Iron Form and Concentration Affect Nutrition of Container-grown \u3cem\u3ePelargonium\u3c/em\u3e and \u3cem\u3eCalibrachoa\u3c/em\u3e

Two experiments were completed to determine whether the form and concentration of iron (Fe) affected Fe toxicity in the Fe-efficient species Pelargonium xhortorum `Ringo Deep Scarlet\u27 L.H. Bail. grown at a horticulturally low substrate pH of 4.1 to 4.9 or Fe deficiency in the Fe-inefficient species Calibrachoa xhybrida `Trailing White\u27 Cerv. grown at a horticulturally high substrate pH of 6.3 to 6.9. Ferric ethylenediaminedi(o-hydroxyphenylacetic) acid (Fe-EDDHA), ferric ethylenediamine tetraacetic acid (Fe-EDTA), and ferrous sulfate heptahydrate (FeSO4·7H2O) were applied at 0.0, 0.5, 1.0, 2.0, or 4.0 mg ·L–1 Fe in the nutrient solution. Pelargonium showed micronutrient toxicity symptoms with all treatments, including the zero Fe control. Contaminant sources of Fe and Mn were found in the peat/perlite medium, fungicide, and lime, which probably contributed to widespread toxicity in Pelargonium. Calibrachoa receiving 0 mg Fe/L exhibited severe Fe deficiency symptoms. Calibrachoa grown with Fe-EDDHA resulted in vigorous growth and dark green foliage, with no difference from 1 to 4 mg·L–1 Fe. Using Fe-EDTA, 4 mg Fe/L was required for acceptable growth of Calibrachoa, and all plants grown with FeSO4 were stunted and chlorotic. Use of Fe-EDDHA in water-soluble fertilizer may increase the upper acceptable limit for media pH in Fe-inefficient species. However, iron and Mn present as contaminants in peat, irrigation water, or other sources can be highly soluble at low pH. Therefore, it is important to maintain a pH above 6 for Fe-efficient species regardless of applied Fe form or concentration, in order to avoid the potential for micronutrient toxicity