Cooked spinach is actually a good source of non-heme iron and
raw spinach contains oxalates (derived from oxalic acid) which IMPAIRS the absorption of iron.
"Oxalates impair the absorption of nonheme iron. Oxalates are compounds derived from oxalic acid and found in foods such as spinach, kale, beets, nuts, chocolate, tea, wheat bran, rhubarb, strawberries and herbs such as oregano, basil, and parsley. The presence of oxalates in spinach explains why the iron in spinach is not absorbed.