Iron (Fe)

Function:

Like magnesium, iron (Fe) also plays a role in the chlorophyll molecule.

Deficiency:

Deficiency symptoms appear as an interveinal chlorosis of the younger leaves, progressing to tip dieback under severe conditions. Deficiencies occur when the root substrate has a high pH (>6.5), root death has occurred, over-irrigation, poor drainage of the root substrate, insect damage (fungus gnat larvae) to the roots, or when there are excessive levels of PO₄, Mn, Cu, or lime are applied.

High pH induced iron chlorosis on poinsettias.
Click on image for larger version. (image ©Brian Whipker, NCSU)

Excess:

Toxicity symptoms are rare. Excess levels of Fe can reduce the uptake of Mn. Fe is more readily available when the pH is <5.5.

Confirm your actual Fe levels by conducting a plant tissue analysis. Root substrate (medium) tests are less reliable for determining Fe levels, but pH determination may be useful in indirectly assessing Fe availability.

Monitoring and Management Strategy for Iron (Fe)

Fertilization Rate:

Apply as part of your constant liquid fertilization program. Maintain the root substrate pH between 5.8 to 6.2 for a soilless root substrate is and 6.0 to 6.5 for a soil-based substrate to maximize Fe availability.

Tissue Concentration:

Normal range is 100 to 300 ppm. Levels below 50 ppm are considered deficient.

Options:

Preplant:

Incorporation of a micro-nutrient mix into your soilless substrate.

Continual Fertilization:

1. Most commercial fertilizers that provide micronutrients including Fe. Remember to add supplemental Fe applications if using calcium nitrate (Ca(NO₃)₂) + potassium nitrate (KNO₃).
2. Maintain the root substrate pH between 5.8 to 6.2 for a soilless root substrate is and 6.0 to 6.5 for a soil-based substrate to maximize Fe availability.

Corrective Fertilization:

1. Correct the causal condition if possible (when the root substrate has a high pH (>6.5), root death has occurred, over-irrigation, poor drainage of the root substrate, insect damage (fungus gnat larvae) to the roots, or when excessive levels of PO₄, Mn, Cu, or lime were applied).
2. Supplemental applications of iron chelate to the root substrate at the rate of 1 ounce per 15 gallons of water to cover 50 square feet of bench area. Visible improvements (darker green leaves) with the use of a corrective Fe fertilization should evident within 2 weeks. It is important to correct Fe deficiency when symptoms first appear because tip dieback can not be reversed. Do not overapply.

Corrective Foliar Spray:

Foliar sprays have a greater risk of plant injury. Early morning applications are preferred. Avoid applications during the midday heat. Plant uptake is enhanced by the increased drying time which occurs during the moist conditions in the morning. Nutrient uptake through the leaves is also greater in the light periods than at night, thus making morning applications more desirable than evening sprays.

To apply a foliar spray of Fe: mix 4 oz. (113.4 g) of iron sulfate per 100 gallons of water. It will give a final concentration of 62 ppm Fe. Test on a few plants first. Do not over apply.

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