Magnesium (Mg)

Function:

Chlorophyll molecule, enzyme activation

Deficiency:

Initially developing as an interveinal chlorosis (yellowing) of older, lower leaves. Upward curl of the leaves possible. With advanced conditions, the leaves turn from chlorotic to necrotic (dead, brown tissue). A corrective fertilization with Mg will return the chlorotic tissue to the normal green color within 1 to 2 weeks. It is important to correct Mg deficiency when symptoms first appear because necrotic spots can not be reversed. Mg is a mobile element within the plant, therefore deficiency symptoms will first appear on the lower, older leaves.

Beginning Mg deficiency symptoms (chlorosis) on poinsettias.
Click on image for larger version. (image ©Brian Whipker, NCSU)

Advanced Mg deficiency symptoms (necrosis) on poinsettias.
Click on image for larger version. (image ©Brian Whipker, NCSU)

Excess:

Can reduce uptake of: calcium (Ca)

Confirm your actual Mg levels by conducting a routine root substrate (medium) test and/or a plant tissue analysis.

Monitoring and Management Strategy for Magnesium (Mg) Fertilization Rate:

50 to 75 ppm. See the Fertilization Rate Table for specific fertilization recommendations based on growth stage and irrigation practices.

Ratio:

Potassium (K) fertilization rates >200 ppm or calcium (Ca) can have an antagonistic effect on Mg uptake by the plant. Supplying the plants with a K : Ca : Mg ratio (ppm) of 4 : 2 : 1 will limit any antagonisms.

Tissue Concentration:

Normal range of 0.3 to 1.0%. Levels below 0.2% are considered deficient.

Options:

Preplant:

Application of dolomitic limestone

Irrigation Water:

Mg in irrigation water (test water to determine available levels) Supplement with additional Mg in your fertilization program.

Continual Fertilization:

1. Use a fertilizer that provides Mg. Examples include Excel® 15-5-15 Cal-Mag and others. Calculate the ppm of Mg provided and make monthly supplemental applications if required.
2. Monthly magnesium sulfate (Epsom Salts) (MgSO₄·7H₂O) applications at the rate of 1 pound per 100 gallons of water. Do not mix with other fertilizers. 3. Common fertilizers like 20-10-20 and 20-20-20 do not provide Mg (or calcium [Ca]). If using 20-10-20 or 20-20-20, alternate monthly with supplemental magnesium sulfate (Epsom Salts) applications as recommended above. (Also apply supplemental Ca.)

Corrective Fertilization:

1. Magnesium sulfate (Epsom Salts) application at the rate of 2 pounds per 100 gallons of water. Do not mix with other fertilizers. A corrective fertilization of Mg will return the chlorotic tissue to the normal green color within 1 to 2 weeks. Do not over apply. It is important to correct Mg deficiency when symptoms first appear because necrotic spots can not be reversed.

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