Activated charcoal in powder form does not typically leach through soil; it remains on the surface, even when watered. Its porous structure allows it to absorb substances, like pesticides, but it doesn't dissolve and move downwards. While it can be mixed into soil for various benefits, it won't leach.
1 gram per 100 ml of liquid to 2-6 grams per liter for solid mediums. The specific ratio may vary depending on the blah blah blah.