i'm running a small aquaponics system, using a growbed to filter my fish tank water.. an australian message board has provided me with all kinds of tips.. one user experimented with just using urea in her system (pee ponics?) and all plants did quite well.. this was on a very small scale compared to 40 acres though..
human urea will take about 2 weeks to convert to ammonia. if you're getting the urea from multiple sources, you might run the risk of chemicals getting into it (if you're on antibiotics it will affect your pee)

a local fish farmer dumps horse poo into his ponds for fertilization