By natural I don't know if you mean organic or not. For cheap, maybe a Miracle Grow brand. Haven't tried it personally, but you can get alot of it for not much money. For an organic option, you can try "Dr. Earth brand" sold at Sears. I would be cautious about the levels you use it at, not sure if they have been tested by people for growing weed, maybe start at 1/4 strength. I usually go by the rule of thumb if its safe to eat stuff you put it on, its probably safe to smoke it. But like any nutes cut it out a couple weeks in advance (or longer if indicated on the labeling) of harvesting so it doesn't effect the flavor.
