Direct determination of potassium-calcium activity ratio in soils with two ion-selective electrodes. II. Interactions of potassium and calcium ions with soils

The interactions of potassium and calcium ions with two variable charge soils and one constant charge soil were examined by measuring the pK-0.5pCa value with two ion-selective electrodes, together with measurements of single ion activities. The constant charge soil showed a much higher preference for calcium relative to potassium than the variable charge soils. The relative affinities of potassium, calcium, sodium and aluminum ions with exchange sites were not constant, but were a function of surface properties of the soil. pH affected the pK-0.5pCa value in both the variable charge soil and the constant charge soil. The strong competition of aluminum ions for exchange sites was the major cause of the higher preference of soils for calcium ions relative to potassium ions at low pH. The pK-0.5pCa value was always smaller in sulfate systems than in chloride systems, with the difference more distinct in variable charge soils. In addition to an initial fast reaction, some potassium ions were immobilized at later times, even for the highly weathered variable charge soil.     

An evaluation of anion exchange resin used to measure nitrate movement through soil

Abstract

The attribute that ion‐exchange resins remove ions from solutions moving through them can be used to measure nitrate transport through soils. The characteristics of nitrate adsorption by resins must be known to interpret nitrate accumulation on ion‐exchange resins embedded in soil. The extent to which anion exchange resins retain NO₃‐ from soil leachate was measured in 15.9 cm diam.by 60 cm long intact cores of Nolin (fine silty mixed mesic Dystric Fluventic Eutrochroept) soil. A NC₃ ^‐‐selective resin and a non‐selective resin were tested. Columns were fertilized at a rate of 300 kg N/ha and 150 kg Br/ha and leached with 50 cm of water. Under these conditions, both resins retained approximately 80% of the NO₃‐ and Br^‐ leached through the soil. This compared with greater than 95% retention in laboratory columns containing only resin. The difference in retention was attributed to different flow through the resin associated with the method of resin emplacement.     

Experimental in situ transformation of vermiculites to study the weathering impact of tree species on the soil

Weathering of soil minerals under forest seems to depend on the species present. To study the influence of tree species we placed unweathered vermiculites in the soil and assessed the impact in terms of saturation index of earth‐alkaline cations and cation exchange capacity in 64 forest stands, dominated by different species and growing side by side at 20 sites on acidic soils. The vermiculites were of two types, one with a large charge and the other with a small charge. Minerals were maintained in the soil for 1 and 3 years. The minerals placed in the topsoil and in soils with low buffering capacity were more acidified and weathered than those deeper in the soil and in less acid conditions. The vermiculites were transformed into hydroxylated interlayered vermiculites, and the formation of hydroxides in the interlayer space decreased the vermiculites' cation exchange capacities. The high‐charge vermiculite had a greater affinity for aluminium than the low‐charge variety. The effect of tree species was significant but small compared with factors such as soil type, depth and duration of incubation. Nevertheless, we can rank the acidifying and weathering caused by the trees in the following order: Picea abies, Abies alba > Pinus sylvestris, Pseudotsuga menziesii > Quercus spp., Fagus sylvatica. This in situ experimental approach enabled us to study potential trends in pedogenesis in few years.     

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.     

Adsorption and transport of nitrate and potassium in an Andosol under banana (Guadeloupe, French West Indies)

A series of batch, soil column and field experiments was carried out in order to study nitrate and potassium transport and to improve fertilizer practices in an Andosol under a banana plantation. The focus of the study was on the influence of the substantial variable charge of the soil on ion transport. Leaching of both K⁺ and NO₃⁻‐N through the Andosol were both retarded, as expected, because of the soil’s cation and anion exchange capacities (CEC and AEC). Under acidic conditions, the B horizon, with the greater allophane content, developed preferential affinity for nitrate. The A horizon, with both organic and mineral adsorption sites, exhibited a stronger affinity for potassium than the B horizon when the soil solution pH was below the zero point of charge (ZPC). Fast leaching of nutrients occurred under the banana stem due to the abundant stemflow derived from rainfall interception by the banana plant. Intensive leaching under the plant questions the concept of fertilizer application at the plant stem.     

Physico‐chemical characteristics of three saprolites in peninsular Malaysia

Abstract

Three highly weathered inland saprolites developed over different parent rocks, namely basalt, granite and schist, were sampled and analyzed for their physico‐chemical properties. The profiles were located along new road cuts in Peninsular Malaysia with depth varying from 10 to 26 m deep. The soils in this study were highly weathered, nutrient‐poor, and contained large amount of variable charge colloids. They exhibited soil pH (1:2.5, soil:water) below 5.0, and were low in CEC [usually <7 cmol(+)kg^‐1 soil]. The base saturation was low, but aluminum (Al) saturation was very high with values >70%. All soils had available water content of <100 mm#lbm^‐1. The results of the study showed that saprolites were generally unsuitable for the cultivation of perennial crops. Saprolites, which were soft and friable, had poor physical properties and were a poor substrate for agriculture. The available water content was higher than the solum, but would not be accessible to plant roots. The fertility status of the saprolite was poorer than the soil. It had high phosphorus (P)‐retention capacity and low net negative permanent charge, and, therefore, had lower cation‐retention capacity. The Al phytotoxic effect was lower in the saprolite than the solum.     

POTASSIUM-CALCIUM EXCHANGE EQUILIBRIA IN SOILS: THE LOCATION OF NON-SPECIFIC (GAPON) AND SPECIFIC EXCHANGE SITES

Several studies (e.g. Bolt et al., 1963; Beckett, 1964a) of the exchange equilibria between K and Ca on soil colloids or pure clay minerals have drawn attention to the presence of sites with different affinities for K. There is so far no evidence to indicate the location of sites with the highest affinity, though several workers (e.g. van Schouwenburg and Schuffelen, 1963) have assumed that they lie on the edge-faces of stacks of clay plates, or in the spaces between the expanded leaves of partially weathered stacks of clay plates. Earlier work has resolved the K: Ca exchange isotherms of soils and clays into a curved part at low values of aK/√aCa, attributed to exchange at sites with a high specific affinity for K, and an upper linear part commonly described by the Gapon equation and attributed to non-specific sites. The present work on soil clays and clay minerals shows that Na hexametaphosphate or changes in pH affect the curved but not the linear part; cetyl trimethyl ammonium bromide and changes in the amount or charge of exchangeable Al affect the linear but not the curved part of the isotherms. At low pH, the linear but not the curved part of the kaolinite isotherm obeys Schofield's Ratio Law. Grinding has more effect on the curved than on the linear part. So the specific sites are attributed to the edges or peripheral interstices of stacks of clay plates, and the non-specific sites to their planar surfaces. The specific sites take up K more slowly from solution than the non-specific sites. The isotherms of completely dispersed bentonites have no curved part. The specific sites are attributed to the wedge-shaped interstices opened between clay plates by weathering, from which exchange is diffusion-controlled. Added organic cations reduce the numbers of both kinds of site; peroxide treatment increases them. This paper confirms that the exchange sites with highest affinity for K are indeed associated with the edges, rather than the faces, of stacks of clay plates.     

A comparison of methods used for determination of cation exchange capacity

Abstract

Cation exchange capacity of ten non‐calcareous and non‐saline soils from Queensland, Australia, has been determined by the ammonium acetate method using different procedures, by an ammonium chloride method at pH8.5, and by three methods which attempt to approximate field conditions. Procedural differences in the ammonium acetate method produced variation in results, and methods using approximate field conditions gave much lower values for those soils considered to have variable charge properties.     

Sorption and fate of perchlorate in arid soils

The mobility of perchlorate in soils depends on several factors, including soil mineralogy and the presence of other oxyanions that compete with perchlorate for the retention sites. Currently, there are no studies that evaluate the mobility of perchlorate in arid soils. The present study evaluated the mobility of ClO₄^– in three arid soils, Canchones, Humberstone and Pica, exposed to two ClO₄^– concentration ranges and different ionic strengths. In Humberstone (non-agricultural) and Pica (agricultural) soils, the sorption processes were not important for both concentration ranges, while Canchones soil (agricultural) showed a decrease in perchlorate concentration associated with microbial degradation processes. The increase of medium ionic strength by addition of Ca(NO₃)₂ only had an important effect on Humberstone soil, associated with the presence of kaolinite and muscovite (variable charge). A competition effect was observed between perchlorate, nitrate and other anions presents in solution by absorption sites generated from variable charge mineral and calcium. Considering the quite low sorption capacity of the soils and the high solubility, perchlorate can be absorbed by fruit and vegetables of export in concentrations over the healthy levels established by international organization.     

Effects of liming,green manuring,and phosphate addition on electrochemical attributes of an Oxisol from central Brazil

Abstract

Highly weathered tropical soils are characterized by having a predominantly variable charge. Many management practices commonly used in the exploitation of these soils (e.g., liming, phosphate application, and manuring) are known to modify their electrical charge and the sorption/desorption behavior of cations and anions. This process is, at least, partially governed by the charges existing in the soil system. Available information on this subject comes mainly from short‐term laboratory and greenhouse experiments. There is a lack of data regarding the cumulative and long‐term effects of those practices used at farm‐scale levels and on the dynamics and availability of nutrients to the plants under field conditions. In the present work, changes in some electrochemical attributes of a variable charge soil (Oxisol) were studied, as influenced by treatments with phosphate + green manure (Cajanus cajan), phosphate + lime, and phosphate + lime + green manure, applied during a six‐year period. In this period, rice, bean, wheat, or corn, were grown in seventeen successive crops. Phosphate (total 334 ppm P) and phosphate + lime (total 5.5 t ha^‐1) were shown to increase net electric charge and soil cation exchange capacity (CEC) at the field pH, and not to affect zero point of charge (ZPC), CEC at pH 7.0, or anion exchange capacity (AEC) of the soil at the field pH. The effects of phosphate + lime were more pronounced than those of phosphate alone. Green manure (total 16 t ha^‐1 dry matter), associated to crop residues and phosphate or phosphate + lime, did not influence electrochemical properties.     

热带地区强风化土壤表面的化学反应以及土壤管理的合理建议

Highly weathered soils are distributed in the humid and wet-dry tropics, as well as in the humid subtropics. As a result of strong weathering, these soils are characterized by low activity clays, which develop variable surface charge and related specific properties. Surface reactions regarding base exchange and soil acidification, heavy metal sorption and mobility, and phosphorus sorption and availability of the tropical highly weathered soils are reviewed in this paper. Factors controlling surface reactivity towards cations and anions, including ion exchange and specific adsorption processes, are discussed with consideration on practical implications for rational management of these soils. Organic matter content and pH value are major basic factors that should be controlled through appropriate agricultural practices, in order to optimise favorable effects of colloid surface properties on soil fertility and environmental quality.     

DIFFUSE DOUBLE LAYER THEORY APPLIED TO Na-Ca EXCHANGE EQUILIBRIA IN TEMPERATE AND SEMI-ARID TROPICAL SOILS

A cation exchange equation based on diffuse double layer (DDL) theory was tested on 26 surface and sub-surface soils from 6 field experiments in temperate and semi-arid tropical regions. Sodification (the fraction of total charge on the surface neutralised by the excess of Na⁺ plus the deficit of Cl^?), or the exchangeable Na percentage, ESP, of these soils in relation to SAR, the molar ratio (Na]/[Ca + Mg] ^1/2, was evaluated from the observed data on Na:Ca exchange in two ways, using the DDL equation: (1) by multiplying the surface charge density of the soil with a mean correction factor f (based on the whole isotherm) assuming no interaction between adjacent clay platelets; this predicted the sodification of the soils satisfactorily between 0–30 ESP, though at the highest SAR values (i.e. > 60), predicted ESP values were significantly smaller than the experimental values for 23 of the 26 soils; (2) by assigning values to the extent of interaction Y_d (directly related to the electric potential Ψ_d midway between adjacent clay platelets) over the whole isotherm; Y_d was found to increase by varying degrees in different soils with SAR and ESP. Also in 12 of the 15 pairs of soils compared, the mean value, _d, over the whole Ca-Na exchange isotherm was appreciably larger for surface soils (which had higher surface charge densities) than for the subsoils from the same sites. The suggestion that the Y_d parameter provides a better criterion than the f parameter for characterising and comparing Na:Ca exchange equilibria in contrasting soils is discussed in relation to the effects of soil components.     

Enhancing Nitrogen Availability,Ammonium Adsorption-Desorption,and Soil pH Buffering Capacity using Composted Paddy Husk

Form of nitrogen present in soils is one of the factors that affect nitrogen loss. Nitrate is mobile in soils because it does not absorb on soil colloids, thus, causing it to be leached by rainfall to deeper soil layers or into the ground water. On the other hand, temporary retention and timely release of ammonium in soils regulate nitrogen availability for crops. In this study, composted paddy husk was used in studies of soil leaching, buffering capacity, and ammonium adsorption and desorption to determine the: (i) availability of exchangeable ammonium, available nitrate, and total nitrogen in an acid soil after leaching the soil for 30 days, (ii) soil buffering capacity, and (iii) ability of the composted paddy husk to adsorb and desorb ammonium from urea. Leaching of ammonium and nitrate were lower in all treatments with urea and composted paddy husk compared with urea alone. Higher retention of soil exchangeable ammonium, available nitrate, and total nitrogen of the soils with composted paddy husk were due to the high buffering capacity and cation exchange capacity of the amendment to adsorb ammonium thus, improving nitrogen availability through temporary retention on the exchange sites of the humic acids of the composted paddy husk. Nitrogen availability can be enhanced if urea is amended with composted paddy husk.     

The retention of nitrate in acid soils from the tropics

Abstract. In most soils of temperate regions nitrate is not held on soil surfaces and moves freely in solution. But when soils carry positive charges, nitrate is held as an exchangeable anion. As a result, leaching of nitrate is delayed relative to the movement of water. The delay can be predicted provided the anion exchange capacity (AEC) can be measured and the concentration of counter-anions is known. For soils with variable charge, the AEC varies with both pH and ionic strength, and the effective AEC should be determined under conditions similar to those in soil solution. A simple leaching method is described which satisfies this requirement. Delays in the leaching of nitrate measured in columns of repacked soil were strongly related to the AEC.     

An evaluation of cation exchange capacity measurements for soils in the tropics

Abstract

Comparisons of CEC and exchange acidity neasurements were made on a group of selected West African soils using three commonly used analytical procedures, namely, neutral‐acetate displacement, BaCl₂‐TEA leaching at pH 8, and unbuffered KCl extraction.

The three methods gave large differences in CEC values which followed the order of BaCl₂‐CEC>>NH₄OAc‐CEC> KCl‐CEC. Results of exchange acidity also followed the same order. The high exchange acidity values obtained by the BaCl₂‐TEA (pH 8) method were mainly due to changes in surface charge characteristics of Fe and Al oxides and hydrous oxides. The effective CEC method is recommended for routine soil analysis for highly weathered soils in the tropics.

Regression analysis of the base saturation values obtained from the three methods indicated the data followed a curvilinear relationship. The acetate method was more highly correlated with the effective CEC method than with the BaCl₂ method.     

配位体交换在减慢由酸雨造成的可变电荷土壤酸化速度中的意义

For the purpose of evaluating the role of ligand exchange of sulfate ions in retarding the rate of acidification of variable charge soils,the changes in pH after the addition of different amounts of HNO3 or H2SO4 to representative soils of China were measured .A difference between pH changes caused by the two kinds of acids was observed only for variable charge soils and kaolinite,but not for constant charge soils and bentonite,The larger the proportion of H2SO4 in the HNO3-H2SO4 mixture,the lower the calculated H^ ion activities remained in the suspension.The difference in H^ ion activities between H2SO4 systems and HNO3 systems was larger for soils with a low base-saturation(BS) percentage than those with a high BS percentage.The removal of free iron oxides from the soil led to a decrease in the difference,while the coating of Fe2O3 on a bentonite resulted in a remarkable appearance of the difference.The effect of ligand exchange on the acidity status of the soil varied with the soil type.Surface soils with a high organic matter content showed a less pronounced effect of ligand exchange than subsoils did.It was estimated that when acid rain chiefly containing H2SO4 was deposited on variable charge soils the acidification rate might be slower by 20%-40% than that when the acid rain chiefly contained HNO3 for soils with a high organic matter content,and that the rate might be half of that caused by HNO3 for soils with a low organic matter content,especially for latosols.     

恒电荷土壤与可变电荷土壤对Cl~-和NO_3~-吸附的差异及其机理

对 3种可变电荷土壤和 4种恒电荷土壤在不同 pH、不同浓度、不同相伴阴阳离子下混合体系中Cl-和NO3-的吸附进行了测定。结果表明 ,在Cl-和NO3- 共存体系中 ,Cl-比例增大使可变电荷土壤Na+吸附量及OH-释放量增加 ,而对恒电荷土壤影响不大。Cl-和NO3-吸附量随平衡Cl-和NO3-浓度增加而增大 ,随pH升高而减少。但恒电荷土壤在上述各种条件下对Cl-和NO3-吸附均相同 ,而可变电荷土壤对Cl-吸附量大于NO3-的吸附量 ;NO3-、Cl-的选择系数为 0.51～0.78,Cl-和NO3-的相对吸附量分别为56.9%和 43.1%。在不同相伴阳离子下 ,可变电荷土壤平衡溶液Cl-/NO3-比值均小于 1,且为Na+K+Ca2+Mg2+Fe3+;而恒电荷土壤Cl-/NO3-比值为 1左右 ,且不受阳离子类型的影响。由此认为 ,Cl-和NO3-在两类土壤中均以电性吸附为主 ,恒电荷土壤对Cl-和NO3-的亲合力及吸附机理相同 ;而可变电荷土壤对Cl-的亲合力 NO3- ,Cl-存在着专性吸附     

Influences of aluminum ions on the determination of zpc (zero point of charge) of variable charge soils

Influence of Al dissolution on soil ZPC (zero point of charge) measured by a potentio-metric titration (PT) method and a modified salt titration (STPT) method was examined using two strongly weathered soils from Thailand and two volcanic ash soils from Japan. The amount of dissolved Al ions increased with the increase in the concentration of a supporting electrolyte for the strongly weathered soils, while the increase was negligible for the volcanic ash soils.

ZPC value of the strongly weathered soils determined by the PT method was lower than that by the STPT method, due to the greater Al dissolution associated with the higher electrolyte concentration used in the PT method. Al ions adsorbed onto the soil surface would shift the ZPC to a higher pH value not as a result of the formation of hydroxy Al polymers, but due to the blocking of permanent negative charge sites, which could otherwise lower the ZPC. The σp value, as a measure of permanent charge or the amount of 11 or O11 adsorbed by a soil required to attain the ZPC, could be used to describe this phenomenon.

In the STPT method, the salt concentration was not high enough to causc a significant Al dissolution at the ZPC, which is considered to be a more suitable condition than in the PT method because the ZPC value can be evaluated at a low salt concentration as in the ease of field conditions for crop production. Thus, the STPT method is rccommendcd for the determination of the ZPC.     

Shrinkage of initially very wet soil blocks, cores and clods from a range of European Andosol horizons

In advanced stages of volcanic ash soil formation, when more clay is formed, soil porosity values and soil water retention capacities are large and the soils show pronounced shrinkage on drying. Soil shrinkage is a key issue in volcanic soil environments because it often occurs irreversibly when topsoils dry out after changes from permanent grassland or forest to agriculture. European Andosols have developed in a wide range of climatic conditions, leading to a wide range in intensity of both weathering and organo‐mineral interactions. The question arises as to whether these differences affect their shrinkage properties. We aimed to identify common physically based shrinkage laws which could be derived from soil structure, the analysis of soil constituents, the selected sampling size and the drying procedure. We found that the final volumetric shrinkage of the initially field‐wet (56–86% of total porosity) or capillary‐wet (87–100% of total porosity) undisturbed soil samples was negatively related to initial bulk density and positively related to initial capillary porosity (volumetric soil water content of soil cores after capillary rise). These relationships were linear for the soil clods of 3–8 cm³, with final shrinkage ranging from 21.2 to 52.2%. For soil blocks of 240 cm³ and soil cores of 28.6 cm³ we found polynomial and exponential relationships, respectively, with thresholds separating shrinkage and nearly non‐shrinkage domains, and larger shrinkage values for the soil cores than for the soil blocks. For a given sample size, shrinkage was more pronounced in the most weathered and most porous Andosol horizons, rich in Al‐humus, than in the less weathered and less porous Andosol horizons, poor in Al‐humus. The Bw horizons, being more weathered and more porous, shrank more than the Ah horizons. We showed that the structural approach combining drying kinetics under vacuum, soil water analysis and mercury porosimetry is useful for relating water loss and shrinkage to soil structure and its dynamics. We also found that the more shrinkage that occurred in the Andosol horizon, the more pronounced was its irreversible mechanical change.     

Determination of surface charge density of soils

Abstract

A simple and precise method for determination of surface charge density of soils is described. It involves saturating the negative and positive exchange sites with NH⁺ ₄ and NO^‐ ₃ ions, respectively, removing the excess solution by centrifugation, and determining the ions on the exchange sites by a steam distillation method. Results showed that the concentration and type of saturating ions and the extractant used significantly affected the surface negative charge densities. The average value of surface negative charge densities of 10 surface soils from Iowa, Chile, and Costa Rica by the proposed method agreed closely with that obtained by the original method proposed by Schofield in 1949 (14.6 vs. 13.7 cmol(‐) kg^‐1 soil). The advantages of the proposed method are no need for the laborious extraction steps and simplicity of the steam distillation method for determination of NH⁺ ₄ and NO^‐ ₃ in soil samples.