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Yellow leaves in landscape can signal problem with iron, soil pH
Kansas
Areas with limestone rocks often have high pH soils, too--which may not seem like a problem unless gardeners want to grow acid-loving plants, such as rhododendrons and azaleas.
But high pH soils also can cause iron chlorosis.
"Kansas soils, for example, normally contain iron--which is a good thing. Iron plays a major role in the production of chlorophyll, the stuff that makes plants green and helps them produce food," said Ward Upham, coordinator of the Master Gardener Program for Kansas State University Research and Extension.
Where Kansas soils have a high pH, however, the pH makes the soil's iron unavailable to plants.
"And that's bad, because it reduces the amount of chlorophyll available, makes leaves yellow and weakens the plants. In the worst cases, it can kill a susceptible plant," Upham said.
The pin oak is notorious for its sensitivity to iron chlorosis. That's why residents of high pH areas have better success planting other types of oak, he said. Amur, red and silver maples are susceptible to the condition, but Norway maples are much less so.
Sweetgum and dawn redwood trees tend to fall victim to iron chlorosis, too. But cottonwood, elm, ginkgo, hawthorn and linden trees are moderately tolerant.
"The classic symptoms of iron chlorosis are yellow leaves with a network of dark green veins," Upham said. "In severe cases, the entire leaf turns yellow, and the edges of the leaf scorch and turn brown."
The general recommendation for soils causing iron chlorosis is to cut their pH by adding sulfur.
"That works well with many soils," the horticulturist said. "But it's not a one-time job. You'll need to retest your soil every few years and get lab recommendations on when to add sulfur again."
Plus, sulfur alone may not do the job in what are called calcareous soils, which contain actual particles of calcium carbonate (limestone).
"Calcareous soils can be difficult or nearly impossible to acidify because the sulfur has to neutralize all of the free limestone before it can affect the pH. In many cases, you'd need well over a pound of sulfur per square foot, just to neutralize the free lime," Upham said.
The sure way to know whether soil is calcareous is to have soil tested for free calcium carbonate.
Kansas counties' Research and Extension office can help homeowners learn how to test their soil, as well as forward their soil sample for testing at K-State (for a small fee). Private labs also can do the test. As a "pre-test," however, homeowners can add a drop or two of vinegar to a sample of dry soil.
"If it fizzes vigorously, you have a calcium carbonate content of 3 percent or more. A mild fizz suggests a content of between 1 percent and 3 percent. If you hear, but don't see a fizz, you've probably got less than 1 percent," Upham said.
Dealing with calcareous soil is the extreme of handling soils with a high pH. The recommended approaches to take can vary with the plants involved.
"In vegetable and annual flower gardens, you can work products into the soil in fall or early spring, when no plants are present. Some research suggests mixing 5 pounds sulfur per 100 square feet will form enough little pockets of acidity to make sufficient iron available the following growing season," he said.
Another option is to use iron chelates, (1) mixed with the soil at planting time, (2) mixed with water and used as a drench, or (3) mixed with water and sprayed on the foliage early in the season.
"Not all iron chelates will do the job, though. For soils with a pH over 7.2, you have to use a chelate that contains FeEDDHA - iron ethylenediamine-di-(ohydroxyphenylacetate)," Upham said.
That compound is in such products as Sequestar 6 percent Iron Chelate WDG, Sequestrene 138 and Millers FerriPlus.
"Trying to rescue trees that are already growing and suffering in high pH soils is another matter, however--even if the soil isn't calcareous," he warned. "Success rate can really vary."
The treatment with the best odds is trunk injection or implantation--introducing ammonium citrate or ferrous sulfate through holes made with a brad-point drill bit (to minimize tree wounding) in the lower trunk or root flare--preferably in spring just after the leaves have fully expanded. Successful applications often last several years.
"Other alternatives include soil treatments in spaced holes, dripline drenches and foliar sprays. The first is a lot of work. The second has to be done before spring growth starts, and the third may burn leaves or provide spotty results," Upham warned. "Plus, all three may need to be repeated yearly.
"If your situation seems serious enough to warrant such treatments, though, I'd strongly suggest that you make soil testing your first step. Then discuss the possibilities with your county Research and Extension agent, a certified arborist or reliable nursery owner."
Date: 4/15/04
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