Extraction of copper,lead, zinc or cadmium ions sorbed on calcium carbonate

The amount of sorbed metal ion released from CaC0₃ by 16 different extractants was found to vary with the chemical nature of the solution and the metal ion involved. In general, acid solutions dissolved a high proportion of both substrate and Cu, Ph, Cd coatings; complexing agents dissolved the same coatings but left most of the calcite; and competing cations (e.g. NH₄ ⁺, Ca²⁺) displaced primarily chemisorbed Cd and Cu. In Zn studies, little metal ion was retrieved by any extractant due to the limited solubility of the coatings formed at pH < 7.7. The diverse behavior observed in the sorption studies has been interpreted in terms of solubility and absorption equilibria. The pH of the CaC0₃ suspensions was high enough to precipitate all added Pb as hydroxy species, and excess Cu tended to precipitate at pH > 6.4 if one increased the soluble carbonate level (e.g. by adding acid). Unlike Cd and Cu, Zn was not chemisorbed; it formed sparingly soluble compounds such as ZnC0₃.2Zn(OH)₂, with excess coming out as Zn(OH)₂ at pH > 7.7. The significance of the results in respect to the mobility of metal ions in calcareous soils, and the evaluation of available levels, has been considered.     

Observation of specific ion effects in humus aggregation process

Specific ion effects play a vital role in a variety of colloidal and interfacial processes. However, few studies have reported the specific ion effects in the humus aggregation process, which strongly influence the transport and fate of environmental pollutants. In this study, soil humus colloids were prepared and characterized, and the specific ion effects on humus aggregation in electrolyte solutions were investigated at a variety of concentrations and p H values using dynamic light scattering methods. Activation energy(?E), which is known to reflect the dynamics and stability of a colloidal system, was used to quantitatively characterize the specific ion effects. The results showed that given ?E value of 2.48 × 10~3 J mol~(-1) at p H 3.0, the electrolyte concentrations were 91.6, 58.2, 3.8, and 0.8 mmol L~(-1) for Na~+, K~+, Mg~(2+), and Ca~(2+), respectively, thus indicating significant specific ion effects in the humus aggregation process. Most importantly, decreasing the electrolyte concentrations increased the differences in the ?E value between two cation species with the same valence(i.e., ?EN_a-?E_K and ?E_(Mg)-?E_(Ca)), while increasing the p H increased the magnitude of ?E_(Mg)-?E_(Ca). However, the classic Derjaguin,Landau, Verwey, and Overbeek(DLVO) theory and the double layer theory, as well as the currently widely used ionic hydration and dispersion effects, failed to predict the experimentally observed increase in the specific ion effects with decreasing electrolyte concentrations in a quantitative sense. These results have implications for the necessity of involving specific ion effects for a better understanding of humus aggregation and interactions in aqueous and soil systems.     

Influence of electrolytes on EDTA extraction of Pb from polluted soil

Because the economics of soil extraction processes depend on conservation and reuse of costly chelating agents, the ability of various electrolytes to modify EDTA extraction of Pb from a grossly-contaminated soil (Pb_T=21%) was investigated using batch equilibration experiments. In the absence of added electrolyte, a single 5-hr. extraction with 0.04 M EDTA (corresponding to 1∶1 Pb_T to EDTA ratio) released 65% of Pb_T over the pH 5 to 9 range. Under these conditions, Na⁺-, Li⁺-, and NH₄ClO₄ salts at 0.5 M increased Pb desorption to nearly 80%, probably from exchange displacement of soilbound Pb²⁺ and increased solubility of Pb-containing phases at higher ionic strength. Because Cl^? and ClO₄ ^? salts were equally effective, chlorocomplex formation was insignificant. Under slightly acidic conditions, Ca(ClO₄)₂ and Mg(ClO₄)₂ at 0.167 M caused roughly the same elevation in Pb recovery as 0.5M of the monovalent electrolytes. However, with progressively higher pH, Ca, and to a lesser extent Mg, suppressed Pb solubilization by competitive chelation of EDTA. Pb recovery by EDTA soil washing could be enhanced by addition of Ca salts at pH 4 to 6. Subsequent pH elevation in the presence of Ca would promote decomposition of Pb-EDTA complexes and separation of Pb as a hydroxide precipitate.     

Solubility of soil phosphorus as influenced by urea

The objective of this study was to investigate possible ways of mobilizing residual fertilizer P as a result of local pH elevation caused by urea hydrolysis. The response of water-soluble P (P_w) and dissolved organic C (DOC) to urea hydrolysis was monitored in three cultivated soils and at two P levels for up to 127–135 d and compared with corresponding changes in soils limed with Ca(OH)₂. Hydrolysis of urea was complete in 8–15d during which soil pH increased by 1–1.5 units at the maximum. Subsequently, the pH decreased to or below the original level owing to nitrification. Mobilization of soil P was enhanced substantially in parallel with the increase in pH, the peak P_w occurring simultaneously with the highest pH value. In all urea-treated soils, P_w remained at an elevated level for at least 60d. As compared to urea, elevation of soil pH with Ca(OH)₂ had only a minor and inconsistent influence on P_w. In mobilization of soil P, the urea-induced increase in pH and a simultaneous production of NH₄⁺ ions proved to be superior to liming with Ca(OH)₂. It was hypothesized that when an acid soil is amended with urea, phosphate is first displaced by OH^? ions, resulting in elevated solution P concentrations. A simultaneous dissolution of organic matter contributes to the persistence of high P concentration by competition for sorption sites on Fe and Al oxides, and thus retards the resorption of P.     

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.     

CALCIUM: ALUMINIUM EXCHANGE EQUILIBRIA IN CLAY MINERALS AND ACID SOILS1

Exchange reactions between 0.0in AlCl₃ solutions of different pH and Ca-saturated montmorillonite, vermiculite, illite, and soils from the Park Grass Experiment at Rothamsted and the Deerpark Experiment, Wexford, Ireland, showed that Al³⁺ and Al(OH)₂⁺ were adsorbed from solutions of pH > 4.0 and Al³⁺ and H⁺ from solutions of pH < 3.0. When Al was adsorbed, the cation exchange capacity of Ca-saturated soils and clays increased. Conventional Ca: Al exchange isotherms showed that Al³⁺ was strongly preferred to Ca²⁺ on all soils and clays. The equilibrium constant for Ca: Al exchange, K, was identical for soils before and after oxidizing their organic matter and did not vary, for any exchanger, with Al-saturation or the initial pH of the AlCl₃ solution. This proved the validity of the procedure used for calculating exchangeable Al³⁺. K values for Ca:Al exchange favoured Al³⁺ in the order: vermiculite > Park Grass soil > Deerpark soil > illite > montmorillonite. The influence of surface-charge densities of the clay minerals on this order is discussed and a method proposed and tested for calculating the K value of a soil from its mineralogical composition.     

Remediation of Lead Contaminated Soil by EDTA. I. Batch and Column Studies

Extraction using ethylenediaminetetraacetic acid (EDTA), and other chelates has been demonstrated to be an effective method of removal of Pb from many contaminated soils. However, column leaching of Pb from alkaline soils with EDTA has been problematic due to extremely low soil permeability. The first purpose of this study was to develop batch extraction procedures and methods of analysis of batch extraction data to provide Pb solubility information which can be used to model the column extraction of Pb from soils. The second purpose was to determine the effect of the addition of KOH and CaCl₂ to K₂H₂EDTA extract solution on both hydraulic conductivity and Pb removal. A Pb-contaminated soil sample was collected from an abandoned battery recycling facility. Both batch shaker extractions and column leaching experiments were completed using 5 different EDTA extract solutions. When only CaCl₂ was added to EDTA no change in the amount of Pb removed by batch extraction was observed. As expected, lead solubility was observed to decrease as pH was increased by the addition of KOH. However, Pb solubility was only slightly decreased by the addition of both CaCl₂ and KOH. The amount of time required to leach 6.0 L of extraction solution through the soil columns varied from 2 to 33 days. The addition of CaCl₂ and/or KOH resulted in increased soil hydraulic conductivity relative to the EDTA-only solution. The hydraulic conductivity was related to residual calcium carbonate content, suggesting that dissolution of CaCO₃ and subsequent production of CO₂ gas in the soil pores was partially responsible for the observed reductions in soil permeability. However, Pb removal was diminished with the addition of CaCl₂ and KOH because of the decreased Pb solubility and also kinetic limitations associated with the shorter residence time of the extract solution in the column.     

The effects of feeding byOniscus asellus (Isopoda) on nutrient cycling in an incubated hardwood forest soil

Summary Oniscus asellus produced changes in the nutrients leached from Oie and Oa horizons of a hardwood forest soil. Soil with isopods lost more K⁺ (54%) from the Oie horizon and more Ca²⁺ (25%), Mg²⁺ (40%), and water-extractable S (23%) from the Oa horizon than soil without isopods. In contrast, soils with isopods lost less Ca²⁺ (39076) from the Oie horizon and less dissolved C-bonded S (33%) from the Oa horizon than soil without isopods. In addition, the Oia and Oa horizons exhibited different nutrient dynamics. When isopods were present, the Oa horizon leachates accumulated more Na⁺ K⁺, Ca²⁺, Mg²⁺, NO₃ ^– , water-soluble SO₄ ^2–, and dissolved C-bonded S, and the Oie horizon retained more of these nutrients. The type of leaching solution also had a major effect on nutrients. Leaching with a simulated soil solution resulted in smaller nutrient losses for K⁺ and Mg²⁺ in both horizons and for Na⁺, Ca²⁺, and NO₃ ^– in the Oa horizon than leaching with distilled water.     

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.     

Ability of sorption–desorption experiments to predict potassium leaching from calcareous soils

When potassium (K⁺) fertilizers are applied to the soil, K⁺ is subject to displacement through the soil profile. Leaching can play an important role in agricultural K⁺ losses that can decrease groundwater quality. To avoid overfertilization, estimation of K⁺ leaching from soil is important. The ability of the soils to retain K⁺ against leaching varies according to the adsorption coefficient of the soils. The aim of this study was to relate the K⁺ leaching from a wide range of calcareous soils to the values obtained from a sorption–desorption experiment. The soil columns were leached with 10 mM CaCl₂ solution and the leachate was analyzed for K⁺. The breakthrough curves for K⁺ were different, and the amounts of K⁺ leached varied considerably between different soils. In these calcareous soils where crops are irrigated with water containing significant concentrations of Ca²⁺ and other cations, large amounts of K⁺ will be leached. Cumulative K⁺ leached after five pore volumes leaching with 10 mM CaCl₂ was significantly (r = 0.776, p < 0.01) related to the equilibrium K⁺ concentration. The results of this study enabled us in many cases to estimate the K⁺ leaching from soil without conducting column experiments, minimizing the laborotary work.     

Canopy leaching of cations in Central European forest ecosystems – a regional assessment

The leaching of Ca, Mg, and K from canopies is a major pathway of these cations into forest soils. Our aim was to quantify rates of canopy leaching and to identify driving factors at the regional scale using annual fluxes of bulk precipitation and throughfall from 37 coniferous and deciduous forests of North and Central Europe. Total deposition of Ca, Mg, K, and H⁺ was estimated with Na as an index cation. The median canopy leaching increased in the order: Mg (0.11 kmol_c ha^–1 a^–1) < Ca (0.31 kmol_c ha^–1 a^–1) < K (0.39 kmol_c ha^–1 a^–1). Canopy leaching of Ca and K was positively correlated with the calculated total H⁺ deposition and H⁺ buffered in the canopy, whereas canopy leaching of Mg was not. With contrasting effects, fluxes of SO₄‐S and NH₄‐N in throughfall explained to 64 % (P<0.001) of the Ca canopy leaching. Fluxes of NH₄‐N and Ca were negatively correlated, suggesting that buffering of H⁺ by NH₃ deposition reduced canopy leaching of Ca. Amount of bulk precipitation and SO₄‐S in throughfall were identified as much weaker driving factors for canopy leaching of K (r²=0.28, P<0.01). Our results show that Ca is the dominant cation in buffering the H⁺ input in the canopy. At the regional and annual scale, canopy leaching of Mg appears to be unaffected by H⁺ deposition and H⁺ buffering in the canopy.     

Determination of exchangeable potassium in soil using ion-selective electrodes in soil suspensions

Summary Methods of determining exchangeable K⁺ of soil by mixing extracting solutions and analysing the soil suspension with ion‐selective electrodes were developed and evaluated on 30 samples of soils. From preliminary comparisons of the K⁺ extracted by BaCl₂ and NH₄OAc solutions and by batch and leaching treatments of soils, we established that suspensions of 5 g soil in 100 ml 0.5 m BaCl₂ and single batch treatments of 1 h should be used. The exchangeable K⁺ was determined with a K‐selective, valinomycin‐based PVC membrane electrode and electrochemical cells that did or did not include a liquid junction (the reference electrode being a double‐junction reference electrode assembly with a 10 m LiOAc salt bridge solution or a Cl‐selective electrode, respectively). The Ba‐exchangeable K⁺ values were sensibly the same whether a liquid junction was involved or not, a result that can be attributed to the beneficial effects of the salt bridge and the ionic strength of the extractant. Comparisons of these Ba‐exchangeable results with those obtained by various combinations of batch or leaching treatments, BaCl₂ or NH₄OAc extractants and filtrate analysis by the ion‐selective electrode method or atomic absorption spectrometry showed they were all highly correlated (r≥ 0.996). The selectivity of the K⁺‐selective electrode (k^pot_KNH4 = 0.004) significantly reduced the interference from indigenous soil NH₄⁺ in the BaCl₂ suspensions. Overall, the results show potentiometric measurements of K⁺ in soil suspensions can provide a simple, rapid, and reliable means of determining exchangeable K⁺ in soils.     

Additional application of aluminum sulfate with different fertilizers ameliorates saline-sodic soil of Songnen Plain in Northeast China

Purpose

Serious soil salinization, including excessive exchangeable sodium and high pH, significantly decreases land productivity. Reducing salinity and preventing alkalization in saline-sodic soils by comprehensive improvement practices are urgently required. The combinations of aluminum sulfate with different types of fertilizer at different rates were applied on rice paddy with saline-sodic soils of the Songnen Plain in Northeast China to improve soil quality and its future utilization.

Materials and methods

Experiments were carried out in a completely randomized block design. Twelve treatments with aluminum sulfate at the rates of 0, 250, 500, and 750 kg hm^?2 with inorganic, bio-organic, and organic-inorganic compound fertilizers were performed. Soil pH, electronic conductivity (EC), cation exchangeable capacity (CEC), exchangeable sodium percentage (ESP), total alkalinity, sodium adsorption ratio (SAR), soil organic carbon (SOC), available nutrients, soluble ions, rice growth, and yield in the saline-sodic soils were measured across all treatments. The relationships among the measured soil attributes were determined using one-way analysis of variance, correlation analysis, and systematic cluster analysis.

Results and discussion

The pH, EC, ESP, total alkalinity, SAR, Na⁺, CO₃^2?, and HCO₃^? in saline-sodic soil were significantly decreased, while CEC, SOC, available nitrogen (AN), available phosphorus (AP), available potassium (AK), K⁺, and SO₄^2? were significantly increased due to the combined application of aluminum sulfate with fertilizer compared with the fertilizer alone. The most effective treatment in reducing salinity and preventing alkalization was aluminum sulfate at a rate of 500 kg hm^?2 with organic-inorganic compound fertilizer. This treatment significantly decreased the soil pH, EC, ESP, total alkalinity, SAR, Na⁺, and HCO₃^? by 5.3%, 28.9%, 41.1%, 39.3%, 22.4%, 23.5%, and 35.9%, but increased CEC, SOC, AN, AP, AK, K⁺, SO₄^2?, rice height, seed setting rate, 1000-grain weight, and yield by 77.5%, 115.5%, 106.3%, 47.1%, 43.3%, 200%, 40%, 6.2%, 43.9%, 20.3%, and 42.2%, respectively, compared with CK treatment in the leaching layer.

Conclusions

The combined application by aluminum sulfate at a rate of 500 kg hm^?2 with organic-inorganic compound fertilizer is an effective amendment of saline-sodic soils in Songnen Plain, Northeast China. These results are likely related to the leaching of Na⁺ from the soil leaching layer to the salt accumulation layer and desalination in the surface soil, and the increase of SOC improved the colloidal properties and increased fertilizer retention in soil. In addition, the environmental impact of aluminum sulfate applied to soil needs to be further studied.

     

Leaching behaviour of a sandy soil amended with natural and NH4+ and K+ saturated clinoptilolite and chabazite

Using saturated or enriched zeolites as slow release fertilizers (SRFs) is considered as an environmental-friendly strategy to enhance use of macronutrients in sandy soils. In this paper, two natural zeolites, clinoptilolite (CLI) and chabazite (CHA) were used as mineral precursors to prepare NH₄⁺/K⁺ saturated clinoptilolite (NH₄⁺/K⁺-CLI) and chabazite (NH₄⁺/K⁺-CHA) as zeolitic nutrient sources (ZNSs). Comparison between the nutrient retention capabilities of these ZNSs was one of the main objectives of this study. The NH₄⁺/K⁺-CLI and NH₄⁺/K⁺-CHA were prepared by soaking the zeolites in NH₄Cl and KCl solutions, respectively. Leaching tests were performed on a sandy soil amended with chemical fertilizers (CFs), NH₄⁺/K⁺-CLI and NH₄⁺/K⁺-CHA to evaluate the leaching properties of them. The results indicated that approximately 84% and 88% of the NH₄⁺ and K⁺ of soils fertilized with CFs were lost during the experiment, respectively. While, the NH₄⁺ and K⁺ losses from soils amended with NH₄⁺/K⁺-CLI and NH₄⁺/K⁺-CHA were 29%, 23%, and 14%, 24%, respectively. Despite of drastic changes in leaching behavior of CFs, nutrient losses from ZNSs were more uniform during the experiments. No significant difference was found between the two studied zeolites on reduction of K⁺ loss. However, CHA was more effective in preventing NH₄⁺ loss during leaching.     

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.     

The influences of temperature on aqueous aluminium chemistry

Temperature affects the solubility of Al(OH)₃(s), the solubility product formed, the hydrolysis and molecular weight distribution of aqueous Al species as well as the pH of the solutions. In the present work, identical solutions of inorganic Al (400, 600, and 800 μg Al L^?1) were stored for 1 mo at either 2 or 25 °C. In the solutions stored at 25 °C pH varied from 4.83 to 5.07, while in the corresponding solutions stored at 2 °C pH varied from 5.64 to 5.78. In spite of the relatively low pH at 25 °C, significant amounts of high molecular weight Al species were precipitated from the solution and the solubility product (log^* K _s ) of (Al(OH)₃) (s) was low (9.0). Substantial amounts of high molecular weight Al species were also formed at 2 °C, but the majority was present as colloids in the solution. The solubility product (converted from 2 into 25 °C) was 10.2, reflecting a solubility product of an amorphous (Al(OH)₃)(s) phase. The different physico-chemical forms of Al present at 2 and 25 °C should have relevance for water/soil chemistry modeling.     

Bioavailability and Rhizotoxicity of Cd to Pea (Pisum sativum L.)

Risk assessment of cadmium (Cd) contamination in soils requires identifying the bioavailable portion of the total Cd, a portion that is determined by environmental conditions such as pH and calcium (Ca) level in soils and by the physiological processes going on in the plant roots. Growth tests in solutions were conducted to develop a terrestrial biotic ligand model to describe uptake and rhizotoxicity of Cd to pea (Pisum sativum L. cv. Lincoln). Inhibition concentration associated with a 50% reduction in root elongation (IC₅₀) values were found to vary with external Ca²⁺ and H⁺ activities. Root-bound Ca was found to reach a plateau of about 63 µmol g^?1 (dry weight) although Ca treatment increased from 0.04 to 2 mmol L^?1. When experimental treatments (e.g., pH 6, Ca 0.2 to 2 mM) resulted in sufficient Ca supply, dose–response curves relating root elongation to root-bound Cd could be modeled with Weibull equations; IC₅₀ values were expressed in terms of root-bound Cd concentration. When the treatments (e.g., pH 4 or 5, Ca 0.04 mM) suggested a low Ca supply, root elongation was more sensitive to Ca content and root-bound Ca concentration became the dominant predictor variable. Cd accumulation was modeled by treating the pea roots as an assemblage of biotic ligands with known site densities (Q _Lj ) and proton binding constants (K _HLj ). The logK _Ca and logK _Cd values were established using measured root-bound ion concentrations and solution chemistry. The logK _Ca values were negatively correlated to root Ca contents. The logK _Cd values were positively correlated to logK _Ca values. Explanations for the changing of constants are discussed.     

Response of acid sulphate soils to different liming treatments II. Exchangeability of soil cations

Liming-induced changes in cation exchangeability were studied in six samples from acid sulphate soils (pH 3.9-4.7) incubated with water or with equivalent quantities of Ca(OH)₂ or KOH. The extractability of acid cation species susceptible to hydrolysis was shown to be affected not only by increased pH but also by the kind of the cation and related electrochemical properties of the base used. Both liming treatments practically eliminated the exchangeable Al. In the virgin soils, however, the polynuclear Al-complexes formed by Ca(OH)₂ treatment seemed to have been hydrolyzed further. The superiority of Ca(OH)₂ was assumed to be due to the higher valency of its cation and its act of provoking a higher increase in ionic strength. The liming agents affected to varying extents also the extractability of base cations. Exchangeable soil K seemed to decrease by the KOH treatment and the soil Ca by the Ca(OH)₂ treatment, whereas K and Ca were only slightly, if any, affected by the Ca(OH)₂ and KOH treatments, respectively. Thus, the reductions were assumed to be attributable to other factors than increased pH. A fixation of K and a possible precipitation of Ca as CaSO₄ were discussed. Ca(OH)₂ decreased in all soil samples the exchangeability of Mg more than did KOH. The depression was not related to the Al polymerization and, thus, cannot entirely be ascribed to specific sorption on Al gel. The results imply that liming may affect base cation exchange reactions by neutralizing exchangeable Al of high bonding strength and by replacing it by cations of the liming agent.     

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.     

The Impact of Acid Rain on Potassium and Sodium Status in Typical Soils of the Wielkopolski National Park (Poland)

A previous investigation of thechemical characteristics of precipitation in theWielkopolski National Park has shown its high acidity,which sometimes drops below pH 3.0. This paper dealswith the leaching of potassium and sodium ions by acidrain from typical soils of the study area. Laboratoryexperiments were conducted on undisturbed soil cores(15 cm in diameter, 50 cm high) with acid solutions of pH 3.0, pH 2.0 and with water of pH 5.6 (control). The sprinkling lasted 30 days simulating a rainfall of 400 mm. The eluates were analysed daily. Soil propertiesand forms of potassium and sodium were determinedbefore and after treatment. The investigations showthat quite significant amounts of K⁺ andNa⁺ can be leached from the soil: in the very acidtreatment (pH 2.0) about 4 mg K⁺ and 3 mgNa⁺ per kg of soil. The leaching of these elementswas smaller in the pH 3.0 and 5.6 treatments.Differences in the dynamics of the process are shownin the leaching curves. In the case of potassium theirshapes are smooth when pH is 5.6 and 3.0, while at pH2.0 the curves rise sharply. The leaching curves inthe case of sodium do not show sharp peaks, whichmeans that the leaching is slow and equalised.