Iron nutrition improves turf's mettle
Under normal turfgrass-growing conditions, it is common to observe a color response in turfgrass after an iron application, especially if the turfgrass is growing on a soil of neutral or alkaline pH. For the most part, turfgrasses are sensitive to iron levels and an application of iron, either to the soil or foliarly, generally imparts a darker green color to the turfgrass. Due to this response, iron is perhaps the most frequently applied micronutrient for turf-grasses. Although iron is not an actual component of the chlorophyll molecule (the green photosynthetic pigment of plants), it is an essential element that plants need to make chlorophyll. That is why iron levels affect foliage color.
The role of iron in turf nutrition Researchers have conducted many studies involving the function and necessity of iron in plants. It turns out that iron is an elusive element, and researching a plant's requirement and making recommendations for iron is difficult. After many years of research, scientists still have not identified the exact roles of iron in plants. Although we know that iron is involved in chlorophyll synthesis and is a component of other plant tissues, questions remain concerning other roles it plays in plants. Further, attempts to correlate tissue-iron status with growth and color responses have not always revealed an obvious relationship. For this reason, few laboratories analyze soils and tissue for iron content. Lacking a strong correlation between soil- and tissue-iron content and turfgrass growth, it is difficult to make meaningful recommendations.
Turfgrasses differ in their sensitivity to iron deficiency. Generally, warm-season turfgrasses are more sensitive to iron deficiencies than cool-season turfgrasses. Differences also exist among the warm-season grasses. Centipedegrass, bahiagrass and bermudagrass tend to be more sensitive to iron levels than St. Augustinegrass and zoysiagrass. This is one reason why bahiagrass and centipedegrass usually grow better on acidic soils where iron is more available, whereas St. Augustinegrass is recommended for neutral and alkaline soils where iron is less available.
Recommended soil- and tissue-iron levels As I mentioned, laboratories usually do not analyze soils for iron status. When they do, they employ chelating compounds such as EDTA (ethylene-diaminetetracetic acid), EDDHA (ethylene-diaminedi[o-hydroxyphenylacetic] acid) and DTPA (diethylenetriaminepentaacetic acid). (Chelating compounds are, in effect, chemical carriers for some plant nutrients, particularly iron.) In previous research, the critical soil level of DTPA-extractable iron appeared to be in the range of 2.5 to 5.0 ppm. However, the availability of iron seems to depend on many factors aside from the extractable amount in the soil. Among these are a high manganese-to-iron ratio, soil pH, the presence of free lime and the soil-moisture level. Some factors affecting iron uptake may even be inherent in the plant, and I doubt that any soil-iron test will result in reliable recommendations until researchers better understand them.
The role that soil pH plays in soil-iron availability is worth emphasizing. The marked effect pH has on the solubility of iron is well-illustrated by the fact that at pH 6.3, soluble iron exceeds 1,000 ppm. At pH 6.5, 352 ppm is available, at pH 7.0 just 35 ppm is available and a mere 3.5 ppm is available at pH 7.5. (Note the 10-fold difference in iron concentration for each 0.5-unit pH change.) Most turfgrass managers recognize that high soil pH limits iron availability, but the data above serve to emphasize the importance of relatively small changes in soil pH and their effects on iron. As you can see, increasing the soil pH by one unit from 6.5 to 7.5 results in a 100-fold change in soluble iron.
Analyses of plant tissue for iron can be trying. First, iron levels in plant tissue can vary over a large range with little change in plant growth or appearance. This makes it difficult to draw meaningful conclusions from tissue-analysis results. Additionally, iron-analysis results may be useless if the technician did not first wash the leaf material in dilute acid or detergent. This is because iron applications often occur in the form of a liquid foliar application to turfgrasses. As a result, iron may accumulate on the surface of the leaves. If the technician does not remove this excess iron before analysis, the tissue-iron-concentration reading may be erroneous. Thus, technicians must take proper care when handling plant tissue for iron analysis.
Analysis of plant roots for iron status also is difficult. Iron can precipitate on the root surface, and if you do not remove it, it will result in the appearance of high levels of iron in the root tissue when, in fact, most of the iron is actually on the exterior of the plant.
As I mentioned above, tissue-iron content can vary considerably. However, in general, when dry-matter iron values are 50 ppm or less, deficiency symptoms are likely to occur. The sufficiency range seems to be from 50 to 250 ppm of iron. The opposite condition-iron toxicity-apparently does not occur in turfgrasses growing under natural conditions. The rapid conversion of applied iron, especially soil-applied iron, to insoluble, unavailable compounds suggests little likelihood of iron-toxicity problems. Some soils contain in excess of 5 percent iron-that's 50,000 ppm-with no apparent toxicity problems.
Correcting deficiencies Conditions for iron deficiency frequently occur on neutral or alkaline soils, but they also exist in acidic sands that have low iron levels or elevated levels of other nutritional elements such as phosphorus or manganese. If you've diagnosed a deficiency, either by a tissue analysis, visual observations of the plants or by soil analysis, you must undertake corrective measures if you wish to maintain acceptable turfgrass growth and quality. Several inorganic forms of iron are available as supplemental sources of iron for plants (see table, page 28 for a list of some inorganic and organic iron sources). If you use a fertilizer that includes iron, be sure to find out what form the iron is in. Often, fertilizer labels provide the percent iron but do not state the form of the iron. If iron is not in the correct form, it does little to correct deficiencies.
* Iron sulfate is highly soluble in water and is the most widely used inorganic-iron source applied in solution form. You typically apply it as a foliar application because soil applications require high rates, and the responses sometimes are marginal. When you apply it to a near-neutral or alkaline soil, ferrous sulfate quickly converts to water-insoluble iron, which has limited plant availability. (However, some products combine ferrous sulfate with acidifying materials to increase soil acidity and maintain the solubility of the iron.) When applying iron sulfate, whether in liquid or granular form, take care to avoid sidewalks and driveways because this material can produce an unattractive, difficult-to-remove stain.
* Iron oxides are concentrated, relatively inexpensive sources of iron that manufacturers frequently include as components of mixed, complete-blend turfgrass fertilizers. However, iron oxides generally are insoluble and ineffective as nutritional sources of iron. Iron deficiencies usually occur on high-pH soils, which are the same conditions in which iron oxides are the least soluble. Thus, only on acidic soils would you expect to obtain a response from the application of iron oxides, and under these conditions iron deficiencies usually do not exist.
* Oxysulfate is another oxide source of iron. It is produced by partially reacting iron oxide with sulfuric acid to produce a material that is a combination of iron sulfate and iron oxide containing approximately 50 percent total iron. In staining tests, oxysulfate-iron sources stained less than sucrate iron (which I discuss below), suggesting that the oxysulfate iron contains less soluble iron. Oxysulfate does supply limited quantities of iron under acid conditions, but due to its predominantly iron-oxide composition, it is of little value on soils having a pH of 6.5 or greater.
* Ferrous ammonium sulfate usually is formulated as a liquid for use as a foliar spray. An application supplies plants with both nitrogen (N) and iron, and usually produces a favorable response. Turf managers often use it before special events to give the turfgrass a dark-green, lush appearance. This iron source generally is more expensive than iron sulfate, but it more reliably produces a favorable response.
* Iron chelates are, as a rule, more effective than soluble inorganic iron sources when you apply them on soils of neutral or high pH. Plus, iron chelates require lower rates than the inorganic sources. The EDTA form of iron generally produces acceptable results on acidic soils and near-neutral soils. The other iron chelates, FeHEDTA, FeDTPA and FeEDDHA, are more effective than EDTA at soil pHs above 7.5.
* Iron sucrate, a relatively new iron source, is a solid material manufactured using iron oxide and molasses to form an iron-containing organic complex with limited water solubility. Tests have shown that only 4 to 5 percent of the iron in this compound is water-soluble. In staining tests, about 35 percent of the granules stained concrete. In a glasshouse study, researchers compared this material with other iron sources on centipedegrass. They maintained acid-washed sand media at pH 8.0 and applied the equivalent of 20 pounds of iron per acre, except EDTA iron, which they applied at 5 pounds per acre. Sucrate iron and EDTA iron produced the largest amount of growth, whereas centipedegrass growth with iron sulfate, iron oxide and iron oxysulfate was not different than the control treatment (see photos, page at left). Centipedegrass-tissue-iron concentration followed the same trend as dry-matter production.
* Iron humate is the term for the material that some wastewater-treatment facilities produce by using ferric sulfate to precipitate fulvic and humic acids during the production of potable water. The organic precipitate, containing 20 to 25 percent iron, is classified as an iron humate, a complex iron source. This product is about 32 percent organic, of which 21 percent is fulvic acid. Its solubility in water is nearly zero. In addition to iron, the product contains about 3.5 percent sulfur.
Researchers have evaluated iron humate as an iron source for turfgrasses. They revealed that unmodified iron humate breaks down slowly compared to other organic sources such as cellulose and sewage sludge. Glasshouse and field studies show that in an unmodified form, iron humate has been of limited benefit to turfgrasses. However, growth studies have revealed that you can increase the iron-release rate of iron humate by adding N to the material. Modified iron humates fortified with N produced positive growth and quality responses in bermudagrass. In one study, a 7-0-0-8 ("8" indicating 8 percent iron) fertilizer applied at 1 pound of iron per 1,000 square feet every 90 days produced a faster-growing and higher-quality bermudagrass than did iron sulfate. Good responses also have been observed on ryegrass.
An additional benefit from the iron-humate application was observed during transition from ryegrass to bermudagrass. Plots receiving at least 1 pound of iron (as 7-0-0-8) per 1,000 square feet from iron humate every 90 days transitioned with less loss of grass. The transition was smooth, with essentially no visible bare ground at any time, and turf quality was good throughout the entire transition period. In contrast, plots receiving iron sulfate exhibited lower quality during transition and patches of bare ground were visible.
Researchers have conducted field studies to determine if you can maintain turfgrass quality at reduced N rates by regularly applying N-modified iron humates. Results indicate that this may be practical. Bermudagrass plots receiving one-third as much N applied as IBDU exhibited equal or better visual quality where N-modified iron humate was used relative to plots receiving only IBDU.
Conclusions Soil conditions-especially if the pH is above 7.0-greatly affect iron availability to plants. Both soil and plant-tissue analyses are of limited usefulness for evaluating iron status, so visual appearance is the most frequently used method of iron-deficiency diagnosis. Fortunately, turfgrasses can accumulate high levels of iron without toxicity, so you pose little risk by applying iron based on mere suspicion of iron deficiency, or simply as "insurance." Soil applications generally require high rates to produce even a modest response. Thus, frequent foliar applications of iron are more effective and more frequently used in eliminating iron deficiency.
Soluble-iron sources are most effective when you apply them foliarly. Iron chelates tend to be more effective than the inorganic iron salts in soil applications, but they, too, are most effective when foliar-applied. In most situations (such as high-pH soils) where iron-deficiency symptoms are likely to occur, iron oxides are not soluble enough to correct the problem. Some relatively new iron sources-iron sucrate and modified iron humate-show promise as iron sources. These granular sources represent a class of new materials that may release iron slowly when soil-applied.
Dr. Jerry B. Sartain is professor of turfgrass science at the University of Florida (Gainesville, Fla.).
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