Iron chlorosis results when green chlorophyll in leaf tissue fails to develop properly. Although iron is not part of the chlorophyll molecule, it is one of the nutrients essential for chlorophyll synthesis. Iron chlorosis first develops in new growth and appears as yellowish-green leaves, usually as an interveinal yellowing, giving the leaf a striped appearance. As the condition worsens, leaves appear yellow or almost white. In severe cases of iron chlorosis, loss of turf occurs in irregular patterns.
Iron chlorosis is attributed to reduced plant availability of iron in calcareous or high pH soils and may also be associated with high levels of bicarbonate and phosphate in plant tissue.
Management practices can also contribute to iron deficiencies. Well-aerated soil is needed for plants to take up iron. Excessive irrigation and soil compaction result in poorly aerated soils and reduced iron uptake. High phosphorus levels resulting from excessive fertilization and high levels of bicarbonate in irrigation water also interfere with iron uptake by plants.
Environmental factors such as temperature, rainfall and light intensity also impact iron uptake and assimilation by plants. Low soil temperatures reduce soil microbial activity, which, in turn, reduces iron uptake. Wet soils, or excessively dry soils, and low light intensities also reduce iron uptake. For example, iron chlorosis is more common in St. Augustine turf under shaded conditions. Magnesium is a central part of the chlorophyll molecule, and factors that affect it also affect chlorosis.
Plant genetics is a dominant factor influencing the plant's ability to take up iron. Grasses of the same species may differ considerably in their ability to take up iron. Floratam St. Augustine grass, for example, is much less likely to show iron chlorosis than other varieties of St. Augu
stine grass. Bermuda varieties also differ in their ability to take up iron.
One approach to correcting iron chlorosis has been to reduce soil alkalinity with acidifying materials such as elemental sulfur. In soils, elemental sulfur is oxidized by microorganisms to form sulfuric acid. Under acid conditions, iron is more soluble and consequently may be more available to the plant. In turf, five to 20 pounds of elemental sulfur per 1,000 square feet are applied to reduce soil pH and iron chlorosis. Also, acidifying-type nitrogen sources, such as ammonium sulfate, should be chosen. Take care not to over acidify the soil and burn roots. This is easy to do with fast-acting products like aluminum sulfate.
The application of products containing iron to the soil or directly to the plant is the most widely used method to correct iron chlorosis. The problem with this method is the short longevity of the effect. Typically, iron applications improve the color of turf grasses for only three to four weeks. In soils, iron is rapidly oxidized to form insoluble iron oxides. In grasses, iron is immobile and is removed with the clippings. Thus, the response is of short duration.
Clippings yield suggest that
iron applications can significantly improve the color of the lawn without increasing the amount of grass clippings produced. With the concern today for disposal of grass clippings, it is important to document the fact that color and yield are not directly related or that an improvement in color does not correspond to an increase in yield (more growth).
On alkaline soils, the recommended rates of application need to be increased to two or three times the lower recommended rates presently shown on product labels. As a homeowner, I would be disappointed in the observed response to lowest rates of application on alkaline soil. Even at the 2x rate, the observed response may be disappointing.