Research outlined in an earlier article showed that the roots of geraniums produce acids when they are exposed to high temperatures. These acids lower the pH of the growing medium, which increases the availability of micronutrients and causes symptoms of micronutrient toxicity in the plants. The effects are worse if the plants are deficient in phosphorous.
In the research described here, zonal geranium cuttings were rooted, planted in a peat:perlite mixture and then grown in growth rooms at four different light levels. The day/night temperatures were 22 degsC/18 degsC. In another, similar experiment, as well as four different light levels, there were five different phosphorous concentrations in the growing media. The pH of the growing media fell if no phosphorous was added, especially at the higher light level.
Geraniums were also grown under two different light levels and in water culture so that phosphorous uptake could easily be measured. The higher light level suppressed phosphorous uptake but it didn't cause acidification if sufficient phosphorous was available.
The results confirm that phosphorous-deficient geraniums will acidify their growing medium, especially if they are grown for long periods under high light. Evidently, growing media should be monitored regularly to ensure that phosphorous concentrations are adequate.
Substrate Acidification by Geranium: Light Effects and Phosphorous Uptake by Taylor, Nelson & Frantz (2008). Journal of the American Society for Horticultural Science 133 (4): 515-520. Members of ISHS can view the Journal of the American Society for Horticultural Science from the website www.ishs.org.