Often there will be yellowing in the leaves between the veins which is termed interveinal chlorosis. In milder situations, the veins and area around them will stay green while the tissue in between will turn various shades of yellow. In very severe cases, the entire leaf becomes pale.
This discoloration can easily be mistaken for a nitrogen or manganese deficiency, but the difference is that iron chlorosis will first show up in newer or younger leaves whereas nitrogen deficiencies will first become visible in the older leaves because it will be mobilized to support the younger portion of the plant. The reason for this is that iron is not very mobile in the plant and generally doesn’t move from the older leaves to the newly produced ones.
As mentioned, most often there is plenty of iron in the soil but even a slightly elevated pH will lead to bonds formed rendering it from being soluble and taken up. In addition, manganese and calcium can interfere with the uptake of iron. Calcium and carbonate concentrations from lime or irrigation water with high levels of carbonates can create an artificial condition where absorption of iron will be reduced.
What makes this situation more troubling is that most soil testing does not accurately show levels of available iron in the soil solution. Other factors for chlorosis include, drought, poor drainage, soil compaction and other situations where root development is poor or impeded.
There are plenty of granular and liquid sources of iron which can be applied to rectify the situation. Most times these iron sources are chelated meaning that complexes have been formed with polymers or organic acids to keep the iron in a more soluble state for a longer period so as to be more available.
Some of the common chelated sources include: DTPA, EDDHA, and EDTA along with humic and fulvic acids. These products will help restore color and vigor to the plant but some consideration should be given to managing soil pH for a longer term answer.
Keep in mind that many iron sources can stain so be sure to recommend sweeping or blowing granular products off of hard surfaces and make sure not to spray liquid sources onto stainable areas. For more information, here is a more technical article regarding the function of iron from the University of Florida IFAS Extension.