THE EQUILIBRIA OF K:Al EXCHANGE IN CLAY MINERALS AND ACID SOILS1

Conventional K: Al exchange isotherms for montmorillonite showed that Al³⁺ was strongly preferred to K⁺ in o-oin solutions. The exchange coefficient, K', calculated using the isotopically exchangeable K, was greater than unity and did not vary with the Al-saturation or with the initial pH of the AlCl₃ solutions. Isotherms for vermiculite, illite, and soils in o·oin solutions also showed Al³⁺-preference but unlike those for montmorillonite were not asymptotic to q_Al/q_o= 1, q_Al being the amount of adsorbed Al and q_o the total adsorbed (Al + K), indicating that some of the isotopically exchangeable K could not easily be exchanged by Al³⁺ ions; this difficultly exchangeable K (DEK) was estimated for each exchanger. K' values for vermiculite, illite, and soils were less than unity and did not vary with Al-saturation or initial pH if the isotopically exchangeable K was corrected for DEK. This showed that K⁺ was adsorbed more strongly than Al³⁺. Strengths of K⁺ adsorption referred to Al³⁺ as the counter-cation were in order: soils > vermiculite, illite > montmorillonite.     

SOLUBLE ALUMINIUM AND CALCIUM-ALUMINIUM EXCHANGE IN RELATION TO THE pH OF DILUTE CALCIUM CHLORIDE SUSPENSIONS OF ACID SOILS

Measurements of pH and A1 concentration were made on 10^-2 M CaCl₂, suspensions of a number of acid soils that had been limed to give 3 range of pH values, and exchangeable A1 and Ca+Mg were determined in 1.0 M NH₄Cl extracts. The variation of pH with A1 concentration did not support the theory that pH is controlled by the solubility of Al(OH)₃. For some of the soils, proton release on hydrolysis of A1₃₊ions in solution accounted for the pH values, and explained quantitatively the variation of pH with the Ca:Al balance of the exchange complex, taking account of the selectivity coefficient for exchange, K_ca→A1 Although K_ca→A1 was smaller for soils containing more humus, their pH values were also less than those predicted by the hydrolysis of A1³⁺ in solution, indicating that they contained other sources of protons, presumably the carboxyl groups in humus.     

Speciation and Solubility Control of Aluminium in Soils Developed from Slates of the River Sor Watershed (Galicia,NW Spain)

The Al species in the soid and liquid phases were studied in eight soils developed from slates in a watershed subjected to acid deposition. From soil solution data the mechanisms possibly controlling Al solubility are also discussed. The soils are acidic, organic matter rich and with an exchange complex saturated with Al. In the solid phase, more than 75% of non-crystalline Al was organo-Al complexes, mostly highly stable. In the soil solutions, monomeric inorganic. Al forms were predominant and fluoro-Al complexes were the most abundant species, except in soil solutions of pH<4.8 and Al L/F ratio >3, in which Al³⁺ predominated and sulphato-Al complexes were relatively abundant. The most stable phases were kaolinite, gibbsite and non-crystalline Al hydroxides. In most samples, Al solubility was controlled by Al-hydroxides. Only in a few cases (solutions of pH 4-5, Al³⁺ activity >40 µmol L-¹ and SO₄ content >200 µmol L-¹), Al-sulphates such as jurbanite also could exert some control over Al solubility. In adition to these minerals, a possible role of organo-Al complexes or the influence of adsorption reactions of sulphate is considered, especially for samples with very low Al³⁺ content (<0.5 µmol L-¹).     

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).     

Equilibria of weak acids and organic Al complexes explain activity of H+ and Al3+ in a salt extract of exchangeable cations

Two sequential extractions with unbuffered 0.1 m BaCl₂ were done to study the release of salt-exchangeable H⁺ and Al from mineral horizons of five Podzols and a Cambisol. Released Al was found to have a charge close to 3+ in all horizons and in both extractions. This finding was supported by the near-equality of the titrated exchangeable acidity (EA_T) and the sum of exchangeable acids (EA = H_e + 3Al_e, calculated from the pH and Al concentration of the extract). The ratio between EA of the second and the first extraction was over 0.50 in the Bs2 and C horizons and smaller in the other horizons. H⁺ was assumed to be in equilibrium with weak acid groups, and the modified Henderson–Hasselbach equation, pK_HH = pH ? n log (α/(1 ? α)), was used to explain pH of the extract. The degree of dissociation (α) was calculated as the ratio between effective and potential cation exchange capacity. Value of the empirical constant n was found to be near unity in most horizons. When the monoprotic acid dissociation was assumed in all horizons, pK_HH had the same value in both extractions. For Al³⁺, two equilibrium models were evaluated, describing (i) complexation reactions of Al³⁺ with soil organic matter, and (ii) equilibrium with Al(OH)₃. Apparent equilibrium constants were written as (i) pK_o = xpH ? pAl³⁺, and (ii) log Q_gibbs= log Al³⁺ ? 3log H^+. The two extractions gave an average reaction stoichiometry x close to 2 in all horizons. Results suggest that an equilibrium with organic Al complexes can be used to express dissolved Al³⁺, aluminium being apparently bound to bidentate sites. The value of log Q_gibbs was below the solubility of gibbsite (log K_gibbs = 8.04) in many horizons. In addition, log Q_gibbs of the second extraction was greater than that of the first extraction in all horizons except the C horizon. This indicates that equilibrium with Al(OH)₃ cannot explain dissolved Al³⁺ in the soils. We propose that the models of pK_HH and pK_o can be used to simulate exchangeable H⁺ and Al³⁺ in soil acidification models.     

Soil solution aluminium activity related to theoretical A1 mineral solubilities in four Australian soils

Four soils were treated with HNO₃, CaCO₃ and K₂SO₄ to enable observation of the response of the soil solution composition and the solution A1 ion activity (Al³⁺) to the treatments and to time. The clay fraction of three of the soils was dominated by illite, kaolinite and quartz. The fourth was minated by kaolinite and iron oxides. The initial pH in 0.01 M CaCl₂ varied between 4.0 and 5.0 and the organic carbon content from 0.7 to 1.1%. The soil solutions from soils dominated by kaolinite, illite and quartz were generally supersaturated with respect to quartz and well ordered kaolinite, and unsaturated with respect to illite. The soil solutions from the soil dominated by kaolin and iron oxide were generally unsaturated with respect to quartz but still saturated with respect to ell crystallized kaolin. Within mineral groups such as Al₂SiO₅ compounds, A1₂Si₂O₅(OH)₄ (kaolinite group), and Al(OH)₃ (A1 oxide) minerals, the more soluble forms became less supersaturated or unsaturated with time for many treatments. Lime treatment usually increased the ion activity product of AI(OH)₃ in all soils, and of minerals with the composition, Al₂SiO₅, in the illite/kaolinite soils. Acid treatment reduced the apparent solubility of Al(OH)₃, and the A1 silicates in the Al₂SiO₅, and Al₂, Si₂, O₅,(OH)₄, mineral groups on all soils. These results are interpreted to indicate that lime treatment led to the formation of trace quantities of more soluble A1 minerals that subsequently controlled (Al³⁺), whereas acid treatment dissolved trace quantities of such minerals leaving less soluble minerals to control (Al³⁺). The results suggest that, in mineral soils such as these, (Al³⁺) is under the control of inorganic dissolution and precipitation processes. These processes conform to expectations given the free energy of various inorganic aluminium compounds. Furthermore the sequence of dissolution and formation processes appears to be governed by the Gay-Lussac—Ostwald step rule.     

Solubility control of KCl extractable aluminum in soils with variable charge

Abstract

Solubility and kinetic data indicated that concentrations of aluminum (Al) extracted with 1 M KCl are determined by the solubility of a precipitated A1(OH)₃ phase in soils dominated by variable charge minerals. Kinetic studies examining the release of Al on non‐treated and KCl treated residues indicated the precipitation of an acid‐labile Al phase during the extraction procedure. The log ion activity products estimated for the KCl extracts ranged between 8.1–8.6 for the reaction Al(OH)₃ + 3H⁺ < = > Al³⁺+ 3H₂O, which was similar to the solubility product of several Al(OH)₃phases. The mechanism proposed for Al precipitation indicated that Al released by exchange with added K⁺ hydrolyzed and released H⁺ that was readily adsorbed on surfaces of variable charge minerals. The increased ionic strength of the extracting solution further increased the amount of H⁺adsorbed to the variable charge surface and reduced the H⁺ concentration in the aqueous phase. Consumption of H⁺ induced further hydrolysis of Al, resulting in supersaturation of the extracting solution and formation of polynuclear hydroxy Al species. It was concluded that the 1 M KCl extraction does not quantitatively extract salt exchangeable Al from variable‐charge soils.     

Growth of and potassium transport in winter wheat and durum wheat as affected by various aluminum exposure times

Effects of various aluminum (AlCl₃) concentration and exposure times (6, 12, and 24 h and 3 d) on growth and potassium (K) transport were studied in two wheat species (Triticum aestivum L. cv. Jubilejnaja 50 and Triticum durum Desf. cv. GK Betadur) grown in low salt conditions hydroponically. In longer (3 d) Al exposure times at pH 4.1, the inhibition of root growth appeared at 10 μM Al³⁺ treatment in GK Betadur, and at 50 μM Al³⁺ treatment in Jubilejnaja 50. Shoot growth was not influenced by Al³⁺ treatment, except at 100 μM in 7 d experiments. In 6, 12, and 24 h Al³⁺ exposure times, at low pH, the K⁺(⁸⁶Rb) influx in roots increased as the Al³⁺ concentration increased in the outer medium in both species. It also appeared in K⁺(⁸⁶Rb) transport toward the shoots, except by higher Al³⁺ treatments of GK Betadur seedlings. At the same time, in longer‐term (3 d) Al³⁺ treatments, a striking inhibition were observed in K⁺(⁸⁶Rb) influx and K⁺ concentration of roots and shoots. The K⁺concentration of roots and shoots measured at the end of 24 h Al³⁺ exposure times was significantly not affected by Al³⁺ treatment. Durum wheat proved to be more sensitive to the Al toxicity than common winter wheat.     

Apparent cation-exchange equilibria in podzolic forest soils

The Gaines–Thomas selectivity coefficient, K, was used to express the relation between the cations in solution and the cations in exchange sites in podzolic forest soils. Soil solution was obtained by centrifuging a fresh bulked soil sample. Exchangeable cations HX, AlX, CaX, MgX and KX and effective cation-exchange capacity, CEC_e, were determined with 0.1 m BaCl₂. Apparent values of K indicated a preference of Ca²⁺ over Mg²⁺ and over Al³⁺ in O, A and B horizons (log K_Al–Ca < 0 and log K_Mg–Ca < 0), whereas log K_K–Ca and log K_H–Ca exceeded zero. The horizons were similar with respect to log K_H–Ca, and the differences in log K_Mg–Ca were small. Log K_K–Ca and log K_Al–Ca increased in the horizons in the order O < A < B. Log K_Al–Ca was not significantly correlated with the fraction AlX/CEC_e. Log K_Mg–Ca was positively correlated with the fractions HX/CEC_e and AlX/CEC_e, and negatively correlated with log (CaX/MgX). The selectivity coefficient of binary cation exchange seemed to be applicable to in situ soil solutions. However, the fraction of each cation on exchange sites should be based on the CEC_e rather than on the sum of the two cations. The latter, also, seemed to be acceptable in cases of exchangeable cations with a large relative content in soil, e.g. in Al³⁺–Ca²⁺ exchange in A and B horizons, and in H⁺–Ca²⁺ exchange in O and A horizons.     

Aluminium release in relation to the determination of cation exchange capacity of some podzolized New Zealand soils

In upper mineral horizons, CEC by compulsive and isotopic exchange methods, using Ba²⁺ as the saturating cation, gave higher values than the effective CEC at natural soil pH, and much higher values than CEC determined with m NH₄OAc at pH 7. Cumulative Al release during leaching was considerably higher using Mg²⁺ and Ba²⁺ chlorides than K⁺ and NH₄⁺ chlorides, and gave a different shape extraction curve. Basal spacing of the dominant dioctahedral vermiculite in the soil clays contracted from 14.5Å to 10.0–10.9 Å when saturated with NH₄⁺ and K⁺, restricting release of interlayer Al. Lower horizons, containing a large proportion of Al-chlorite in the clay fraction, which did not contract with any of the cations, showed more normal exchange behaviour. On leaching, Al release was slightly greater with K⁺ and NH₄⁺, than with Mg²⁺ and Ba²⁺, chlorides. The implication of the results for CEC measurements is discussed.     

Comparison of the early response to aluminum stress between tolerant and sensitive wheat cultivars: Root growth,aluminum content and efflux of K+

In order to characterize the mechanism of Al tolerance (Atlas 66) and Al sensitivity (Scout 66) in two cultivars of wheat (Triticum aestivum L.), the early responses to Al stress under acidic conditions were investigated. Marked inhibition of root elongation of Scout was observed upon treatment with 10 μM AlCl₃ for less than 3 h. The inhibition of root elongation of Scout was reversed within 3 days when the treated samples were transferred to a solution without Al. However, treatment for 6 h with AlCl₃ repressed root elongation almost completely and irreversibly. Root elongation of Atlas was only partially inhibited by the treatment with 10 μM AlCl₃ for more than 6 h. Levels of Al in two portions of roots, namely, portions 0–5 mm and 5–10 mm from the tip, were lower in Atlas than those in Scout. In Atlas the levels of Al on a fresh weight basis in both portions were very similar, while the level of Al in the portion 0–5 mm from the tip was almost double than that in the 5–10 mm portion in Scout. A distinct increase in levels of Al in the 0–5 mm portion over that in the 5–10 mm portion of Scout was observed even after 3 h of treatment with AlCl₃.

Both Atlas and Scout were preloaded with K⁺ at pH 5.5 and transferred to distilled water at various pH values to monitor the efflux of K⁺. A reduction in the pH induced increases in the efflux of K⁺ in both cultivars, and the rate of efflux in Scout was twice that in Atlas at pH 4.2. AlCl₃ at concentrations as low as 5 μM markedly repressed K⁺efflux at pH 4.2 and this effect was more pronounced in Scout. Ca²⁺ also had a repressive effect on K⁺ efflux, while EGTA increased K⁺ efflux. Vanadate increased K⁺ efflux, a result that suggests the involvement of a H⁺ pump in K⁺ efflux. Ca²+ failed to repress the increased efflux of K⁺ caused by vanadate while Al repressed the K⁺ efflux even in the presence of vanadate. These results suggest that a low extracellular pH may cause an increase in the cytoplasmic concentration of H⁺ that is followed by depolarization of the plasma membrane, which may be modified by the efflux of K⁺ and H⁺. The characteristic difference in terms of K⁺ efflux between Atlas and Scout at low pH may be caused by differences associated with plasma membrane potentials, as follows. The net influx of H+ at low pH, which causes depolarization of the plasma membrane, is higher in Scout than in Atlas. The difference in the net influx of H⁺ may be regulated in part by Ca2⁺, that either repress the influx of H⁺ or the activate of the H⁺ pump. Inhibition of K⁺ efflux by Al, which tends to depolarize the plasma membrane at low pH, may be an important factor in determining sensitivity and/or tolerance to Al.     

pH and solubility of aluminium in acidic forest soils: a consequence of reactions between organic acidity and aluminium alkalinity

Data from two Podzol O and E horizons, sampled in 1-cm layers at 13 points within 2 m × 2 m plots, were used to test the hypothesis that the composition of hydrogen ions (H) and aluminium (Al) adsorbed to the solid-phase soil organic matter (SOM) determines pH and Al solubility in organic-rich acidic forest soils. Organically adsorbed Al was extracted sequentially with 0.5 m CuCl₂ and organically adsorbed H was determined as the difference between total acidity titrated to pH 8.2 and Al extracted in 0.5 m CuCl₂. The quotient between fractions of SOM sites binding Al and H (N_Al/N_H) is shown to determine the variation in pH and Al solubility. It is furthermore shown that models in which pH and Al solubility are linked via a pH-dependent solubility of an Al hydroxide and in which cation exchange between Al³⁺ and Ca²⁺, rather than cation exchange between Al³⁺ and H⁺, is the main pH-buffering process cannot be used to simulate pH or Al solubility in O and E horizons. The fraction of SOM sites adsorbing Al increased by depth in the lower O and throughout the E horizon at the same magnitude as sites adsorbing H decreased. The fraction of sites binding the cations Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ remained constant. It is suggested that a net reaction between Al silicates (proton acceptors) and protonated functional groups in SOM (proton donors) is the long-term chemical process determining the composition of organically adsorbed H and Al in the lower part of the O and in the E horizon of Podzols. Thus, in the long term, pH and Al solubility are determined by the interaction between organic acidity and Al alkalinity.     

EFFECT OF SOIL pH AND ORGANIC MATTER ON LABILE ALUMINIUM IN SOILS UNDER PERMANENT GRASS

The rates of extraction of Na, K, Mg, Ca, and Al with 1M NH₄ NO₃ from the mineral-and organic-rich layers of some Park Grass (Rothamsted) soils were measured at the pH of the soil. Below pH 3.7 exchangeable Al, derived from the kinetics curve, increases with decreasing soil pH and is less in the organic-rich layer. The sum of the basic exchangeable cations, ∑(Na + K + Mg + Ca), increases with increasing soil pH and is more in the organic-rich layer. The extraction of exchangeable Al obeys first order kinetics, the rate constant being similar for all the soils (mean value 36 ± 7 × 10^?6|s^?1), which implies that exchangeable Al is released from surfaces with similar properties for the adsorption of Al, and that the rate is not affected by soil pH and organic matter. The rate of extraction of non-exchangeable Al is the same in the mineral-and organic-rich layers of each soil, and is maximal at about pH 3.7, decreasing sharply at more and less acid pH values.     

Influence of Aluminum Hydroxide on Potassium Release from Two Colombian Soils

Abstract

The effect of sesquioxides on the mechanisms of chemical reactions that govern the transformation between exchangeable potassium (Kex) and non‐exchangeable K (Knex) was studied on acid tropical soils from Colombia: Caribia with predominantly 2∶1 clay minerals and High Terrace with predominantly 1∶1 clay minerals and sesquioxides. Illite and vermiculite are the main clay minerals in Caribia followed by kaolinite, gibbsite, and plagioclase, and kaolinite is the major clay mineral in High Terrace followed by hydroxyl‐Al interlayered vermiculite, quartz, and pyrophyllite. The soils have 1.8 and 0.5% of K₂O, respectively. They were used either untreated or prepared by adding AlCl₃ and NaOH, which produced aluminum hydroxide. The soils were percolated continuously with 10 mM NH₄OAc at pH 7.0 and 10 mM CaCl₂ at pH 5.8 for 120 h at 6 mL h^?1 to examine the release of Kex and Knex. In the untreated soils, NH₄ ⁺ and Ca²⁺ released the same amounts of Kex from Caribia, whereas NH₄ ⁺ released about twice as much Kex as Ca²⁺ from High Terrace. This study proposes that the small ionic size of NH₄ ⁺ (0.54 nm) enables it to enter more easily into the K sites at the broken edges of the kaolinite where Ca²⁺ (0.96 nm) cannot have access. As expected for a soil dominated by 2∶1 clay minerals, Ca²⁺ caused Knex to be released from Caribia with no release by NH₄ ⁺. No Knex was released by either ion from High Terrace. After treatment with aluminum hydroxide, K release from the exchangeable fraction was reduced in Caribia due to the blocking of the exchange sites but release of Knex was not affected. The treatment increased the amount of Kex released from the High Terrace soil and the release of Knex remained negligible although with Ca²⁺ the distinction between Kex and Knex was unclear. The increase in Kex was attributed to the initially acidic conditions produced by adding AlCl₃ which may have dissolved interlayered aluminum hydroxide from the vermiculite present, thus exposing trapped K as exchangeable K. The subsequent precipitation of aluminum hydroxide when NaOH was added did not interfere with the release of this K, and so was probably formed mostly on the surface of the dominant kaolinite. Measurement of availability of K by standard methods using NH₄ salts could result in overestimates in High Terrace and this may be a more general shortcoming of the methods in kaolinitic soils.     

ALUMINUM SPECIATION OF AMENDED ACID TROPICAL SOIL AND ITS EFFECTS ON PLANT ROOT GROWTH

Exchangeable and soluble soil aluminum (Al) is limiting plant growth in many soils worldwide. This study evaluated the effects of increasing rates of dolomite and magnesium carbonate (MgCO₃) on Al³⁺, pH, dissolved organic carbon, cations, anions, and Al speciation on oil palm Deli dura × AVROS pisifera root growth. Dolomite and MgCO₃ additions significantly raised linearly soil solution pH, magnesium (Mg²⁺), nitrate (NO₃ ^?) and chlorine (Cl^?) concentrations; exponentially decreased the activity of phytotoxic Al species [aluminum (Al³⁺), aluminum sulfate (Al₂SO₄), and aluminum fluoride (AlF₃)]; and reduced manganese (Mn) concentration and activity. High activity of those species exponentially reduced root dry weight. Optimum oil palm growth was achieved at: <50 μM monomeric Al, < 30 μM Mn, and <0.20 unit of the ratio Al+Mn to calcium (Ca)+Mg. High activity of Al species and Mn in acidic soil solution cause significant reduction of the root growth. Soil acidity alleviation either with dolomite or MgCO₃ mitigates the toxic effect of Al and Mn.     

Soil Solution as Affected by Plant Residues and Nitrogen Rates

Cation mobility in acidic soils with low organic‐matter contents depends not only on sorption intensity but also on the solubility of the species present in soil solution. In general, the following leaching gradient is observed: potassium (K⁺) > magnesium (Mg²⁺) > calcium (Ca²⁺) > aluminum (Al³⁺). To minimize nutrient losses and ameliorate the subsoil, soil solution must be changed, favoring higher mobility of M²⁺ (metal ions) forms. This would be theoretically possible if plant residues were kept on the soil surface. An experiment was conducted in pots containing a Distroferric Red Latosol, with soil solution extractors installed at two depths. Pearl millet, black oat, and oilseed radish residues were laid on the soil surface, and nitrogen (as ammonium nitrate) was applied at rates ranging from 0 to 150 mg kg^?1. Corn was grown for 52 days. Except for K⁺ and ammonium (NH₄ ⁺), nitrogen rates and plant residues had little effect upon the concentrations and forms of the elements in the soil solution. Presence of cover crop residues on soil surface decreased the effect of nitrogen fertilizer on Ca leaching. More than 90% of the Ca²⁺, Mg²⁺, and K⁺ were found as free ions. The Al³⁺ was almost totally complexed as Al(OH₃)⁰. Nitrogen application increased the concentrations of almost all the ions in soil solution, including Al³⁺, although there was no modification in the leaching gradient.     

SOIL ALKALINITY. I. EQUILIBRIA AND ALKALINITY DEVELOPMENT

Theoretical relationships between pH, CO₂ partial pressure and alkalinity (bicarbonate + carbonate concentrations) have been shown to apply to solutions and calcite and soil suspensions. The exchange of Na onto three Ca clays shows that Ca is preferred but with negative free energies of exchange. With decreasing total electrolyte concentration, the preference for Ca increases, so that only when the concentration is above 10^-3M will significant amounts of exchangeable Na be found in soils. The preference for Ca is illite > montmorillonite > vermiculite. Dilution of the mixed Na-Ca clay suspensions causes exchange, desorption of Na and. in some cases Ca. This desorbed or ‘alkaline’ Na (and Ca) is replaced by H some of which attacks the clays. The apparent hydrolysis coefficient, KG = H adsorbed (Na)/Na adsorbed (H), varies between 5 and 9 × 10⁵, increases with increase in electrolyte concentration and varies in the order vermiculite > illite > montmorillonite. The concentration of alkaline Na + Ca increases with increasing ESP, and with clay type in the same order as the KG values. The pH calculated from the alkaline Na + Ca, assumed equal to the alkalinity was equal to measured values except for montmorillonite where the calculated values were low. Magnesium release by acid attack of the montmorillonite may explain the differences. Vermiculite rich soils will be most likely to accept exchangeable Na and to hydrolyse and develop alkalinity.     

Proton alleviation of growth inhibition by toxic metals (Al, La, Cu) in rhizobia

Free-living rhizobia are sensitive to soils and artificial media that are acidic. Both excessive H⁺ and Al released from acid-soluble minerals appear to be toxic. The complex, heterotrophic nutrient requirements of rhizobia and the joint occurrence of Al³⁺ and hydroxo-, sulphato-, phosphato-, fluoro-, and other Al species have prevented a precise attribution of toxicity to the Al species. In the present study, a medium composed of 0.3 mM MgSO₄, 2 mM CaCl₂, and 10 mM sucrose (the basal medium) enabled a 1000-fold cell increase at pH 4.6 or above. Additions of 1 μM AlCl₃ to the basal medium were highly intoxicating, especially at higher pH: below pH 5.0 cell numbers increased slightly; at pH 5.0 cell numbers did not change from the inoculum; at higher pH values the cell numbers declined. Similar trends were observed for La³⁺ and Cu²⁺ intoxication. Uptake of methylene blue, a positively charged dye useful as a probe of cell-surface electrical potential, was inhibited by pH reductions between pH 3.5 and 6.0. Factors that decrease cell-surface negativity (such as lower pH) reduce the intoxication by cations in plant roots, but the pH responsiveness of the rhizobia in our system was much greater than the pH responsiveness of plants. Although plant-root intoxication by mononuclear hydroxo-Al species has been discounted, rhizobia may be sensitive to those species. These results have implications for the management of rhizobia in acidic soils and for the development of resistant strains.     

Influence of Different Forms of Iron and Aluminum on the Nature of Soil Acidity of Some Inceptisols,Alfisols, and Ultisols

Abstract

Fifteen acid soils of Mizoram representing Ultisols and Inceptisols, and Madhya Pradesh, representing Alfisols, were studied to characterize the nature of acidity in relation to different forms of iron (Fe) and aluminum (Al). The mean contents of Fe and Al were extracted by various extracting reagents and were found to be in descending order as followed: dithionite>oxalate>pyrophosphate>ammonium acetate>KCl. The electrostatically bonded EB‐H⁺ and EB‐Al³⁺ acidity comprised 28.3 and 71.7% of exchangeable acidity whereas EB‐H⁺, EB‐Al³⁺, exchangeable, and pH‐dependent acidities comprised 9.8, 30.7, 40.5, and 59.5% of total potential acidity. All forms, of acidity showed significant correlation with pH_k and organic carbon. Among the different forms, Fe and Al caused most of the variations in different forms of soil acidity but the effect of different forms of Al are more active and directly participate in the formation of EB‐H⁺, EB‐Al³⁺, and exchangeable acidity.     

Cation exchange in forest soils: the need for a new perspective

The long‐term sustainability of forest soils may be affected by the retention of exchangeable nutrient cations such as Ca²⁺ and the availability of potentially toxic cations such as Al³⁺. Many of our current concepts of cation exchange and base cation saturation are largely unchanged since the beginnings of soil chemistry over a century ago. Many of the same methods are still in use even though they were developed in a period when exchangeable aluminium (Al) and variable charge were not generally recognized. These concepts and methods are not easily applicable to acid, highly organic forest soils. The source of charge in these soils is primarily derived from organic matter (OM) but the retention of cations, especially Al species, cannot be described by simple exchange phenomena. In this review, we trace the development of modern cation exchange definitions and procedures, and focus on how these are challenged by recent research on the behaviour of acid forest soils. Although the effective cation exchange capacity (CECe) in an individual forest soil sample can be easily shown to vary with the addition of strong base or acid, it is difficult to find a pH effect in a population of different acid forest soil samples. In the very acidic pH range below ca 4.5, soils will generally have smaller concentrations of adsorbed Al³⁺. This can be ascribed to a reduced availability of weatherable Al‐containing minerals and a large amount of weak, organic acidity. Base cation saturation calculations in this pH range do not provide a useful metric and, in fact, pH is modelled better if Al³⁺ is considered to be a base cation. Measurement of exchangeable Al³⁺ with a neutral salt represents an ill‐defined but repeatable portion of organically complexed Al, affected by the pH of the extractant. Cation exchange in these soils can be modelled if assumptions are made as to the proportion of individual cations that are non‐specifically bound by soil OM. Future research should recognize these challenges and focus on redefining our concepts of cation retention in these important soils.