Alfalfa sprouts actually have very little nutrition. So, they are not a good food for turtles. The FULLY-GROWN alfalfa plant is a good food for juvenile turtles, though, having much protein and calcium.
Alfalfa sprouts are also sources of food-borne illness. They are frequently found to be contaminated with salmonella and e.coli, which can sicken or even kill you and your turtles.
Mature greens are almost always higher in the nutrients that your turtle needs, than are their sprout counterparts. In the case of alfalfa sprouts, you might as well be feeding iceberg lettuce, for all the nutritional value they are receiving.
Please see the following links for more information:
http://www.anapsid.org/iguana/alfamat.html
http://www.anapsid.org/fdasalm1.html
http://www.anapsid.org/sprouts.html
On the last page, you can see that alfalfa sprouts are the least nutritious of all commercial sprouts, and that they (along with other commercial sprouts) have a lower Calcium to Phosphorus ratio than does lettuce. This is important because Phosphorus binds with calcium and carries it out of the bodies of reptiles. Reptiles should be fed mostly foods with a high Ca:P ratio, and supplemented calcium to make up for the rest.
I used to be a reptile expert. Now I'm just an old turtle lover.