Acid-loving plants such as gardenia, azalea, and holly prefer a soil pH of 5.5 to 6.0. The two main reasons new growth becomes chlorotic (yellow) are if the soil is alkaline (above pH 7.0) or if excessive phosphorus fertilizer was applied to the soil. Other reasons, such as poor root growth due to poorly drained soil and herbicide application, can also account for leaf chlorosis.
Chlorosis occurs when the plant is not getting enough iron. There is seldom a deficiency of iron in the soil; rather the iron is made unavailable to the plants because of soil pH. If the new planting is near newly poured concrete, then lime may be leaching from the concrete into the soil, changing the soil pH. High soil pH can occur when soil fill is brought on site during home construction and for landscaping. Take a soil sample to determine your soil pH. If the pH is out of range, multiple applications of an acidifying soil amendment can be used to provide temporary relief. If you know the soil pH is alkaline, request recommendations to specify how much acidifying material is required to reduce the soil pH to a desirable range. Use with caution; overapplication can kill plants. Choose between ground sulfur (flowers of sulfur), iron sulfate (copperas), or aluminum sulfate. Always water well after application. Liquid products are also available to acidify the soil and to supply iron for rapid uptake by the plant for immediate response.
Contact your local Extension office for information on soil testing and amendments.
