.net/worm-gold-plus/
^
Hey man, I'm not sure if its more cost effective, but the above castings are amazing. They have rock phosphate and fossilized kelp amended in. Read on if you're interested.
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"Our ultra-premium worm castings are produced by worms fed a specific diet...
Soil sounds like it will be fine for now. Not too hot (too hot meaning having too much nutrients), just a light touch of Nitrogen to encourage healthy vegetative growth. I've not used Black Gold, possibly someone who has can speak better about it.
Suggestion, raise the pots closer, about 4-5...