This acid occurs in many plants and is poisonous in excessive amounts. Some of the plants that contain a measurable amount of oxalic acid are sorrel, spinach and rhubarb. Because it forms insoluble compounds with calcium and iron, inhibiting their absorption by the human body, oxalic ascid greatly diminishes the purported nutritional punch of spinach.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.