Potassium (K)

Function:

Involved as a metabolism catalyst, for stomata function, and disease resistance.

Deficiency:

Medium to older leaves first develop chlorosis of interveinal areas, progressing to necrosis (browning) of the leaf margins (edges), and plants develop weak stems and stalks. K is a mobile element within the plant, therefore deficiency symptoms will first appear on the lower, older leaves.

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

Advance symptoms of K deficiency.
Click on image for larger version. (image ©Brian Whipker, NCSU)

Excess:

Can reduce uptake of: calcium (Ca), magnesium (Mg), manganese (Mn), ammonical-nitrogen (NH₄^-N), or zinc (Zn).

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

Monitoring and Management Strategy for Potassium (K) Fertilization Rate:

150 to 250 ppm. See the Fertilization Rate Table for specific fertilization recommendations based on growth stage and irrigation practices.

Ratio:

Potassium (K) fertilization rates >200 ppm can have an antagonistic effect on Ca or 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 1.5 to 3.5%. Levels less than 1.0% are considered deficient and greater than 4.0% are considered toxic.

Options:

Preplant:

Incorporation of potassium nitrate (KNO₃) into the root substrate (medium) for a starter charge.

Continual Fertilization:

1. Use a fertilizer that provides K. Examples include potassium nitrate (KNO₃), 20-10-20, 15-5-25, Excel® 15-5-15 Cal-Mag, and others.
2. Remember when calculating K fertilization rates, the numbers on the fertilizer bag are expressed as a percent of K₂O. Therefore multiply the bag number by 0.83 for the percentage of K.

Corrective Fertilization:

1. Applications include potassium nitrate (KNO₃) or 15-5-25 at the rate of 300 to 400 ppm K. One or two corrective K fertilizations will return the chlorotic tissue to the normal green color within 1 to 2 weeks. Do not overapply. It is important to correct K deficiency when symptoms first appear because necrotic leaf margins can not be reversed.

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