Greenhouse - Low pH Can Lead to Toxicities

Low pH in greenhouse media can be detrimental to plant performance. Two potential problems are iron and manganese toxicity. The following is an article on the subject from the New England Greenhouse Update.

Zonal geranium, 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 leaves of geranium and marigold. Target pH to prevent toxicity is within the range of 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 the plants.

Common causes of iron/manganese toxicity include: acidic growing medium pH, use of acid-forming fertilizers (e.g., 20-10-20, 15-6-17, 15-15-15), inappropriate 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 are not the actions to take on species sensitive to low pH and iron toxicity.

Preventing iron/manganese toxicity starts with adequate liming of the growing medium to the pH range of 5.8-6.5 before planting. Higher pH reduces the availability of iron and manganese. After planting, fertilizers with low potential acidity (e.g.,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, liquid limestone can be used to raise the pH of the growing medium. Many geranium growers treat their plants with liquid limestone as a routine preventive treatment. The label for Cleary’s Limestone F suggests using a 100:1 injector to dilute and apply 1 gallon of product. The effectiveness of liquid limestone depends on the volume applied/depth the solution reaches. Foliage should be rinsed after the liquid limestone is applied to remove any residue.

Reprinted from the article "Iron/Manganese Toxicity to Annuals" by Douglas Cox in the April 10, 2006 edition of the New England Greenhouse Update. Go to http://www.negreenhouseupdate.info/greenhouse_update/?m=200604 for more information