A reminder that zonal and seed geraniums, American marigold, and all types of impatiens are susceptible to iron/manganese toxicity, a nutritional disorder associated with low growing medium pH. This problem is sometimes called "bronze speckle" due to the appearance of numerous small brown spots on the edges of the leaves of geranium and marigold. The target pH range to prevent toxicity is 5.8-6.5. If the pH is lower than the target range the risk for toxicity increases because at low pH too much iron and manganese becomes available to these plants.
Common causes of iron/manganese toxicity include: Acidic growing medium pH, use of acid-forming fertilizers (e.g., petunia fertilizer, 20-10-20, 15-6-17, 20-20-20), incorrect use of acid injection, and unnecessary use of supplemental micronutrient fertilizers. Watering with acidified water and fertilizing with iron chelates benefit plants like petunia and calibrachoa, but should not be used on plants sensitive to low pH and iron toxicity.
Fertilizers with low potential acidity (e.g., 15-0-0, Cal-Mag 15-5-15, 15-5-25, 15-0-15) should be applied to susceptible species to raise or maintain desirable pH. Regular pH monitoring should be done and, if necessary, flowable limestone (Limestone F) can be used to raise the pH of the growing medium. Foliage should be rinsed after the flowable limestone is applied to remove any residue. Some growers have also used or CalOx (liquid calcitic lime).
Webinar (30 min.) by Rosa Raudales, University of Connecticut Extension:
How to Identify and Manage Iron Deficiency and Toxicity in Spring Greenhouse Crops
Fact sheet: "Iron Out" by Paul R. Fisher University of Florida Extension and William R. Argo, Blackmore Co.
Commonly Used Limestones for Adjusting pH in Greenhouse Mixes by Heidi Lindberg and W. Garrett Owen, Michigan State University.
Fact sheet: pH and Fertility Review for Vegetatively-Propagated Annuals, University of Massachusetts Extension.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension