Adsorption of sulphate and fluoride by variable charge soils

The adsorption of sulphate and fluoride by three variable charge soils was studied. Adsorption increased with increase in the amount of sulphate added at constant _pH, and decreased with increase of _pH.
The ratios of the amount of released OH⁻to that of the adsorbed SO^2-₄ at pH 5–0 were 0.12, 0.14 and 0.20 for the three soils, respectively, much lower than the corresponding OH⁻/F⁻ ratios which ranged from 0.3 to 1.0. For a ferric acrisol the OH⁻ released accounted, on average, for only 15% of the SO^2-₄ adsorbed, leaving more than 60% to be explained by the decrease in positive charge and the increase in negative charge carried by the soil.     

Correlations between extractable Na, K, Mg, Ca, P & N from fresh and dried samples of two Aquults

Significant increases in extractable ions resulted from air-drying and grinding samples of two infertile Aquults. Effects of the sample preparation differed markedly between ions and between the two soils. Regression equations were calculated to predict extractable ions in dried, ground samples from extractable ions in fresh, unground samples and the relationships were compared between the two soil series. Regressions were significantly different between soils for extractable PO³₄, Mg⁺⁺, and K⁺, but not for Ca⁺⁺ and Na⁺. Extractable NH ⁺₄ and NO-₃ in fresh, unground samples were not correlated with those in air-dry, ground samples of either soil. Differences in response to preparation between soil types appeared to be related to the oxidative status of these soils in the field, wherein constituents of more poorly-drained soils may be less stable to the oxidizing conditions of air-drying and grinding. Such complexities suggest that effects of sample preparation should be considered when interpreting soil nutrient data for studies of forest nutrient cycling and forest soil fertility.     

Ion Transport and Permeability in an Allophanic Andisol at Low pH

Allophanic Andisols have a significant pH-dependent charge. The positive charge increases and the negative charge decreases as pH decreases; therefore, anion movement becomes slower and cation movement becomes faster as pH decreases in the soil. At low pH, soil dispersion occurs easily due to electric repulsive force. The permeability of the soil then decreases because of structural changes that occur when dilute HCl or HNO₃ is percolated in the soil. However, soil permeability does not decrease when dilute H₂SO₄ is percolated in the soil. This is because SO₄^2- strongly adsorbs on the soil surface at low pH and the soil remains flocculated.     

Charge characteristics in relation to free iron and organic matter of soils from Bambouto Mountains, Western Cameroon

We have examined the charge characteristics, with special emphasis on the role of free Fe and organic matter, of humid tropical soils from Bambouto Mountains, Western Cameroon. The soils, which are formed from tuff, basalt and trachyte, are dominated by kaolinite and sesquioxides. The amounts of Fe oxides in them increase somewhat with depth. Open 2:1 phyllosilicates are present in trace amounts. The point of zero charge of the variable charge components, pH₀, is around 4 in the topsoil (0–20 cm) and around 6 at 100–150 cm depth. In the subsoils, pH₀ exceeds soil pH presumably because of large quantities of Fe oxides. Deferration increases both soil pH and pH₀, but diminishes the anion exchange capacity. Oxides and oxyhydrates of Fe have positive surface charge, so their removal from the soils would result in overall loss of positive charge. Increases in soil pH would bring about an increase in the cation exchange capacity of the soils. Hence, management practices that reduce soil acidity should reduce loss of essential basic cations via leaching.     

Acid inputs from the atmosphere in the United Kingdom

Abstract. Inputs of acidity to the ground arise through two distinct routes: wet deposition which includes all acidity deposited in rain and snow and dry deposition, the direct sorption of SO₂, NO₂ or HNO₃ gases by vegetation or soil surfaces. The acidity from dry deposition of SO₂ and NO₂ is created during the oxidation of deposited SO₂ and NO₂ to SO²₄ and NO₃⁻ respectively. The areas of Britain experiencing the largest wet deposition of acidity are the high rainfall areas of the west and north, in particular the west central highlands of Scotland, Galloway and Cumbria where inputs exceed 1 kp H⁺ ha⁻¹ annually. Wet deposited acidity in the east coast regions of Britain is in the range 0.3–0.6 kg H⁺ ha⁻¹ a⁻¹. Monitoring data for rainfall acidity at rural sites throughout northern Britain show a decline in deposited acidity of about 50% during the last six years. Dry deposition is largest in the industrial midlands and southeast England and in the central lowlands of Scotland, where concentrations of SO₂ are largest. In these regions the dry deposition of SO₂ following oxidation may lead to acid inputs approaching 3 kg H⁺ ha⁻¹ a⁻¹ and greatly exceeding wet deposition.     

A multiple ion uptake model

A model is developed which describes uptake of Ca, Mg, K, NO₃, Cl, and SO₄. The electrical neutrality of plant and soil are maintained through exchange of H or OH at the root-soil interface, constant partial pressure of CO₂ and non-exchangeable H reaction with the soil cation exchange complex.
An important innovation in this model is the inclusion of electrical neutrality as a condition for plant, soil and soil solution. The uptake of cations is a function of both concentration of anions in solution and the suite of exchangeable cations. The model emphasizes an important role for CO₂ in soil chemistry and plant nutrition. Presently, the model is most useful for generating research hypotheses. Perhaps the most important hypothesis is that something about as complicated as the present model will be required to model multiple ion uptake and crop yields.     

The nature and kinetics of organic matter release from soil by salt solutions

Laboratory experiments in soil columns were performed to study the influence of dissolved salts on the amount and composition of organic matter (OM) released from soil. Samples of two surface soils from former wastewater infiltration sites were leached with solutions containing dissolved salts (NaH₂PO₄, NaNO₃, CaCl₂) and by deionized water. The NaH₂PO₄ solution induced strongest release with 0.6% of soil organic carbon (C_org) with 700 ml for 100 g of soil, while CaCl₂ released the least, summing to 0.1–0.2% of C_org. The OM released was characterized by UV absorbance (aromaticity), ultrafiltration (molecular size distribution) and solid-phase extraction (polarity). The results suggest that CaCl₂ preferentially released readily soluble OM. For the other solutions we assume solubilization by enhanced electrostatic repulsion (water), sodium exchange (NaNO₃), and sodium exchange and calcium decomplexation and displacement of sorbed organic anions (NaH₂PO₄) to be the major mechanisms of release. In all experiments a phase of spontaneous desorption was observed, followed by a phase of steady-state desorption. Activation energies for steady-state release were estimated from kinetic investigations and suggest that the release is controlled by diffusion towards the phase boundary. These investigations emphasize the influence of dissolved salts on the nature and quantity of organic matter released from soil. The method presented seems able to characterize soil organic matter with respect to its availability and its mode of association with the soil matrix.     

Effects of sorbed orthophosphate on zinc status in three soils of eastern Canada

P-Zn interactions can affect fertilizer use and produce Zn deficiencies with certain crops. Phosphorus-Zn sorption-desorption reactions were studied in topsoil and subsoil samples from three Quebec soils. Soils were equilibrated with P solutions, then with Zn solutions, and finally with solutions containing no P or Zn. The first equilibration evaluated P sorption (P_s), the second evaluated Zn sorption (Zn_s) after P sorption (P_s), and the third evaluated Zn desorption (Zn_D) as related to added P. Subsequently, Zn fractions were extracted sequentially with KNO₃ (Zn _{kno 3}), NaOH (Zn_NaOH) solutions and concentrated HN0₃+ H₂0₂(Zn_HNO,).
One mmole sorbed P resulted in increases of 0.5 to 1.0 meq (mean = 0.72) increases in cation exchange capacity (CEC). Increased Zn_s with added P was equivalent to 4 to 5% of the increase in CEC induced by P_s in the Uplands (sand) and St. Bernard (loam) soils, and 0.4 to 0.9% in the Dalhousie (clay) soils, while one meq increase in CEC resulted in 1.5-3.5% decrease in Zn_D. There existed positive correlations between P_s and extractable soil Fe materials. Phosphate sorption enhanced associations between Zn_s, Zn_D or Zn fractions and soil organic or crystalline Fe contents, confirming that P addition increased specific sorption of Zn on Fe components. Other mechanisms including precipitation, P-induced negative charge and 'bridge' effects are also discussed.     

Methane oxidation kinetics differ in European beech and Norway spruce soils

Coniferous forest soils often consume less of the greenhouse gas methane (CH₄) than deciduous forest soils. The reasons for this phenomenon have not been resolved. It might be caused by differences in the diffusive flux of CH₄ through the organic layer, pH or different concentrations of potentially inhibitory compounds. Soil samples were investigated from three adjacent European beech ( Fagus sylvatica ) and Norway spruce ( Picea abies ) stands in Germany. Maximal CH₄ oxidation velocities (V_max(app)) and Michaelis Menten constants (K_M(app)), retrieved from intact soil cores at constant CH₄ concentrations, temperature and matric potential, were twice as great in beech as in spruce soils. Also atmospheric CH₄ oxidation rates measured in homogenized soil samples displayed the same trend. Greatest atmospheric CH₄ oxidation rates were detected in the O_a horizon or in the upper 5 cm of the mineral soil. In contrast to the beech soils, the O_a horizon of the spruce soils consumed no CH₄. A differential effect due to divergent diffusive flux through the litter layer was not found. pH and ammonium concentration were similar in samples from both forest soil types. Ethylene accumulation in all soils was negligible under oxic conditions. These collective results suggest that the different atmospheric CH₄ uptake by beech and spruce soils is caused by different CH₄ oxidizing capacities of methanotrophic communities in the O_a horizon and top mineral soil.     

Aluminium speciation and pH of an acid soil in the presence of fluoride

The aim was to determine whether the addition of F to an acid soil reduces the concentration of free Al³⁺ and other forms that have been shown to be toxic to plants. The ability of two different extracts to reflect Al speciation in the soil solution was also investigated. Addition of F (0-5.2μmolg⁻¹) to an acid soil (pH 4.15, soil solution) increased the pH and total concentrations of Al and F in the soil solution whereas Al³⁺ remained constant or decreased. Soil solution pH, total soluble Al and Al extracted by 0.01 m CaCl₂ are not good predictors of the likelihood of aluminium toxicity in soils containing soluble fluoride.     

Ion chromatographic determination of nitrate in 2 M KC1 soil extracts

A method for the analysis of NO₃-N in a 2 m KCL soil extract, using ion chromatography, is discussed. It involves the treatment of the KCl extract with Ag⁺-activated cation exchange resin to remove excess Cl^–, followed by analysis using a Dionex ion chromatograph. The results of NO₃-N in soil extracts agreed closely with those obtained by steam distillation. The method was linear up to 20 mg N dm^–3 with a detection limit of 0.1 mg Ndm^–3. Quantitative recovery of NO_3-N added to soil extracts over the linear range were observed.
Standard deviations of samples containing 2.57 and 15.11 mgN dm^–3 were found to be 0.06 and 0.11 respectively.     

Factors affecting formation of methyl nitrite in soils

The formation of CH₃ONO in 11 soils treated with HNO₂ or NaNO₂ in a closed system, was studied by measuring the concentration in the gas space above the soil and by absorbing CH₃ONO in HI. The gaseous concentration of CH₃ONO increased and then decreased following additions of HNO₂ or NaNO₂, and the production of CH₃ONO increased with increasing concentrations of HNO₂ or NaNO₂ added to soils.
The amounts of CH₃ONO trapped in HI were 13.5 to 20.4 times higher than those determined by integrating under the net production curves. The evolved CH₃ONO amounted to 0.4 to 3.5% of added NO₂⁻, and 4.2 to 50% of the gaseous forms of N absorbed by acidic KMnO₄ solution. The CH₃ONO evolved from soils was positively correlated with the methoxy content of the soils, and inversely related to soil pH, with negligible amounts being evolved from alkaline soils. The results show that CH₃ONO is a product of NO₂⁻ decomposition in soils, and indicate that small concentrations of the gas may be produced in N–fertilized soils in which NO₂⁻ accumulates.     

Prolonged leaching of mineral forest soils with dilute HCl solutions: the solubility of Al and soil organic matter

Long-term acidification has been shown to result in a considerable decrease in the amount of organically bound soil Al and in a gradual decrease in the solubility of Al. We examined the solubility of soil organic matter (SOM) and Al in four acid mineral soils (one Arenosol Ah, two Podzol Bh, and one Podzol Bs) as they were leached sequentially using a solution containing 0.001 m HCl and 0.01 m KCl. The acid leaching resulted in relative decreases in Al that were 2–6 times greater than for organic C. The organic C and Al dissolved by the acid leaching originated mainly in the pyrophosphate-extractable fraction of the elements. Protonation seems to be a major mechanism in stabilizing the residual SOM, as indicated by small changes in effective cation exchange capacity with the degree of acid leaching. In the samples of Podzol Bh and Arenosol Ah soils the solubility of Al (defined as log₁₀{Al³⁺} + 1.5pH) in equilibrium suspensions (0.01 m KCl) was closely related to the ratio of pyrophosphate-extractable Al to pyrophosphate-extractable organic C. The Podzol Bs sample probably contained a small amount of a surface-reactive Al(OH)₃ phase, which rapidly became depleted by the acid leaching.     

The importance of exchangeable cations and resin-sink characteristics in the release of soil phosphorus

The significance of exchangeable cations in the release of phosphorus by sequential extraction with water was evaluated in 11 acid (_pH 5.0–6.3) New Zealand soils contrasting in P status and P retention. The release of P from Na-saturated soil exceeded that from the original Ca-dominated soils by up to four-fold. Possible explanations for the larger P release in the Na system include: (i) desorption of P induced by increased surface negative potential associated with the exchange of Na for Ca/Mg, and/or (ii) accelerated dissolution of Ca phosphate compounds or complexes resulting from the creation of a sink for Ca.
The potential of a series of anion- and cation-exchange resin systems (AER and CER, respectively) as sinks for labile soil P was also examined. For all soils studied, P extracted by AER-HCO₃ 3 and that removed by sequential extraction with water. Also, the amounts of P extracted by AER-OH/CER-H and NaCl/ H₂O were closely correlated ( r = 0.95**), suggesting similar release mechanisms. The results obtained indicate that charge-balancing cations, particularly Ca which is the predominant exchangeable cation in the majority of soils, exerts a more significant control on soil P equilibria in acid soils than is commonly recognized.     

Quasi-thermodynamic basis of competitive-adsorption coefficients for anionic mixtures in soils

Equilibrium competitive-adsorption coefficients derived from the application of the multi-component Freundlich-type Sheindorf-Rebhun-Sheintuch (SRS) equation appeared to have a soil-specific, quasi-thermodynamic basis. Using three soils, single-solute adsorption isotherms at 288, 298, and 308 K were constructed with arsenate, phosphate, and molybdate. A Clausius-Clapeyron-type equation was used to estimate apparent isosteric heats of adsorption values ( q _iso). The adsorption of the three anions was characterized by different q _iso values, and was generally in the order of H₂PO₄->H₂AsO₄->MoO₄^2-. The magnitudes of the SRS competitive coefficients derived from binary-solute mixtures were in agreement with the distributions of the heats of adsorption of the individual anion-soil systems. Higher competitive coefficients, indicating the preferential adsorption of solute i in a binary-solute mixture containing solutes i and j, tended to associate with higher q _iso values for solute i relative to solute j. However, the q _iso values for the adsorption of molybdate by one soil underestimated the extent of competitive interactions between arsenate and molybdate.     

Decomposition in a peaty soil improved for pastoral agriculture

Abstract. The rates of CO₂ production and decomposition of ¹³C-enriched Lolium perenne leaves and roots in soil from the surface five cm of two upland stagnohumic gley soils were measured in laboratory experiments. One of the soils had been limed (pH 6.8) 13 years earlier. The other was unlimed (pH 3.7). Liming increased the rate of CO₂ release from soil to which no L. perenne had been added. About 30% of the ¹³C in L. perenne leaves remained in both limed and unlimed soil after 224 days. By contrast, less ¹³C-remained in the limed soil amended with L. perenne roots (44%) than in the limed soils (55%). Although the daily rate of CO₂ from the plant material-amended soils was initially greater in the improved than in the unimproved soil, it subsequently declined more rapidly.     

Methane fluxes on agricultural and forested boreal organic soils

Abstract. Annual methane fluxes from an organic soil in eastern Finland, originally drained and planted with birch ( Betula pendula ) and then later cultivated, were studied for two years using a chamber technique. The agricultural soils growing grass or barley or without vegetation, generally acted as sinks for CH₄. Surprisingly, the agricultural soils emitted CH₄ during a warm dry summer. The CH₄ oxidation capacity and CH₄ uptake rate of the forested site was three times that of agricultural soils. Also, the forest soil better retained its capacity to take up CH₄ during a dry summer. Despite periods of CH₄ emission, the agricultural soils were annual sinks for CH₄, with uptake rate of CH₄-C varying from 0.1 to 3.7 kg ha⁻¹ yr⁻¹. The forested soil had a methane uptake rate of 3.9 kg CH₄-C ha⁻¹ yr⁻¹. All the soils acted as sinks for CH₄ during winter, which contributed up to half of the annual CH₄ uptake. The capacity of soils to transport gases did not explain the larger CH₄ uptake rate in the forest soil. At the same gas filled porosity, the forest soil had a much larger CH₄ uptake rate than the agricultural soil. Neither the soil acidity (pH 4.5 and 6.0) nor high ammonium content appeared to limit CH₄ uptake. The results suggest that CH₄ oxidation in agricultural organic soil is more sensitive to soil drying than CH₄ oxidation in forested organic soil.     

Causes of soil acidification: a summary

Abstract. A review of recent data shows that (i) dissolved CO₂ has its greatest acidifying effect in soils with pH values above about 6.5, (ii) fertilizers containing NH₋₁⁺ ions or urea will acidify soil whether the ions are taken up directly by plants or are first nitrified, (iii) oxidation of nitrogen and sulphur in soil organic matter causes acidification especially after deforestation, and (iv) the acidifying effect of rainfall and dry deposition is due to sulphuric and nitric acids, SO₂ and NH₋₁⁺ ions. A table is given showing the order of magnitude of each source of acidification.     

The effect of soil type on phosphorus sorption capacity and desorption dynamics in Irish grassland soils

Abstract. The phosphorus (P) sorption and desorption dynamics of eleven major agricultural grassland soil types in Ireland were examined using laboratory techniques, so that soils vulnerable to P loss might be identified. Desorption of P from soil using the iron-oxide paper strip test (Pfeo), water extractable P (Pw) and calcium chloride extractable P (Pcacl₂) depended on soil P status in all soils. However, soil types with high organic matter levels (OM), namely peat soils (%OM >30), had lower Pfeo and Pw but higher Pcacl₂ values compared to mineral soils at similar soil test P levels. Phosphorus sorption capacity remaining (PSCr) was measured using a single addition of P to soils and used to calculate total P sorption capacities (PSCt) and degree of P saturation (DPS). Phosphorus sorption capacities correlated negatively with % OM in soils indicating that OM may inhibit P sorption from solution to soil. High organic matter soils exhibited low P sorption capacities and poor P reserves (total P, oxalate extractable P) compared to mineral soils. Low P sorption capacities (PSCt) in peat soils were attributed to OM, which blocked or eliminated sorption sites with organic acids, therefore, P remained in the soil solution phase (Pcacl₂). In this work, peat and high organic matter soils exhibited P sorption and desorption characteristics which suggest that these soils may not be suitable for heavy applications of manure or fertilizer P owing to their low capacities for P sorption and storage.     

Release of potassium from some benchmark soils of India

Release of potassium from 15 surface samples of benchmark Alluvial, Red and Black soils of India to 0.01 M solutions of BaCl₂, CaCl₂, NH₄Cl and NaCl was studied in soils either untreated or pretreated with 5 × 10⁻³ M KCl. In the untreated soils, the efficacy of the extractants declined in the sequence: BaCl₂ > NH₄Cl > CaCl₂ > NaCl. Cumulative K-release was greatest from Black soils, followed by Red and Alluvial soils. From soils pretreated with 5 * 10⁻³ M KCl, more K was released than retained, and more 'native' K was released than that from untreated soils. Increase in the release of 'native' K decreased in the sequence: Red > Alluvial > Black soils. The amounts of surface and internal K, desorption rate constants and parabolic diffusion constants were calculated from K release to the various electrolytes.