It is a slow-release, balanced material. In addition, alfalfa contains triacontanol, a root-growth stimulant, which I'm told earthworms love.
Earthworms are a valuable tool in the garden as they aerate and fertilize the soil.
Rosarians like to use alfalfa pellets. Blood meal is a quick source of nitrogen. Bone meal has nitrogen and is a quick phosphorus source. Bone meal also slightly increases soil pH, making it more alkaline. Cottonseed meal, too, is a good and rapid nitrogen source.