I have been given different accounts on the edible-ness of sorrel. Some people/sellers/books do not give you any information about how much it is safe to eat, whilst others tell you to just use a few leaves in a salad.
In addition to this, sorrel soup was apparently the bane of my father's poor childhood, so I wondered if cooking it (as a soup or otherwise) neutralised the semi-poisonous chemical it may or may not contain.
If there are different varieties of sorrel, mine grows leaves up to about 20-30cm long, and the plant is about 30-40cm high before making flower stalks, the tallest of which gets to maybe 1.5m.
Best Answer
There are indeed different varieties of sorrel,* but they're both/all high (though I can't determine how high) in the substance of concern, which is oxalic acid. This acid is found in lots of other green leafy vegetables, notably spinach and parsley, and is the reason that you shouldn't eat rhubarb leaves.**
It seems to have a reasonably high expected lethal dose; the internet echo chamber puts it at 600 mg/kg, which would mean eating pounds and pounds of whatever leaf you've got. The necessary amount to cause sickness is obviously much lower, however. There's also a long-term risk for kidney stones -- oxalic acid combines or reacts with calcium to form calcium oxalate, which is a major component of those stones (and this is the mechanism for more immediate poisoning, whether lethal or not). I have also seen reports of simple mouth and digestive irritation, again caused by the calcium salt.
Unfortunately, it seems that cooking does not destroy oxalic acid. It may be possible to reduce its presence by boiling and discarding the water, however. It's unclear to me whether this is because the acid itself dissolves into the water, or whether the oxalates (Harold McGee says that the potassium and sodium salts are water-soluble) form during cooking and are dissolved.
In all, I think the only recommendation that can be made is simple moderation. It's incredibly unlikely that you'll kill anyone with a salad, but don't eat the stuff in large amounts every day.
*"Common" (looks like elongated spinach) and "wood" (looks a bit like clover, but the leaves are cardiod) are the two that I'm most familiar with.
**Although it seems that there's possibly at least one other poisonous substance in the leaves.