Connie Weaver, PhD, Distinguished Professor of Food and Nutrition at Purdue University reported in her 2001 book chapter "Calcium," in Present Knowledge in Nutrition:

“Calcium absorption from milk and other dairy products is about 32%, whereas calcium absorption from vegetables ranges from about 5% in spinach to more than 60% in some brassica vegetables such as broccoli. However, the high bioavailability of calcium from some vegetables cannot overcome their low calcium content. One would have to consume 2 1/4 cups of broccoli to obtain the same amount of calcium absorbed from one cup of milk.”