Phosphorus (P)

Function:

Involved in energy transfer (ADP), nucleic acids, enzymes, and membrane structure. It also plays an important role in root and floral development and stimulates rapid plant growth.

Deficiency:

First expressed as extensive stunting with the leaves turning dark green. Under advanced P deficiency conditions the lower leaves will yellow, then become reddish-purple, chlorotic, and ultimately necrotic. It is important to correct P deficiency when symptoms first appear because lower leaf necrosis can not be reversed. P is a mobile element within the plant, therefore deficiency symptoms will first appear on the lower, older leaves.

Possible phosphorus deficiency of poinsettias. 0022.jpg missing
Click on image for larger version. (image ©Brian Whipker, NCSU)

Phosphorus deficiency of poinsettias.
Click on image for larger version. (image ©Doug Bailey, NCSU)

Excess:

Excessive levels of P will reduced plant growth, reduce bract size, and can induce deficiencies of iron (Fe), copper (Cu), manganese (Mn), and zinc (Zn).

Phosphorus toxicity of poinsettias induced by 16 times the normal rate of superphosphate.
Click on image for larger version. (image ©Robert Eddy, Purdue Univ.)

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

Monitoring and Management Strategy for Phosphorus (P) Fertilization Rate:

10 to 25 ppm. (Remember 20-10-20 applied at the rate of 200 ppm N will supply 44 ppm of P.) See the Fertilization Rate Table for specific fertilization recommendations based on growth stage and irrigation practices.

Tissue Concentration:

Normal range of 0.3 to 0.6%. Levels below 0.15% are considered deficient and levels greater than 0.9% toxic.

Options:

Preplant:

Incorporation of super phosphate or treble-super phosphate in the root substrate (medium) for a starter charge.

Continual Fertilization:

1. Use a fertilizer that provides P. Examples 20-10-20, 15-5-25, Excel® 15-5-15 Cal-Mag and others.
2. If using 20-10-20 or 20-20-20, remember to supply Ca and Mg to the plants.
3. If using calcium nitrate (Ca(NO₃)₂) with potassium nitrate (KNO₃), remember to supply P, Mg, and micro-nutrients to the plants.
4. If injecting phosphoric acid for alkalinity control, remember to adjust your P fertilization rate.
5. Remember when calculating P fertilization rates, the numbers on the fertilizer bag are expressed as percent of P₂O₅. Therefore multiply the bag number by 0.437 for the percentage of P.

Corrective Fertilization:

1. Applications of 20-10-20 at the rate of 200 ppm N will supply 44 ppm of P. A corrective P fertilization will return the lower leaves to the normal green color within 1 to 2 weeks. Do not over apply. It is important to correct P deficiency when symptoms first appear because lower leaf necrosis can not be reversed.

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