"The idea occured to me that alfalfa pellets might work,
but what else would I need to add to balance out the
nitrogen?"
According to
http://www.organicgardening.com/feat...21-112,00.html
alfalfa pellets are 40.5% carbon and 2.7% nitrogen
(in the real world, it probably varies with where the alfalfa
was grown, how mature it was when harvested, etc).
That would be about a 15-1 C-N ratio, and 30 is supposed
to be ideal for compost, so maybe a garbage-bag full of
dry leaves or loose straw to 25 lbs of alfalfa? (Just a guess.)