Water-Soluble Fertilizer Concentration and pH of a Peat-Based Substrate Affect Growth, Nutrient Uptake, and Chlorosis of Container-Grown Seed Geraniums

Abstract

Two experiments were conducted to evaluate effects of water-soluble fertilizer concentration (WSF) and substrate-pH on growth, foliar nutrient content, and chlorosis of seed geranium (Pelargonium × hortorum) “Ringo Scarlet.” Geraniums were grown for 21 days in a 70% peat-30% perlite substrate. Experiment 1 included four pre-plant lime rates (pH 3.8, 4.3, 4.8, and 5.5), and plants were irrigated using 1X, 2X, 3X, and 4X rates of a WSF containing 75N-11P-84K-72Ca-17 Mg-23S-0.375Fe-0.185 Mn-0.019Zn-0.028Cu-0.058B-0.006Mo. Iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) were supplied as EDTA-chelated micronutrients. Experiment 2 included six lime rates (initial pH 3.2, 4.3, 5.2, 6.3, 7.0, and 7.5) at 2X and 4X WSF concentrations. Two forms of chlorosis were observed, consistent with micronutrient toxicity at moderately-low pH (4.3–5.2) and micronutrient deficiency at pH above 6.3. At very low substrate-pH, below 4.0, cation content decreased in leaf tissue, anion content increased, and plants appeared healthier than at pH 4.3–5.2. The decline in cation uptake at low pH was possibly the result of low substrate Ca, low-pH stress on membrane or cation channel activity, or competition between H+ and cations for root binding sites. At pH 4.3–5.2, plants were stunted, with chlorotic and necrotic spotting, necrotic leaf margins, and high tissue levels of Fe and Mn. At pH 5.5 (Experiment 1) or 6.3 (Experiment 2), plants appeared healthy. Iron and Mn declined at pH above 6.3, and interveinal chlorosis was observed. WSF concentration affected the pH range at which chlorosis occurred, intensifying toxicity symptoms at pH 4.3–5.2, and ameliorating deficiency at pH > 6.3. Results emphasize that an acceptable pH range for healthy growth can be affected by the applied fertilizer concentration.