For The Answer
PRIMETIME NEWSPAPERS WEEKLY COLUMN
Week of May 21, 2001
By Calvin Finch, Conservation Director, San Antonio Water System, and Horticulturist
TREATING IRON CHLOROSIS
Yellow St. Augustine grass is a common problem in San Antonio. The popular grass is more comfortable in acid soils in regions of high rainfall. The yellowing is usually caused by iron deficiency. Our soil has plenty of iron but it is in a form that acid-loving plants cannot pull from the soil, especially when the soil is cold, soggy or dry. When soil is in such a state, the roots are injured or inactive and the mechanism for iron uptake is not at full capacity. pH is a measure of soil chemistry. Depending on how high the pH of your soil is determines the frequency and severity of the iron chlorosis problem. Neutral soil is pH 7.0 and ideal soil for plant growth is around 6.5. Most South Texas soils are between 7.5 and 8.5 pH which is pretty high.
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 acidification projects 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).
Add organic material to your lawn by top dressing with compost ½ to 1-inch deep. It has even more impact when you aerate before you top dress.
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.
Jerry Parsons, my colleague on KLUP radio, recommends that you spread generous amounts of copperas (iron sulfate) on the soil around 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.
My favorite method to treat iron chlorosis is with foliar sprays. It is the least expensive and fastest treatment.
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.