Calcium-Ammonium Selectivity of Two Benchmark Soils from Botswana as Assessed by Competing Semi-empirical Ion Exchange Equations

The effectiveness of lime-ammonium-nitrate (LAN) as a nitrogen (N) fertilizer in weathered soils depends on the respective selectivity for ammonium (NH₄) and calcium (Ca) by the soils. The study assessed Ca²⁺/NH₄⁺ exchange selectivity of two benchmark soils from Botswana and examined the soil fertility management implications. Surface horizons (0–20 cm) of Pellustert and Haplustalf were equilibrated with 50 ml stock solution containing variable concentrations of Ca²⁺ and NH₄⁺. The Ca²⁺/NH₄⁺ exchange data were fitted into the Vanselow (K_V), Gaines and Thomas (K_GT), Davies (K_D), and the regular solution (K_RS) equations. The selectivity coefficients for the Ca²⁺/NH₄⁺ exchange reactions varied widely with the soil exchanger composition except for the relatively stable K_RS. The selectivity coefficients indicated strong preference for NH₄⁺ to Ca²⁺. The thermodynamic exchange constant, K_ex, was 5.75 ± 1.24 in the Pellustert, indicating preferential adsorption of NH₄⁺, but not in the Haplustalf with K_ex = 0.92 ± 0.27. The free energy for Ca²⁺/NH₄⁺ exchange (ΔG°_ex) was negative (?4.26 ± 0.59 kJ mol^?1) in the Pellustert but slightly positive in the Haplustalf (0.34 ± 0.87 kJ mol^?1). In conclusion, the soil-NH₄ complex was more stable than soil-Ca complex in the Pellustert, indicating LAN as a N fertilizer would have greater potential effectiveness in the Pellustert than in the Haplustalf.     

Interactions of allophane with humic acid and cations

Allophanic soils are known to accumulate organic matter, but the underlying mechanism is not well understood. Here we have investigated the sorption of humic acid (HA) by an allophanic clay in the presence of varied concentrations of either CaCl₂ or NaCl as background electrolytes. Both the HA and the clay were separated from New Zealand soils. Much more HA was sorbed in CaCl₂ than in NaCl of the same ionic strength. Apparently Ca²⁺ ions were more effective than Na⁺ ions in screening the negative charge on HA. In CaCl₂ the HA molecule might also assume a more compact configuration than in NaCl. In the presence of CaCl₂ sorption increased, reached a maximum, and then declined as the concentration of HA in solution was increased. This behaviour was not observed in NaCl where sorption showed a gradual and steady increase with HA concentration. We propose that ligand exchange occurs between the surface hydroxyl groups of allophane and the carboxylate groups of HA. As a result, the allophane–HA complex acquires negative charges, requiring the co‐sorption of extraneous cations (Ca²⁺ or Na⁺) for charge balance. The Ca²⁺ co‐sorbed can attract more HA to the complex possibly by a cation‐bridging mechanism, giving rise to a maximum in sorption. The decline in sorption beyond the maximum may be ascribed to a decrease in the concentration of free Ca²⁺ ions through binding to HA molecules in solution. The increase in supernatant pH may be attributed to a ligand exchange reaction between the surface hydroxyls of allophane and the carboxylate groups of HA, and proton binding to the allophane–HA complex.     

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.     

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.     

High monosilicic acid supply rapidly increases Na accumulation in maize roots by decreasing external Ca2+ activity

Both calcium (Ca²⁺) and silicon (Si) improve plant performance under salt (NaCl) stress. Although these two mineral elements share numerous similarities, the information on how their extracellular interactions in the root apoplast affect uptake of sodium (Na⁺) is still lacking. Here, we investigated the effect of high Si supply in the bioavailable form of monosilicic acid (H₄SiO₄) on the activity of Ca²⁺ in the external root solution, and subsequent root uptake and compartmentation of Na in maize (Zea mays L.). In the short‐term experiments (6 h), 14‐d‐old maize plants were exposed to various concentrations of Ca²⁺ at three different pH‐values (6.5, 7.5, and 8.5) and two Si concentrations, i.e., low (1 mM) and high (4 mM) supply of H₄SiO₄. The activity of Ca²⁺ and Na⁺ in the external solution as well as the root concentrations of total and cell sap and BaCl₂‐exchangeble apoplastic fractions of both elements were analyzed. The pH of the nutrient solution affected neither the ion activities nor the root accumulation of both Ca²⁺ and Na⁺. At higher pH values (7.5 and 8.5) the interactions of Ca²⁺ and Si at high Si supply led to a decrease of Ca²⁺ activity and, hence, an increase of Na⁺ : Ca²⁺ activity ratio in the external root solution. Concomitantly, despite the elevated exchangeable apoplastic fraction of both Ca²⁺ and Na⁺, the total and cell sap concentrations were remarkably decreased for Ca²⁺ and increased for Na⁺ by the addition of 4 mM H₄SiO₄. This work demonstrates that at high Si supply extracellular Ca‐Si interactions leading to lowered activity of Ca²⁺ might rapidly compromise the ameliorative effect of Ca²⁺ on Na⁺ accumulation in roots. Practically, Si over‐fertilization of saline and, in particular, sodic soils may further promote the accumulation of Na⁺ in root tissues hours after Si application and, hence, increase a potential risk of Na⁺ toxicity.     

THERMODYNAMICS AND THERMOCHEMISTRY OF THE EXCHANGE REACTION BETWEEN NH4+ AND Mn2+ IN A MONTMORILLONITE CLAY

The exchange reaction between NH₄⁺ and Mn³⁺ was studied on a montmorillonite clay at several temperatures and different ionic strengths. Manganese was preferred to ammonium; this preference increased with the temperature and dilution of the dialysate. Comparison with published data concerning exchanges involving NH₄⁺ and the alkaline-earths showed that in the sequence of increasing selectivity: Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺, Mn²+ lies between Mg²⁺ and Ca²⁺. The enthalpy change was measured calorimetrically and calculated by application of the van't Hoff law to the temperature coefficient of the equilibrium constants. Both values were in good agreement. The excellent recoveries of Mn²⁺ at the end of the exchange reaction and the constancy of the cation exchange capacity over the whole range of surface composition ruled out the possibility of significant adsorption in the MnOH⁺ form. The behaviour of manganese was very similar to that of the alkaline-earth cations.     

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.     

Problems in CEC determination of calcareous clayey sediments using the ammonium acetate method

Cation‐exchange–capacity (CEC) results of calcareous soils and clays can be erroneous if the ammonium acetate method is used. In this study, a model is proposed to explain the process for systematic underestimation of the CEC. Seven clayey sediments from Germany with varying calcite and low organic‐C content were studied. After several exchange treatments with concentrated ammonium acetate (NH₄Ac) solutions, the exchange population is assumed to be in homoionic ammonium form. Throughout the cation‐exchange experiment, calcite reacts with the NH₄Ac exchange solution generating Ca²⁺ cations. During the necessary washing steps to remove excess salt, calcite dissolution is lower but still occurs. The permanently added Ca²⁺ ions compete successfully with NH , especially during the washing steps. This leads to a more or less partial biionic exchange population resulting in an underestimation of the CEC which is calculated based on NH concentration of the clay by Kjeldahl analysis. The biionic exchange population was proven using the new silver thiourea technique with presaturation of calcite, AgTU calcite . The clay with 148 g kg^–1 calcite had a fraction of 16.4 cmol₊ kg^–1 exchangeable Ca²⁺. This is ca. 50% of the CEC of this clay being 31.8 cmol₊ kg^–1_. For clays with similar mineralogical composition, this trend is proportional to the calcite content.     

Uptake of cations by annual ryegrass as related to cations adsorbed onto root exchange sites

Abstract

Previously published results on exchange capacities for Ca²⁺, Mg²⁺, Mn²⁺, and K⁺ in the Donnan free space of roots of two ryegrass cultivars (Lolium multiflorum Lam. cv. Marshall and Wilo) grown at two Al levels in the nutrient solution (0 and 74 μM) were correlated with the average net uptakes of the same cations. For Al‐treated plants regressed separately, significant correlations r=0.906 and r=0.963 were found for Mn²⁺ and Ca²⁺/Mg²⁺, respectively. No significant correlations were observed for these cations in control plants. In contrast, when data of control and Al‐treated plants were combined, significant linear correlations r=0.805, r=0.924, and r=0.968 were found for Ca²⁺, K⁺, and K⁺/(Ca²⁺+Mg²⁺)^1/2, respectively. The influence of cations adsorbed onto the root exchange sites and the effect of Al on the cation uptake processes were discussed.     

Chemical Composition of Precipitation, Throughfall and Soil Solutions at Two Forested Sites in Guangzhou, South China

Rain water at two forested sites in Guangzhou (south China) show high concentrations of SO₄ ^2?, NO₃ ^? and Ca²⁺ and display a remarkable seasonal variation, with acid rain being more important during the spring and summer than during the autumn and winter. The amount of acid rain represents about 95% of total precipitation. The sources of pollutants from which acid rain developed includes both locally derived and long-middle distance transferred atmosphere pollutants. The seasonal variation in precipitation chemistry was largely related to the increasing neutralizing capacity of base cations in rainwater in winter. Soil acidification is highlighted by high H⁺ and Al³⁺ concentrations in soil solutions. The variation in elemental concentration in soil solution was related to nitrification (H⁺, NH₄ ⁺ and NO₃ ^?) and cation exchange reaction (H⁺, Al³⁺) in soil. The negative effect of soil acidification is partly dampened by substantial deposition of base cations (Ca²⁺, Mg²⁺ and K⁺) in this area.     

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.     

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.     

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.     

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.     

Effects of nitrogen source and aluminum on growth of tropical tree seedlings adapted to low pH soils

The effects of N-source and Al on the growth of seedlings of Melastoma malabathricum, Acacia mangium, and Melaleuca cajuputi, which are tropical woody plants and are very tolerant to Al, and barley (Hordeum vulgare), which is a typical Al-sensitive plant, were investigated. The Al and N treatments consisted of the application of either 0 or 0.5 mM Al, and 2 mM NH₄ ⁺ or N0₃ ^-, respectively. Growth of the tropical plants was enhanced by Al and NH₄ application. In all the plant species, the pH of the culture solution decreased and the concentrations of soluble Al and P increased with the + NH₄ treatment, which positively affected the growth of the tropical plant species. Excised roots of M. malabathricum dissolved insoluble Al with NH₄ application and absorbed Al mainly from root tips. Al did not affect the leaf N concentration except in the case of barley. Roots of M. cajuputi exuded a large amount of citrate, which slightly increased by the + Al treatment. In A. mangium, the reactivity of soluble Al to PCV (pyrocatecholviolet) decreased in the culture solution of the + Al + NH₄. treatment and Al concentration of roots in this treatment was very low. Roots of M. malabathricum released H+ along with Al uptake as well as NH₄ ⁺ uptake. It is concluded that Al and NH₄ ⁺ exert beneficial effects on the growth of tropical tree seedlings.     

Element Budgets in a Forested Watershed in Southern China: Estimation of a Proton Budget

We evaluated the element budgets in a forested watershed in Jiulianshan, southern China. The element input in bulk precipitation was characterized by high depositions of H⁺, NH₄ ⁺, Ca²⁺, and SO₄ ^2?, i.e., 400, 351, 299, and 876 eq/ha/yr, respectively. The outputs of H⁺, NH₄ ⁺, and SO₄ ^2? from the watershed were very low, while those of Ca²⁺ and Mg²⁺ were high, 712 and 960 eq/ha/yr, respectively. The element budgets suggested that i) the net retentions of H⁺, NH₄ ⁺, and SO₄ ^2? in this watershed were high, and ii) the net release of Mg²⁺ from this watershed was high mainly due to weathering. The net release of Ca²⁺ was not so high because of the high atmospheric deposition, while atmospheric deposition of Mg²⁺ was not so high (130 eq/ha/yr). Decrease of acid neutralizing capacity in the soil, i.e., net soil acidification, was caused mainly by the net release of Mg²⁺. Moreover, the net retention of SO₄ ^2? also contributed to soil acidification.     

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.     

钾对铵离子在蛭石矿物表面吸附与层间固定的影响

在K+、NH4+不同施用顺序的情况下研究了蛭石对NH4+的晶层固定 ,证实了K+的存在对NH4+的矿物晶层固定不具有影响。在Ca2+-NH4+二元系统和Ca2+-NH4+-K+三元系统中 ,对NH4+在蛭石表面的吸附进行了研究 ,结果同样显示 ,K+对NH4+在蛭石表面的吸附位点不具有竞争作用。     

Cation exchange capacity measurements

Abstract

Soil cation exchange capacity (CEC) measurements are important criteria for soil fertility management, vaste disposal on soils, and soil taxonomy. The objective of this research was to compare CEC values for arable Ultisols from the humid region of the United States as determined by procedures varying widely in their chemical conditions during measurement. Exchangeable cation quantities determined in the course of two of the CEC procedures were also evaluated. The six procedures evaluated were: (1) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity; (2) N Ca(OAc)₂ (pH 7.0) saturation with Mg(OAc)₂ (pH 7.0) displacement of Ca²⁺; (3) N NH₄OAc (pH 7.0) saturation with NaCl displacement of NH₄ ⁺; (4) N MgCl₂ saturation with N KCl displacement of Mg²⁺; (5) compulsive exchange of Mg²⁺ for Ba²⁺; and (6) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus N KCl exchangeable AJ. The unbuffered procedures reflect the pH dependent CEC component to a greater degree than the buffered methods. The compulsive exchange and the summation of N NH₄OAc exchangeable cations plus N KCl exchangeable Al procedures gave CEC estimates of the same magnitude that reflect differences in soil pH and texture. The buffered procedures, particularly the summation of N NH₄OAc exchangeable cations plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity, indicated inflated CEC values for these acid Ultisols that are seldom limed above pH 6.5. Exchangeable soil Ca and Mg levels determined from extraction with 0.1 M BaCl₂ were consistently greater than values for the N NH₄Oac (pH 7.0) extractions. The Ba²⁺ ion is apparently a more efficient displacing agent than the NH₄ ⁺ ion. Also, the potential for dissolving unreacted limestone is greater for the Ba₂ ⁺ procedures than in the NH₄ ⁺ extraction.     

EFFECTS OF CATION EXCHANGE MATERIAL ON ZINC ADSORPTION BY SOILS

The adsorption of Zn by soils which are different in their major cation-exchange materials was measured at equilibrium Zn concentrations up to 10^?2 M in 10^?2 to 10^?3 M CaCl₂. The results are interpreted on K^Zn_Ca[Zn]_soil plots, where K^Zn_Ca is the selectivity coefficient defined by the equation All natural samples except those containing halloysite exhibited no or very small specific Zn adsorption. All Ca-saturated samples exhibited specific Zn adsorption dependent on cation-exchange materials. The cation-exchange sites with high selectivities for Zn (K^Zn_Ca > 10) constitute more than 40 per cent of the total exchange sites in soils containing allophane, imogolite, and halloysite, whereas those with moderate to low selectivities for Zn (K^Za_Ca < 10) predominate in montmorillonitic, vermiculitic, and humic soils. Differences in the contribution of the respective cation-exchange materials to specific Zn adsorption are discussed relating to differences in the origin of their negative charge.