The best way to treat the soil? (the dirt stuff. not store bought) BUY GOOD SOIL. This is the best advice I can give you. Buy the good fluffy stuff. Soil is a huge factor when it comes to yeild and potentcy... don't take short-cuts. Look for Pro-mix or Foxfarm or something and get some bone meal for flowering and blood meal for vegetative.

The best fertilizers and nutrients to add (and at what stages?)I like the GH 2 part (FLoraNova), Bonticare has a line out called pure blend. That one is all organic and has good feedback.

The best additives (if any) for the water? There are a bunch of additives. Superthrive is a good one. Just a couple drops per gallon.

The addition of gibberalic acid (a plant growth hormone) to a seedling? (This one I am particularly worried about trying, since its growing faster and taller, wouldn't that make the buds smaller, less potent, and less dense? Or would it help?) No... Gibberalic acid is an extremely poisonious chemical used for inducing hermies' so you can make feminized seeds. DON"T USE THIS STUFF...