Nutritional factsheets: Gerbera, Calibrachoa, dahlia cutting
Gerbera
Geberas require medium levels of fertilization, growing best with 150 to 200 ppm N. Optimal substrate pH values are between 5.8 and 6.2. Gerberas are susceptible to both low and high pH disorders, developing iron (Fe) and manganese (Mn) toxicity at low pH and Fe deficiency at high pH.
Gerberas require medium levels of fertility. Growers should maintain fertilization at 150 to 200 ppm N. Low soluble salts [referred to as electrical conductivity (EC)] can cause yellowing (chlorosis) or reddening on the lower foliage. High EC can lead to marginal chlorosis or browning (necrosis) of the lower leaves. To lower the EC, apply clear water to leach excess salts.
Substrate pH should be maintained between 5.8 and 6.2. Lower or higher values beyond this range commonly lead to low or high pH induced nutritional disorders (Whipker et al., 2011). High pH inhibits iron (Fe) uptake, causing interveinal chlorosis of the upper foliage. Low pH causes excessive uptake of Fe and manganese (Mn), which leads to toxicity symptom development. Toxicity of Fe and Mn exhibits symptoms of a lower leaf purplish coloration with black spotting. [This contrasts with phosphorus deficiency which has purple leaves.]
Foliar Fe and Mn concentrations should remain below 250 ppm (Bryson and Mills, 2014). Previous reports of low pH induced Fe and Mn toxicity document foliar concentrations of these two elements that were ~23× higher for Fe [3080 ppm] and 4× higher for Mn [1240 ppm] than in asymptomatic plants (Whipker and Henry, 2016). Monitoring substrate pH and periodic tissue sampling can help to determine if symptoms are due to high or low Fe and Mn. Iron deficiency can be remedied with an application of iron chelate, while Fe and Mn toxicity can be remedied by flowable lime application to adjust (raise) the pH.
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Calibrachoa
Calibrachoa require a medium to high fertility level. Fertilizer concentrations of 150 to 250 ppm N during active growth should be applied in the South, while slightly lower levels of 150 to 200 ppm can be used in the North. Calibrachoa prefer a pH within the range of 5.5 to 5.8.
This range prevents low substrate pH induced iron (Fe) and manganese (Mn) toxicities that may occur if the pH drifts lower than 5.2 to 5.5. Substrate pH values above 6.2 inhibit Fe availability and result in the upper foliage developing interveinal chlorosis. Elevated substrate pH induced interveinal yellowing (chlorosis) is the primary nutritional problem associated with calibrachoa.
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Dahlia cutting
Dahlias, produced from vegetative cuttings, require low to medium fertility of 100 to 200 ppm N. They prefer a pH within the range of 5.8 to 6.2. This range prevents low substrate pH induced iron (Fe) and manganese toxicities which occurs if the pH drifts lower than 5.5. Substrate pH values above 6.5 can also inhibit Fe availability and induce interveinal chlorosis (yellowing).
Dahlias, produced from vegetative cuttings, should be grown with a pH range of 5.8 to 6.2. Tissue nutrient levels found in healthy, newly expanded leaves and critical tissue values of ‘Maxi Morelia’ dahlias will enable growers to avoid low and high pH nutritional disorders.
Substrate pH below 5.8 causes increase uptake of iron (Fe) and manganese (Mn) to toxic levels which will accumulate in leaf tissue. Plants exhibiting Fe and/or Mn toxicity exhibit lower leaf chlorosis and black speckling or flecking of the entire leaf. Corrective procedures for low substrate pH should begin within the range of 5.5 to 5.8.
High substrate pH above 6.5 can inhibit Fe uptake causing newly developed leaves to become deficient in Fe and exhibit interveinal chlorosis. If plants become severely Fe-deficient, interveinal chlorosis intensifies and leaves become white or bleached. Corrective procedures for high substrate pH should begin within the range of 6.2 to 6.4.
Cutting dahlias are highly sensitive to soluble salts [referred to as electrical conductivity (EC)] under short photoperiods (≤ 14 hours; Nau, 2011) and are considered to require low to medium fertility.
Maintain substrate EC below 0. 9, 2.0, or 3.0 mS/cm, based on the 1:2 Extraction, SME, or PourThru methods, respectively. To avoid high EC, it is recommended to leach with clear irrigation water. Fertilizing with excessive ammoniacal-nitrogen (NH 4 –N) has been reported to promote undesirable soft growth and stem elongation (Nau, 2011; Barnes et al., 2015; Gaydos et al., 2003).
Providing too little fertility during production can cause lower leaf chlorosis (yellowing) and leaf drop. Overfertilization or high EC will cause leaves to exhibit chlorosis and marginal leaf necrosis. Plants that produce vegetative growth with few to no flowers may be a result of excessive applications of ammoniacal-based fertilizers, overfertilization under low light conditions, short days/cool temperatures, and/or low light combined with overwatering or wet substrate (Nau, 2011). Therefore, it is important to provide dahlias with low to medium (100 to 200 ppm N) fertility during crop production and to limit ammoniacal-based fertilizers.
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