This year I used blood meal. I saw no difference w.r.t. synthetic or natural urea.

Any organic fertilizer will work about the same. You can use the protein meals if you like, a prepackaged organic blend or even just compost. Just apply it early in the season before planting.

Both alfalfa meal and blood meal tend to be rather fast acting for an organic product and direct contact with plant roots can result in fertilizer "burn". Mix them in well before planting.

Alfalfa meal is also known to have other materials that stimulate plant growth besides the N. I find that alfalfa pellets added to my seedling starting pots really stimulate good growth.
Renais