长期施钾肥水稻土对铵离子吸附特征及影响因素的研究

以长期施 K 肥的水稻土为研究对象,以不施肥处理为对照,应用Langmuir 等温吸附方程对比研究两种处理水稻土对铵离子(NH4 )的吸附特征及影响因素.结果表明:本研究中水稻土对NH4 的吸附属于受浓度梯度扩散控制的物理吸附.土壤对NH4 的吸附量随吸附溶液NH4 浓度的增大而增加,但增加的幅度随吸附溶液NH4 浓度的增加而减小.不同大小团聚体对NH4 的吸附能力存在差异.土壤对NH4 的吸附特征通过拟合Langmuir方程发现,和不施肥处理相比,长期施用K肥提高了土壤对NH4 的最大吸附量,表明K肥的施用提高了土壤对NH4 的吸附潜力,可能的原因在于长期施K肥土壤伊利石含量和pH值的增加,固定态NH4 含量的降低.     

植茶年限降低土壤团聚体稳定性并促进大团聚体中钾素释放

  【目的】   研究植茶年限对土壤团聚体稳定性以及供钾和释钾能力的影响,以期为指导茶园土壤施肥、促进茶园生态系统可持续发展提供科学依据。   【方法】   采集四川省雅安市草坝镇茶园农业生态区植茶年限分别为5 a、10 a、15 a和30 a的原状土 (0—15和15—30 cm),利用湿筛法分离出粒径> 2 mm、0.25～2 mm、0.053～0.25 mm和 < 0.053 mm的土壤样品,分析其稳定性、各粒级土壤速效钾和缓效钾含量,并采用四苯硼钠 (NaTPB) 浸提法探讨了其有效钾释放特征。   【结果】   各植茶年限土壤均以大团聚体 (粒径> 0.25 mm) 为主,质量占比为75.87%～95.75%,但随植茶年限增加,土壤大团聚体比例显著减少 (P < 0.05),且各土层土壤团聚体平均重量直径 (MWD) 均明显减小。15 a和30 a的茶园土壤中同一粒级团聚体的土壤速效钾和缓效钾含量均高于5 a和10 a茶园。5 a和10 a茶园土壤中各粒级团聚体中速效钾含量分布较为均匀,15 a和30 a的茶园土壤速效钾含量随大粒级团聚体的增加而增高。土壤缓效钾在各年限茶园均表现为微团聚体 (粒径< 0.25 mm) 高于其他团聚体组分。各茶龄土壤不同粒级团聚体有效钾累积释放量在102.3～236.5 mg/kg,且呈现前期快、后期较稳定的趋势,释放过程均以扩散模型拟合程度最好。进一步比较表明,植茶15 a和30 a的土壤团聚体有效钾累积释放量明显高于植茶5 a和10 a时的土壤。   【结论】   茶园土壤中以大团聚体 (粒径> 0.25 mm) 的比例最高,但随植茶年限的增加,微团聚体 (粒径< 0.25 mm) 比例增加,土壤结构稳定性降低,特别是15—30 cm土层土壤。大团聚体的减少促进了土壤速效钾、缓效钾的释放,因而,植茶15 a和30 a的土壤速效钾含量较5 a和10 a的茶园高,但是会耗竭土壤钾库,不利于茶园的可持续利用。     

Soil structure and potassium supply. II. The effect of ped size and root density on the supply to plants of exchangeable and non-exchangeable potassium from a chalky boulder clay soil

A model based on measurements of potassium release from spherical aggregates (Heming & Rowell, 1985, Part I) has been used to predict release from larger aggregates, for longer times, drier soils and varying diffusion coefficients. The model has been adapted to predict supply to a single root from a soil cylinder and compared with measured uptake by onions. A ‘supply index’ shows likely restrictions in the availability of exchangeable K and in the rate of supply of non-exchangeable K resulting from diffusion limitations over a 4 week period for (i) aggregates covered with a dense roots system, and (ii) roots growing at known densities. For this period diffusion alone should not limit exchangeable K uptake in the field if the root density is at least 3.5 cm/cm³: at 0.9 cm/cm³ about 50% may be unavailable. The supply rate of non-exchangeable K increases with root density and may be important for high-demanding crops in dry seasons.     

Modelling of colloid leaching from unsaturated, aggregated soil

The migration of colloids in soils can enhance the leaching of strongly sorbing contaminants. We present a model for the simulation of colloid leaching from unsaturated, aggregated soil media under stationary flow. Transport in the intra-aggregate pores is simulated by convection–dispersion, and transport in the interaggregate pores, and a stagnant layer of water surrounding the aggregates, is simulated by diffusion. The model describes the release of colloids from soil aggregates, sorption and desorption processes at the air–water interfaces, and flocculation and subsequent straining from the flowing water. All three processes were simulated as functions of ionic strength. Transport of ions in intra-aggregate pores was simulated by Fickian diffusion. The model was calibrated against experimental results of colloid leaching from columns packed with natural soil aggregates. The aggregates were of two soils differing in organic matter content. On each soil a single calibrated parameter set could describe the experiments with the three ionic strengths. The parameters for release of colloids from the aggregate surface and the sorption properties of the air–water interface were different for the two soils. The key parameters for leaching were the thickness of the stagnant layer of water surrounding the aggregates, the mechanical dispersion, the maximum concentration of colloids at the surface of the aggregates, the sorption capacity and rate coefficient of the colloids at the air–water interface, and the colloid diffusion coefficient. Simulations were also done with two additional irrigation intensities at one ionic strength. Simulated leaching was greater than measured leaching at both irrigation intensities, but the diffusion-controlled release of colloids from the aggregates was simulated correctly.     

稻草覆盖和香根草篱联合调控红壤坡耕地土壤团聚体粒组碳钾分配规律

土壤有机碳可以通过影响土壤矿物钾释放以及解钾菌特性而对土壤钾素产生影响,本文基于稻草覆盖和香根草篱控制红壤坡耕地水土流失长期定位试验,通过对比常规管理(CK)、稻草覆盖(S)、稻草覆盖+香根草篱(S+V)处理,研究稻草覆盖及香根草篱措施对土壤主要化学性质的影响,并分析土壤团聚体粒组分布及碳和钾在土壤团聚体粒组中的分配规律。结果表明,在所有处理中,S+V处理的土壤pH、有机碳、全钾和速效钾含量显著较高,与CK处理相比,S+V处理的土壤pH提高了0.23,有机碳、全钾和速效钾含量分别提高了6.52%、11.60%和11.49%。不同处理下土壤团聚体粒组分布无显著差异,所有土壤团聚体粒组中均以>0.25 mm土壤团聚体粒组的比例最高。与CK处理相比,S和S+V处理的>2 mm土壤团聚体粒组有机碳含量分别提高了9.42%和40.03%。与CK处理相比,S+V处理下>2 mm、0.25～0.053 mm和<0.053 mm土壤团聚体粒组中全钾含量分别提高了3.67%、4.92%和5.14%。同时,S+V处理下>2 mm和<0.053 mm土壤团聚体粒组中速效钾...     

Eine Methode zur Ermittelung der wasserspannungsabhängigen Änderung des Sauerstoffpartialdruckes und der Sauerstoffdiffusion in einzelnen Bodenaggregaten

A method to determine oxygen partial pressure and oxygen diffusion in single soil aggregates as a function of soil moisture tension Anaerobic zones occur even in unsaturated soils of silty or clayey texture, that are aerated sufficiently in their macropore system. These zones can be related to the inner parts of soil aggregates. To describe the oxygen balances in soils it is necessary to measure not only in soil profiles but as well in single soil aggregates within a range of soil matrix potentials. Therefore oxygen partial pressure in single soil aggregates of different texture was measured continuously as a function of soil matrix potential. For that purpose we developed an oxygen sensitive microelectrode with a tip diameter of 0.5 mm, that is sturdy enough to measure even in sandy soils. One microtensiometer (diameter of the tip < 0.5 mm) and one oxygen microelectrode were placed in water saturated soil aggregates. Soil water potential and oxygen partial pressure were measured continuously during soil drying. The results show an aeration of primarily anoxic soil aggregates at different soil matrix potentials due to different texture and structure. The clayey polyhedral aggregates of the Vertisol were aerated at significantly lower soil matrix potentials than the loamy prisms of the Fluvisol. These show higher values of oxygen partial pressure even at soil water potentials less than 150 hPa. In the aggregates of the Vertisol, that have a fine texture, values of rel. aparent diffusion D_s/D_o were in the range of 1 · 10^?3 at soil water potentials < ?     

Aggregate characterization as compared to soil bulk properties

The aim of this paper is to clarify the effect of soil aggregation on the physical and chemical properties of structured soils and as compared with the homogenized material. Aggregation and aggregate strength do not only depend on biological activity and organic exudates, but also on the intensity, number and time of swelling, and drying events. Such aggregates are not only more dense than the structured bulk soil, the intra-aggregate pore distribution consists not only of finer pores, but they are also more tortuous. Thus, water and ion fluxes by mass flow as well as ion transportation by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. Futhermore, the chemical composition of the percolating soil solution differs more from that of the corresponding homogenized material the stronger and denser the aggregates are. From the mechanical point of view, the strength of single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. However, internal soil parameters, like grain size distribution or chemical composition, further affect the strength. The more structured the soils are, the higher is the proportion of the effective stress on total stress, but even in single aggregates neutral stresses can be revealed. This is true because of the relationship to the smaller value of the hydraulic conductivity and higher tortuosity. Finally, some dynamic effects on aggregation and aggregate deterioration are discussed.     

Untersuchungen über den Einfluß des Bodengefüges für den Eindringwiderstand in Böden

The influence of soil structure on penetration resistance Penetration resistance depends strongly on the soil structure. However, because roots may either penetrate aggregates or grow around them, the value determined for the bulk soil can only be used as a first approximation. If e.g. unconfined aggregates are penetrated, the penetration resistance increases with increasing size. If however, the aggregates are confined by being embedded in gypsum then the penetration resistance is higher but is independent of aggregate size. Thus, the outer skin is stronger than the inner part of the aggregates. When single prisms are penetrated horizontally, then the penetration resistance is smaller than the resistance to vertical penetration. On the other hand, for polyhedral structure the penetration resistance is of the same order of magnitude in both directions.     

有机物料及无机氮对耕地黑土团聚体水稳性的影响

用室内模拟试验研究了有机C、无机N添加对耕地黑土不同粒级团聚体水稳性的影响。结果表明 ,在不添加任何物料的湿培养条件下 ,随团聚体粒径增大抵抗浸水分散能力减弱 ;添加葡萄糖和玉米根干粉对 1mm水稳团聚体有良好保护作用 ,但不能保护小粒径的团聚体。单纯添加无机N会引起耕地黑土团聚体水稳性下降。     

Soil Water Relations and Aeration Status of Single Soil Aggregates,Taken from a Gleyic Vertisol

Clayey soils have the potential to swell and to shrink depending on their hydraulic and hydrological status. Thus bulk density values vary in a range of 1.0 to 2.0 g cm^?3 in the case of a gleyic Vertisol, by which also other soil physical properties e.g. the pore size distribution of the bulk soil as well as of the soil aggregates are affected. Intraaggregate airfilled porosities are reduced by shrinkage and are relatively low. Thus it appeared to be difficult to determine the airfilled porosity of the aggregates below pF 1.5. For that reason and because of the influence of pore forms we were not able to get a clear relation of diffusion constant K with airfilled porosity. Regarding soil aeration status, the existence of anoxic microsites in the interior of unsaturated soil aggregates has been proved by microelectrode measurements of oxygen partial pressure and redox potential distribution in single soil aggregates. We verified restrained oxygen supply to the aggregate center as well as reduced redox potentials only for aggregates of the A horizon. There the microbial activity, measured as soil respiration as well as the source for C and N was by a factor 2 to 4 higher than in the subsurface horizons.     

Soil physical properties related to soil structure

The aim of this paper is to clarify the effect of soil aggregation on soil physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Such aggregates are, most of all, more dense than the aggregated bulk soil. The intra-aggregate pore distribution consists not only of finer pores but these are also more tortuous. Thus, water fluxes in aggregated soils are mostly multidimensional and the corresponding water fluxes in the intra-aggregate pore system are much smaller. Furthermore, ion transport by mass flow as well as by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. The chemical composition of the percolating soil solution differs even more from that of the corresponding homogenized material the stronger and denser the aggregates are.

The rearrangement of particles by aggregate formation also induces an increased apparent thermal diffusivity as compared with the homogenized material. The aggregate formation also affects the aeration and the gaseous composition of the intra-aggregate pore space. Depending on the kind and intensity of aggregation, the intra-aggregate pores can be completely anoxic, while the inter-aggregate pores are already completely aerated. The higher the amount of dissolved organic carbon in the percolating soil solution, the more pronounced is the difference between the gaseous composition in the inter- and in the intra-aggregate pore system.

From the mechanical point of view, the strength single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. The more structured soils are, the higher the proportion of the effective stress on the total stress is, but even in single aggregates positive pore water pressure values can be revealed. Dynamic forces e.g. due to wheeling and/or slip processes can affect the pore system as well as the composition of the soil by: (1) a rearrangement of single aggregates in the existing inter-aggregate pore system resulting in an increased bulk density and a less aerated and less rootable soil volume, (2) a complete homogenization, i.e. aggregate deterioration due to shearing. Thus, the smaller texture dependent soil strength coincides with a more intensive soil compaction due to loading. (3) Aggregate deterioration due to shearing results in a complete homogenization, if excess soil water is available owing to kneading as soon as the octahedral shear stresses and the mean normal stresses exceed the stress state defined by the Mohr-Coulomb failure line. Consequently, normal shrinkage processes start again.

Thus, the rearrangement of particles and the formation of well defined single aggregates even at the same bulk density of the bulk soil both affect, to a great extent, various ecological parameters. Environmental aspects can also be correlated, or at least explained with the processes in soils, as a major compartment of terrestial ecosystems, if the physical and chemical properties of the structure elements and their composition in the bulk soil are understood.     

Effects of bituminous emulsions on retention and release of some chemical elements in two soil samples

The effects of bituminous emulsions on the retention and release of some nutrient elements and heavy metals were investigated in the laboratory for a pair of different soil samples, one from each of Egypt and Belgium. The emulsion was applied at several rates to promote the formation of aggregates, to stabilize them, and to make the soil samples more hydrophobic. The quantities of extractable potassium and trace elements were determined for the untreated and treated samples, as well as for aggregates more than 2 mm in diameter before and after destruction.The results show that addition of bituminous emulsion increases aggregation percentage and aggregate stability, and decreases the extractable amounts of elements. This effect increases with increasing concentration of bituminous emulsion. The extractable amounts of the nutrient elements after destruction of the aggregates were higher than those before destruction. The mechanisms of retention and release of nutrient elements before and after destroying the aggregates are schematically presented and discussed.     

Die Bestimmung der Wasserspannungs- /Wasserleitfähigkeits-Beziehung von Bodenaggregaten

Determination of the unsaturated hydraulic conductivity of soil aggregates by use of microtensiometers The hydraulic properties of single aggregates were measured with the use of microtensiometers. They are small enough (tip diameter 1 mm, length 1–2 mm) that two of them can be installed inside an aggregate within a distance of 1–3 mm. The changes of water suction are measured with pressure transducers and recorded by a micro-computer. Results obtained for different aggregates show, that at the same water suction, the hydraulic conductivity of single aggregates is up to 2 orders of magnitude smaller than that of the bulk soil. The cross-over-suction value for aggregates can also be derived.     

Water-drop stability of PVA-treated natural soil aggregates from different land uses

Soil erodibility is a function of land use as it affects the stability of soil aggregates. The use of soil conditioners like polyvinyl alcohol (PVA) may help in reducing the soil erodibility, but it is important to economize the use of PVA. A study was carried out to evaluate the interactive effects of land use and PVA concentration on the water-drop stability of natural soil aggregates collected from eroded, forest, agricultural and grass lands. The water-drop stability of these aggregates was monitored using single raindrop simulator. The water-drop stability was lowest in eroded soils, followed by soils from agriculture, forest and grass lands. The smaller aggregates were more stable than the bigger ones. The water-drop stability of aggregates of different sizes and from different lands increased with the application of polyvinyl alcohol (PVA). The mean water-drop stability increased with the application of PVA at the rate of 0.05% by 40% in 2–5 and 5–10 mm aggregates. Increasing the PVA concentration to 0.1 and 0.2% increased water-drop stability value by 71–73% and 87–88%, respectively. The PVA application at the rate of 0.1% could increase the water-drop stability of soils under eroded land equivalent to that of the untreated grassland soils.     

Availability in Soil and Acquisition by Plants as the Basis for Phosphorus and Potassium supply to Plants

This report summarizes research aimed at describing the processes and quantifying the factors affecting transfer of P and K from soil into plants. Soil properties related to availability and plant properties reflecting nutrient acquisition were determined. Their interactions in the rhizosphere and their importance for nutrient supply of plants were studied by a combination of measurements and calculations using a simulation model. Phosphorus and potassium uptake by roots decreased P and K concentration at the root surface and caused characteristic depletion profiles in the adjacent soil. The shape of the profiles depended on the effective diffusion coefficient, the concentration of the nutrient in soil, morphological properties of the roots and on influx into roots. The degree of depletion at the root surface indicated the proportion of the nutrient potentially available in the soil. The shape of the depletion profiles reflected the amount of the nutrient taken up by a root section. The parameters found to describe nutrient acquisition are (i) influx per unit root length, (ii) root length per unit shoot weight (root/shoot ratio), and (iii) the period of time a root section absorbs nutrients. Plant species differed considerably in these properties. In order to integrate the processes involved and to evaluate the importance of individual factors, the Claassen-Barber model was used. Depletion profiles and nutrient uptake calculated with this model were in good agreement with measured values in a number of cases. However, at low P supply, plants absorbed substantially more P than the model predicted. This indicates that influx in this case is supported by mechanisms not properly taken into account yet. Influx per unit root length depends on morphological properties of and nutrient mobilization by roots. Root hairs increase root surface area per unit root length. In addition, because of their small diameter and geometric arrangement in soil, root hairs are specially apt to gain from diffusion when concentration gradients are small. This applies even more to VA-mycorrhizae. Their hyphae are longer and thinner than root hairs and can thus deplete larger volumes of soil per unit root length. Root-induced changes of soil pH increased the size of P depletion profiles, indicating that roots can mobilize soil P by this mechanism. Both acid and alkaline phosphatase enzyme activities were found to be markedly increased at the soil-root interface suggesting that soil organic P may contribute to the P supply of plants.     

Availability of phosphate and potassium as the result of interactions between root and soil in the rhizosphere

A number of findings are summarized in order to show the significance of individual plant properties and soil factors on the availability of phosphate and potassium to plants growing in soil. The flux of a nutrient into a given plant root depends directly on the concentration of the nutrient in the adjacent solution. In nutrient solution, P and K influx follows Michaelis-Menten kinetics. Almost maximum rates of influx have been observed in the range of soil solution concentrations usually found in German arable soils. Roots exhaust P and K from solutions to about 0.2 μmol P and 1 μmol K 1^?1 if not replenished. At the root surface P and K concentrations in soil decrease rapidly within one day; small changes occur after this period. Initially, the extent of the depletion zone is very small but it extends radially with time. After the initial phase therefore, P and K supply to the plant depends on transport from more remote parts of the soil and also on release from undissolved sources. The degree of depletion and the extent of the depletion zone are related to the diffusion coefficient; they decrease with increasing clay content of soil. Root hairs penetrate the soil and extend the volume of soil supplying nutrients to a unit of root. P and K influx therefore increase with the length of root hairs. Proton release of roots mobilize P and K in soil. This is clearly detected by the HCl-soluble P and K fractions within 2 mm of the root surface. The activity of acid and alkaline phosphatases strongly increase in the soil in the vicinity of the root surface of several plant species. It is supposed that organic P compounds can therefore be utilized by plants. P and K influx per unit of root length and root length per unit of shoot weight differed widely between species. The product of these two parameters however was closely related to the P and K concentration of the shoots. Calculations from a mathematical model were in good agreement with measured K depletion profiles and K uptake by plants. It is therefore concluded that the main factors influencing the P and K availability of plants growing in soil have been accounted for in the mathematical model and that the parameters have been accurately measured.     

Development and testing of a model for predicting tillage effects on nitrate leaching from cracked clay soils

Both water movement and nitrate leaching in structured soils are strongly influenced by the nature of the macro-porosity. That macro-porosity can however also be manipulated by choice of tillage operations. In order to investigate the potential impacts of tillage on rates of nitrate leaching from structured soils, a model specific to these soils, CRACK-NP was developed. The model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone Farm, Oxfordshire, UK, is described. The model considers the soil as a series of aggregates whose size is also the spacing of the macro-porosity. Water and solutes move in the macro-pores, but within the peds they move only by diffusion, internal infiltration and root uptake (evaporation). The model reflects the influence of diffusion limitation in the release of solutes to by-passing water. The model was then used to investigate the influence of variable ped spacings which were created by variations in tillage practices. The results both from the model and from the field data demonstrated that finer soil structures, which have larger surface contact areas and shorter diffusion path lengths, present greater opportunities for interaction between peds and the water moving around them, and so release more nitrates through the drainage waters.     

南亚热带丘陵湿润铁铝土结构退化及其机理

本文探讨了南亚热带丘陵湿润铁铝土结构退化过程特征及其机理。结果表明,与地带性自然植下丘陵湿润铁铝土比较,次生植被或人工植被下丘陵湿润铁铝土结构退化现象明显。表现在表土砂化,即砂粒含量相对增加,粘粒含量相对减少;土壤结构稳定性下降,主要全现在较大粒径水稳性团聚体减少;土壤孔隙性变差,主要体现容重增另和较大孔径的孔隙减少。对湿润铁铝土而言,蛔、较大孔隙减少为明显;对耕型湿润铁铝土而言,土壤结构稳定性下     

Soil structure and potassium supply. I. The release of potassium from soil aggregates to Ca-resin

Potassium release from saturated spherical aggregates of three clay soils to Ca-resin was measured and compared with release in suspensions (Talibudeen et al., 1978). The diffusion coefficient for K was calculated from measurements of the tortuosity factor using ³⁶Cl, volumetric water content and buffer power. Release of K is in the order: suspensions >2–3 mm > 6–8 mm aggregates. A computer model incorporating diffusion analysis and release of K shows that in the aggregates the rapidly diffusing exchangeable K, K_ex, is best measured by the extrapolated intercept of a desorption isotherm (24 h equilibration), and is 8–55% less than that extracted by NH₄OAc leaching. Release of K within the aggregates (K_nex, a fraction approximating to Talibudeen's peripheral K) is delayed until after about 95% of K_ex has been depleted which takes 1–2 days. The effective release rate of K_nex inside the aggregates is ～ 17% less than in soil suspensions. Diffusion imposes only small limitations on the release from aggregates of K_nex. This rate of release depends on the frequency of resin changes.     

Measuring interlayer potassium release rates from soil materials. II. A percolation procedure to study the influence of the variable ‘solute K’ in the < 1…10 μM range

Release rates of nonexchangeable K from Ap material of a Luvisol (‘Eckerde’ loess, 15% clay) are determined using a percolation procedure which avoids the common artifacts due to shaking or stirring soil suspensions and thus provides less biased kinetic data. CaCl₂ solution (10 mM_c. 20 °C. pH 5.8) is percolated through packages of soil aggregates (0.5–1 mm grain size, 0.5 g samples) with 0.02 to 25 ml h^?1. Solute K (C_K) was varied between < 1 and > 10 μM and is shown to have a dominant influence on the rates of interlayer K release. These increase exponentially below 3.5 μM (no steady state but steady decrease of release rates in each sample). The difference between the related C_K between moderate and high release rates is as small as 1 μM K. The average rates, of a 10-day-interval, starting after 1.3 times the exchangeable K had been removed, are 40 μmol K kg^?1 soil d^?1 at 4 μM C_K and 240 μmol kg^?1 d^?1 at 3 μM C_K, respectively. It is concluded that larger quantities of interlayer K become plant available in the studied soil if the of soil solutions gets below 3.5 μM (for 10 mM_c Ca, 20 °C, pH 5.8), probably because the dominant dioctahedral illites start to join the release process below this critical limit. The higher K concentration range was accounted for by K-Ca exchange isotherms which, by alteration of shape, indicate that K_nex release becomes measurable below 10 to 20 μM K. It is further argued that existing diffusion or reaction kinetics approaches towards K release are incomplete because the influence of solute K is not considered.