Aluminum and iron(III) species in the soil solution including organic complexes with citrate and humic substances

The solubility of Al and Fe in soil is of relevance for their toxicity and availability, respectively, to plant roots. Humic substances as the main part of stable soil organic matter and citrate which is often excreted by P deficient plants are strong complexants of Al and Fe(III). Therefore, equations were developed to calculate the Al and Fe(III) species distribution in the soil solution in the presence of humic substances and citrate as organic ligands. Calculations in the pH range 4.0–7.0 showed that at higher pH humic-Al complexes were the most important species whereas AlOH-citrate^? dominated between pH 4.0 and 5.4. Free monomeric Al and AlSO₄⁺ were of minor relevance. Iron(III) species calculations showed that humic-Fe complexes were the main species in the pH range 4.0–7.0. But if mugineic acid, a Fe complexing phytosiderophore released into the rhizosphere by graminaceous plant species, was present in the soil solution (10^?6 M), Fe-mugineic acid complexes accounted for most of the Fe in solution. Fe-citrate^? was relevant at lower pH but contributed little to Fe(III) species at pH > 6.0. The results demonstrate the strong importance of the considered organic ligands for Fe and Al in the soil solution.     

A Study of Diffusive Gradients in Thin Films for the Chemical Speciation of Zn(II), Cd(II), Pb(II), and Cu(II): The Role of Kinetics

The lability and mobility of Zn(II)–, Cd(II)–, Pb(II)–, and Cu(II)–humic acid complexes were studied using diffusive gradients in thin films (DGT). A unique feature of this research was (1) the use of DGTs with diffusive layer thicknesses ranging from 0.4 to 2.0 mm to study lability and mobility of Zn(II)–, Cd(II)–, Pb(II)–, and Cu(II)–humic acid complexes, combined with (2) the application of a competing ligand exchange (CLE) method using Chelex 100, the same chelating resin that is used in DGT, to study the kinetic speciation. The CLE experiments were run immediately after the completion of the DGT experiments, thereby allowing effects of the competing ligand to be separated from the effects introduced by the use of the polyacrylamide gel that is used in DGT. The results indicate that Zn(II) and Cd(II) tend to form more labile and more mobile complexes with humic acid than Pb(II) or Cu(II). The dissociation rate constants of Zn(II), Cd(II), and Pb(II) were found to increase with the ionic potential of the metal, suggesting that the binding between some trace metals and humic acid has a significant covalent component. Furthermore, the results suggest that the Eigen mechanism may not be strictly obeyed for metals such as Cu(II) which have high rate constants of water exchange, k _w. Consequently, the markedly slow kinetics of Cu(II)-HA species suggests that the usual equilibrium assumption may not be valid in freshwaters.     

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.     

Some structural and electronic features of the interaction of phosphate with metal-humic complexes

Previous studies demonstrated the formation of stable phosphate-metal-humic complexes in solution. These studies, however, indicated that the proportion of complexed metal that intervenes in phosphate fixation is rather low. In this study we investigate the possible structural and electronic features of the binding site involved in phosphate fixation in metal-humic complexes that could explain this fact. To this end, we have studied phosphate-metal-humic complexes involving Fe(III), Al(III), and Zn(II) using three complementary techniques: infrared spectroscopy (FTIR), fluorescence, and molecular modeling. The FTIR study indicated that, in the case of those complexes involving Fe and Zn phosphate, fixation is associated with a stabilization of the metal-carboxylate bond. In the case of Al this effect is less clear. This effect of phosphate fixation on the characteristics of the metal-humic binding site was also supported by the results obtained in the Fluorescence study, which showed significant changes in the quenching effect normally associated with metal complexation in humic substances upon phosphate fixation. Finally, the molecular modeling study revealed that the stability of phosphate-metal-humic complexes is inversely related to the stability of the metal-humic interaction. This result could explain why only a relatively low proportion of humic complexed metal is involved in phosphate fixation.     

Iron(III) reduction by natural humic acids: a potentiometric and spectroscopic study

The reduction of iron(III) by natural humic acid (NHA) was studied in aqueous solution as a function of pH, time and iron(III) concentration. The information gained from FTIR and ESR spectroscopies as well as potentiometric data suggests that redox reactions occur at a low pH due to the involvement of phenolic groups and radicals. At pH values higher than 3.5 the reaction is strongly inhibited by the formation of iron(III)–humate complexes.     

On the Probable Nature of Biological Activity of Humic Substances

It is common knowledge that humic substances extracted from different sources are characterized by high biological activity (BA), though the nature of this phenomenon is not quite clear up to now. To expand our understanding of the BA of humic substances, we studied the effect of humates prepared from humic acids of different origin on the germination of seeds. The efficiencies of seed treatment by humate solutions obtained from preliminary purified humic acids extracted from peat, coal, and soil differed insignificantly. At the same time, the solutions of salts of humic substances obtained via alkaline extraction from peat without subsequent purification did not lead to statistically significant biological effects. The analysis of literature and our own data allowed us to conclude that the biological activity of humic acids could be related to their capacity to regulate the growth processes via binding growth inhibitors released into the solution upon seed swelling into the supramolecular complexes.     

A Study on Al(III) and Fe(II) Ions Sorption by Cattle Manure Vermicompost

Cattle manure vermicompost has been used for the adsorption of Al(III) and Fe(II) from both synthetic solution and kaolin industry wastewater. The optimum conditions for Al(III) and Fe(II) adsorption at pH?2 (natural pH of the wastewater) were particle size of ≤250?µm, 1 g/10 mL adsorbent dose, contact time of 4 h, and temperature of 25°C. Langmuir and Freundlich adsorption isotherms fitted reasonably well in the experimental data, and their constants were evaluated, with R ² values from 0.90 to 0.98. In synthetic solution, the maximum adsorption capacity of the vermicompost for Al(III) was 8.35 mg g^?1 and for Fe(II) was 16.98 mg g^?1 at 25°C when the vermicompost dose was 1 g 10 mL^?1, and the initial adjusted pH was 2. The batch adsorption studies of Al(III) and Fe(II) on vermicompost using kaolin wastewater have shown that the maximum adsorption capacities were 1.10 and 4.30 mg g^?1, respectively, at pH?2. The thermodynamic parameter, the Gibbs free energy, was calculated for each system, and the negative values obtained confirm that the adsorption processes were spontaneous.     

Reactions of fulvic acid with metal ions

Fulvic acid is a water-soluble humic material that occurs widely in soils and waters and that tends to form water-soluble and water-insoluble complexes with a variety of metal ions, some of which are toxic. This paper presents information on the conditions under which the different types of FA-metal complexes are formed. The solubility in water, separately and after mixing, of FA (2 to 30 mg/100 ml) and eleven metal ions (Fe(III), Al, Cr(III), Pb, Cu, Hg(II), Zn, Ni, Co, Cd and Mn; 1 × 10^?5 moles of each metal ion) was investigated over the pH range 4 to 9. After mixing, the solubility of the components was significantly affected by pH only when less than 20 mg of FA was present. As the systems became richer in FA (22 to 30 mg), most of the metal ions remained in the aqueous phase, likely due to the formation of FA-metal complexes, inhibiting the formation of metal hydroxides. The order in which the eleven metal ions tended to form water-insoluble FA-metal complexes depended on the pH. At pH 6 it was: Fe = Cr = Al > Pb = Cu > Hg > Zn = Ni = Co = Cd = Mn. This order appeared to correlate with the valence, 1st hydrolysis constants and effective hydrated ionic diameters of the metal ions. In general, FA/metals weight ratios of > 2 favored the formation of water-soluble FA-metal complexes; at lower ratios, water-insoluble complexes, which could accumulate in soils and sediments, were formed.     

Effect of acid deposition on the displacement of Al(III) in soils

This study represents an assessment of some of the key factors influencing the mobility of Al(III) and its displacement in acid soils. This assessment is based on the effect of pH and other solution variables on the solubility of Al(III) and its complex formation with OH^?, F^? and organic ligands (fulvates and humates). Above all, the adsorption behavior of Al(III) on iron(III) (hydr)oxides and on SiO₂ on one hand, and the adsorption of organic acids and of humic substances on mineral surfaces on the other hand was investigated. Adsorption is interpreted in terms of surface complex formation equilibria; the mass law constants derived permit the modeling of adsorption as a function of solution variables. It is illustrated that the distribution of oxides and hydroxides in the soil profile affects the pH buffering, determines the mobility of the organic acids while the mobility of Al(III) is primarily governed by the formation and dissolution of Al(III) (hydr)oxides.     

Competition from Cu(II), Zn(II) and Cd(II) in Pb(II) Binding to Suwannee River Fulvic Acid

This is a study of trace metal competition in the complexation of Pb(II) by well-characterized humic substances, namely Suwannee River Fulvic Acid (SRFA) in model solutions. It was found that Cu(II) seems to compete with Pb(II) for strong binding sites of SRFA when present at the same concentration as Pb(II). However, Cd(II) and Zn(II) did not seem to compete with Pb(II) for strong binding sites of SRFA. These two metals did compete with Pb(II) for the weaker binding sites of SRFA. Heterogeneity of SRFA was found to play a crucial role in metal–SRFA interactions. The environmental significance of this research for freshwater is that even at relatively low Pb(II) loadings, the metals associated with lead in minerals, e.g. Cu(II), may successfully compete with Pb(II) for the same binding sites of the naturally occurring organic complexants, with the result that some of the Pb(II) may exist as free Pb²⁺ ions, which has been reported to be one of the toxic forms of Pb in aquatic environment.     

Co-adsorption of Cd(II) and Sb(III) by ferrihydrite: a combined XPS and ITC study

Purpose

Heavy metal and metalloid commonly coexist in soils and sediments, and interact frequently with various minerals. The coexistence of Sb and Cd is commonly observed in Sb mine area, but their co-adsorption behaviors to soil minerals still remain poorly understood. This study aimed to elucidate the co-adsorption characteristics of Cd(II) and Sb(III) by ferrihydrite (Fh) under anoxic condition.

Materials and methods

Batch experiments were performed to determine the sorption capacity of Cd(II) and Sb(III) in both single and binary systems. The major functional groups that were responsible for Cd(II) and Sb(III) sorption were determined by X-ray photoelectron spectroscopy (XPS), while the thermodynamic sorption mechanisms were elucidated using isothermal titration calorimetry.

Results and discussion

Cd(II) sorption on Fh increases with increasing pH levels (4–8) whereas Sb(III) sorption shows less variation with pH level variations. The Langmuir adsorption capacity is 55.54 mg/g for Cd(II) and 188.19 mg/g for Sb(III). In Cd–Sb binary systems, Cd(II) sorption is significantly diminished whereas Sb(III) uptake is close to single Sb(III) sorption. XPS indicates the Fe–OH groups are mainly responsible for the binding of Cd and Sb, possibly through the formation of inner-sphere complexes. This hypothesis is further confirmed by the positive entropy (ΔS) after Cd and/or Sb binding. A larger ΔS in the binary Cd–Sb titration than in their single titrations implies the formation of a ternary Fh–Sb–Cd complex, which results in a higher disorder of the sorption system.

Conclusions

The presence of Sb(III) reduces Cd(II) sorption whereas Cd(II) has a negligible effect on Sb(III) sorption to ferrihydrite; moreover, Sb(III) and Cd(II) might form surface ternary complexes in binary systems. These new findings have important implications for predicting the sequestration, migration, and fate of Cd and Sb in soils.

     

Chemical characteristics and microbial degradation of humate

Abstract

Humates are often used in agriculture as a source of organic matter. This study was conducted to characterize a commercial humate and to evaluate its chemical and decomposition characteristics. Characterization methods included fractionization of humic and fulvic acids of the humate, based on their alkali/acid insolubility; elemental analysis; acidic functional group analysis; and E₄/E₆ ratio determinations. The humate consisted of the following: 58% organic matter, 32% ash, and 10% moisture. Humic fraction was mostly humic acid (76%), with some fulvic acid (18%). Organic elemental composition [59% carbon (C), 5% hydrogen (H), and 36% oxygen (O)] also suggests a humic‐acid nature. Inorganic elemental content of this humate, which was primarily aluminum (Al) [4.9%] and iron (Fe) [0.46%], reflects its spodic origin. Much of the Al present, however, results from the flocculant (alum) used at the mining site to precipitate the humate. The relatively low total acidity of this humate (250 cmol/kg) suggests the blockage of some of its functional groups by Al, Fe, and associated clay minerals. After purification, total acidity increased to 510 cmol/kg and acidity associated with carboxyl groups increased to 280 cmol/kg. Results of E₄/E₆ determinations for the humate (2.5) and the humic‐acid fraction (4.8) also suggested that the organic fraction was predominately humic acid. Decomposition of the humate was estimated by measuring the quantity of carbon dioxide (CO₂) evolved during a four‐week incubation. Results suggested a relative resistance to microbial degradation. However, results also suggested the presence of some readily decomposable C compounds associated with humate. Agricultural use of this humate reguires some modifications to produce a more reactive material. This may include a change of flocculant and a lowering of its ash content.     

Cation binding by acid-washed peat, interpreted with Humic Ion-Binding Model VI-FD

A sample of ombrotrophic peat from Moor House in northern England was extensively extracted with dilute nitric acid (pH 1) to free it of bound cations. Suspensions of the acid‐washed peat (5–30 g l^?1), prepared with different concentrations of background electrolyte (NaCl and KCl), were used to conduct batch acid–base titrations. A strong dependence of proton release on ionic strength (I) was observed, the apparent acid dissociation constant (pK_app) being found to decrease by approximately 1.0 for each tenfold increase in I. This behaviour could not be explained satisfactorily with Humic Ion‐Binding Model VI, a discrete‐site/electrostatic model of cation binding by humic substances, parameterized with data from laboratory studies on isolated samples. More success was obtained by abandoning the impermeable‐sphere electrostatic submodel used in Model VI, and instead assuming the peat to consist of aggregates with fixed internal volume, and with counterion accumulation described by the Donnan model, as proposed by Marinsky and colleagues. The fixed‐volume Donnan model (Model VI‐FD) could also approximately explain other reported results from acid–base titrations of peat, including the effects on the titrations of complexing cations (Al, Ca, Cu). Copper titrations of the Moor House sample were performed using an ion‐selective electrode, with peat suspensions in the acid pH range, at two ionic strengths, and in the presence of Al and Ca. The measured concentrations of Cu²⁺ were in the range 10^?13?10^?5 m . Model VI‐FD provided reasonable fits of the experimental data, after optimization of the intrinsic binding constant for Cu, the optimized value being close to the default value derived previously from data referring to isolated humic substances. The optimized constants for Al and Ca, derived from their competition effects, were also close to their default values. Additional experiments were performed in which the centrifugation‐depletion method was used to measure the binding of a cocktail of metals (Al, Ni, Cu, Zn, Cd, Eu, Pb) at a single pH. The model correctly predicted strong binding of Al, Cu, Eu and Pb, and weaker binding of Ni, Zn and Cd. For the strongly binding metals, the dissolved forms were calculated to be mainly due to complexes with dissolved humic matter, whereas the free ions (Ni²⁺, Zn²⁺, Cd²⁺) dominated for the weakly binding metals. Acid‐washed soil appears to provide a valuable intermediate between isolated humic substances and untreated soil for the investigation of cation binding by natural organic matter in the natural environment.     

Characterization of Al/Fe—humus complexes in dystrandepts through comparison with synthetic forms

The Al/Fe—humus complexes in A₁ horizons of Dystrandepts from several parts of Japan were compared with synthetic complexes prepared from hydroxy ions and from two kinds of humic substances and at several pH values. Samples represented 26 A₁ horizons of soils differing in age, including some that had been buried. The comparisons were based chiefly on extractions of the synthetic complexes and the soil samples with solutions of sodium hydroxide-tetraborate and sodium pyrophosphate.The Al—humus complexes in the Dystrandept samples appeared to be similar to those of synthetic complexes prepared at pH 4–5. Those have OH/Al molar ratios of 0.7–2.5 1. Distribution of the data for the soil samples suggested further that the proportions of polymeric hydroxy Al ions increased as soils became older and also if they were buried. The Fe—humus complexes in the soil samples seemed to be like the synthetic complexes prepared with Fe and humus at pH 4–5 for the most part.     

Effect of Metal Ions on the Entomopathogenic Nematode Heterorhabditis Bacteriophora Poinar (Nematoda: Heterohabditidae) Under Laboratory Conditions

The effect of sixteen metal ions: Al, Cd, Co(II), Cr(III), Cr(VI), Cu(II), Fe(III), Li, Mg, Mn(II), Mo(VI), Ni(II), Pb(II), Se(IV), V(V), and Zn on the mortality and infectivity ofHeterorhabditis bacteriophora were observed over a 96 hr period. All ions except Pb(II) even at naturally unrealistic concentrations did not cause the mortality of the nematodes. A weak vitalizing effect could eventually be observed with Mn(II), Mg, Fe(III) and Ni(II) (Table 1). However, such treatment generally lowered infectivity of the nematodes with respect to wax moth caterpillars.Galleria mellonella. This effect was particularly significant with Ni(II) and Pb(II).     

Activite des complexes acides humiques-invertase: Influence du mode de preparation

During the preparation of insoluble humic acid-invertase-Ca²⁺ complexes, the humic acid, enzyme and Ca²⁺ concentrations were varied as well as the time of contact between the organic and the cation, and the order in which these three substances were added. The enzymatic activities of these complexes are very different. In complexes obtained at pH 7.0, adding humic acids first, then invertase and finally Ca²⁺, the enzyme molecules probably occur in two forms: some may be fixed on the surface, either by adsorption or covalence, the others may be trapped in a micellar net. The initial catalytic activity and lifetime of the complexes depends on the relative abundance of the three substances and on the time of contact. The kinetics of the first type of complex is divided into three parts. During the first very short period, a strong inactivation is noticed which can be explained in terms of three phenomena: separation of some micellae bound to enzyme molecules, discharge of trapped invertase molecules because of destructuring of the net after loss of micellae, desorption of the enzyme. The intermediate phase corresponds to a greater cohesion of the net since the loss of organic matter is reduced and the inactivation is slower; its length depends on the preparation. The final phase begins when the complex is free from labile fractions: it represents the residual activity of the complex and develops very slowly.If insoluble humate suspensions are used rather than humic acids solutions, the formation of humateinvertase complexes proceeds essentially by adsorption at the surface of the support. The kinetics of the catalytic activity of such enzymatic complexes is similar to that of desorption.     

Characterization of the Fe(III)—fulvic acid reaction by Mössbauer spectroscopy

Fulvic acids have been isolated from a sandy loam (Countesswells series) and a clay soil (Tipperty series) and the products of their reaction with different amounts of iron over a range of pH from 0.5 to 11 analyzed by Mössbauer spectroscopy. Three distinct types of spectral component were detected at 77 K, a sextet from magnetically dilute Fe(III) and doublets from Fe(II) and Fe(III), the last arising from both organic complexes and poorly crystalline oxide species. In iron-fulvic acid mixtures the proportion of iron as Fe(II) increased as the pH was lowered from 5 to 1 by the addition of hydrochloric or nitric acid at all Fe to fulvic acid ratios (1:5 to 1:500). When the pH was lowered below 1 the amounts of Fe(II) decreased with the lower Fe to fulvic acid ratios, but increased with the higher ratios. The amounts of the Fe(III) component contributing to a doublet signal decreased with decreasing Fe:fulvic acid ratios. At low iron concentrations the iron appears to be strongly bound to the fulvic acid, but when the iron content is of the order of 1–2% uncomplexed Fe(III) species can be present. At pH > 2 these are hydrolysed ions which form poorly-crystalline oxides at higher pH. This was confirmed by analysis of spectra at 4.2 K. At pH < 2 free ions are present in solution. In solutions with high fulvic acid contents (greater than 100-fold excess) the reactions with iron are completely reversible, but in solutions with a lower proportion of fulvic acid to iron, where free ions are present, there is a lack of reversibility.     

Aluminum status of synthetic Al–humic substance complexes and their influence on plant root growth

Abstract

Aluminum (Al)–humus complexes are abundant in the A horizons of non-allophanic Andosols and contribute to the unique properties of volcanic ash soils, such as high reactivity with phosphate ions and a low bulk density. Natural non-allophanic Andosols commonly show Al toxicity to plant roots. There have been very few studies examining the contribution of Al–humus complexes to the Al toxicity of plant roots, although the complexes are the probable source of the toxic Al. We extracted humic substances from the A horizon of a non-allophanic Andosol using NaOH solution and reacted the humic substances and partially neutralized the AlCl₃ solution at three pH conditions (pH 4.0, 4.5 and 5.5) to prepare pure Al–humic substance complexes. The Al solubility study (equilibrium study in 10^?2 mol L^?1 CaCl₂) and the Al release study (a stirred-flow method using 10^?3 mol L^?1 acetate buffer solution adjusted to pH 3.5) indicated that all the synthetic complexes easily and rapidly release monomeric Al into the liquid phase with slight changes in pH and ion strength, although the Al contents and their extent of polymerization are considerably different among the complexes. A plant growth test was conducted using a medium containing the Al–humic substance complexes and perlite mixture. Root growth in burdock (Arctium lappa) and barley (Hordeum vulgare L.) was reduced equally by all three complex media, and the roots showed the typical injury symptoms of Al toxicity. These results indicate that in soils dominated by Al–humus complexes the Al released from the Al–humus complexes, as well as the exchangeable Al adsorbed by soil minerals, is definitely toxic to plant roots.     

The metal — Metal interactions in biological systems Part III. Daphnia magna

The toxicity of single metal ions: Al, Co(II), Cr(III), Cu(II), Fe(III), Mg, Mn(II), Mo(VI), Ni(II), Se(VI), V(V) and Zn and the following pairs of them: Al-Co, Al-Mg, Al-Mo, Al-Se, Al-Zn, Cr-Co, Cr-Mg, Cr-Mo, Cr-Se, Cr-Zn, Cu-Co, Cu-Mg, Cu-Mo, Cu-Se, Cu-Zn, Fe-Co, Fe-Mg, Fe-Mo, Fe-Se, Fe-Zn, Mn-Co, Mn-Mg, Mn-Mo, Mn-Se, Mn-Zn, Ni-Co, Ni-Mg, Ni-Mo, Ni-Se, Ni-Zn, V-Co, V-Mg, V-Mo, V-Se, V-Zn, Zn-Co, Zn-Mg, Zn-Mo, and Zn-Se on Daphnia magna was examined. The most prominent antagonism in the toxicity was observed in the following ion pairs: Al-Mo(VI), Cr(III)-Co(II), Cr(III)-Mg, Cr(III)-Mo(VI), Cr(III)-Se(VI), Cr(III)-Zn, Fe(III)-Se(VI), Mn(II)-Mg, Mn(II-Se(VI), Zn-Mg and Zn-Mo(VI). The strong synergism was found for the following ion systems: Cr(III)-Se(VI), Cr(III)-Zn, Fe(III)-Se(VI), Mn(II)-Zn, Mn(II)-Se(VI), Ni(II)-Co(II), Ni(II)-Mo(VI), Ni(II)-Se(VI), Ni(II)-Zn, V(V)-Co(II), V(V)-Mo(VI), V(V)- Se(VI), and V(V)-Zn. Synergism and antagonism in toxicity were dependent on water hardness as well as on the ion concentration. Adaptive procesess of the animals to the toxic environment could also be observed. Thus, the toxicity of the single ions and their pairs was not linear with respect to time.     

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.