Iron Toxicity in Geraniums

By Dr. Paul Fisher, University of New Hampshire and William R. Argo, Blackmore Co.
Excerpted with permission from "Iron-Out",a University of New Hampshire Cooperative Extension publication.

A common problem with geraniums is iron/manganese toxicity. According to Dr. Paul Fisher, University of New Hampshire, ceratin plant species are very efficient at taking up the soluble iron and manganese into the plant tissue. It is typically the combination of low media-pH and production of a sensitive species that results in iron/manganese toxicity.

According to Dr. Fisher, once plants show toxicity symptoms of necrotic spots and marginal burn, the affected leaves do not completely heal, and the only option is to produce healthy new growth that will cover the older foliage.

The best approach is therefore to prevent iron/manganese toxicity (and low pH) from occurring. One of the steps that, according to Dr. Fisher, you can take to avoid the problems is: train a nutrition-scout in your business. This may be the grower or another member. Their responsibilities will be:

  1. To learn about this problem and discuss the fertilizer program for the coming season with the main grower.
  2. Maintain and calibrate pH and EC meters as needed to keep reliable measurements.
  3. Measure pH, EC, and alkalinity of irrigation water and fertilizer solution every week.
  4. Measure pH and EC of fresh media out of the bag, and weekly tests of media from crops out in the greenhouse.
  5. Inspect plant health, including nutritional problems, but also pest and disease.
  6. Enter this information and observations into the tables and charts provided as soon as soil is tested.
  7. Communicate these results to the grower (who make fertility decisions) as soon as information is tabulated.
  8. Ensure that any changes in fertility management have been put into action.

Be sure that the nutrition-scout reads the instructions on how to use the meters and is confident enough in its use to question abnormal readings. For example, if the pH- meter measures a soil sample at pH 2.0, they should know to retest the sample and check the meter. The scout should also know about nutrition and plant health to help identify problems.

* For more information on this topic read "Iron-Out": A nutritional program for geraniums and other crops prone to iron and manganese toxicity at low media pH, by Paul R. Fisher, University of New Hampshire Cooperative Extension and William R. Argo, Blackmore Co.