Grains and beans: soaking in lemon juice and water breaks down phytic acid. This acid prevents absorption of some minerals hence is for the most part an undesirable. You can also sprout and accomplish the same thing. Here is a website with much more detail on this subject http://www.phyticacid.org/.
Oxalic acid, which is present in spinach, also limits absorption of minerals by chelating them out. This can be prevented by eating baby spinach. (You can get organic baby spinach at Sam’s for a great price.)
Soy also contains phytic acid. It, however, is a much more complex subject. Jen has researched it and her synopsis is so helpful in understanding what part, if any, soy should play in our nutrition. I will post her paper as soon as she emails it to me.