I use alfalfa meal and soybean meal for slow release N. Both are good for ~6 weeks.
Alfalfa can really heat up your mix, enough to burn roots if you're careless. I just mixed up some alfalfa, soybean and castings for a top dress of some long term plants. The following morning I went out to give it another swirl and damn near burnt my fingertips! It was that hot, well over 100*. This was ~1 1/2 gallons in a 5 gal bucket.
I use the soybean at ~2x the amount of alfalfa, even though the soybean has ~3x the N, because of this and I've learned to not use more than 1cup/cf. LOL
Wet