unsulphured black strap molasses, 1 tablespoon per gal of water. And it will not make them sweeter , only feeds the soil.
^ = win, especially for autos.
However, it does colour the water, so if you use a liquid-based pH testing kit, make sure to test the water beforehand. You must also be aware that black strap molasses lowers pH by about 2-3pts per tbsp/gal, depending on your base pH. Again, testing is necessary.
Don't let this fact deter you, because it really is a great additive. Experiment a little to start, and when you have it, you have it.
Note that molasses is best for soil-grows, and not hydro. Hydro systems have their own sugar-based additives you can buy. (Unsulphured BSM can be purchased in your local bulk foods store for about a buck or two.)
Hope that helps!