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Two experiments were completed to determine whether the form and concentration of iron (Fe) affected Fe toxicity in the Fe-effi cient species Pelargonium ×hortorum 'Ringo Deep Scarlet' L.H. Bail. grown at a horticulturally low substrate pH of 4.1 to 4.9 or Fe defi ciency in the Fe-ineffi cient species Calibrachoa ×hybrida 'Trailing White' Cerv. grown at a horticulturally high substrate pH of 6.3 to 6.9. Ferric ethylenediaminedi(o-hydroxy- phenylacetic) acid (Fe-EDDHA), ferric ethylenediamine tetraacetic acid (Fe-EDTA), and ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) 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 defi ciency 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 FeSO 4 were stunted and chlorotic. Use of Fe-EDDHA in water-soluble fertilizer may increase the upper acceptable limit for media pH in Fe-ineffi cient 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-effi cient species regardless of applied Fe form or concentration, in order to avoid the potential for micronutrient toxicity. The pH of the substrate in container grown crops has a major infl uence on micronutrient solubility (Lindsay, 1979; Peterson, 1981) and their subsequent uptake by plants (Marschner, 1995). If the substrate pH is too high, solubility of the micronutrients, boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) can decrease to levels unacceptable for growth, causing a defi ciency. If the substrate pH is too low, solubility of these micronutrients can increase to levels that cause toxicity. For most species grown in substrates without soil, a pH range of about 5.8 to 6.2 provides adequate nutrient solubility to meet plant requirements without causing defi ciency or toxicity, although recommendations vary ±0.2 pH units (Bailey and Nelson, 1998; Argo and Fisher, 2002). When Fe uptake is inadequate (usually induced by high substrate pH), the shoot tip can rapidly become chlorotic followed by ne- crosis as the defi ciency becomes more severe (Argo and Fisher, 2002; Miller et al., 1984). In contrast, excess accumulation of Fe (usually induced by low substrate pH) occurs in the lower leaves of the plant. Toxicity symptoms can include chlorotic and necrotic speckling of leaves, leading to complete leaf death (Albano et al., 1996; Argo and Fisher, 2002) through production of free radicals and resulting oxi- dative damage of membranes (Bartosz, 1997; Marschner, 1995). Bedding plants can be classifi ed Fe-effi cient or Fe-ineffi cient based on their ability to ac- cumulate Fe under conditions of limited sup- ply (Romheld, 1987), usually caused by high substrate pH. At high pH (low Fe solubility), Fe-effi cient plants can increase Fe uptake into their tissue using various strategies including acidifi cation of the root zone, extrusion of chelating agents and Fe(III) reductases, and morphological changes to root structure with increased growth of fi ne root hairs (Gueri- not and Yi, 1994; Bienfait, 1988; Romheld, 1987). Iron defi ciency induced by high pH is therefore not generally a problem with Fe-ef- fi cient plants. Instead, low pH induced iron and manganese toxicity is the major concern in Fe-effi cient fl oricultural species, including Tagetes species (marigold) and Pelargonium

Iron Form and Concentration Affect Nutrition of Container-grown Pelargonium and Calibrachoa