I just dug a large whole filled with compost. You can find good compost very cheap for a very large bag. Mine included Vermiculite and Sphagnum peat moss. The peat moss itself gives the plant enough nitrogen when it is vegging and when you need to add more nitrogen just add another inch or two of the compost on top of the soil. When my soil would look old and stale it would be right around when my plant would start to yellow from nitrogen deficiency, and the compost perked it right up every time. Along with the compost some feedings of fish emulsion.
When it is flowering keep adding your inch or two of compost every now and then but start to use molasses and if you would like you could even put a little wood ash in there.
Every time I have done this the plants have exploded and the roots love it. Plus everyone knows all bud comes out better with all organic rather then synthetic fertilizers
