This has actually been a bug since carrots were introduced. The attached image shows the issue the clearest, but in text form: Unlike wheat (not sure about potatoes, I don't grow those), which displays green on a map, carrots do not display and just appear brown (the color of the farmland).

Edit: I should add that the screenshot is from 42b so that's why the map is transparent. However, the bug persists in 1.7.1.