Metal micronutrient deficiencies can be effectively treated with the use of synthetic chelates.
This article presents the different types and factors on which its efficacy depends, such as its stability, reactivity in the soil and influence in the process of absorption of the element by plants. In spite of its high efficiency its high cost does not always be an adequate solution so it is also studied the use of metal complexes. The micronutrient complexes (lignosulfonates, humates, citrate, gluconates, heptagluconates and amino acids) aim to keep the complexed elements in soluble form. Since they are usually complexes of less stability in the soil than chelates, their main route of action is in nutritive solution or foliar application. Thus its efficiency depends not only on the complexing capacity of metals but also on other factors such as the ability of foliar penetration.
The chelates are products of high stability capable of maintaining the metal ions surrounded by an organic molecule (chelating agent) so that they are safeguarded from the environment that would favor its precipitation in the form of insoluble hydroxide and not available for the plant (Chen & Barak, 1982, Lucena, 2006). Types of chelates Among them the most used for iron they are the ones who have EDDHA (Álvarez-Fernández et al, 2005 – see figure 1). This compound can present two positional isomers, ortho-ortho (o,o-EDDHA- ethylenediamine acid-N,N’-di[(orthohydroxyphenyl )acetic], which in turn presents optical isomerism: meso isomers and dl-racemic mixture) high stability and long-term efficacy and ortho-for (o,pEDDHA- ethylenediamine acid-N,-[(ortho-hydroxyphenyl)acetic]-N’-[(parahydroxyphenyl)acetic] less stable but faster response (García-Marco et al., 2006a). There are products with a high presence in ortho-for and others in whose composition there is only ortho-ortho (García-Marco et al., 2006b).
Figure 1: Chemical structure of the racemic and meso geometric isomers of Fe-o, oEDDHA and the positional Fe-o, pEDDHA. Note the protection of the central ferric ion, surrounded by 6 donor groups in the oxyda and five in the o, pEDDHA.
A as of place is very important to take into account the wealth in the isomers and not in the data of soluble Fe, since if it is not chelated it will precipitate and will not be used by the plants (Hernández-Apalolaza et al., 2000; García-Marco & Lucena, 2009). EDDHMA- ethylenediamine acid-N,N’-di(orthohydroxymethylphenylacetic), EDDHSA- ethylenediamine acid-N,N’- di[(2-hydroxy-5-sulfophenylacetic acid] y EDDCHA- ethylenediamino-N, N ‘- [(5-carboxy-2-hydroxyphenyl) acetic] also form high stability ferric chelates. The last two are the most soluble so they can be used in liquid fertilizers (Lucena et al., 2004). HBED – acid N,N’-bis (2-hydroxybenzyl) ethylenediamino-N, N’-diacetic and DCHA- 2- (2 – ((2-hydroxybenzyl) amino) ethylamino) – 2-(2-hidroxiphenil) are two new solutions under study. For non-sensitive cultures, or non-calcareous substrates and with frequent addition of chelate, it is possible to use chelants of lower stability for Fe such as EDTA- etlylenediamine tetraacetic acid, HEEDTA- N- (2-hydroxyethyl) ethylenediaminetriacetic acid and DTPA- diethylenetriaminepentaacetic acid. Recently, the use of IDHA – imidodisuccinic acid as biodegradable chelating agent (Lucena et al., 2008). The most commonly used chelates for the micronutrients Mn, Zn and Cu are DTPA, EDTA, HEEDTA (Lindsay & Sommers 1997) and also IDHA (Lucena et al., 2008). Although its effectiveness in acid soils conditions is high, however none is a completely satisfactory solution, especially for the Mn, if the soil limestone content is very high.
Figure 2: Schematic of the action of the chelating agent
The efficacy of chelates is sufficiently proven, although it must be taken into account that the products that are suitable for each agronomic condition and element must be used at the correct dosage and with products that have a proven commercial quality.More effort is needed in the understanding and evaluation of both the products and their mode of action.
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