Nitrogen (N)

Function:

Involved in the synthesis of amino acids, proteins, enzymes, and nucleic acids.

Deficiency:

Plants exhibit slow growth, stunting, or with advanced conditions, substrate to lower leaf chlorosis (yellowing) and leaf abscission in some plants. N is a mobile element within the plant, therefore deficiency symptoms will first appear on the lower, older leaves.

Smaller growth of poinsettia due to low electrical conductivity / nitrogen levels.
Click on image for larger version. (image ©Brian Whipker, NCSU)

Excess:

Excess levels of N will result in reduced plant growth and delayed flowering. Can reduce uptake of potassium (K).

Poor plant growth of poinsettia due to excessive EC / nitrogen levels.
Click on image for larger version. (image ©Roy Larson, NCSU)

Confirm your actual N levels by conducting a routine root substrate (medium) test (including both nitrate-nitrogen [NO₃^-N] and ammonium-nitrogen [NH₄^-N] analysis) and/or a plant tissue analysis.

Monitoring and Management Strategy for Nitrogen (N) Fertilization Rate:

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

Ratio:

Poinsettias are susceptible to ammoniacal-nitrogen (NH₄^-N) toxicity. Symptoms appear as an interveinal V-shaped chlorosis with green veins, and downward cupping of the younger leaves. Ammoniacal-nitrogen toxicity can be avoided by supply >75% of N in the nitrate (NO₃^-) form. (Categorize urea-nitrogen with ammonical-nitrogen when calculating the ratio.) Ammonical-nitrogen promotes vegetative growth and is beneficial during the first half of the growing season. After November 1st, discontinue the application of ammonical-nitrogen due to the promotion of growth (stretch) and that high levels of ammonical-nitrogen have been linked to increased incidents of bract edge burn and a shorter post-harvest life of poinsettia plants.

(insert xx16 Ammoniacal-nitrogen (NH₄-N) toxicity symptoms. © )

Excessive growth resulting from a high ammonical-nitrogen type fertilizer.
Click on image for larger version. (image ©P. Allen Hammer, Purdue Univ.)

Tissue Concentration:

Normal range of 4.0 to 6.0%. Levels less than 3.5% are considered deficient and greater than 7.3% toxic.

Options:

Preplant:

Incorporation of calcium nitrate or potassium nitrate in the root substrate (medium) for a starter charge.

Continual Fertilization:

1. Use a fertilizer that provides N. Examples include calcium nitrate (Ca(NO₃)₂) with potassium nitrate (KNO₃), 20-10-20, 15-5-25, Excel® 15-5-15 Cal-Mag, and others.
2. If using calcium nitrate (Ca(NO₃)₂) with potassium nitrate (KNO₃), remember to supply P, Mg, and micro-nutrients to the plants.
3. If using 20-10-20 or 20-20-20, remember to supply Ca and Mg to the plants.

Corrective Fertilization:

1. Applications include calcium nitrate (Ca(NO₃)₂), potassium nitrate (KNO₃) or Excel® 15-5-15 Cal-Mag at the rate of 300 to 400 ppm N. A corrective N fertilization will return the lower leaves to the normal green color within 1 to 2 weeks. Do not overapply. It is important to correct N deficiency when symptoms first appear because lower leaf drop may occur under severe conditions.

Suggested constant liquid fertilizer rates for poinsettias (ppm) based on growth stage.

Element

Growth Stage

Early

(Weeks 0 to 2)

Mid

(Weeks 2 to 12)

Late

(Weeks 12 to 15)

Total N^1,2

100 - 150

150 - 250

0 to 100

% NO₃-N

70 - 80

80 - 100

100

% NH₄-N

20 - 30

0 - 20

0

P¹

10 - 25

10 - 25

0 - 10

K^1,3

100 - 150

150 - 250

0 to 100

Ca^1,3

50

75 - 125

75 - 125

Mg^1,3

25 - 50

50 - 75

50 - 75

S¹

25

25 - 64

25

Mo⁴

0.1

0.1

0.1

B, Cu, Fe, Mn, Zn

There is a fine line between deficient and toxic levels. Use a fertilizer that provides a measured dose of micronutrients.

¹ Optimal range is 25% to 50% lower with flood irrigation or reduced leaching fertilization programs.

² Ammoniacal-nitrogen toxicity can be avoided by supply >75% of N in the nitrate (NO₃-N) form. (Categorize urea-nitrogen with ammonical-nitrogen when calculating the ratio.) Ammonical-nitrogen promotes vegetative growth and is beneficial during the first half of the growing season.

³ Supply poinsettias with a K : Ca : Mg ratio (ppm) of 4 : 2 : 1 to limit any antagonisms.

⁴ Poinsettias have a high requirement for Mo and excessive rates up to 60 ppm are reported to cause little or no plant damage. Mo toxicity is possible with other greenhouse crops and some plants are sensitive at >0.5 ppm.