Iron chlorosis refers to a loss of green color in leaves due to a lack of iron. Of course, a number of factors cause leaves to yellow--but if chlorosis is the problem, leaves will tend to turn yellow from the edges inwards, with their veins usually remaining green, and new leaves will be affected more than old ones.
While chlorosis may be due to iron-poor soil, it is more likely that iron is present but simply cannot be absorbed by plants. The problem may be alkaline soil, since a high soil pH renders iron insoluble in water. Poorly drained soil that remains soggy for long periods also retards the release of iron from minerals.
To remedy iron chlorosis, check soil drainage and pH and make any necessary corrections. To aid afflicted plants quickly, add chelated iron to the soil or apply it (in spray form) directly to leaves. In this form, the iron will be available to plants at once.