Iron (Fe) is the fourth most abundant element in the earth’s crust. It is vital for important plant functions such as photosynthesis, DNA synthesis, protein formation, biological nitrogen fixation, and respiration. There are three main classes of iron fertilizers include inorganic iron compounds, synthetic iron chelates and natural iron complexes.
A general chlorosis of young trees is the most telling symptom of iron deficiency. At first the veins may remain green, but in most species in which the deficiency has been observed the veins also become chlorotic eventually. The deficiency is common in fruit trees. Where it is associated with high levels of calcium carbonate in the soil, it is called "lime-induced" chlorosis.
The number one cause of iron deficiency is having a high soil pH. So, under some circumstances, lowering soil pH can resolve iron deficiency if the soil pH can easily be changed (desert soils are buffered, meaning it will be hard to change soil pH). Traditionally, adding elemental sulfur or sulfates can change soil pH, but if too much is applied soil can become anaerobic.
To quickly correct iron deficiency chelated iron should be foliar applied, but even though this is a very quick way to fix deficiency, it will need be be reapplied to keep adequate levels of iron within the plant.