By Gary Hall, Colorado State University Cooperative Extension, Horticulture
If you are noticing yellow leaves on your aspen and shrubs, In some cases iron chlorosis could be the problem.
Iron chlorosis can turn aspen trees and other plants yellow due to high pH soils (alkaline soil). Though Colorado soils normally contain adequate amounts of iron, a high pH makes that iron unavailable to the plant. Iron plays a major role in the production of chlorophyll. Thus, a lack of iron reduces the amount of chlorophyll and results in yellowing of leaves. Iron chlorosis weakens, and in severe cases, may kill a susceptible plant.
A popular recommendation for high pH soils is adding sulfur to lower pH, but many of the soils in Colorado are calcareous. Calcareous soils are those that contain actual particles of calcium carbonate (limestone). Calcareous soils can be difficult to almost impossible to acidify because the sulfur must neutralize all the free limestone before the pH is affected. In many cases you would need well over a pound of sulfur per square foot just to neutralize the free lime.
What you can do to remedy the problem depends on the situation. Vegetable gardens and annual flowerbeds allow us to work products into the soil during the time of year when there are no plants present. One recommendation is to mix five pounds of sulfur per 100 square feet into the soil before planting. The idea is to form little pockets of acidity that result in enough iron availability for the plants during the year of application. Note that this must be done each year.
Another possibility is to use iron chelates. Iron chelates hold the iron in such a way that the plant can get to it. However, not all iron chelates will work in high pH soils. For soils with a pH over 7.2, use a chelate that contains FeEDDHA (iron ethylenediamine-di- (ohydroxyphenylacetate)). This can be found in the products Sequestar 6% Iron Chelate WDG, Sequestrene 138 and Millers FerriPlus. Chelates can either be mixed into the soil at planting or sprayed on the foliage early in the season. Reapply as needed and reduce the yellow plants in your landscape.
Trade or brand names mentioned are used only for the purpose of information; Cooperative Extension does not guarantee nor warrant the standard of the product, nor does it imply approval of the product to the exclusion of others, which also may be available, nor does it intend discrimination or criticism of products or providers that are mentioned or not mentioned.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
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Date last revised: 01/05/2010