Is substrate-pH too high?

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.

Symptoms of high substrate-pH

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.

Calibrachoa Million Bells pH after 20 days

Evaluate root health

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.

Iron deficiency at high pH

Corrective actions for pH problems

  1. Apply an acid reaction (high ammonium) water-soluble fertilizer such as 21-7-7 or 20-10-20 at 200 to 250 ppm nitrogen (N) for plugs and liners, and 300 to 400 ppm N for finished plants. Make sure that substrate-EC is not already high, or you can run into salt damage of roots at those fertilizer rates. High ammonium may also result in soft lush growth.
  2. Correct micronutrient deficiencies: 
    • Drench with iron chelate at up to 5 oz/100 gal (or 37.5 g/100L) of either Iron-EDDHA (such as Sprint 138TM) or Iron-DTPA (such as Sprint 330TM). The solutions should be applied with leaching, followed immediately by washing of foliage to avoid leaf spotting. Plants usually respond within 7 days.
    • Don't apply iron drenches to iron-efficient plants, such as seed or zonal geranium, lisianthus, marigold, or New Guinea impatiens.
  3. Acidify your water to neutralize alkalinity: If alkalinity in the irrigation water is above 80 ppm calcium carbonate (CaCO3) equivalents, consider injecting a mineral acid (such as sulfuric acid) into the irrigation water to bring water-pH down to 4.5 – 5.0 (a low pH level where alkalinity is close to zero).
  4. Consider an iron sulfate drench (at 2 lb/100 gal or 2.4 g/L) if substrate-pH is still not dropping. One application of iron sulfate can lower substrate-pH within a day or two. Immediately wash the foliage with clear water to avoid phytotoxicity. One week after the drench, leach the pot generously with clear water, followed by irrigation with a complete fertilizer solution to restore the nutrient balance.

Iron-inefficient (petunia) group

Use best management practices (BMPs) in your fertilizer program for a stable pH

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.

Many factors affect substrate-pH

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.