http://www.cloudnet.com/~djeans/Asides/OxalicAcid.htm
Although oxalic acid binds to calcium (producing calcium oxalate crystals), and making calcium unavailable for absorption, many vegetables that contain oxalic acid also contain calcium. The amount of oxalic acid is what determines how much calcium is bound. So if there is more calcium than oxalic acid present, the calcium that has not been bound will be available for use by the body.