Geranium growers may occasionally find that their zonal plants are exhibiting symptoms of iron and manganese toxicity. The increased prevalence of this problem in some areas can be at least partly attributed to starting with media with a low pH (4.0 to 5.3).
Michigan State University Extension experts recommend growers continuously monitor the pH and EC of crops so trends or problems can be detected early. Growers should also calibrate pH meters prior to first use during the season.
Here are some commonly asked questions when it comes to pH monitoring (courtesy of Michigan State University).
What substrate pH is recommended for zonal geraniums?
The ideal pH for growing zonal geraniums is between 6.0 and 6.5. They are the poster child of plants that readily absorb iron and thus are referred to as iron-efficient crops. In contrast, other crops such as petunia and calibrachoa are iron inefficient and require a lower substrate pH of 5.5 to 5.8 to prevent an iron nutrient deficiency.
What symptoms do you see when the pH of the substrate is too low?
• Brown speckling of leaves, especially toward the outer margin
• Leaf chlorosis or yellowing around the edges
• Upward cupping of leaves in some varieties
• Stunted growth
How can I increase media pH quickly to prevent toxicity symptoms on new growth?
Growers can increase the substrate pH by drenching with a suspended limestone (flowable lime), potassium bicarbonate, or calcium carbonate. The recommended rates will strongly depend on your initial pH. Drenching the growing media with one of these products is generally only recommended if the pH of the substrate is excessively low (less than 5.5).
If the pH is only slightly lower than recommended (5.6 to 5.9), consider changing to a more alkaline fertilizer. Growers who have highly alkaline water and normally inject acid to neutralize the bicarbonates could stop acidification to raise the pH of the media. However, these strategies can be difficult to implement when growing a wide variety of iron efficient and iron inefficient crops together.
What should I do if some geraniums have a low pH while others in the greenhouse do not?
It is recommended that growers sort out the geraniums that are exhibiting iron and manganese toxicity symptoms and have a low pH from those that are not. Because the growing media pH may have varied between bales, some growers may see that only some plants are exhibiting symptoms. Growers should apply a corrective drench only to those plants with a low pH.
Check out the complete article on the Michigan State Extension page for more tips and links to additional resources.