I start feeding at the first transplant, when the plant has about 4 sets of adult leaves, when I put the plants in 3" pots (I start them in six-pax). Before that, they get no ferts at all, including from the soil.
While you're at teh grow shop, some useful items would be a small bottle of liquid pH indicator (it's super cheap, like $7), a solid 'grow' fert with micros (try Power Plant by Botanicare, or Pure Blend Pro if you like a more organic-leaning grow), and a micronutrient supplement for those occasions that you find yourself treating a deficiency (the GH Micro works well, as does AlgoFlash, and all the fertilizer companies have their own supplement- ask the shop staff to recommend one for you). Or ask the shop staff if your RainGrow will be better if you add a micronute supplement to it when you mix your ferts.
You'll also want a bloom fert for later.
stinkyattic Reviewed by stinkyattic on . PURPLE Stems? Hey everyone, Now I'm aware of red stems and nitrogen deficiencies but I Have a month old indoor that has become dark PURPLE between a section of two nodes down the stem. Is this some sort of extreme N deficiency, or is this a whole 'nother beast?!:( Rating: 5