During the life of a marijuana plant it goes through two cycles. These cycles are Vegatative and Flowering. When the plant is in these cycles they need different nutrients to grow to maxium potential. Vegatative growth needs a solution that contains equal amounts of each nutrient. Such as 20-20-20 which means 20 parts nitrogen, 20 parts phosphorous,and 20 parts potassium. This will provide a vegatative plant with all the nutrients it needs. When a plant is in flowering mode you should provide a fertilizer with a high Phosphorous number.
When growing indoors a grower will have the choice of when he wants the plant to flower. This can be anywhere from 12 inches to 12 feet all depends on the space you have for growing. Once you have decided you plant is big enough to flower you will have to change the fertilizer to somthing like 5-50-5 just something with a high middle number. When growing outdoors you do not have a choice of when your plant flowers there is a hormone released in the air which will cause your marijuana to bud automatically this happens around June 21. The plant relizes the days are getting shorter so the plant starts to flower.
The thing with fertilizer is it can be a great thing or it can be a deadly thing. When fertilizing your plant you must relize that you can kill your plant if you over fertilize it. I recommened that you use fertilizer every 4 waterings (4 days). When your plant is in the flowering mode and coming to the end stop the fertilizing about two weeks before you pick the buds. If you dont your weed will taste like fertilizer.