THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS

Diffusion coefficients of soil phosphate calculated from flux to chloride form of anion-exchange resin-paper, assuming total depletion of the labile pool at the boundary, were too small and unrealistic. Thus not all the phosphate in the labile pool contributed to the diffusion process while being desorbed at constant pH in the presence of an indifferent anion—Cl—from the resin-paper. Desorption relationships under these conditions, using CaCl₂ at atmospheric CO₂ pressure, were markedly different from the relationship between the long-term ³²P-exchangeable phosphate and the corresponding pore-solution concentrations. In the same soil containing different amounts of labile phosphate, a different desorption relationship was found for each phosphate level. The constant proportionality between amount of phosphate diffusing into resin-paper and square root of time was indicative of a constant concentration at the boundary. The resin-paper: soil system was therefore considered as an infinite composite medium in which diffusion through both resin-paper and soil were rate-limiting. The constant boundary concentrations were estimated by the use of the diffusion coefficients in the resin-paper, the phosphate adsorption isotherm for the resin-paper and the desorption relationship for each phosphate level in the soil. Diffusion coefficients, calculated using the boundary concentration appropriate to each phosphate level, were related to the slopes of the corresponding desorption relationships, resulting in values of the impedance factor similar to those found for K diffusion under similar conditions. The resin-paper method, however, does not provide an accurate enough measure of the diffusion coefficient of soil phosphate to be of any practical use. Until better and simpler methods are found, the diffusion coefficient may be calculated using the slopes of the desorption relationship and the separately determined value of the impedance factor.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS

The diffusive flux of soil cations and anions to an absorbing surface may be measured by a simple, quick, and accurate method using ion exchange resin paper as a sink maintaining, initially, zero concentration of the diffusate at the soil surface. The quantity of ions diffusing from the soil of semi-infinite thickness is directly proportional to the square root of the diffusion time until about half the counter-ions originally on the resin have been exchanged. Values of average effective diffusion coefficients are calculated using this proportionality constant and the concentration of total exchangeable ions. The effect of the counter-ions of the resin on soil-ion flux is small but measurable.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS

Integral diffusion coefficients for K in two contrasting soils, containing different levels of exchangeable K, are calculated from measurements of diffusive flux to a hydrogen resin paper. The diffusion coefficients are larger, the higher the exchangeable K level or moisture content. Diffusive flux of K in these soils can be accounted for entirely by diffusion through the soil solution in the pores. Impedance factors calculated compare satisfactorily with values for similar soils reported by other workers. The concentration dependence of the diffusion coefficient is shown to be causally related to the nature of the sorption isotherm. A method for estimating the differential diffusion coefficient at any concentration is described. It can also be calculated from the slope of the sorption isotherm at the chosen concentration, if the value of the impedance factor at the relevant moisture content is known and if diffusion other than through solution only is negligible.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS V. DIFFUSION OF HYDROGEN ION IN SOILS

The linear diffusion of H ion from a series of soil blocks, containing essentially H and Ca only as exchangeable cations, into a sink solution maintained at pH 5·2, has been studied. In three separate series of experiments, first the total concentration of diffusible H, then the amount of diffusible H in the pore solution, and finally the amount of exchangeable H associated with the solid phase have been successively varied. Account has also been taken of the variation in activity ratio: a_H/√a c_a. The effect on the resulting flux and diffusion coefficients of the ion has been related to the H buffer power of the soil. The impedance factor of the ion through the liquid pathway is slightly lower than that for other cations determined in soil. The diffusive flux of H occurs through the liquid pathway, and the contribution of solid-associated H is small, except possibly at low H ion solution concentrations.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOIL

If an exchangeable ion in soil diffuses along a liquid and solid pathway, its diffusion coefficient may be expressed as where D, v, f, C are diffusion coefficient, volume fraction, impedance factor, and concentration terms and the suffixes _l,_S refer to liquid and solid. The self-diffusion coefficient of the ion is then where D′, D′_t, and D′_s, are self-diffusion coefficients. D and D′ will vary with concentration. In diffusion out of the soil to a zero sink, the appropriate average diffusion coefficient is, approximately, the self-diffusion coefficient in the undisturbed soil. Diffusion of one ion species is influenced by other ions diffusing in the system through the diffusion potential set up. When ions are diffusing to plant roots, the diffusion potential is likely to be small. A more likely, though more complicated, expression for D than the first equation above is derived by assuming the ion to follow solid and liquid pathways in series as well as in parallel.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS III. THE EFFECT OF MOISTURE CONTENT AND SOIL-SOLUTION CONCENTRATION ON THE SELF-DIFFUSION OF IONS IN SOILS

The self-diffusion coefficients of Cl, Na, Sr and phosphate have been measured in Upper Greensand, sandy clay loam (CEC 7.45 me/100 g.) between pF 1.8 and 5.4 and, for Na and phosphate, over a range of soil-solution concentrations. An attempt has been made to estimate the relative contributions of solution- and exchangeable-ions to diffusion. There is some indication that exchangeable Na may contribute to diffusion at the lowest moisture contents used, but little indication of this at pF 2.1 (15 per cent moisture content) when the proportion of ions on the solid was as high as possible. Exchangeable Sr appears to make no significant contribution to diffusion at any moisture content when the soil-solution concentration is 0.1M. Exchangeable phosphate appears to make no contribution to diffusion at pF 2.1 between 5 × 10^?6M and 9 × 10^?4M.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS. VIII —A THEORY FOR THE PROPAGATION OF CHANGES OF pH IN SOILS

The way pH changes in soil are propagated by movement of acids and bases is described. In acid soils the H₃O⁺-H₂O acid-base pair is most important, while in alkaline soils the H₂CO₃-HCO₃^? pair is always dominant, its effect depending directly on the pressure of CO₂. In neutral and slightly acid soils, soluble organic matter and the H₂PO₄^?-HPO²₄^? pair may also contribute. A soil acidity diffusion coefficient is derived, and defined as: where v_l= the volume fraction of the soil solution, f_l= the impedance factor for the liquid diffusion pathway, b_HS= the pH buffer capacity of the soil, b _HB= the pH buffer capacity of each mobile acid-base pair, Dl _HB= the diffusion coefficient of each mobile acid-base pair in free solution, and the sum is taken over all mobile acid-base pairs. The soil acidity diffusion coefficient may be used to predict the course of pH equilibration in practical situations. It is high in acid and alkaline soil, and at a minimum in slightly acid soil. It is little affected by variation of the ionic strength of the soil solution at concentrations less than 0.01M. When the pH buffer capacity of the soil is constant, and only the H₃O⁺-H₂O and H₂CO₃-HCO₃^? pairs are important, the soil acidity diffusion coefficient varies as cosh{2.303(pH—pH₀)}, where pH₀ is the pH at which the soil-acidity diffusion coefficient is a minimum.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS. IX. CHANGES IN SOIL ACIDITY NEAR A SOURCE OF BICARBONATE IONS

To simulate processes tending to raise the pH of soil near a plant root, a stack of moist HCO^-₃saturated ion exchange resin papers was placed in contact with non-calcareous soil of varying pH, containing HC1 + CaCl₂ in the soil solution. Changes in the concentration profiles of Ca²⁺, Cl^- and H⁺ in the soil were followed. HCO^-₃ released from the resin in exchange for soil Cl^- reacted with the soil and raised its pH. Increases of up to 1 unit were measured near the interface. The zone of pH increase extended into the soil much less than the zone of chloride depletion. It was shown that Ca²⁺ could accumulate near the interface in the absence of mass flow in this experimental system.     

土壤中离子扩散的基本方程与实验验证

土壤是一个非均一的带电体系,因此土壤中离子扩散的推动力来自两个方面：一是浓度梯度,二是电位梯度,同时土壤作为一个非均一的带电体系,其离子的分布极为不均,可见土壤中的离子扩散不能直接用Ｆｉｃｋ扩散定律来描述,有必要建立起适合于土壤这个不均一的带电体系的离子扩攻的基本方程,本文从热力学及物质扩散动力学的基本原理出发,导出一组描述土壤中离子扩散的基本方程,并在实验的给定条件下,通过求解基本方程,对土壤中     

MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOIL X. PREDICTION OF SOIL ACIDITY GRADIENTS IN ACID-BASE TRANSFERS

The concentration gradient of soil acid neutralised when a block of moist soil was placed in contact with a source of HCO₃- was predicted from an equation for diffusion of HCO₃-, and its simultaneous reaction with the soil. The reaction was defined by empirical rate constants describing the changes of pH with time in a soil: M/100 CaCl₂+ KHCO₃ solution, suspension system. Values of these constants, determined independently, were too large resulting in prediction of steeper and leas extensive concentration profiles of soil acid neutralised than measured in the diffusion system. The true pH of the soil pore solution in the block would have been considerably greater than a predicted ‘equilibrium’ pH. The consequences for rhizosphere processes are discussed.     

THE SIMULATION OF GASEOUS DIFFUSION IN SOILS

Computer simulation techniques are presented as a means of applying diffusion theory to two problems concerning the dynamics of oxygen and carbon dioxide distributions in soils. It is demonstrated that these techniques facilitate both problem formulation and solution, and require less mathematical skill than the equivalent analytic techniques. For problems with no analytic solution, simulation can still yield results with an accuracy equal to other numerical methods. Using simulation techniques, it was computed that stationary state conditions for carbon dioxide in a 0·7 cm layer of soil were established 24 hours after waterlogging. Similarly it was computed that for oxygen diffusing through a 0·4 cm layer of soil, equilibrium was attained within six minutes. These data were useful in that they justified the assumptions regarding equilibrium conditions which had necessarily been made in previous experiments. Simulation techniques may be of widespread use, not only for solving diffusion problems, but also for applying other fundamental laws to specific situations.     

THE MEASUREMENT AND PREDICTION OF AVAILABLE WATER CAPACITY OF FERRALLITIC SOILS IN UGANDA

Field capacity was measured directly, and soil moisture characteristics were determined on undisturbed cores, for a wide textural range of ferrallitic soils in Uganda. The initial moisture conditions of the soils were shown to affect results and thus standardized procedures were adopted for field and laboratory determinations. Laboratory estimates of field capacity for undisturbed and disturbed samples were shown to be unreliable, but a correction factor was found which improved them. There was no single moisture tension for undisturbed core samples that corresponded to field capacity. Particle-size composition could be related to field capacity, permanent wilting-point, and available water capacity by multiple regression equations having correlation coefficients of 0.96, 0.98, and 0.88 respectively. The relationships predicting available water capacity were different from those found for soils in England, but those for field capacity and permanent wilting-point were similar. Particle-size analyses were carried out using three methods of dispersion of different efficiencies. Particle-size composition following gentle dispersion by shaking soil with distilled water was poorly correlated with moisture-holding properties. There was little advantage in using vigorous ultrasonic dispersion compared with overnight shaking with sodium hexametaphosphate. Two soils formed on alluvial deposits with kaolinite the only clay mineral, and one soil with montmorillonite the dominant clay mineral, showed markedly different relationships between moisture-holding properties and particle-size composition.     

THE DETERMINATION OF IONIC DIFFUSION COEFFICIENTS IN FIELD SOILS. I. DIFFUSION COEFFICIENTS IN SIEVED SOILS IN RELATION TO WATER CONTENT AND BULK DENSITY

A method for measuring diffusion coefficients in large cores of structured field soils has been developed and tested, in the first instance using sieved soil. Bromide-chloride and bromide-nitrate counter diffusion coefficients have been determined using ion exchange membranes as sinks for the diffusing ions. The effects of membrane selectivity, soil surface preparation, solution concentration, water content and bulk density have been investigated. The organic colloid content appears to have a considerable influence on the impedance factor-water content relationship of a soil. Increasing the bulk density at constant volumetric water content linearly reduced the impedance factor by up to 30 per cent. The relationship between impedance factor and bulk density is discussed.     

pH对土壤吸持磷酸根的影响及其原因

本文选择了浙江、江苏15个性质变化范围较大的土壤样品,研究在两种支持电解质、不同pH条件下对磷酸根的吸持反应。结果表明,加碱提高强酸性土壤的pH值,导致交换性铝的水解和羟基铝聚合物的生成,增加对磷的吸持。磷酸根同酸性土壤的反应,可促进交换性铝的水解,释放出H⁺,降低体系的pH。在CaCl₂介质中,当pH>6时,可能有磷酸钙类盐形成,使溶液中磷浓度显著降低。有机质对土壤吸持磷有重要影响。在低pH下有机质通过与Al³⁺形成络合物,阻碍溶液中A1³⁺的水解,并与磷酸根竞争羟基铝化合物表面的反应点位,从而降低酸性土壤对磷酸根的吸附量。     

pH对酸性土壤中铝的溶出和铝离子形态分布的影响

ＰＨ对酸性土壤中铝的溶出和土壤溶液中铝离子形态分布的影响的研究结果表明,土壤中铝的溶出量随ＰＨ降低而增加,ＰＨ对不同土壤中铝的溶出的影响不同,三种土壤中铝的溶出量受ＰＨ影响的大小顺序是：红壤〉赤红壤〉砖红壤,说明不同类型土壤中铝的溶出对外来酸的敏感和程度不同。     

模拟水田的土壤磷素溶解特征及其流失机制

磷素在农业生产中是不可缺少的。包括土壤成土矿物、氧化物等在内的土壤固磷介质对磷有明显的固定作用。但越来越多的研究发现：土壤磷素以地表径流、明渠或暗渠等排水途径流失进入环境，引发水体富营养化［1，2］。从地表径流［3～5］和地下暗管［2，6］排水发现土壤磷素的流失主要归因于低能量吸附点位占优势的富磷土壤。土壤磷素流失又受到多种因子的影响，当旱作土壤改为淹灌土壤时，土壤磷素的有效性显著提高［7］；干-湿交替的土壤磷素有效性及溶解特性又受到土壤水分状况和干湿交替时间的限制［8，9］；在厌氧条件下的湿地环境能降低土壤对磷的固定能力，提高磷素的溶解活性［10］。因此，对于这种受水分影响的情形不能简单地判断土壤磷素的流失潜能。由于土壤磷素的溶解特性与土壤当时的氧化还原状况直接相关［8］，水稻田的磷素溶解及淋溶特征规律可能与上述旱地的情况有所不同，也可能与水旱轮作或干-湿交替下的土壤不同。鉴于此，我们采取了三种典型浙北水稻土进行室内模拟水田环境，探讨磷素的溶解及其流失规律。     

旱地土壤微生物磷测定方法研究

介绍了国外关于土壤微生物磷测定方法的研究进展 ,讨论了常用的几种方法所存在的问题 ,介绍了主要操作过程要求。对我国 5种主要母质类型的土壤 (pH 3 .3～ 7.4,1molL- 1KCl)的对比研究表明 ,我国土壤采用氯仿熏蒸、0 .5molL- 1NaHCO3在 1∶2 0土水比提取测定无机磷 (Pi)、并以同时测得的培养土壤微生物的磷的回收率作为计算常数得到的结果最佳。测定的大多数南方土壤的微生物磷占土壤全磷的比例小于 1 .5 % ,微生物碳磷比值大于 3 0∶1 ,反映南方土壤磷的生物活性较低 ,土壤微生物对磷的作物供应调节能力不强。     

土壤对铜离子的专性吸附及其特征的研究

供试土壤专性吸附铜的等温线均符合Langmuir方程。红壤吸附量最低,砖红腹与黄泥土最大吸附量相近,但在铜浓度低时砖红壤吸铜量远低于黄泥土,而在高浓度则反之。土壤专性吸附铜是在溶液中Na⁺浓度比Cu²⁺高8.3—100倍条件下,Na⁺离子仍不足以与之竞争的那些专性吸附点所吸持的铜。按其解吸条件区分为松结合铜(可为N NH₄Cl解吸)和紧结合铜(仅能为0.1 N HCl解吸)两种。紧结合铜受平衡溶液铜浓度影响很小,所占据的吸附点对Cu²⁺有较强亲和力。松结合铜则随平衡铜溶液浓度增大而增加,符合Langmuir方程。对于砖红壤和黄泥土,在铜浓度低时紧结合铜>松结合铜;浓度高时则反之。红壤专性吸附铜始终以松结合铜为主。三种土壤比较,紧结合铜是砖红壤>黄泥土>红壤;松结合铜则是黄泥土>砖红壤>红壤。造成这些差别的原因可能与土壤性质、氧化物、有机质和粘土矿物组成等不同有关。用平衡法研究三种土壤专性吸附铜在不同浓度NH₄Cl和HCl溶液中的解吸表明,可进一步区分为三或四种不同的结合状况。红壤对铜吸附容量最小,且最易解吸。