In 2015, it was reported in e-GRO Alert 4.39 about low substrate pH-induced iron toxicity on Christmas cacti. During a recent extension trip, they noticed two operations with plants exhibiting symptoms. Since it has been 10 years since the last Alert, it was decided to revisit this situation.
A group of Christmas cacti (Schlumbergia spp.) was observed in a greenhouse, with mature phylloclades exhibiting chlorotic blotches and necrosis. Based on the symptoms, one may initially have thought it to be an iron (Fe) deficiency or magnesium deficiency. Iron deficiency can be a problem if the substrate pH is too high (>6.5), but symptoms should be more pronounced on the new growth. It is also reported that Christmas cacti have a high demand for magnesium. The symptoms were similar enough that a magnesium deficiency could be a possibility. On the opposite end of the pH spectrum, iron and manganese toxicities are reported if the substrate pH is too low.
© Brian Whipker
Plants were tested using the PourThru technique, and the substrate pHs of the tested plants ranged from 4.4 to 4.7 in 2015, and in 2025, the pH was higher at 5.4. This clearly implicated a low substrate pH problem.
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