
From _Reader's Digest North American Wildlife_, copyright 1982, page 402:
"Several plants with pleasantly sour foliage are known as sorrel (from the German for "sour"), and in fact, the wood sorrels are called sour grasses or sour clovers in many places. Popular salad ingredients for centuries, they were also used by old-time herbalists to treat various stomach ailments and to cure scurvy (they are rich in vitamin C). It is now known, however, that eating too much oxalic acid, the chemical responsible for the sourness, tents to inhibit the body's absorption of calcium. The special flavor of wood sorrel should thus be enjoyed only occasionally."
I should add here that my kids find yellow sorrel very frequently. According the above book, sorrels in Iowa may also have flowers that are white with pink stripes and violet/pink flowers.
In the words of Robert Krampf (also here), Have a wonder-filled day!