Nutritional factsheet geraniums

Geraniums require medium levels of fertilization, growing best with 150 to 200 ppm N. Optimal substrate pH values are between 5.8 and 6.5. Geraniums are susceptible to both low and high pH disorders, developing iron (Fe) and manganese (Mn) toxicity at low pH and Fe deficiency at low pH.

Geraniums require medium levels of fertility. Growers should provide fertilization at 150 to 200 ppm N. Low soluble salts [referred to as electrical conductivity (EC)] from inadequate fertilization can cause yellowing (chlorosis) or reddening on the lower foliage. High EC is an issue during propagation, as cuttings and seedlings are sensitive to root burning (plasmolysis) and lower leaf browning (necrosis). For this reason, fertilization should be maintained at 100 ppm N during propagation (Dole and Wilkins, 2005). Excess salts may build up in the substrate during stock plant production, leading to plasmolysis and lower leaf necrosis. Frequent leaching with clear water should be implemented to control EC and prevent symptom development (Dole and Wilkins, 2005).

Substrate pH should be maintained between 5.8 and 6.5. Values beyond this range commonly lead to high or low pH induced nutritional disorders (Whipker and Henry, 2017). 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. Symptoms of Fe and Mn toxicity include a lower leaf bronzing with dark brown to black spotting.

Foliar Fe and Mn concentrations should remain below 500 ppm (Dole and Wilkins, 2005). Previous reports of low pH induced Fe and Mn toxicity document foliar concentrations of these two elements to be ~10× higher than in asymptomatic plants (Henry and Whipker, 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 Fe chelate, while Fe and Mn toxicity can be remedied by providing flowable lime to raise the pH.

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