Greenhouse and Nursery - Managing High Media pH

The following is a good article on dealing with high pH media problems in greenhouses and nurseries.

High media pH can lead to yellowing and stunting of petunias, calibrachoas, and gerberas as well as many other greenhouse and nursery crops. A desirable pH range for these, and many other “acid-loving” plants, is 5.4 to 5.8. When the pH is higher, iron in the root zone is relatively unavailable to these plants. There are two actions that can be taken to gradually lower the media pH: acidify your water or use an acid fertilizer.

Acidify your water

If the alkalinity of your water is too high (greater than 150 ppm), inject an acid into the water to neutralize the alkalinity down to around 100 ppm. If you are already injecting acid, consider increasing the amount. Remember that the water’s alkalinity, not its pH, is what drives the pH of a media up or down (along with other factors). If the media pH is above 7.0, consider using more acid so that the water alkalinity is down to zero. Once the media pH returns to a desirable level, scale back on the acid. Be careful because if too much acid is added, phytotoxicity is likely.

Use an acid feed

Use a fertilizer that has an acidic reaction. Fertilizers that contain a moderate or high percentage of ammonium can drive down the pH over time (a few weeks). Some common fertilizers with a moderately acidic reaction and other desirable attributes (similar amounts of nitrogen and potassium and lower amounts of phosphorus) are 20-10-20, 21-8-18, 20-2-20, and 21-5-20.

For a quick fix, drench with iron-EDDHAIf you don’t have a few weeks to correct the media pH, one option is to add iron that is soluble at a high pH. The most effective form of iron as a drench at a high pH is iron-EDDHA, which is sold as Sequestrene 138 or Sprint 138. A suggested application rate of iron-EDDHA is 10 ounces per 100 gallons of water, which provides 45 ppm of iron. This solution should be applied as a drench with generous leaching, followed immediately by rinsing off the foliage to avoid leaf burn.

Information from "What to do if your media pH is too high" by Erik Runkle, Horticulture, in the Michigan State University Greenhouse Crop Advisory Team Alert website:
http://ipmnews.msu.edu/greenhouse/