Although the use of Fe-efficient plants is generally the best approach to preventing Fe availability problems (Goos and Johnson, 2000; Hansen et al., 2003; Jolley and Brown, 1994), several fertilizer and cultural management strategies are also available for use alone or in combination, to prevent/correct Fe deficiency, including: 1) lowering soil pH, 2) applying foliar Fe fertilizer or acid sprays, 3) applying chelated/complexed Fe fertilizers to the soil, 4) applying Fe fertilizer materials in a concentrated band near roots, 5) companion cropping with Fe efficient species, and 6) altering management of irrigation and drainage, fertility and seeding practices (Hopkins et al., 2005).
Artificially lowering the soil pH would seem to be a viable alternative for correcting Fe deficiency (Lucena, 2003; Olson, 1950). For example, a pH drop from 7.5 to 6.5 would result in a ten fold increase in hydrogen ion activity and an associated 1000 fold increase in the solubility of soil Fe minerals. However, the amount of acidifying material required to lower the pH in a highly buffered calcareous soil is so great that this option is not cost effective, except in the case of very high value crops or in situations where cost is less of an issue. It may take as little as one half metric ton per hectare of elemental S to lower pH of a poorly buffered sandy soil from the alkaline range to a neutral pH, but a highly buffered (high clay and/or lime content) soil may require several tons to achieve the same result (Tisdale et al., 1993). Furthermore, irrigation water high in lime may serve to raise the pH back into the alkaline range over time. Elemental S is the most common acidifying agent due to cost, availability, and handling advantages (Tisdale et al., 1993). Many other solid and liquid compounds can be used to lower soil pH, including: sulfuric acid, phosphoric acid, aluminum sulfate, ammonium polysulfide, and any other non-toxic, acidic material readily available (Horneck et al., 2005). In soils of low buffer and relatively low excess lime, addition of ammoniacal fertilizers could result in lowering of pH and more available Fe (Whitney et al., 1991).
Liquid solutions containing low rates of chelated or non-chelated Fe are often used to temporarily correct Fe deficiencies (Anderson, 1982; Godsey et al., 2003; Goos and Johnson, 2000; Mengel, 1995; Pestana et al., 2001; Randall, 1981; Reed et al., 1988; Zaiter et al., 1992). If these materials are applied in a fashion that results in adherence to leaf surfaces, the Fe is absorbed for use by the plant (Mengel, 1995). The main advantage of foliar applied Fe is that the fixation reactions of Fe in alkaline/calcareous soils are avoided (Mengel, 1995).
Foliar sprays should be applied with some type of adjuvant that will aid in even distribution and adsorption of the Fe on the leaf surface, where it can then be absorbed internally. Applying Fe with excessively high amounts of water will result in poor leaf adherence. Iron sprays that fall from the leaf surface to the soil will form insoluble precipitates that are not highly bioavailable (Olson, 1950). Supplying Fe into the irrigation water may be equally ineffective, although chelated Fe (EDDHA and analogous) proven to work in calcareous soil may be the exception if the Fe rates are relatively high and water-application rate is low. Although excessive water tends to reduce the effectiveness of a foliar application, enough water should be applied with a foliar Fe spray to insure adequate leaf coverage for uniform surface treatment to achieve maximum effect and to avoid spotting.
Although foliar Fe application readily supplies Fe into plant leaf tissue, this method of correcting Fe deficiency is temporary (Anderson, 1982; Godsey et al., 2003, Romheld and Marschner, 1986) and may have an overall negative effect by inhibiting the plant's natural ability to overcome Fe deficiency by minimizing the activity of the Fe stress response (Romheld and Marschner, 1986). No residual benefit in terms of Fe availability in the soil is realized with foliar sprays. In fact, the effect is limited to the tissue to which Fe is applied due to the immobility of Fe once it becomes a part of the cellular structure (Vose, 1982). New leaves emerging after foliar application will be Fe deficient, unless the plant's ability to solubilize soil Fe by root activity is enhanced by application or the soil and/or environmental conditions change in favor of Fe availability (Anderson, 1982).
Foliar sprays generally correct visual Fe deficiency symptoms readily. In some cases, the foliar spray also results in a yield increase for field crops, but, in other cases, the effect is minimal. Randall (1981) states that a foliar spray of FeEDDHA effectively corrects Fe chlorosis in soybean, but the result may not always provide an increase in yield. Goos and Johnson (2000) found that foliar sprays of FeEDTA generally corrected Fe deficiency chlorosis in Fe-inefficient soybean varieties, resulting in yield increases at some, but not all locations tested. A yield increase was also observed in the Fe-efficient soybean variety at one location with a foliar spray. However, their conclusion was that selecting varieties not susceptible to Fe deficiency was a better management tool than use of foliar sprays.
Caution is needed when using foliar Fe sprays. Excessively high rates will result in tissue necrosis. Ferrous sulfate is a common source of non-chelated foliar Fe, but many other Fe-anion combinations may also be used.
Many other forms of complexed (citrates, fulvates, lignosulfates, gluconates, etc.) and chelated (EDDHA, EDHA, HEDTA, DTPA, etc.) Fe sprays are also available (Lucena, 2003). Foliar acid sprays (such as dilute sulfuric acid) have also been used to correct Fe deficiency by releasing physiologically inactive Fe already found in the leaf tissue (Sahu et al., 1987). However, this method is not always as effective as foliar sprays containing Fe (Pestana et al., 2001). Although only field-tested materials from reliable companies with adequate labeling instructions should be used for any fertilizer material, injuries from foliar sprays are relatively common and caution is particularly important with this management approach.
Single foliar Fe sprays can be cost effective, but there is often a need for repeated applications to make significant yield impacts on field crops (Anderson, 1982; Godsey et al., 2003). Multiple applications may make this management approach impractical, especially for relatively low value field crops (Anderson, 1982). Mixing Fe fertilizer with pesticides or other fertilizer materials that are already planned to be foliarly applied makes the Fe application more cost effective (Mallarino et al., 2001). However, caution must be exercised when tank mixing Fe with other materials, as the combination may result in inactivation of pesticides or precipitation of compounds that may result in plugging of the spray equipment.
As mentioned previously, applying inorganic Fe fertilizer to soil is generally ineffective. Iron combines with anions in the soil to form compounds that are highly insoluble (such as ferric hydroxide) under alkaline pH conditions. Combining Fe with chelates or organic complexing compounds can reduce the rate of formation of these insoluble Fe compounds (Lindsay, 1974; Lucena, 2003; Vempati and Loeppert, 1988). An effective chelate essentially grasps Fe and keeps it in soil solution until it is either taken up by plants or microbes. Unfortunately, it may also be chemically or microbially decomposed, strongly bound by soil minerals or organic matter, or inactivated through the exchange of Fe with another cation (Álvarez-Fernández et al., 2002; Lindsay, 1974; Lucena, 2003; Winkelmann et al., 1999). The effectiveness of an Fe chelate compound may also be limited due to leaching, as synthetic chelates tend to be negatively charged and, as a result, are repelled by the negatively charged soil (Abadía et al., 2004; Álvarez-Fernández et al., 2002; Lucena, 2003; Winkelmann et al., 1999). If excessive irrigation and rainfall does not leach the chelate from the root zone, the effectiveness of chelated Fe often lasts long enough to provide the plant with the Fe that it needs during the early part of the growing season when Fe deficiencies are generally most severe
(Álvarez-Fernández et al., 2002; Lindsay, 1974; Reed et al., 1988; Sahu et al., 1988). In addition to the impact of leaching, decomposition, and inactivation, the effectiveness of synthetic chelates is influenced by factors such as amount and type of clay and salt in the soil (Siebner-Freibach et al., 2004; Vempati and Loeppert, 1988).
Iron chlorosis is generally a problem in alkaline/calcareous soil, but most Fe chelates are not effective under these soil conditions. Only the EDDHA and analogous chelate forms (over 100 types are commercially available) have been shown to be effective for application to soils in which Fe deficiency is a problem (Abadía et al., 2004; Goos and Germain, 2001; Lucena, 2003). Most other chelates (EDHA, HEDTA, DTPA, etc.) and complexes (citrates, fulvates, lignosulfates, gluconates, etc.) can maintain Fe in soil solution only temporarily and are not as effective as EDDHA and analogous chelates under calcareous conditions (Goos and Germain, 2001; Lucena, 2003). These other chelates may be used as foliar sprays or under mild deficiencies in non-calcareous conditions. The possibility of using Fe-chelates in slow-release forms has been evaluated for its ability to limit leaching losses and reduce Fe deficiency chlorosis (Goos et al., 2004, Yehuda et al., 2003). However, in a greenhouse evaluation, polymer coating for controlled release of Fe did not improve the effectiveness of many chelates and actually reduced the effectiveness of FeEDDHA (Goos et al., 2004). Slow release technology likely has more potential for use with mineral Fe fertilizers, such as ferrous sulfate (Morikawa et al., 2004; Singh et al., 2004).
Although not as effective as EDDHA in calcareous soil, many organic compounds can complex Fe and potentially increase its concentration in soil solution in non-calcareous conditions or when applied at very high rates in calcareous soil (Lucena, 2003). These Fe complexing materials include both those specific Fe complexes, as mentioned previously, as well as nonspecific humic substances found in biosolid amendments and in the fabric of the soil organic matter (Cesco et al., 2000; Chen, 1996; Dahiya and Singh, 1979; Lucena, 2000; Mathers et al., 1980; Olson, 1950; Pinton et al., 1999; Pinton et al., 2004; Nikolic et al., 2004; Siebner-Freibach et al., 2004; Thomas and Mathers, 1979). Similar to chelates, these organic materials contain carbon and, as such, are subject to chemical and microbial degradation over time. The rate of degradation of organic materials is highly dependent on soil temperature and is influenced by the activity and type of microbial species present and by soil moisture and oxidation state (Tisdale et al., 1993).
Raw or composted biosolid materials (animal, human or industrial waste) generally contain large quantities of organically complexed Fe that can alleviate Fe deficiency if applied in large quantities (Anderson, 1982;
Mathers et al., 1980). Inorganic industrial wastes may also serve as a source of Fe, although their plant availability is more akin to non-organic fertilizer materials (Wallace et al., 1976). These organic biosolids are not as concentrated as most inorganic fertilizer but are applied in such large quantities to land as to serve as an excellent means of increasing complexed Fe in soil solution. However, unlike commercially available Fe chelates/-complexes, the availability of the Fe and other nutrients found in biosolids may be variable and dependent on microbial degradation of the material. In temperate climates, microbial activity is generally very low in the early part of the season when soil temperatures are cool and Fe deficiencies likely occur. Consequently, the availability of Fe from biosolid materials may be inadequate for early season use. It also is critical to manage biosolids properly to avoid N immobilization, toxicities to salt or specific ions/molecules, and/or problems associated with introduced pathogens and weeds.
The effect of organic matter in alleviating or preventing Fe deficiency is complex and not consistent. In addition to organic matter serving as a source of Fe, organic matter applied in large quantities can also result in "loosening" of the soil. Soil with lower bulk density has higher oxygen content and lower carbon dioxide concentration, which results in less negative impact of bicarbonate on Fe availability (Lucena, 2000). However, addition of easily degradable organic matter can temporarily have the opposite effect as the carbon dioxide level spikes as a function of microbial respiration during the mineralization process (Lucena, 2000). This results in the formation of bicarbonate in the soil, which reduces the bioavailability of iron.
Increased microbial activity is another potential benefit of carbon added via organic matter. Many microorganisms are known to release chelates known as siderophores which can mobilize Fe (Siebner-Freibach et al., 2004). These siderophores are effective in solubilizing Fe for plant uptake and do not leach or decompose as readily as synthetic chelates (SiebnerFreibach et al, 2004).
Although application of chelated or complexed Fe can effectively correct or prevent Fe deficiency, these materials are relatively costly to apply. Applying FeEDDHA chelates to soil is quite effective even at low rates. However, the cost of FeEDDHA is high and consequently is an option only for relatively high value crops. Similarly, application of biosolids and other sources of organically bound Fe require relatively high rates to be effective and transportation and application costs are prohibitive unless a local source is readily available and cost effective (Anderson, 1982).
The cost of inorganic Fe fertilizer is relatively low, but, as discussed previously, broadcast application to the soil is generally ineffective. However, concentrating inorganic fertilizer in a band or point in the soil can effectively slow the rate of formation of insoluble Fe compounds within the concentrated fertilizer area, especially if it is acidic in nature (Godsey et al., 2003; Goos and Johnson, 2000; Hergert et al., 1996; Mathers, 1970). Studies in corn have shown that high rates of ferrous sulfate (~85 kg ha-1) strategically placed in the seed row (Godsey et al., 2003; Goos and Johnson, 2000; Hergert et al., 1996) or in the path of roots effectively prevented Fe deficiency and resulted in significant yield increases (Goos and Johnson, 1999; Mathers, 1970; Mortvedt and Giordano, 1970). However, effectiveness is unpredictable (Godsey et al., 2003; Mortvedt and Giordano, 1970) and likely depends on variable soil factors, including pH, lime content, salt concentration, organic matter, texture, temperature, moisture, and oxygen content (Bloom and Inskeep, 1986; Chen and Barak, 1982; Dahiya and Singh, 1979; Franzen and Richardson, 2000; Inskeep and Bloom, 1986, 1987; Moraghan and Mascagni, 1991; Morris et al., 1990; Vempati and Loeppert, 1988).
The use of controlled-release Fe fertilizers applied as a band near or with the seed has been shown to ameliorate Fe chlorosis and improve yield of rice relative to similar placement of concentrated fertilizers without release control (Morikawa et al., 2004; Singh et al., 2004). This benefit may relate to a concentration of fine roots growing in direct contact with the controlled-release fertilizer granules, suggesting that fixation of released Fe by the soil is prevented by rapid root uptake. Controlled-release ferrous sulfate fertilizer did not improve growth of Fe deficient soybean, but a controlled release fertilizer with ferrous sulfate-ammonium sulfate-citric acid did (Goos et al., 2004). Controlled release fertilizers may play an important role in managing Fe deficiency in some field crops, but the relatively high cost will limit its use.
It is entirely possible that a point injection below and to the side of each seed could also prevent Fe deficiency with a lower rate of fertilizer than reportedly used previously. Use of chelated/complexed Fe fertilizer sources would also be expected to work at much lower rates due to their relatively high efficiency. In theory these practices may be beneficial, but should be verified with research before adoption.
Impregnating chelated (FeEDDHA) Fe fertilizer directly on the seed has also shown some benefit in preventing Fe deficiency (Goos and Johnson, 2000; Karkosh et al., 1988; Wiersma et al., 2005) but the effect is not always realized (Goos and Johnson, 2000). Goos and Johnson (2000) found that seed applied Fe effectively reduced Fe chlorosis and improved grain yield for soybean when applied in wide (76 cm) but not in narrow (15 cm) rows. However, their conclusion was that selecting varieties resistant to Fe deficiency was a better management tool than use of seed applied Fe.
Direct seed contact of any fertilizer salt requires low rates and caution, particularly with salt sensitive species and/or with soil or water already high in salts (Ayers and Westcot, 1985). Application of concentrated Fe fertilizer on or near the seed has been tested only on a limited number of crops, and rate/placement trials should be conducted to evaluate effectiveness and toxicity with other cropping scenarios.
Was this article helpful?