What to do if your media pH is too high?
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
In the past few weeks, our Floriculture AoE Team has been receiving phone calls and emails about chlorotic petunias, calibrachoas, and gerberas. In most cases, it appears that the pH of the media is just too high. 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-EDDHA
If 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.
For more information, contact your local Extension educator or fertilizer supplier. REditor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included. ecent articles with more in-depth information on this topic, written by Paul Fisher and Bill Argo, can be found on the Greenhouse Grower magazine website.
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