Dr. Guy Witney, Former WSU Area Extension Faculty
Heavy winter precipitation in many areas resulted in wetter than normal soils in orchards at the start of the growing season. As a consequence, many trees and even entire sections of orchards may show symptoms of iron deficiency in early summer.
The signs are easily recognized as yellowing of new leaves (chlorosis) over the entire tree, or in just a few limbs. On closer inspection, the leaf veins normally remain green while the blade itself turns a bright yellow to white color. In the most severe situations, leaves may have burned margins and limbs or trees may defoliate. Iron is required for the production of chlorophyll and so a deficiency results in the absence of this green pigment in leaves.
Iron deficiency in fruit trees is expressed in new growth when a simultaneous restriction in root iron uptake occurs. The absorption of iron has been shown to be limited to the growing root tips of trees and so healthy roots are essential. Any restriction in root growth, such as waterlogged oxygen depleted soil conditions, will result in reduced uptake. It has also been shown that carbonate (HCO3-) in the soil restricts the uptake and movement of iron. High levels of carbon dioxide (CO2) are required for the formation of HCO3-. Under conditions of poor aeration, such as in waterlogged soils, CO2 accumulates from root and microbial respiration. This in turn results in elevated HCO3- and restricted iron uptake.
Carbonates are also formed in high pH calcareous soils which are common in some fruit growing areas of the West. Orchards grown on these soils are likely to exhibit iron deficiency symptoms, particularly under wet conditions. Excessive lime applied to raise the pH of soils may also result in iron deficiency. The term “lime induced iron chlorosis” is frequently used for symptoms on trees grown under high soil pH conditions.
While all of this may sound complicated it boils down to a few main points. Under excessively wet conditions root growth is restricted and HCO3- accumulates. Both of these factors result in restricted iron uptake. Any growth occurring at this time will likely show iron deficiency symptoms.
Foliar and soil applications of iron may reduce the severity of deficiency symptoms but are considered only temporary measures. The application of inorganic iron salts to the soil is generally ineffective because the iron is rapidly oxidized and becomes insoluble. Even foliar applications of inorganic iron salts is frequently ineffective. Sequestrene iron chelate has been shown to be effectively taken up, both as a soil dressing or a foliar spray, but is generally an expensive remedy. EDTA iron chelate has been shown effective in some cases, but because of poor stability this form of iron may rapidly oxidize and become unavailable to the plant.
The best way to manage iron deficiency is to manage irrigation and soil pH. Excessive soil moisture resulting from heavy winter precipitation or spring irrigation is most often the cause of a temporary iron deficiency. If excessive irrigation is applied every spring chronic iron deficiency will result and production will suffer. Where soil pH is high, the use of acid fertilizers particularly ammonium fertilizers, can lower the soil pH, improve iron availability and depress deficiency symptoms. Remember, however, the uptake of other nutrients may be restricted if the pH becomes too low.