Interactions of aluminium and fulvic acid in moderately acid solutions: stoichiometry of the H+/Al3+ exchange

Complexation with organic matter controls the activity of dissolved Al³⁺ in many soils. The buffering intensity of these soils is largely dependent on the H⁺/Al³⁺ exchange ratio, i.e. the number of protons consumed by the solid phase when one Al³⁺ is released. Here, the H⁺/Al³⁺ exchange ratio was determined from batch titrations using solutions of fulvic acid (FA) as a model for soil organic matter. Aluminium was added, from 1.04 to 6.29 mmol Al per g FA, which is within the range of humus‐bound Al found in the upper B horizon of podzolized soils. Furthermore, pH was varied with NaOH to give values between 3.5 and 5.0. The H⁺/Al³⁺ exchange ratio ranged between 1.49 and 2.23 with a mean of 1.94. It correlated positively with pH and the total concentration of Al present. Theoretically, this can be explained with a partial hydrolysis of bound Al. The slope of logAl (log₁₀ of Al³⁺ activity) against pH generally underestimated the actual exchange ratio, which can partly be attributed to the systems being diluted (100 mg FA l^?1). However, where 4 mmol Al or more had been added per g FA, the logAl slope gradually approached ?3 between pH 4.5 and 5.0. This might be the result of a shift from Al³⁺ activity control by humus complexation to control by Al(OH)₃(s).     

Solubility characteristics of proto-imogolite sols: how silicic acid can de-toxify aluminium solutions

Proto-imogolite sols can be considered as highly dispersed forms of proto-imogolite allophane, the most widespread type of allophane in volcanic and non-volcanic soils world-wide. The solubility characteristics of such sols define the conditions of precipitation of allophanes in soils, and the maximum concentrations of aluminium released during acidic episodes from soils, such as podzols, that contain allophane. Direct measurement of Al, Si and pH values in equilibrium with proto-imogolite sols, approached from higher and lower pH, indicated a solubility equation: where log*K_so lay in the range 7.14 to 7.23 after equilibration for 4–24 weeks at 22 + 2°C in 17 of the 20 systems studied. The mean value of log *K_SO at 298 K was calculated as 7.02. This value indicates that proto-imogolite will be more stable than amorphous aluminium hydroxides at H₄SiO₄ concentrations above 5 × 10^?6m , but less stable than bayerite below 10^?3m H₄SiO₄, and than gibbsite below 10^?2m . Proto-imogolite is more stable than micro-crystalline gibbsite in 10^?4m H₄SiO₄, a typical minimum concentration in soil solutions and streams in landscapes where podzols are present. The rapid formation of proto-imogolite effectively prevents the formation of gibbsite seeds in soil, except in highly leached and warm environments, i.e. in older landscapes in the tropics. Although the presence of 10^?4m silicic acid has been found to eliminate the acute toxicity to fish exhibited by solutions containing 6–7 μm Al at pH 4.96, little or no proto-imogolite would form under these conditions. Silicic acid would, however, prevent the precipitation of aluminium hydroxides, and could inhibit the formation of the A1₁₃ polycation. These polymeric species are a likely cause of the increased toxicity exhibited by partially neutralized aluminium solutions.     

The H+/M2+ exchange stoichiometry of calcium and zinc adsorption by ferrihydrite

The specific adsorption of Ca²⁺ and Zn²⁺ by ferrihydrite results in the net release of H⁺. The rate and H⁺/M²⁺ exchange stoichiometry of this reaction were monitored with a pH-stat. A rapid reaction of less than 6 min was followed by a slower reaction which continued at a diminishing rate for at least 2 days. Adsorption of Ca²⁺ at pH 7.8 and Zn²⁺ at pH 5.4 resulted in the net release of 0.92 and 1.70 mol H⁺/mol M²⁺ adsorbed, respectively. For Zn²⁺ adsorption, this stoichiometry was shown to be independent of pH. These estimates agree well with independent estimates based on the pH dependence of adsorption. The difference between the Ca²⁺ and Zn²⁺ stoichiometries was related to the differing acidity of the –OH₂ ligands attached to the adsorbed ions.     

Evidence that fungi can oxidize NH4+ to NO3− in a grassland soil

The contribution of bacteria and fungi to NH₄⁺ and organic N (N_org) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g^?1 oven‐dry soil. The size and enrichment of the mineral N pools from differentially (NH₄¹⁵NO₃ and ¹⁵NH₄NO₃) and doubly labelled (¹⁵NH₄¹⁵NO₃) NH₄NO₃ were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH₄⁺‐N and NO₃^?‐N at 3.15 μmol N g^?1 oven‐dry soil. The N treatments were enriched to 60 atom % excess in ¹⁵N and acetate was added at 100 μmol C g^?1 oven‐dry soil, to provide a readily available carbon source. The oxidation rates of NH₄⁺ and N_org were analysed separately for each inhibitor treatment with a ¹⁵N tracing model. In the absence of inhibitors, the rates of NH₄⁺ oxidation and organic N oxidation were 0.0045 μmol N g^?1 hour^?1 and 0.0023 μmol N g^?1 hour^?1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH₄⁺ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH₄⁺ and organic N.     

Chemical and colloidal stability of sols in the Al2O3-Fe2O3-SiO2-H2O system: their role in podzolization

Concentrations of dialysable silica in equilibrium with Al₂O₃-SiO₂-H₂O sols at pH 4.5–5.0 confirm the formation of a poorly ordered non-dialysable proto-imogolite species with an Al : Si ratio near 2, close to that of imogolite. Sols with Al : Si>2 give nearly constant levels of free silica in solution in the range 2–6 μg/cm³, indicating equilibrium between proto-imogolite and aluminium hydroxide species. These findings indicate that imogolite-like precipitates in acid soils will buffer silica in solution to within this range during leaching episodes. Imogolite is more stable than a previous estimate suggested, and a revised value for its free energy of formation is proposed: ΔG⁰_f(298.15) = -2929.7 kJ/mol. In Fe₂O₃-SiO₂-H₂O sols, the Fe : Si ratio of the non-dialysable species varies smoothly from 11 to 3 as free silica in solution ranges from 4 to 35 μg/cm³. Such sols are much less colloidally stable than hydroxyaluminium silicate sols, but mixed Al₂O₃—Fe₂O₃—SiO₂—H₂O sols are almost as stable as iron-free sols up to a Fe : Al ratio of 1.5. Thus migration of Al and Fe as mixed hydroxide sols can account for the almost constant ratio of Al to Fe with depth in oxalate extracts from Bs horizons of podzols.     

SYNTHETIC ALLOPHANE AND IMOGOLITE

In order to prepare allophane and imogolite in the laboratory, solutions containing l–2× 10^–3 M orthosilicic acid and 4–0.5 × 10^–3 M A1C1₃ (SiO₂/Al₂O₃ molar ratio; 0.5, 1.0, 2.0, 4.0 and 8.0) were heated at 95–100°C for 113 hours after addition of NaOH (NaOH/Al molar ratio; 1.0, 2.0, 2.8 and 3.0). Boehmite was found in the precipitates from all solutions with initial SiO₂/Al₂O₃ ratios of 0.5. Imogolite was found with allophane II in the products from solutions with SiO₂/Al₂O₃ ratios of 1.0 or greater and with NaOH/Al ratios of 2.8 or less (final pH 5.0), whereas allophane I was found in the precipitates from solutions with the same SiO₂/Al₂O₃ ratios but with the NaOH/Al ratio of 3.0 (final pH = 5.0–6.3). The mode of formation, chemical composition, infrared spectra, electron micrographs, electron diffraction patterns and differential thermal analysis curves of synthetic imogolite and allophanes (I and II) were compared with those of their natural counterparts.     

KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER. IV. ADSORPTION AND DESORPTION OF Pb2+, Cu2+, Cd2+, Zn2+ AND Ca2+ BY PEAT

The equilibria as well as the rates of adsorption and desorption of the ions Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺, and Ca²⁺ by soil organic matter were determined in batch experiments as a function of the amount of metal ions added to an aqueous suspension of HCl-washed peat. Simultaneous determination of the metal ions and hydrogen ions in the solution by atomic absorption spectrophotometry and pH-measurements showed that the adsorption of one divalent metal ion by peat was coupled with the release of two hydrogen ions. Since this equivalent ion-exchange process causes a corresponding increase of the electric conductivity of the solution, the rates of the adsorption and desorption processes were determined by an immersed conductivity electrode. The distribution coefficients show that the selective order for the metal adsorption by peat is Pb²⁺ > Cu²⁺ > Cd²⁺≌ Zn²⁺ > Ca²⁺ in the pH range of 3·5 to 4·5. The slope of -2, as observed in a double logarithmic plot of the distribution coefficients versus the total solution concentration confirms the equivalence of the ion-exchange process of divalent metal ions for monovalent H₃O⁺ -ions in peat. The absolute rates of adsorption, as well as the rates for the fractional attainment of the equilibrium, increase with increasing amounts of metal ions added. This behaviour is also observed for the subsequent desorption of the metal ions by H₃O⁺-ions. At a given amount of metal ions added, the absolute rates of adsorption decrease in the order Pb²⁺ > Cu²⁺ > Cd²⁺ > Zn²⁺ > Ca²⁺, while the rates for the fractional attainment of the equilibrium decrease in the order Ca²⁺ > Zn²⁺≌ Cd²⁺ > Pb²⁺ > Cu²⁺. The half times for adsorption and desorption were in the range of 5 to 15 sec.     

Aluminum compounds in calcium chloride extracts from podzolic soil and their possible sources

Aluminum concentrations in organoaluminum complexes, mineral polymers, Al(H₂O) ₆ ³⁺ , Al(OH)(H₂O) ₅ ²⁺ , Al(OH)₂(H₂O) ₄ ⁺ , AlH₃SiO ₄ ²⁺ , and Al(OH)₃(H₂O) ₃ ⁰ extracted with 0.001 M CaCl₂ from the main genetic horizons of a podzolic soil on two-layered deposits were determined experimentally and calculated from thermodynamic equations. It was found that aluminum bound in organic complexes was predominant in extracts from the AE horizon, and mineral polymer aluminum compounds prevailed in extracts from the E and IIBD horizons. In the AE horizon, organoaluminum compounds were a major source of aluminum, which passed into solution predominantly by exchange reactions. In the E horizon, aluminum hydroxide interlayers in soil chlorites were the main source of aluminum, which passed into solution by dissolution reactions. In extracts from the IIBD horizon, aluminum was solubilized by the dissolution of aluminosilicates inherited from the parent rock.     

Electrokinetic properties of variable charge clays in the presence of an anionic xanthan polysaccharide and inorganic electrolytes

The effect of the electrolytes (MA: NaCl, MgCl₂ , PbCl₂ , and NaH₂PO₄ ) on the polymeric (a) weak-stabilization (PWST), (b) flocculation (PFL); and (c) stabilization (PST) of allophane in the presence of xanthan polysaccharide (GX), was investigated by electrophoretic mobility (EPM) measurements. At pH 6.5 (i.e.p.), with further addition of chlorides, the decrease in the absolute value of the negative EPM (|—EPM|) of allophane in 0.01 mm MA solutions indicated the suppression of PWST and PST, respectively, probably due to the decrease in the repulsive force originating from the negatively charged GX-chains with the addition of MA. By first addition of MA, the decrease in the |—EPM| value of allophane in 0.01 mm NaCl and MgCl₂ solutions also showed the suppression of PWST and PST. In 0.01 mm PbCl₂ and NaH₂PO. solutions, the increase in the EPM and |—EPM| values of allophane reflected the dispersion due to the specific adsorption of Pb₂₊ and H₂PO₄ - ions on the negatively and positively charged sites of the surface, respectively. At pH 4.5, the decrease in the |—EPM| value of allophane in 0.1 mm NaH₂PO₄ solution indicated the suppression of PST due to the specific adsorption of H₂PO₄ - ion on the positively charged surface of the particles. The absence of (i) PWST in the presence of GX and (ii) high stability in the presence of cationic lead species for the flocculated imogolite at pH 8.5 can be attributed to the tubular structure proposed by Cradwick et al. (1972: Nature (London) Phys. Sci., 240, 187-189), namely, to the difficulty in the development of a negative charge on the outer surface of its unit particle.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS. VIII —A THEORY FOR THE PROPAGATION OF CHANGES OF pH IN SOILS

The way pH changes in soil are propagated by movement of acids and bases is described. In acid soils the H₃O⁺-H₂O acid-base pair is most important, while in alkaline soils the H₂CO₃-HCO₃^? pair is always dominant, its effect depending directly on the pressure of CO₂. In neutral and slightly acid soils, soluble organic matter and the H₂PO₄^?-HPO²₄^? pair may also contribute. A soil acidity diffusion coefficient is derived, and defined as: where v_l= the volume fraction of the soil solution, f_l= the impedance factor for the liquid diffusion pathway, b_HS= the pH buffer capacity of the soil, b _HB= the pH buffer capacity of each mobile acid-base pair, Dl _HB= the diffusion coefficient of each mobile acid-base pair in free solution, and the sum is taken over all mobile acid-base pairs. The soil acidity diffusion coefficient may be used to predict the course of pH equilibration in practical situations. It is high in acid and alkaline soil, and at a minimum in slightly acid soil. It is little affected by variation of the ionic strength of the soil solution at concentrations less than 0.01M. When the pH buffer capacity of the soil is constant, and only the H₃O⁺-H₂O and H₂CO₃-HCO₃^? pairs are important, the soil acidity diffusion coefficient varies as cosh{2.303(pH—pH₀)}, where pH₀ is the pH at which the soil-acidity diffusion coefficient is a minimum.     

Complexation of Cu2+ and Pb2+ by peat and humic acid

The binding of metal to humic substances is problematical. The approaches for studying metal binding to organic matter are briefly reviewed. Ion-selective electrodes (Cu²⁺ and Pb²⁺) were used to measure metal complexation by a whole peat and an extracted humic acid (HA) fraction. Scatchard plots and calculation of incremental formation constants were used to obtain values for the binding constants for the metals onto both peat and HA. Both the peat and the humic acid had a larger maximum binding capacity for Pb²⁺ than for Cu²⁺ (e.g. at pH = 5 HA gave 0·188 mmol Cu²⁺ g^?1 and 0·564 mmol Pb²⁺ g^?1: peat gave 0·111 mmol Cu²⁺ g^?1 and 0·391 mmol Pb²⁺ g^?1). Overall, the humic acid had a larger metal binding capacity, suggesting that extraction caused conformational or chemical changes. The binding constants (K₁) for Cu²⁺ increased with increasing pH in both peat and humic acid, and were larger in the peat at any given pH (e.g. at pH = 5 HA gave log K₁= 2·63, and peat gave log K¹= 4·47 for Cu²⁺). The values for Pb²⁺ showed little change with pH or between peat and humic acid (e.g. at pH = 5 HA gave log K¹= 3·03 and peat gave log K¹= 3·00 for Pb²⁺). In the peat, Cu²⁺ may be more able to bind in a 2:1 stoichiometric arrangement, resulting in greater stability but smaller binding capacity, whereas Pb²⁺ binds predominantly in a 1:1 arrangement, with more metal being bound less strongly. Whole peat is considered to be more appropriate than an extracted humic acid fraction for the study of heavy metal binding in organic soils, as this is the material with which metals introduced into an organic soil would interact under natural conditions.     

Blocking the release of potassium from clay interlayers by small concentrations of NH4+ and Cs+

To understand the process and the kinetics of potassium release from the clay interlayer in natural and arable soils in more detail, I tested the hypotheses that large, monovalent cations, especially NH₄⁺ and Cs⁺, can reduce the release rates of K⁺ which is exchanged by Ca²⁺, even if these monovalent cations are present in concentrations of only a few μm . Percolation experiments were carried out with different illitic soil materials, some containing vermiculite, with 5 m m CaCl₂ at pH 5.8 and 20°C, in some cases for over 7000 h. NH₄⁺ and Cs⁺ both caused a large decrease in the rate at which K⁺ was released, Cs⁺ especially. Suppression began at 5 μm NH₄⁺ Blocking by 20 μm NH₄⁺ was easily reversible: the release rates readily increased when NH₄⁺ was omitted from the exchange solution. Blocking by 2 μm Cs⁺ was equal to approximately 90% of that at 10 μm Cs^+. Larger concentrations of Cs⁺ than 10 μm did not further reduce release but rather caused a slight increase, probably because of enhanced exchange of K⁺ by Cs⁺ without exfoliation of the interlayer space. Blocking by Cs⁺ was not reversible within > 7000 h of percolation by 5 m m CaCl₂. The blocking effect was reproduced in several different soil materials using 10 μm Cs⁺ but was most pronounced in vermiculite-rich samples. As NH₄⁺ is present in most arable soils, at least in concentrations of a few μm , I conclude that the observed effects are of significance in the K dynamics processes in soils, for example near the roots of plants. Further, very small concentrations of Cs⁺ in exchange solutions containing a large background of Ca²⁺ appear to be useful for suppressing K⁺ release from the interlayer in laboratory studies, probably without significantly altering the exchange at outer mineral surfaces.     

Diffusion of NH+4 and NO−3 mineralized from organic N in soil

The mineralization of native soil organic matter and the simultaneous diffusion of zero NH⁺₄ and NO^?₃ to a solution sink of zero N concentration was analysed experimentally and theoretically for a fine sandy loam soil. Experimentally, the NH₄ and NO₃ ions produced in an incubated unsaturated soil column were allowed to diffuse through a sintered glass plate into a stirred solution sink. The distribution of NH⁺₄ and NO^?₃ in the soil column was measured after various incubation times. The rate of ammonification was measured directly during incubation and the rate of nitrification modelled from nitrifier growth kinetics. A Freundlich equation was used to describe the equilibrium between soluble and exchangeable NH⁺₄ in the soil. Terms for the microbial transformation of N and the adsorption-desorption of NH⁺₄ were combined with diffusion equations which were solved numerically using finite difference methods. The model constructed was used to predict the NH⁺₄ and NO^?₃ con-centration distributions in the soil column, and good agreement was obtained between the experimental and predicted concentration profiles. The use of the model for predicting the diffusive flux of mineral N to the outer surfaces of soil peds, where it is vulnerable to leaching, was demonstrated.     

ELECTRIC CHARGE CHARACTERISTICS OF ANDO A, AND BURIED A, HORIZON SOILS

The electric charge characteristics of four Ando soils (A₁ and μA₁) and a Chernozemic soil (Ap) were studied by measuring retention of NH₄⁺ and Cl^– at different pH values and NH₄Cl concentrations. No positive charge appeared in the Ando soils at pH values 5 to 8.5 except for one containing allophane and imogolite. The magnitude of their negative charge (CEC; meq/l00g soil) was dependent on pH and NH₄Cl concentration (C; N) as represented by a regression equation: log CEC =a pH +b log C +c, where the values of a and b were 0.113–0.342 and 0.101–0.315, respectively. Unlike the Chernozemic soil, Ando soils containing allophane, imogolite, and/or 2:1–2:1:1 layer silicate intergrades and humus showed a marked reduction of cation retention as pH decreased from 7 to 5. This was attributed to the charge characteristics of the clay minerals and to the carboxyl groups in humus being blocked by Al and Fe.     

The potentiometric measurement of stability constants of soil polycar boxy late-Cu2+ chelates

Stability constants describing Cu²⁺ combination with three natural and one synthetic polycarboxylates were calculated from pH measurements alone, and using a Cu²⁺ ion selective electrode. Good agreement between the two methods justified the theory for polymer-cation association. The results were consistent with the formation of a single complex CuL₂ over a fairly wide range of metal-ligand (L) ratios. The stability constant (β^H_Cu) that was independent of polymer charge allowed the prediction of copper speciation in soil solution as a function of pH and soluble organic matter.     

Aluminum speciation in soil solutions: Equilibrium calculations

A computer simulation was done to illustrate how the equilibrium solubility and speciation of Al in well-aerated soil solutions may be affected by pH (from 2.0 to 10.0), organic acids (citric, oxalic, phthalic, and salicylic acid), metal ions (K, Mg, Ca, Al, Fe), inorganic ligands (F, OH, SO₄, PO₄, CO₃, and SiO₃), and type of Al-containing solid [kaolinite, gibbsite, or amorphous Al(OH)₃] thought to be present. The simulation indicated that the type of Al-oxide/hydroxide considered has a substantial influence on the inorganic and organic equilibrium composition of the soil solution, and on the occurrence (or non-occurrence) of other Al-minerals such as KA1₃(SO₄)₂(OH)₆ (alunite) and Al(SO₄)(OH)-5H₂O (jurbanite).     

铁铝氢氧化物对两种土壤和高岭石表面K+和NH4+解吸的影响

Potassium (K) and nitrogen (N) are essential nutrients for plants. Adsorption and desorption in soils affect K+ and NH + 4 availabilities to plants and can be affected by the interaction between the electrical double layers on oppositely charged particles because the interaction can decrease the surface charge density of the particles by neutralization of positive and negative charges. We studied the effect of iron (Fe)/aluminum (Al) hydroxides on desorption of K+ and NH + 4 from soils and kaolinite and proposed desorption mechanisms based on the overlapping of diffuse layers between negatively charged soils and mineral particles and the positively charged Fe/Al hydroxide particles. Our results indicated that the overlapping of diffuse layers of electrical double layers between positively charged Fe/Al hydroxides, as amorphous Al(OH) 3 or Fe(OH) 3 , and negatively charged surfaces from an Ultisol, an Alfisol, and a kaolinite standard caused the effective negative surface charge density on the soils and kaolinite to become less negative. Thus the adsorption affinity of these negatively charged surfaces for K+ and NH + 4 declined as a result of the incorporation of the Fe/Al hydroxides. Consequently, the release of exchangeable K+ and NH +4 from the surfaces of the soils and kaolinite increased with the amount of the Fe/Al hydroxides added. The greater the positive charge on the surfaces of Fe/Al hydroxides, the stronger was the interactive effect between the hydroxides and soils or kaolinite, and thus the more release of K+ and NH + 4 . A decrease in pH led to increased positive surface charge on the Fe/Al hydroxides and enhanced interactive effects between the hydroxides and soils/kaolinite. As a result, more K+ and NH + 4 were desorbed from the soils and kaolinite. This study suggests that the interaction between oppositely charged particles of variable charge soils can enhance the mobility of K+ and NH + 4 in the soils and thus increase their leaching loss.     

Interactions of fulvic acid with aluminium and a proto‐imogolite sol: the contribution of E‐horizon eluates to podzolization

The podzolization process is examined in the light of measurements of the solubility characteristics of aluminium fulvate, the extent of dissolution of a proto‐imogolite sol by fulvic acid, the adsorption capacity of proto‐imogolite for fulvic acid and aluminium fulvate, and published evidence. Fulvic acid at 500 mg l^?1 acting on a proto‐imogolite (PI) preparation containing 0.95 mmol l^?1 Al as PI did not bring enough Al into solution at pH 4.5–5.0 over 4–15 months to cause significant precipitation of the fulvic acid. As allophanic Bs horizons of podzols typically have pH ≥ 4.8, fulvic acids entering them in drainage water cannot be quantitatively precipitated by dissolution of Al from the allophane. They are, however, strongly absorbed on the allophane, and this must be the mechanism that removes most of the fulvic acid at the top of the Bs horizon, and which contributes, along with colloidal humus and root decomposition, to the formation of a Bh horizon. We conclude that fulvic acid plays no active role in podzolization, but only recycles Al and Fe, that have been transferred by biological processes to the O horizon, back to the Bh horizon. The podzolization process, which leads to the formation of an allophanic Bs horizon underlying a progressively deepening E horizon, requires the dissolution of Al‐humate and allophanic precipitates at the Bh–Bs interface as well as progressive attack on the more readily weatherable minerals. Inorganic acids, particularly episodic fluxes of nitric acid, could play a major role in this, as well as attack by readily metabolized complexing acids such as oxalic and citric acids released by roots and fungi. In addition to throwing light on the podzolization process, the experimental results provide an explanation of the lower limit to C:Al ratios reported in natural waters, and a check on the applicability of the WHAM chemical equilibrium model to Al–fulvate–proto‐imogolite equilibria. In Ca‐containing fulvate solutions, Al‐fulvate begins to precipitate when C:Al falls below 50, which is also the limiting ratio observed in natural waters. WHAM calculations overestimate by 70–85% the amount of Al‐fulvate formed over 4 months at pH 4.5–5.0 in Ca‐containing fulvate–imogolite systems.