I think the bbq char should be ok to use. Just make sure nothing has been added to make it burn better i reckon.
Its as easy as pissing on it and soaking in wormcasting slurry for at least 24 hours or any of the other ideas in the thread. Then when its used in the garden its bio-char.
I think im gonna use it as mulch this spring to soak up the sun in a few spots.
there are some folks who do no-till outdoors by layering cardboard and/or newspaper on the ground flat over the area they want to improve ~it's mulch
this tech encourages worms and retains moisture
The guy that has this one figured out is Microman I think his handle is . I think he did a book.. good luck headband 707
your question about the alfalfa meal...make a slurry and charge your char with it.