hmm, a cup a cubic foot is a lot of alfalfa, not to mention it'll take a while to break down, i'd cut that soil when you pot them, maybe a third by weight? You might be ok, but it'd be a bitch to have to wait a month after planting to find out, and it's not easy to fix afterwards (being non-soluble, mostly).
I know I keep sayin it, but molasses, manures, and alfalfa are the easiest organic amendment that you can overdo on, and all three can lock your plant out, and piss her off.
Don't get me wrong I LOVE all three of those amendments, but ya got use a lil.