I agree with z, experience and envrionmental control are the key things that limit plant growth. Budget counts too because you need to have good gear to do the job right. So many times i see posts from new growers wanting to get 2lbs from a 150w hps, and a few cf's, using miracle grow.

A good ph meter and ppm meter will go a LONG ways toward a succesfull first crop, but they are fairly expensive to a new grower, and most try to get by without them at first. Most 1st crops fail or are pretty poor, mainly because the new grower did not start out with good equipment. A startup cost of a thousand dollars is not unreasonable for meter/s, a good light, hydro nutes, Ph up, ph down, calibration fluid, a timer/pump or two, and the actual grow system of your choice along with related hardware like airpumps/airline/air stones. Mylar/panda plastic? Lightmover? Good reflector? And a LOT more misc stuff.

But consider the fact that you never will need to ask someone for help again. And over time, and with experience, the very best smoke in the entire world will be yours. So anybody who is really serious about providing for themselves forever needs to consider the startup cost as an investment in your own future that will pay you back many times over. Good luck