Theoretical speciation of ethylenediamine-N-(o-hydroxyphenylacetic)-N'-(p-hydroxyphenylacetic) acid (o,p-EDDHA) in agronomic conditions

The presence of ethylenediamine-N-(o-hydroxyphenylacetic)-N'-(p-hydroxyphenylacetic) acid (o,p-EDDHA) as the second largest component in commercial EDDHA iron chelates has recently been demonstrated. Here is reported the speciation of o,p-EDDHA by the application of a novel methodology through the determination of the complexing capacity, protonation, and Ca(2+), Mg(2+), Cu(2+), and Fe(3+) stability constants. The pM values and species distribution in solution, hydroponic, and soil conditions were obtained. Due to the para position of one phenol group in o,p-EDDHA, the protonation constants and Ca and Mg stability constants have different values from those of o,o-EDDHA and p,p-EDDHA regioisomers. o,p-EDDHA/Fe(3+) stability constants are higher than those of EDTA/Fe(3+) but lower than those of o,o-EDDHA/Fe(3+). The sequence obtained for pFe is o,o-EDDHA/Fe(3+) >/= o,p-EDDHA/Fe(3+) > EDTA/Fe(3+). o,p-EDDHA/Fe(3+) can be used as an iron chelate in hydroponic conditions. Also, it can be used in soils with limited Cu availability.     

Chromatographic determination of Fe chelated by ethylenediamine-N-(o-hydroxyphenylacetic)-N'-(p-hydroxyphenylacetic) acid in commercial EDDHA/Fe3+ fertilizers

EDDHA/Fe3+ chelates are the most common fertilizers used to solve Fe chlorosis in established crops. Commercial products contain two regioisomers, ethylenediamine-N,N'-bis(o-hydroxyphenylacetic) acid (o,o-EDDHA)/Fe3+ and ethylenediamine-N-(o-hydroxyphenylacetic)-N'-(p-hydroxyphenylacetic) acid (o,p-EDDHA)/Fe3+. Although several chromatographic methods exist for the determination of Fe3+ chelated by the o,o-EDDHA isomer, no method has been described for the quantification of Fe3+ chelated by o,p-EDDHA. In this work, factors that affect the behavior of o,p-EDDHA/Fe3+ in ion pair chromatography are reviewed: pH, ion pair reagent, and organic modifier. The best chromatographic performance was obtained with an aqueous mobile phase at pH 6.0 containing 35% acetonitrile and 5 mM tetrabutylammonium hydroxide under isocratic elution conditions. This method was applied to the quantification of commercial samples.     

Kinetic behavior of Fe(o,o-EDDHA)-humic substance mixtures in several soil components and in calcareous soils

Ferric ethylenediamine- N, N'-bis-(o-hydroxyphenylacetic)acid chelate (Fe(o, o-EDDHA)) is one of the most effective Fe fertilizers in calcareous soils. However, humic substances are occasionally combined with iron chelates in drip irrigation systems in order to lower costs. The reactivity of iron chelate-humic substance mixtures in several soil components and in calcareous soils was investigated through interaction tests, and their behavior was compared to the application of iron chelates and humic substances separately. Two commercial humic substances and two Fe(o, o-EDDHA) chelates (one synthesized in the laboratory and one commercial) were used to prepare iron chelate-humic substance mixtures at 50% (w/w). Various soil components (calcium carbonate, gibbsite, amorphous iron oxide, hematite, tenorite, zincite, amorphous Mn oxide, and peat) and three calcareous soils were shaken for 15 days with the mixtures and with iron chelate and humic substance solutions. The kinetic behavior of Fe(o, o-EDDHA) and Fe non-(o,o-EDDHA) (Fe bonded to (o,p-EDDHA) and other polycondensated ligands) and of the different nutrients solubilized after the interaction assay was determined. The results showed that the mixtures did not significantly reduce the retention of Fe(o, o-EDDHA) and Fe non-(o,o-EDDHA) in the soil components and the calcareous soils compared to the iron chelate solutions, but they did produce changes in the retention rate. Moreover, the competition between humic substances and synthetic chelating agents for complexing metal cations limited the effectiveness of the mixtures to mobilize nutrients from the substrates. The presence of Fe(o, p-EDDHA) and other byproducts in the commercial iron chelate had an important effect on the evolution of Fe(o, o-EDDHA) and the nutrient solubilization process.     

Structure and fertilizer properties of byproducts formed in the synthesis of EDDHA

The synthesis of commercial EDDHA produces o,o-EDDHA as the main reaction product, together with a mixture of regioisomers (o,p-EDDHA and p,p-EDDHA) and other unknown byproducts also able to complex Fe3+. These compounds have been obtained by direct synthesis, and their structures have been determined by ESI-MS analysis as oligomeric EDDHA-like products, formed by polysubstitution in the phenolic rings. Short-term experiments show that the iron complexes of samples enriched in these oligomeric byproducts have adequate stability in solution, but a significant amount of them is lost after interaction with soils and soil materials. Mildly chlorotic cucumber plants are able to reduce iron better from o,p-EDDHA/Fe3+ than from the iron complexes of the oligomeric byproducts. In hydroponics, the chlorotic soybean susceptible plants have a lower potential for Fe absorption from these byproducts than from o,o-EDDHA/Fe3+ and from o,p-EDDHA/Fe3+. In the studied conditions, the iron chelates of EDDHA byproducts do not have the long-lasting effect shown by o,o-EDDHA/Fe3+ and present a less efficient fast-action effect than the o,p-EDDHA/Fe3+.     

Interaction of iron chelates with several soil materials and with a soil standard

Frequently the effectiveness of iron (Fe) chelates is low because they can be retained or destroyed by soil materials. The high cost of these Fe fertilizers makes it necessary to study soil material reaction with Fe chelates. Commercial Fe chelates with EDTA, EDDHA, and EDDHMA as ligands and their standards, prepared in the laboratory, were shaken for one hour with various soil materials [amorphous Fe(III) oxide, acid peat, calcium (Ca)‐montmorillonite and calcium carbonate (CaCO₃)] and with a soil standard made in the laboratory. After agitation, the chelate‐soil mixtures were filtered and the micronutrients and chelated Fe that remained in solution were determined. Among the soil materials used, amorphous Fe(III) oxide and acid peat had the greatest affect on the amount of chelated Fe remaining in solution. The type of chelating agent was the next major factor that affected the availability of soluble Fe following reaction with the soil materials. Another factor was the commercial formulation of the Fe chelates. The chelates comprised of EDDHA or EDDHMA maintained the highest percentages of chelated Fe in solution after interaction with the solid phases, except for the acid peat. The last soil material, acid peat, retained more chelated Fe for the Fe chelates with EDDHA or EDDHMA than with EDTA as the chelating agent. The commercial Fe‐EDDHA chelates had greater losses of chelated Fe than their standard after interaction with all the solid phases. The commercial Fe‐EDDHA chelate (Sequestrene) and the commercial Fe‐EDDHMA chelate (Hampirón) solubilized the highest amount of copper (Cu) from soil standard. This was attributed to the presence of by‐products in the commercial formulations since the Fe‐EDDHA standard did not have Cu in solution after the interaction. Therefore, the commercial Fe chelate by‐products are able to form Cu‐complexes which could affect chelated Fe and its availability to plants.     

NMR analysis of the iron ligand ethylenediaminedi(o-hydroxyphenyl)acetic acid (EDDHA) employed in fertilizers

The exceptional efficiency of the iron chelate of ethylenediaminedi(o-hydroxyphenyl)acetic acid (o,o-EDDHA) in correcting iron chlorosis in plants and the medical applications of various metallic chelates of this compound have long been recognized. As commercial preparations of o,o-EDDHA usually contain impurities, a method for their detection is proposed. By using one- and two-dimensional nuclear magnetic resonance two impurities were identified. The structure of one of these compounds was assigned to an isomer of EDDHA containing at least one p-hydroxyphenyl moiety. The structure of the other impurity was tentatively assigned to a byproduct of the EDDHA synthesis: 2,6-di[CH(COOH)NHCH(2)CH(2)NHCH(COOH)Ar]phenol (Ar = hydroxyphenyl). Both compounds were also detected in the EDDHA extracted from a commercial iron fertilizer.     

Influence of the soil/solution ratio, interaction time, and extractant on the evaluation of iron chelate sorption/desorption by soils

Synthetic Fe chelates are the most efficient agricultural practice to control Fe deficiency in crops, EDTA/Fe3+ and o,o-EDDHA/Fe3+ being the most commonly used. Their efficacy as Fe sources and carriers in soils can be severely limited by their retention on it. The aim of this work is to evaluate the possible bias introduced in the studies of the iron chelate retention by soils. For that purpose, results obtained for EDTA and EDDHA iron chelates from two batch studies with different soil/solution ratios were compared with data obtained for a leaching column experiment. Moreover, different extractants were tested to study the o,o-EDDHA/Fe3+ and o,p-EDDHA/Fe3+ desorption from a calcareous soil, and also the effect of the interaction time in their retention process has been evaluated. In summary, the mobility through a calcareous soil of the studied iron chelates differs greatly depending on the type of iron chelate and also on the procedure used to evaluate the retention and the soil/solution ratio used. In general, the leaching column method is preferred because the achieved conclusions are more representative of the natural conditions, but batch methods are very useful as a preliminary experiment, especially one with a high soil/solution ratio. The iron chelate desorption could be quantified by using a sequential extraction with water, sodium sulfate, and DTPA as extractants. Under the experimental conditions used in this study, o,o-EDDHA/Fe3+ retention increased with interaction time.     

Nature of impurities in fertilizers containing EDDHMA/Fe(3+), EDDHSA/Fe(3+), and EDDCHA/Fe(3+) chelates.

Iron chelates derived from ethylenediaminedi(o-hydroxyphenylacetic) acid (EDDHA), ethylenediaminedi(o-hydroxy-p-methylphenylacetic) acid (EDDHMA), ethylenediaminedi(2-hydroxy-5-sulfophenylacetic) acid (EDDHSA), and ethylenediaminedi(5-carboxy-2-hydroxyphenylacetic) acid (EDDCHA) are remarkably efficient in correcting iron chlorosis in plants growing in alkaline soils. This work reports the determination of impurities in commercial samples of fertilizers containing EDDHMA/Fe(3+), EDDHSA/Fe(3+), and EDDCHA/Fe(3+). The active components (EDDHMA/Fe(3+), EDDHSA/Fe(3+), and EDDCHA/Fe(3+)) were separated easily from other compounds present in the fertilizers by HPLC. Comparison of the retention times and the UV-visible spectra of the peaks obtained from commercial EDDHSA/Fe(3+) and EDDCHA/Fe(3+) samples with those of standard solutions showed that unreacted starting materials (p-hydroxybenzenesulfonic acid and p-hydroxybenzoic acid, respectively) were always present in the commercial products. 1D and 2D NMR experiments showed that commercial fertilizers based on EDDHMA/Fe(3+) contained impurities having structures tentatively assigned to iron chelates of two isomers of EDDHMA. These findings suggest that current production processes of iron chelates used in agriculture need to be improved.     

Efficacy of Fe(o,o-EDDHA) and Fe(o,p-EDDHA) isomers in supplying Fe to strategy I plants differs in nutrient solution and calcareous soil

The FeEDDHA [iron(3+) ethylenediamine di(o-hydroxyphenylacetic) acid] is one of the most efficient iron chelates employed in the correction of iron clorosis in calcareous soils. FeEDDHA presents different positional isomers: the ortho-ortho (o,o), the ortho-para (o,p), and the para-para (p,p). Of these isomers, the p,p cannot chelate Fe in soil solution in a wide range of pH values, while both o,o and o,p can. The objective of this work was to compare the efficiency of both isomers (o,o and o,p) to provide Fe to two Strategy I plants (tomato and peach) in nutrient solution (pH approximately 6.0), as well as in calcareous soil (pH approximately 8.4; CALCIXEREPT). For this, chelates of both o,o-EDDHA and o,p-EDDHA with 57Fe (a nonradioactive isotope of Fe) were used, where the 57Fe acts as a tracer. The results obtained showed that the o,o isomer is capable of providing sufficient Fe to plants in both nutrient solution and calcareous soil. However, the o,p isomer is capable of providing sufficient Fe to plants in nutrient solution but not in calcareous soil.     

Fe Chelates for Remediation of Fe Chlorosis in Strategy I Plants

Abstract

Iron chlorosis is a mineral disorder due to low Fe in the soil solution and the impaired plant uptake mechanism. These effects increased with high pH and bicarbonate buffer. The solution to Fe chlorosis should be made by either improving the Fe uptake mechanism or increasing the amount of Fe in the soil solution. Among Fe fertilizers, only the most stable chelates (EDDHA and analogous) are able to maintain Fe in the soil solution and transport it to the plant root. In commercial products with the same chelating agent, the efficacy depends on the purity and the presence of subproducts with complexing activity, that can be determined by appropriate analytical methods such as HPLC. In commercial products declaring 6% as Fe‐EDDHA, purity varied from 0.5% to 3.5% before 1999, but in 2002 products ranging 3–5.4% chelated Fe are common in the Spanish market. Fe‐o,p‐EDDHA, as a synthesis by‐product with unknown efficacy, is present in all Fe‐EDDHA formulations. Commercial Fe‐EDDHMA products also contain methyl positional isomers. Fe‐EDDHSA synthesis produces condensation products with similar chelating capacity to the Fe‐EDDHSA monomer that can account for more than 50% of the chelated iron in the commercial products. Chelates with different molecules should be compared for their efficacy considering firstly their ability to maintain Fe in solution and secondly their capacity to release iron to the roots. Accepting the turnover hypothesis, their efficacy is also dependent thirdly on the ability of the chelating agent to form the chelate using native iron from the soil. The 1st and 3rd points are related to the chemical stability of the chelate, while plants make better use of iron from the less stable chelates. Plant response is the ultimate evaluation method to compare commercial products with the same chelating agent or different chelates.     

Partial replacement of Fe(o,o‐EDDHA) by humic substances for Fe nutrition and fruit quality of citrus

The most widely used Iron (Fe) fertilizer in calcareous soils is the synthetic chelate Fe(o,o‐EDDHA). However, humic substances are occasionally combined with Fe chelates in drip irrigation systems in order to lower costs. We investigated the effect of various mixtures of Fe(o,o‐EDDHA) and a commercially available humic substance on Fe availability in a calcareous soil from Murcia, Spain (in vitro experiment) and on leaf Fe content and fruit‐quality attributes of Citrus macrophylla (field experiment). In the in vitro experiment, a calcareous soil was incubated for 15 d with solutions of sole Fe(o,o‐EDDHA) and humic substance and of a mixture of humic substance and Fe(o,o‐EDDHA) to determine the dynamics of available Fe. While the mixture did not significantly increase the available soil Fe, it did decrease the rate of Fe retention in the surface soil compared to sole Fe(o,o‐EDDHA). In the field experiment, the substitution in the application solution of 67% of Fe(o,o‐EDDHA) by commercial humic substance increased leaf P in lemon trees from 0.19% with sole Fe(o,o‐EDDHA) to 0.30% and leaf Fe from 94 mg kg^–1 to 115 mg kg^–1. Some quality parameters like vitamin C content and peel thickness were also improved with a partial substitution of Fe(o,o‐EDDHA) by humic substances. We conclude that a partial substitution of commercial Fe chelates by humic substance can improve crop Fe uptake and may thus be economically attractive. The underlying physiological mechanisms and ecological implications require further studies.     

Effectiveness of iron chelates and FeSO4 for correcting iron chlorosis of peanut on calcareous soils

The effectivenness of different Fe chelates to correct lime induced chlorosis of peanut (Arachis hypogaea) was tested on calcareous soils in Cyprus. Among the chelates tested, Fe‐DTPA and Fe‐EDTA were less effective than Fe‐EDDHA. In one experiment. Fe‐EDDHMA Was less effective while in another experiment was equally effective compared to Fe‐EDDHA . Three different commercial chelates of the form Fe‐EDDHA were equally effective in correcting iron chlorosis. The stability of the Fe‐EDDHA chelate in the soil does not affect its efficiency in curing chlorosis of peanut due to the short growing season for this crop. Ferrous sulfate applied without an acidifying soil amendment was not effective to correct iron chlorosis of peanut.     

Evaluation of the potential impact of Cu competition on the performance of o,o‐FeEDDHA in soil applications

Ferric ethylene diamine‐N,N′‐bis(hydroxy phenyl acetic acid) (FeEDDHA)‐based iron (Fe) fertilizers are commonly applied to plants grown on calcareous soils and comprise a mixture of FeEDDHA components. Upon application to the soil, the pore water concentrations of the active ingredients racemic and meso o,o‐FeEDDHA show a gradual decline unrelated to plant uptake or biodegradation. In the present study, the potential of soil copper (Cu) to reduce the effectiveness of FeEDDHA‐based fertilizers in calcareous soils by displacing Fe from o,o‐FeEDDHA has been evaluated through modelling and experiments. Predictions with mechanistic multi‐surface models show that there is a thermodynamic basis for assuming that under equilibrium conditions a certain fraction of o,o‐EDDHA ligands in soil solution can be chelated to Cu, in particular for meso o,o‐EDDHA. The large affinity of o,o‐CuEDDHA for binding to the soil solid phase, demonstrated in a batch interaction experiment, greatly increases the potential impact of Cu competition on the o,o‐FeEDDHA solution concentration; for a given quantity of o,o‐CuEDDHA in soil solution, a much larger quantity of o,o‐CuEDDHA is adsorbed to the solid phase. Finally, evidence for the actual displacement of Fe from o,o‐FeEDDHA by Cu was found in a soil incubation study. With these results, the boundary conditions are met for explaining the observed gradual decline in o,o‐FeEDDHA concentration with Cu competition.     

Demetalation of Fe, Mn, and Cu chelates and complexes: application to the NMR analysis of micronutrient fertilizers

The application of nuclear magnetic resonance (NMR) for the quality control of fertilizers based on Fe(3+), Mn(2+), and Cu(2+) chelates and complexes is precluded by the strong paramagnetism of metals. Recently, a method based on the use of ferrocyanide has been described to remove iron from commercial iron chelates based on the o,o-EDDHA [ethylenediamine-N,N'bis(2-hydroxyphenylacetic)acid] chelating agent for their analysis and quantification by NMR. The present work extended that procedure to other paramagnetic ions, manganese and copper, and other chelating, EDTA (ethylenediaminetetraacetic acid), IDHA [N-(1,2-dicarboxyethyl)-d,l-aspartic acid], and complexing agents, gluconate and heptagluconate. Results showed that the removal of the paramagnetic ions was complete, allowing us to obtain (1)H NMR spectra characterized by narrow peaks. The quantification of the ligands by NMR and high-performance liquid chromatography showed that their complete recovery was granted. The NMR analysis enabled detection and quantification of unknown impurities without the need of pure compounds as internal standards.     

The use of iron chelates in compound fertilizers containing trace elements

Abstract

The chemical behaviour of iron chelates, incorporated in compound fertilizers with inorganic micronutrient compounds, was studied. Pot experiments were conducted to evaluate the effectiveness of these products in controlling iron deficiency. A commercial compound fertilizer, containing 0.09% Fe as Fe‐DTPA, 0.12% Cu, 0.16% Mn and 0.04% Zn as sulfates, proved to be ineffective in preventing iron chlorosis in Chamaecyparis lawsoniana ’Alumii’, growing on sphagnum peat at two lime levels. The same fertilizer formulation with 0.09%‐ Fe as Fe‐EDDHA did prevent iron chlorosis at both lime levels. Fe‐EDTA, incorporated in the compound fertilizer, gave good results at the lower but not at the higher lime level, due to the low stability of Fe‐EDTA at high pH. Data from laboratory experiments showed that copper replaced the chelated iron in the compound fertilizer containing Fe‐DTPA, causing the iron to precipitate. The strong competition between copper and iron for the organic ligand is due to a specific affinity of copper for DTPA, resulting in a copper chelate with high stability constant and a molar ratio of copper to chelating agent of 2 : 1.

In the case of Fe‐EDDHA and Fe‐EDTA the competition between iron and copper is much weaker. In contrast to Fe‐DTPA, these chelates remain rather stable when incorporated in fertilizers containing micronutrients.     

Efficacy of commercial Fe(III)‐EDDHA and Fe(III)‐EDDHMA chelates to supply iron to sunflower and corn seedlings

Use of synthetic iron (Fe) chelates is the most common and effective way to treat Fe chlorosis in plants. Most commercial products contain Fe‐EDDHA or Fe‐EDDHMA but their efficacy can be quite different. Commercial products with EDDHA or EDDHMA as active components were chosen based on the data obtained by Lucena et al. (1992) in their chemical test. The chelates present extreme differences in behavior in the mentioned chemical tests. The analysis of the products revealed that the total Fe concentration is greater than the one indicated by the manufacturer in spite of a lesser amount of FeY^‐ present. The plant response to these commercial products was tested using short‐term greenhouse hydroponic cultures. Sunflower and corn were chosen because of their different behavior under Fe‐stress conditions. No significant difference between plants treated with Fe‐EDDHA or Fe‐EDDHMA chelates were observed. Since the purity index indicates there are too many differences between commercial formulations of the same type of chelate, the differentiation between groups cannot be determined with commercial products. Index I3, described by Lucena et al. (1992), does not correlate with the plant response because it did not consider the purity percentage of the products.     

Evaluation of Commercial Fe(III)‐Chelates Using Different Methods

Abstract

A collaborative assay among three laboratories was made in order to compare both the ion (CEN. EN 13368‐2:2001 E. Determination of chelating agents in fertilizers by ion chromatography. Part 2: EDDHA and EDDHMA, 2001a) and the ion‐pair (Lucena, J.J.; Barak, P.; Hernandez‐Apaolaza, L. Isocratic ion‐pair high‐performance liquid chromatographic method for the determination of various iron(III) chelates. J. Chromatogr. A 1996, 727, 253–264) high performance liquid chromatography (HPLC) methods as well as the soluble and complexed Fe (CEN. EN 13366:2001 E. Treatment with a cation exchange resin for the determination of the chelated micronutrient content and of the chelated fraction of micronutrients, 2001b) methods. Fifteen and ten samples of commercial fertilizers of Fe‐EDDHA, Fe‐EDDHMA, respectively were analysed by three laboratories using these methods. No significant differences were observed between the results obtained for the Fe‐EDDHA content using the Lucena et al. or CEN method. The first method makes it possible to distinguish between the meso and DL‐racemic diasteroisomers of Fe‐o, o‐EDDHA. For the Fe‐EDDHMA formulations, the CEN method gives higher values than the ion‐pair method, since in the first one Fe‐EDDH4,6MA coelutes with FeEDDHMA. Also the CEN method does not makes it possible to distinguish between Fe‐EDDHMA and Fe‐EDDH5MA products. The variability among laboratories was larger for the CEN method than for the Lucena et al. method.     

Methodology to Screen New Iron Chelates: Prediction of Their Behavior in Nutrient Solution and Soil Conditions

Abstract

Iron chelates analogous to ethylenediamino‐di(o‐hydroxyphenyl)acetic acid (EDDHA) are the fertilizers chosen to treat iron chlorosis of crops grown on calcareous soils. Characterization of these synthetic ligands should be made to establish their chemical behavior and efficiency as chlorosis correctors. The aim of this research was to develop an appropriate methodology to screen new iron chelates using analytical determinations and chemical equilibrium concepts. Fe‐EDDHA, Fe‐EDDH4MA, Fe‐EDDH5MA, and Fe‐PDDHA chelates, were compared to check the proposed methodology. Titrimetric purity, protonation and Ca, Mg, and Fe(III) stability constants, pFe and species distribution in nutrient solution and soil conditions were determined. The iron chelate stability constants were in order EDDHA > EDDH4MA > EDDH5MA > PDDHA. When pFe was calculated, the larger value corresponds to Fe‐EDDHA chelate at pH below 8; but at pH above 8 the Fe‐EDDH4MA shows the larger pFe values. When the species was plotted against pH, the dominant species was FeL^? at the physiological pH range in all cases. The pH at a FeL/L_T ratio of 80% in both Fe(OH)_3amorp and Fe_soil systems was considered as an iron chelate stability index. This index was EDDH4MA > EDDH5MA > EDDHA > PDDHA in both systems, but shows that all of the chelates tested were sufficiently stable in most soil and nutrient solution conditions. In conclusion, the proposed procedure is adequate for the preliminary evaluation of the synthetic chelating agents, using important parameters such as analytical and speciation properties to predict their chelating behavior and efficiency in nutrient solution and soil conditions.     

Characterization of commercial iron chelates and their behavior in an alkaline and calcareous soil

Iron deficiency is a common problem for many plants grown in alkaline and calcareous soils. To correct this problem, iron is supplied to plants as chelates. Several iron chelates are sold under diverse trademarks with different characteristics. This work evaluated 18 commercial products containing the most representative chelated iron sources used in agricultural practice in Spain when the study was done, namely the ferric chelates of EDDHA, EDDHMA, EDDCHA, EDDHSA, EDTA, and DTPA. The chelates were comprehensively characterized and quantitated by several techniques, including several chromatographic methods. Iron and chelate dynamics in soil were also studied in a model alkaline and calcareous soil. Results indicate that, in this model soil, among the different iron compounds studied only FeEDDHA and analogues have the capacity to maintain soluble iron in soil solution over time. These results are in agreement with general experience under field conditions. Furthermore, among the different ortho-ortho isomers of FeEDDHA's, FeEDDHSA and FeEDDCHA showed greater capacity than FeEDDHA and FeEDDHMA to maintain the chelated iron in soil solution over time.     

Analysis of iron chelates in commercial iron fertilizers by gel chromatography

A gel chromatographic method for the quality control of iron chelate fertilizers is described. The iron chelates are separated on a column of Sephadex G-10 and the eluates are analysed for iron. Using a sample quantity of 25 mg in a volume of 5 ml water and eluting with 0.15 M soidum chloride solution, a separation was achieved of commercial products of Fe-EDDHA or Fe-EDDHMA. The chromatographic analysis of Fe-EDTA or Fe-DTPA calls for a better resolution. This was obtained by decreasing the sample quantity and eluting with 0.035 M calcium chloride solution of pH 7.0. In this way it is possible to eliminate the interference of iron chelates of moderate stability which can be present in commercial products containing Fe-EDTA or Fe-DTPA.