If your substrate-pH is too high (above 6.4), plants can develop iron deficiency symptoms within 1-2 weeks for iron-inefficient crops (such as calibrachoa, diascia, nemesia, pansy, petunia, scaevola, snapdragon, and vinca) with chlorosis of new growth and overall stunting. The following outlines provide corrective actions to rescue your crops. Test corrective actions on a small group of plants first, and as a long-term strategy rely on prevention rather than cure.
Test substrate-pH and substrate electrical conductivity (EC). Typical symptoms of iron deficiency at high pH (above 6.4) are yellowing (chlorosis) between veins in new leaves, or over the entire leaf. If you think that these symptoms result from high pH, confirm with an onsite test using a standard protocol and a calibrated meter. Checking the pH and EC will tell you whether the problem is nutritional, and also if it is caused by inadequate fertilizer (low EC) or a high substrate-pH. Symptoms from low magnesium can also be confused with iron deficiency.
Root damage (brown color, no root hairs, damaged tips) caused by overwatering, fungus gnats, or root pathogens such as Pythium can result in leaf symptoms similar in appearance to iron deficiency at high substrate-pH, because roots can't take up nutrients.
Understanding how these factors interact can help you design a preventative strategy and avoid future problems. For iron-inefficient crops such as petunia, the lime rate may be too high, a more acidic-reaction (higher ammonium) fertilizer may be needed, or acid injection may be necessary to reduce water alkalinity.
For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension, and Dr. Bill Argo of Blackmore Co. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fine Americas, Greencare Fertilizers, Klasmann-Deilmann, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. The University of Florida does not endorse any product, and our research focuses on quality testing on these and competing products to assist grower success. June 30 2015.