Greenhouse - Iron & Manganese Toxicity

Iron and manganese toxicity can be a problem in some greenhouse crops if pH drops too low. The following is an article on the subject.

Check the pH of growing media to adjust pH if necessary. Zonal geranium, American marigold, and all types of impatiens are susceptible to iron/manganese toxicity, a nutritional disorder associated with low growing medium pH. Symptoms show as speckling (numerous very small brown spots) on the leaves of geraniums and marigolds, usually beginning on the edges. Maintain the pH within the range of 6.0-6.6 to prevent toxicity. If the pH drops below this range, the risk for toxicity increases because at low pH too much iron and manganese becomes available to the plants. Note that ivy geraniums require a lower pH (5.5-6.0) and more iron than zonal geraniums.

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 recommended and will cause problems 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 6.0-6.6 before planting. “High” 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. Two applications of 2 qt./100 gal. flowable lime applied 1 week apart has a similar effect to a single drench with 4 qt./100 gal. 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.

Research at North Carolina State University indicated that there are three possible causes for the sudden pH decline associated with micronutrient toxicities. The causes are phosphorus deficiency, high production temperature and high light intensity. For more information see the recently published article: Sudden Substrate pH Decline.




Reprinted from "pH, Iron/Manganese Toxicity Spring Crops" by Douglas Cox, University of Massachusetts; Leanne Pundt, University of Connecticut; and Tina Smith, University of Massachusetts in the New England Greenhouse Update http://www.negreenhouseupdate.info/greenhouse_update/?p=2560