I don't think quantity is written in stone. I'd use a cup or two.

Not a lot of growers bother with charcoal anymore. We should mention to those who do use it to use horticulture charcoal.

What does it do? They say fumes go up to the plant and beg roots to come and get it, forcing roots to go deep down into the soil and it sweetens the soil when things go sour.

Nancy