How to Prevent Iron Toxicity in Christmas Cacti
December 2, 2025 
A red Christmas cactus. | Lana2011 vis iStock
In a recent e-Gro Alert, Luke Finn, Patrick Veazie, and Brian E. Whipker of North Carolina State University revisited a past work of theirs covering low substrate pH-induced iron toxicity in Christmas cacti all the way back in 2015 and updated their findings for the modern day. Here’s a look at what’s changed:
Determining the Cause Behind Christmas Cacti Symptoms
The group reports that Christmas cacti (Schlumbergia spp.) are very sensitive to low substrate pH conditions, “which can lead to mature phylloclades developing chlorotic blotches and necrosis.” However, these symptoms can often be mistaken for nutrient deficiencies, such as iron or magnesium.
Testing Christmas cacti with these symptoms using the PourThru technique, the substrate pHs ” ranged from 4.4 to 4.7 in 2015, and in 2025, the pH was higher at 5.4.” This is far lower than the recommended (5.5 to 6.5) and optimal (6.0 to 6.5) ranges for the Christmas cactus.
Low pH Problems in Floriculture Crops
For the majority of floriculture crops, “low pH problems begin to occur at levels less than 5.4, with the majority of symptoms being evident at a pH of 4.8 or lower.” Additionally, the group “observed elevated substrate pH-induced iron (Fe) deficiencies when the pH is above 6.5.” This has lead to their proposal that the the lower recommended range level for substrate be increased to 5.8, while the high recommended level be decreased to 6.3. This creates a much tighter window, but can help curtail nutrient and toxicity problems, and, therefore, prevent the need for corrective procedures.
Relating to the Christmas cacti sample displaying leaf chlorosis and necrosis, it was tested at the Agronomic Division Lab of the North Carolina Department of Agriculture. Results showed “an iron (Fe) concentration of 538 ppm in 2015 and 672 ppm in 2025,” which is up to five times higher than the Dole and Wilkins recommended range. Meanwhile, manganese concentration was tested at 359 ppm in 2015 and 79.3 ppm in 2025, with the former being nearly three times higher than the Dole and Wilkins recommended range.
Monitoring and Correcting the Substrate pH of Christmas Cacti
The group recommends the following corrective procedures when low substrate pH levels are found in Christmas cacti, with application details available from Fert, Dirt, and Squirt:
- Flowable lime
- Hydrated lime
- Potassium biocarbonate
For additional information on diagnosing and improving Christmas cacti low and high substrate pH problems, including visual examples of symptoms and a full data table of testing results, please read the full e-Gro (Electronic Grower Resources Online) alert “Christmas Cactus: Low pH-Induced Iron Toxicity.” Additional and current e-Gro alert pieces from Volume 14 (2025) can be found online.
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