Essential Nutrients

Iron (Fe)

Form used by plants:

Fe²⁺ (ferrous) or Fe³⁺ (ferric). Fe³⁺ must, however, be reduced to the ferrous form before uptake can occur.

Important functions:

• 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:

40-400 ppm

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Ideal fruit concentration:

Ideal soil range:

4.5+ ppm 

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. 

Best indicators:

Mobility in plant:

Not remobilized within plant

Deficiency symptoms: 

• 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)

• apply multiple sprays of Fe-chelates at labeled rates during growing season

Trunk injection with Fe-compounds

• can be effective if done properly but usually is very time consuming and requires specialized equipment. It can cause severe tree injury and death if done improperly.

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.

 

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