Vermiculite and calcined clay components decrease iron/manganese toxicity symptoms in seed geranium at low pH in a peat-based substrate
The objective was to evaluate whether increasing the cation exchange capacity in a peat-based substrate could reduce the incidence of iron/manganese toxicity symptoms in seed geranium (Pelargonium × hortorum L.H.Bail.) LSQUORingo ScarletRSQUO at low substrate pH levels. In two experiments, 70% peat was combined with various combinations of 15-30% of perlite, vermiculite, and/or calcined clay by volume, and hydrated lime was incorporated to provide pH levels of 3.3-6.3. Addition of vermiculite or clay increased the cation exchange capacity in the substrates by up to 44% on a substrate volume basis compared with the peat/perlite substrate. In Experiment 1, a substrate with 70% peat/15% perlite/15% vermiculite reduced tissue Fe and Mn levels in the pH range of 4-5 compared with 70% peat/30% perlite, resulting in lower micronutrient toxicity symptoms (chlorosis and necrotic spotting). In Experiment 2, combinations of peat with 15-30% vermiculite or calcined clay also increased SPAD chlorophyll index and reduced leaf necrosis compared with the 70% peat/30% perlite substrate between pH 4-5.5, with the exception of 70% peat/30% vermiculite at pH 4.3. The trends in observed micronutrient uptake, plant growth, and toxicity symptoms indicate that increasing the substrate CEC buffered the level of Fe and Mn available for plant uptake. A pH level above 6.0 is the recommended method to reduce iron/manganese toxicity in iron-efficient plants, however the choice of substrate components can also affect risk of toxicity symptoms at low pH.
Fisher, P.R. and Smith, B.R. 2017. Vermiculite and calcined clay components decrease iron/manganese toxicity symptoms in seed geranium at low pH in a peat-based substrate. Acta Hort. (ISHS) 1168:253-260
cation exchange capacity, CEC, micronutrient, nutrient availability, perlite, pH management