If you check the “USDA National Nutrient Database for Standard Reference”, you may be surprised to find that spinach does not contain an unusual amount of iron: it has less than most breakfast cereals, about a quarter as much as raw clams, and about the same as canned pork and beans (not counting the can).
Here’s how the spinach-iron connection got started.
Near the end of the nineteenth century, German scientists correctly found that the amount of iron in spinach is comparable to that in meat: some 3 milligrams per 100 grams, or 30 parts per million. But in the report of their findings somebody put the decimal point (actually, in Europe, a comma) in the wrong place, making the yield of iron appear to be ten times as great.
The error was corrected some forty years later, but not before Popeye decided to adopt spinach as his power food. After all, iron is strong, right? If Bluto only knew that Popeye’s cans of spinach were a bluff!
The final irony (pun intended) in all this is that whatever iron spinach does contain is not readily absorbed by the body because spinach also contains a small amount (1 percent) of oxalic acid, which ties up the iron into an insoluble form, ferrous oxalate. So only a fraction of spinach’s modest amount of iron is available for our metabolism.
What if you did plant spinach in your back yard and there wasn’t enough iron in the soil? That’s really very unlikely, because iron is needed by plants in only trace amounts and iron is a widely distributed element, constituting about 5 percent of Earth’s crust.
Hypothetically, though, without any iron at all your spinach would grow, but it would show symptoms of a nutritional deficiency, just as you would if you were deficient in a vitamin. The leaves would be a sickly yellow instead of green, because plants use iron in synthesizing chlorophyll.