Express-News Weekly Column

Saturday, May 5, 2001

Submitted by Calvin Finch, Ph.D., Director of Conservation, SAWS, and Horticulturist

TIPS ON TREATING IRON CHLOROSIS

            Some St. Augustine lawns and many other plants have a tendency to yellow in San Antonio soils. The yellowing is called chlorosis and it is usually caused by an iron deficiency in the plant. It is not as if we have a lack of iron in our soil, there is lots of it there, but it is locked up in compounds with phosphorous and calcium from which plants cannot obtain the iron. The compounds form because of our alkaline soil. In more acid soil, iron is readily available to plants.

            The problem is exasperated when the roots are not functioning at full capacity. Iron chlorosis is most prevalent in spring when the grass wants to grow because the air temperature is right and moisture is available but the soil is cool and root activity has not caught up to the growth on top. The same situation can happen in the fall after a droughty summer. The roots are limited because of damage from the drought period, and suddenly water and mild temperatures are available for top growth. The plant can get water and other easier-to-retrieve nutrients to the leaf, but not sufficient amounts of iron.

            You may have also noticed chlorosis after soggy periods for the same reasons—injured roots unable to obtain sufficient supplies of iron.

            Many ambitious gardeners have declared that they would solve the iron problem by acidifying the soil with massive amounts of sulfur or organic material. It is true that both of these treatments temporarily acidify our soil or at least reduce the effects of the soil alkalinity, but neither is permanent. Our native soils are very alkaline to begin with and are highly buffered. Buffering is a measure of potency. Our heavy soils have great reserves of alkalinity to overcome acidic treatments and almost all change the nature of the soil project’s end in failure.

            There is a large group of desirable plants that show chlorosis at times but grow out of it, even if you do not treat them. Yaupon hollies, nandinas, bridle wreath, crepe myrtles, and pyracantha come to mind.

            Some acid loving plants require constant attention if they are going to survive iron deficiency even for a short time. Azaleas and acid loving pines are in this category. Roses, fruit trees, blackberries, magnolias, photinias and other plants can often survive without constant attention but live longer and produce more fruit or color if they get attention for the problem.

            Tactics that generally help to reduce the impact of iron chlorosis are organic material in the soil or organic mulches over the root system. Organic acids and the microorganisms in organic material release some of the iron from the inorganic compounds in our soil so they can be picked up by the plant. Iron also finds a place in organic compounds. Many of these organic sites are chelates (a term that indicates plants can pick up the iron).

            In addition to mulch and incorporation of organic material to the soil, iron chlorosis can be treated in the short term by adding iron to the soil or leaf surface.

            Iron chelate soil treatments are very effective but are expensive. Sprint 330, Sprint 138, and Carl-Pool Iron Chelate are iron chelates sold in most nurseries and garden centers. You can make your own chelate treatment by mixing one cup of iron sulfate per bushel of compost and spreading it as a top dressing for St. Augustine grass or over plant roots as part of the mulch.

            My colleague on the KLUP gardening show spreads generous amounts of iron sulfate (copperas) directly on the soil of chlorotic plants. It is an inexpensive iron source that eventually loses its iron to soil sites but the initial blast often does the trick.

            Some good gardeners rely on granular products like Ironite, but I have never had good performance from the product.

            The least expensive and fastest way to treat chlorosis is to use a foliar spray. That being said, prepared foliar sprays are hard to find. There used to be one from a company called Ruffin. Ironite also had a spray. If you can find either product, use them. If not, make your own. Dissolve iron sulfate or a chelated product (one cup of iron sulfate or ½ cup of chelated material to five gallons of water) in a bucket and then apply it generously with your hose end sprayer. You can also use a backpack sprayer to cover the foliage of the chlorotic plant with iron.