The 17 essential elements
Form used by plants:
Fe2+ (ferrous) or Fe3+ (ferric). Fe3+ must, however, be reduced to the ferrous form before uptake can occur.
Required component in the formation of chlorophyll (80% of Fe in plants is located in the chloroplasts)
Activator of many biochemical processes (found in ferredoxin, and enzymes such as peroxidase, catalase, and cytochrome oxidase which participate in oxidation-reduction processes)
Ideal foliage range for apple leaves:
Ideal fruit concentration:
Ideal soil range:
However, most test methods don't distinguish between the forms of iron and therefore, have little meaning for plant nutrition.
Iron is strongly sorbed by soil components and is relatively immobile.
Mobility in plant:
Symptoms increase with increase in soil pH.
Vegetation symptoms: Younger leaves are affected first and will show interveinal chlorosis. Veins remain green except in extreme cases. Shoot growth is stunted and twig dieback may occur. Necrosis of tips and margins as deficiency progresses. All or part of a tree may be affected. May occur sporadically throughout orchard.
Increased risk of Fe deficiencies on:
Soils receiving excessive amounts of irrigation water, especially in early spring when soils may still be frozen (see Extension paper)
Alkaline soils (pH>7)
Lack of poor soil aeration (explain this chemically)
Application of N fertilizer may increase the chlorosis
Correcting Iron Deficiency
Soil and water management
Improve soil drainage by soil profile modification or installing tile drains; and/or
optimize irrigation water amounts and scheduling; and/or
subsurface banding of Fe-containing chemical amendments
Airblast foliar sprays (effect usually only temporary)
Trunk injection with Fe-compounds
Excess problems/Interactions with other elements:
- Excess zinc, manganese, copper, molybdenum, or phosphate encourages iron deficiency.
- Excess iron may reduce manganese absorption.
- In neutral to alkaline soils with low available iron, increased acidity from ammonium fertilizer forms may enhance the availability of ferrous (Fe2+) iron by promoting the reduction of the unavailable ferric (Fe3+) iron.
July 13, 2004