Fertility Management for Geraniums

6/98 HIL-504

Brian E. Whipker
Floriculture Extension Specialist

Geraniums require an adequate supply of the essential nutrients and a slightly acidic pH. This leaflet covers some of the basic considerations for fertilizing zonal, ivy, and regal geraniums. Optimal pH, electrical conductivity (EC), and nutrient levels for the root medium are listed in Table 1. These values are based on the saturated-paste extract method. Differences are listed for zonal, ivy, and regal geraniums. More specifics for each element as to the function, deficiency and toxicities, and fertilization strategies are covered below. The fertilization and EC strategies are based on an irrigation practice with 20% leaching. Fertilization levels would be 25% to 50% lower with a subirrigation or reduced leaching irrigation program, in order to achieve similar root medium nutrient levels as with a 20% leaching program. Nutrient deficiency and toxicity symptoms for zonal geraniums are listed in Table 2, and Table 3 lists leaf-tissue analysis standards for zonal, seed, ivy, and regal geraniums. In certain cases, specific cultivars have been listed below. These cultivated varieties may have particular merit due to exceptional ornamental features or due to a unique form or habit. Many of the upright or columnar selections are particularly useful in situations where there is limited space for canopy development as is often the case along urban streets.

Table 1. Optimal substrate nutrient levels for geraniums.

Optimal rangea

Test

Units

Zonal geranium

Ivy geranium

Regal geranium

pH

5.8 to 6.3

5.5 to 6.0

5.5 to 6.0

EC

mS/cm

1.5 to 2.5

1.0 to 2.0

1.5 to 2.5

Nb

ppm

200 to 250

200 to 250

150 to 250

P

ppm

5 to 19

5 to 19

5 to 19

K

ppm

150 to 250

150 to 250

150 to 250

Ca

ppm

50 to 100

50 to 100

50 to 100

Mg

ppm

25 to 50

25 to 50

25 to 50

a Ranges based on the saturated-paste extract method.

b Nitrate-nitrogen should compose >75% of the total.

N, with the remainder being ammoniacal-N or urea-N.

Table 2. Nutrient deficiency and toxicity symptoms of zonal geraniums.

Element

Deficiency symptoms

Toxicity symptoms

Nitrogen (N)

Slow growth, stunting, or with advanced conditions, lower leaf chlorosis (yellowing)

Reduced plant growth and delayed flowering. Ammoniacal-N toxicity is expressed as a curling of the older leaves, leaf chlorosis and necrosis

Phosphorus (P)

Stunting of plant growth with the leaves turning dark green. Under advanced P deficiency conditions, the lower leaves will become reddish-purple and ultimately necrotic (die)

Reduced plant growth and high P levels can induce deficiencies of Fe, Zn, Cu, and Mn

Potassium (K)

Necrosis (death) of lower leaf margins and plants develop weak stems and stalks

Excess levels of K can reduce the uptake of Ca, Mg, Mn, and Zn

Calcium (Ca)

Expressed as death (blackening) of growing points of terminal buds and roots

Excess levels of Ca can reduce the uptake of K, Mg, and B

Magnesium (Mg)

Interveinal chlorosis of older leaves and the leaves may have an upward curl on the edges

Excess levels of Mg can reduce the uptake of Ca

Iron (Fe)

Interveinal chlorosis of the younger leaves, progressing to tip dieback under severe conditions. Deficiencies occur when the substrate has a high pH, root death has occurred, or when there are excessive levels of P, Mn, or Cu

Chlorotic and necrotic speckling of the lower leaves. Excess levels of Fe can reduce the uptake of Mn. Toxicity symptoms mainly occur when the substrate pH is too acidic

Table 3. Leaf tissue analysis standards for geraniums.

Element

Zonal geranium

Seed geranium

Ivy geranium

Regal geranium

Nitrogen (%)

3.8 to 4.4

3.7 to 4.8

3.4 to 4.4

3.0 to 3.2

Phosphorus (%)

0.3 to 0.5

0.3 to 0.6

0.4 to 0.7

0.3 to 0.6

Potassium (%)

2.6 to 3.5

3.3 to 3.9

2.8 to 4.7

1.1 to 3.1

Calcium (%)

1.4 to 2.0

1.2 to 2.1

0.9 to 1.4

1.2 to 2.6

Magnesium (%)

0.2 to 0.4

0.2 to 0.4

0.2 to 0.6

0.3 to 0.9

Iron (ppm)

110 to 580

120 to 340

115 to 270

120 to 225

Manganese (ppm)

270 to 325

110 to 285

40 to 175

115 to 475

Zinc (ppm)

50 to 55

35 to 60

10 to 45

35 to 50

Copper (ppm)

5 to 15

5 to 15

5 to 15

5 to 10

Boron (ppm)

40 to 50

35 to 60

30 to 280

15 to 45

From Biamonte et al.

pH: The optimal pH varies by the type of geranium and root medium used. For zonal geraniums, the range for a soilless root medium is 5.8 to 6.2 and for a soil-based medium is 6.0 to 6.5. The optimal range is up to 0.3 units lower for ivy and regal geraniums. If modifications are required, the pH can be lowered with an acid-based fertilizer or acid injection. The pH can be increased with dolomitic limestone or hydrated lime.

Electrical Conductivity (EC): The optimal EC range is 1.5 to 2.5 mS/cm for zonal and regal geraniums. Slightly lower levels are required by ivy geraniums. To lower EC levels, growers can apply a clear water leach or, more optimally, simply decrease their fertilization rate. If EC levels are low, increasing the fertilization rate will increase the root medium EC.

Nitrogen (N): The function of nitrogen is in the synthesis of amino acids, proteins, enzymes, and nucleic acids. Deficiency symptoms are exhibited as slow growth, stunting, or with advanced conditions, lower leaf chlorosis (yellowing) and leaf abscission in some plants. Excess levels of N will result in reduced plant growth and delayed flowering. Geraniums are susceptible to ammoniacal-N (NH4-N) toxicity which is expressed as a curling of the older leaves, leaf chlorosis, or necrosis. Ammoniacal-N toxicity can be avoided by supply >75% of N in the nitrate (NO3-) form. N should be supplied at the rate of 200 to 250 ppm for zonal and ivy geraniums. Regal geraniums require less N. Excellent sources for N would be calcium nitrate, potassium nitrate, ammonium nitrate, 20-10-20, or 15-5-25.

Phosphorus (P): The function of phosphorus in plants is 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 symptoms are first expressed as extensive stunting with the leaves turning dark green. Under advanced P deficiency conditions the lower leaves will become reddish-purple, then chlorotic, and ultimately necrotic. Excessive levels of P will reduced plant growth and can induce deficiencies of Fe, Zn, Cu, and Mn. Supply P at the rate of 5 to 20 ppm. P promotes stem growth, therefore limit applications to avoid stretch. P can be supplied as a preplant amendment of triple superphosphate incorporated into the root medium mix or as a constant liquid feed via phosphoric acid, monopotassium phosphate, ammonium phosphate, 20-10-20, or 15-5-25. 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.

Potassium (K): Potassium is involved as a metabolism catalyst, for stomata function, and disease resistance. Deficiency symptoms appear as necrosis of lower leaf margins and plants develop weak stems and stalks. Excess levels of K can reduce the uptake of Ca, Mg, Mn, NH4-N, and Zn. K should be applied at the rate of 150 to 250 ppm. To insure that K does not interfere with Ca and Mg uptake, a K:Ca:Mg fertilizer ratio of 4:2:1 should be used (similar to poinsettias). Excellent sources for K are potassium nitrate, 20-10-20, or 15-5-25. 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.

Calcium (Ca): Calcium is a major constituent of cell walls. Deficiency symptoms are expressed as death (blackening) of growing points of terminal buds and roots. Ca is a non-mobile element and uptake is by the root tips. Excess levels of Ca can reduce the uptake of K, Mg, and B. A fertilization rate of 50 to 100 ppm Ca should be used, remembering to maintain the K:Ca:Mg fertilizer ratio of 4:2:1. Ca can be supplied from your irrigation water (if adequate levels exist), dolomitic limestone, or calcium nitrate. Remember that Ca uptake into the plant and transportation within the plant is through the water flow, so promoting good root growth so water uptake can occur and good shoot growth that aids in transpiration will assist in Ca uptake.

Magnesium (Mg): Magnesium is an important element in the chlorophyll molecule and in enzyme activation. Deficiency symptoms appear as interveinal chlorosis of older leaves and the leaves may have an upward curl. Excess levels of Mg can reduce the uptake of Ca. Mg fertilization rates of 25 to 50 ppm should be adequate, remembering to maintain the K:Ca:Mg fertilizer ratio of 4:2:1. Sources of Mg are dolomitic limestone, Mg in water supply (if adequate levels exist), and magnesium sulfate (Epsom salts). To correct a Mg deficiency, magnesium sulfate can be mix at the rate of 1 lb in 100 gal of water and applied as a drench. Do not mix magnesium sulfate with other fertilizers. To prevent Mg deficiency, magnesium sulfate can be applied monthly.

Iron (Fe): Like magnesium, iron also plays a role in the chlorophyll molecule. Deficiency symptoms appear as an interveinal chlorosis of the younger leaves, progressing to tip dieback under severe conditions. Deficiencies occur when the root medium has a high pH, root death has occurred, or when there are excessive levels of P, Mn, or Cu. Excess levels of Fe can reduce the uptake of Mn. Toxicity symptoms occur when the root medium pH is too acidic. Toxicity symptoms frequently appear with seed geraniums at a pH <5.5. Symptoms appear as chlorotic and necrotic speckling of lower leaves. Ventanovetz and Knaus found Fe toxicity occurs more readily when the pH is less than 6.0, when Fe is 1.0 ppm or above (based on a saturated paste extract), and when the Fe-to-Mn ratio is >3:1. Supply Fe in your postplant fertilization program or by preplant incorporation into the root medium, but do not provide excessive levels. Iron can also be supplied from iron chelates, ferrous sulfate, or fritted iron. Recommended rates and considerations for correcting iron deficiencies are contained in Horticultural Information Leaflet #553, Managing Micronutrients in the Greenhouse. Fe toxicity or deficiency can be avoided by providing adequate amounts of Fe and maintaining root medium pH between 5.8 and 6.5.

Example Fertilizer Regime: The following example recipe can be used to meet the nutritional requirements of zonal geraniums. Mix together the following amounts per gal of concentrate for a 1:100 injector: 9 oz calcium nitrate + 7 oz potassium nitrate + 6 oz Excel® 15-5-15 Cal-Mg. Also provide monthly applications of magnesium sulfate at 1 lb per 100 gal of water. Using the above recipe would provide (in ppm): 214 NO3-N, 21 NH4-N, 10 P, 246 K, 138 Ca, 9 Mg (value does not account for Mg supplied from the monthly MgSO4 applications), and micronutrients. Remember to conduct routine root medium tests to monitor nutrient levels.

Water Quality: Peat-based media are more susceptible to chemical property changes than soil-based media. Well water which is high in alkalinity will gradually cause the pH to become basic. At pH's above 7.0, iron availability decreases, resulting in iron deficiency (iron chlorosis). Conduct a water test to determine the pH and alkalinity of your irrigation water and consider acid injection if needed.

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Published by

North Carolina Cooperative Extension Service


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