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.     

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.     

Performance of soil-applied FeEDDHA isomers in delivering Fe to soybean plants in relation to the moment of application

FeEDDHA (iron(3+) ethylenediamine-N,N'-bis(hydroxyphenylacetic acid) products are commonly applied to mend and prevent Fe deficiency chlorosis in soil-grown crops. Plants mainly take up Fe in the progressed vegetative and in the reproductive stages. This study examined which of the principal constituents of FeEDDHA products (the isomers racemic o,o-FeEDDHA, meso o,o-FeEDDHA, and o,p-FeEDDHA), most effectively meets the Fe requirements of soybean plants (Glycine max (L.) Merr.) grown on calcareous soil in the aforementioned growth stages. FeEDDHA isomers were applied once, separately or in mixtures, at t = 0, in the progressed vegetative stage or in the reproductive stage. o,p-FeEDDHA did not significantly contribute to Fe uptake in either growth stage. Both racemic and meso o,o-FeEDDHA were effective in supplying plants with Fe, approximately to the same extent. The moment of application had a significant effect on yield and FeEDDHA pore water concentrations at harvest, but not on Fe uptake. To optimize yield while minimizing FeEDDHA dosage, FeEDDHA is best applied to soybean plants prior to the onset of chorosis.     

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+.     

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.     

Synthesis of o,p-EDDHA and its detection as the main impurity in o,o-EDDHA commercial iron chelates

Ethylenediamine-N,N'bis(o-hydroxyphenyl)acetic acid (o,o-EDDHA) is one of the most efficient iron chelates employed to relieve iron chlorosis in plants. However, the presence of positional isomers of EDDHA in commercial iron chelates has been recently demonstrated, and among them, it has been claimed that ethylenediamine-N(o-hydroxyphenylacetic)-N'(p-hydroxyphenylacetic) acid (o,p-EDDHA) is the main impurity present in EDDHA fertilizers. Here we report the preparation of o,p-EDDHA, a compound whose synthesis had not been previously reported. The synthetic o,p-EDDHA is able to form ferric complexes, and it has been used as a standard in the analysis of the impurities of commercial iron fertilizers. The presence of o,p-EDDHA/Fe(3+) in commercial samples has been unambiguously demonstrated by HPLC.     

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.     

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.     

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.     

Fe(III)-EDDHA and -EDDHMA sorption on Ca-montmorillonite, ferrihydrite, and peat.

The effectiveness of Fe chelates as Fe sources and carriers in soil can be severely limited by the adsorption of Fe chelates or chelating agents in the solid phase. To study this phenomenon, well-characterized peat, Ca-montmorillonite, and ferrihydrite were used as model compounds, and the adsorption of Fe-EDDHA and Fe-EDDHMA chelates were studied. Sorption isotherms for the meso and racemic isomers of these chelates on the soil materials are described. The variability of sorption with pH in peat and ferrihydrite was also determined because both have variable surface charge at different pH values. In montmorillonite, at low concentrations, the retention of Fe from the Fe-EDDHMA chelate is greater than the one of the Fe-EDDHA chelate. As well as the concentration increased, the inverse situation occurs. The behavior of both meso and racemic isomers of chelates in contact with Ca-montmorillonite is similar. The Fe-meso-EDDHA isomer was highly adsorbed on ferrihydrite, but the racemic isomer is not significantly retained by this oxide. For Fe-EDDHMA isomers, the racemic isomer was more retained by the oxide, but a small sorption of the racemic isomer was also observed. Results suggest that Fe-EDDHA chelates were more retained in peat than Fe-EDDHMA chelates. The most retained isomer of Fe-EDDHA was the meso isomer. For Fe-EDDHMA, the adsorption was very low for both racemic and meso isomers.     

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.     

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.     

Interaction of Oxidation Products from Caffeic Acid with Fe(III) and Fe(II)

Abstract

Phenolic substances in the soil–plant system can be oxidized by metal ions, inorganic components, molecular oxygen as well as by phenoloxidases, giving rise to the formation of products of low or high molecular weight. Interactions of these products with iron, in both reduced and oxidized form, can affect the iron mobility in soil and rhizosphere, and thus its availability to plants. Here we report the results of a study on the complexing and reducing activity of the oxidation products from caffeic acid (CAF), obtained via electrochemical means, towards Fe(III) and Fe(II) in aqueous solution in the 3.0–6.0 pH range. The HPLC analysis of the filtered solutions after the CAF oxidation showed the formation of two main groups of products: (i) CAF oligomers formed through radicalic reactions which do not involve the double bond of the CAF lateral chain and (ii) products where this bond is involved. These oxidation products (COP) were found to interact with both Fe(III) and Fe(II) with formation of soluble and insoluble Fe(III)‐, and Fe(II)‐COP complexes. The COP were found to be able to reduce Fe(III) to Fe(II) mainly at pH < 4.0. A low redox activity was observed at pH ≥ 4.5 due to Fe(III) hydrolysis reactions as well as to the decrease in the redox potential of the Fe(III)/Fe(II) couple. Formation of hydroxy Fe(III)‐COP polymers occurs at pH > 3.5.     

Lactic acid decreases Fe(II) and Fe(III) retention but increases Fe(III) transepithelial transfer by Caco-2 cells

Lactic acid (LA) has been proposed to be an enhancer for dietary iron absorption, but contradictory results have also been reported. In the present study, fully differentiated Caco-2 cell monolayers were used to evaluate the effects of LA (1-50 mmol/L) on the cellular retention and transepithelial transport of soluble non-heme iron (as ferric nitrilotriacetate). Our data revealed a linear decline in Fe(III) retention with respect to the concentration of LA added. In the presence of 50 mmol/L LA, retention of Fe(III) and Fe(II) decreased 57% and 58%, respectively. In contrast, transfer of Fe(III) across the cell monolayer was doubled, while Fe(II) transfer across the cell monolayer decreased 35%. We conclude that LA reduces cellular retention and transepithelial transport of Fe(II) by Caco-2 cells in a dose-dependent manner. However, while LA also reduces retention of Fe(III) by Caco-2 cells, the transfer of Fe(III) across cell monolayers is enhanced, possibly due to effects on paracellular transport.     

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.     

Alleviation of Alkalinity-Induced Fe Deficiency in Eggplant (Solanum melongena L.) by Foliar Application of Different Fe Sources in Recirculating System

This study was carried out to investigate the effects of foliar sprays of different iron (Fe) sources on eggplant grown in alkaline aquaponic solutions. Four treatments were used, untreated control, foliar application of iron sulfate (FeSO₄), ferric ethylenediaminetetraacetic acid (Fe-EDTA) and ferric ethylenediamine bis(2-hydroxyphenyl)acetic acid (Fe-EDDHA). The results showed that overall growth was significantly increased by foliar Fe application, and the highest values of vegetative growth parameters were recorded in plants treated with FeSO₄. The Fe treatment led to a significant increase of shoot Fe concentration, and the highest Fe was observed in plants sprayed with FeSO₄, compared to Fe-EDTA and Fe-EDDHA. The lowest chlorophyll content was observed in untreated plants. The highest SPAD index, maximal quantum yield of photosystem (PS II) photochemistry (F_v/F_m) and performance index (PI) values of young and old leaves were found with FeSO₄ treatment. It is concluded that application of foliar Fe must be performed in the aquaponic system, to overcome Fe deficiencies in alkaline conditions.     

Response of Cucumber Plants to Low Doses of Different Synthetic Iron Chelates in Hydroponics

The factors that control the use of iron (Fe) provided by iron chelates in strategy I plants are not well known. In this paper, the effectiveness of low concentrations of a series of pure Fe chelates to supply Fe to cucumber plants in hydroponics was studied. The Fe Chelate Reductase (FCR) of the roots was measured using Fe- ethylene diamine tetraacetic acid (EDTA) as substrate. Despite the differences found in SPAD and biometric indexes among the treatments, FCR and Fe in xylem sap were only significantly larger for the Fe- Ethylene diamine di-(o-hydroxy-p-methylphenyl) acetic acid (EDDHMA) treatment. The trend in nutritional indexes was the opposite to the trend in the stability of the chelates, except for Fe-EDTA that gave the poorest results. A mechanism describing the uptake process, considering the re-oxidation of the Fe (II) reduced by the FCR and the formation of the Fe (II) complex is proposed.     

活性铁作为植物铁营养状况诊断指标的相关研究

通过研究铁供应状况和不同形态氮素对植物铁营养状况的影响,比较分析组织中全铁及活性铁含量与叶片叶绿素含量、过氧化物酶及过氧化氢酶活性的相关性,认为活性铁含量比全铁含量能更准确地反映植物的铁营养状况,尤其是对于双子叶植物,有望发展成为诊断植物铁营养状况的可行指标。     

Interaction of Different Iron Chelates with an Alkaline and Calcareous Soil: A Complementary Methodology to Evaluate the Performance of Iron Compounds in the Correction of Iron Chlorosis

Abstract

A great number of studies have shown that the stability of iron chelates as a function of pH is not the unique parameter that must be considered in order to evaluate the potential effectiveness of Fe‐chelates to correct iron chlorosis in plants cultivated in alkaline and calcareous soils. In fact, other factors, such as soil sorption on soil components or the competition among Fe and other metallic cations for the chelating agent in soil solution, have a considerable influence on the capacity of iron chelates to maintain iron in soil solution available to plants. In this context, the aim of this work is to study the variation in concentration of the main iron chelates employed by farmers under field conditions—Fe‐EDDHA (HA), Fe‐EDDHMA (MA), Fe‐EDDHSA (SA), Fe‐EDDCHA (CA), Fe‐EDTA (EDTA), and Fe‐DTPA (DTPA)—in the soil solution of a calcareous soil over time. To this end, soil incubations were carried out using a soil:Fe solution ratio corresponding to soil field capacity, at a temperature of 23°C. The soil used in the experiments was a calcareous soil with a very low organic matter content. The variation in concentration of Fe and Fe‐chelates in soil solution over time were obtained by measuring the evolution in soil solution of both the concentration of total Fe (measured by AAS), and the concentration of the ortho‐ortho isomers for Fe‐EDDHA and analogs or chelated Fe for Fe‐EDTA and Fe‐DTPA (measured by HPLC). The following chelate samples were used: a HA standard prepared in the laboratory and samples of HA, MA, SA, CA, Fe‐EDTA, and Fe‐DTPA obtained from commercial formulations present in the market. The percentage of iron chelated as ortho‐ortho isomers for HAs was: HA standard (100%); HA (51.78%); MA (60.06%); SA (22.50%); and CA (27.28%). In the case of Fe‐EDTA and Fe‐DTPA the percentages of chelated iron were 96.09 and 99.12, respectively. Results show that it is possible to classify the potential effectiveness of the different types of iron chelates used in our experiments as a function of two practical approaches: (i) considering the variation of total iron in soil solution over time, MA is the best performing product, followed by HA, CA, SA, DTPA, EDTA, and ferrous sulfate in the order listed and (ii) considering the capacity of the different iron chelates to maintain the fraction of chelated iron (ortho‐ortho isomers for HA, MA, SA, and CA and total chelated iron for EDTA and DTPA) in soil solution, the order is: SA > CA > HA > MA > EDTA ≈ DTPA. This result, that is related to the nature of the chelate and does not depend on the degree of chelated Fe in the products, indicates that SA and CA might be very efficient products to correct iron chlorosis. Finally, our results also indicate the suitability of this soil incubation methodology to evaluate the potential efficiency of iron compounds to correct iron chlorosis.     

Resistance of mineral soils to Fe(III) reduction

Gleying and enhancement of hydromorphism in wetland soils due to Fe(III) reduction entail a series of degradation processes. The resistance of wetlands to degradation can be calculated from the content of potentially reducible iron, Fe(III)_pr, which is found from the van Bodegom equation taking into account the contents of oxalate-soluble iron Fe_ox and dithionite-soluble iron Fe_dit in the soil. In addition, this makes it possible to distinguish relict and actual gleysols. The van Bodegom equation is applicable to soils from which the oxalate solution extracts only amorphous and poorly crystallized iron compounds, which are quickly reduced by Fe-reducing bacteria. These soils have a low proportion of Fe(II) (no more that 15% of the total iron), as well as an accumulative profile distribution of Fe_ox. The van Bodegom equation is unsuitable for calculating the Fe(III)_pr content in soils with a high proportion of Fe(II) and a nonaccumulative profile distribution of Fe_ox.