Geraniums with brown speckling and yellowing leaves were submitted to the UMass Extension Plant Diagnostic Lab last week. The pH of the growing medium was measured at 5.5. Geraniums are “iron efficient” plants and their health and growth are optimal when the pH of the growing medium is approximately 6.0-6.5. Lower pH levels increase plant availability of both iron and manganese, and visual symptoms and the low substrate pH indicated iron/manganese toxicity. Older leaves showed bronzing coloration with necrotic spots and yellowing around the leaf margins.
The pH can be raised by switching from a high ammonium fertilizer to one that is based on nitrate. Switch to nitrate-based fertilizers (e.g. 13-2-13 or 15-0-15). If pH has not risen within a week, drench with potassium bicarbonate at 1-2 lbs/100 gal or flowable (liquid) lime at 2-4 qts/100 gal water. An alternative or additional amendment is to topdress with a very fine grind of dolomitic limestone.
For more information, see:
- https://ag.umass.edu/greenhouse-floriculture/fact-sheets/growing-geraniums
- https://www.canr.msu.edu/news/low_ph_induces_iron_and_manganese_toxicity_in_geraniums
- https://www.growertalks.com/Article/?articleid=16852
- https://www.greenhousegrower.com/production/fertilization/understanding-plant-nutrition-fertilizers-and-media-ph/
- Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab
