CALCIUM: ALUMINIUM EXCHANGE EQUILIBRIA IN CLAY MINERALS AND ACID SOILS1

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

Transformation of clay minerals in nanoparticles of several zonal soils in China

Purpose

Clay minerals significantly affect the physical, chemical, and biological processes of soils. They undergo spontaneous modification and transformation depending to the climatic conditions. Information concerning the compositions and transformation of clay minerals in nanoparticle colloids (NPs) (25–100 nm) is severely lacking. Studying clay mineral transformation is important approach to understand soil formation. This study was conducted to determine the transformation sequence of clay minerals in several zonal soil NPs.

Materials and methods

Four soils (Haplustalf, Alf-1; Hapludalf, Alf-2; Hapludults, Ult-1 and Ult-2) were collected from B horizons developed under three different climatic zones of China. Alf-1 (36° 05′ N and 117° 24′ E) was located under a warm temperate zone and Alf-2 (30° 38′ N and 115° 26′ E), Ult-1 (29° 13′ N and 113° 46′ E), and Ult-2 (19° 27′ N and 109° 17′ E) under a subtropical zone. The clay particles (<?2000 nm) (CPs) and nanoparticles (25–100 nm) (NPs) of tested soils were separated. The element composition of CPs and NPs was identified by microwave digestion method. The mineralogy and chemical bonding of clay minerals were studied by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR).

Results and discussion

With decreasing latitude, NPs and CPs showed that the molar ratio of SiO₂ to Al₂O₃ trends to diminish, indicating the phenomenon of desilication and allitization in the tested soils. XRD analysis revealed that the main clay mineral of Alf-1 NPs was illite, followed by vermiculite, kaolinite, and kaolinite interstratified minerals (KIMs). The clay minerals of Alf-2, Ult-1, and Ult-2 NPs were dominated by kaolinite (and KIMs), followed by illite, with a little content of hydroxyl-interlayered vermiculite (HIV) in Ult-1 NPs and trace content of gibbsite in Ult-2 NPs. With decreasing latitude, vermiculite and HIV decreased in NPs. When compared to CPs, smectite as well as illite-vermiculite mix-layer mineral (I-V) and illite-HIV mix-layer mineral (I-HIV) were not detected in NPs. The analysis of d060 region by XRD showed that with decreasing latitude, the main clay minerals in NPs were dioctahedral minerals (e.g., illite or kaolinite). These clay minerals resulted from the transformation of trioctahedral minerals in CPs. The disappearance of 2:1 swelling minerals and trioctahedral minerals showed that the NPs were more susceptible to weathering than CPs.

Conclusions

With decreasing latitude, the transformation of clay minerals followed the sequence of illite?→?HIV?→?kaolinite?→?gibbsite in tested NPs.

     

不同时期形成的铁质富铝土特性及其在发生上的意义

本文在其他成土因素相对一致的前提下研究时间因素对土壤性状的影响,并以此为依据探讨分类中的一些问题.随成土时间变长(Q₄-Q₁),土壤的微形态特征,化学性质和粘土矿物组成都发生较大变化.如原生矿物含量减少;三水铝石逐渐增多;高岭石结晶程度变高;粘粒硅铝率和交换量分别由1.91、57.5减至1.25、12.9;游离铁含量增加.分析资料的统计结果表明:成土时间对土壤性状的影响一般小于生物气候的影响;全新世火山灰发育的土壤的粘土矿物、交换量、游离铁含量等性状与红壤截然不同,不应归属于红壤而应在高级分类单元中加以区分;许多在发生上有意义的指标,如硅铝率等,由时间因素引起的变化(Q₃-Q₁)已超过了同一母质发育的不同土类间的变化,因此把Q₃-Q₁玄武岩发育的高富铝土归属于同一土属也是不妥的.     

昆明地区不同母质对红壤发育的影响

昆明地区位于云南省中部,为我国西南高原红壤的主要分布区。关于本区土壤形成过程及发生分类等同题,虽然曾有人做过不少工作,但至今仍存在着一些分歧;有人认为本区土壤的形成过程以砖红壤化为主,土壤类型为砖红壤及铁质砖红壤性土^[1,2];有人认为棕壤化为本区土壤的主要成土过程,土壤应命名为棕色森林土^[4];另有人认为本区土壤属红壤,目前的成土过程为红壤化^[5],所有这些意见,均因资料不足而难取得统一。     

The nature of smectites and associated interstratified minerals in soils of the Gharb plain of Morocco

The mineralogies of ‘Tirs’ (Typic Pelloxererts), and ‘Debs’ (Typic Haploxerolls and Typic Xerochrepts) soils of the Gharb plain in north-western Morocco are investigated, with special attention given to the determination of the nature of the smectitic phase using the lithium test (Li test) and the alkylammonium method. The sand and silt mineralogy of Tirs soils is dominated by quartz with small amounts of feldspars and kaolinite. The sand and silt fractions of Dehs soils also contain significant amounts of mica, chlorite, and interstratified phyllosilicates. The clay minerals of Tirs soils are predominantly a high-charge smectite. The estimated interlayer charge for this phase is 0.61 mol(c)/O₁₀(OH)₂ and the fraction of tetrahedral charge varies from 38 to 44%. Although the percentage tetrahedral charge is less than 50%, the smectitic phase behaves as beidellite with the Li test. Dehs clays are more heterogeneous, consisting of smectite, vermiculite, illite, kaolinite, chlorite, and interstratified illite/smectite and illite/vermiculite. The Li test and the alkylammonium method demonstrate that a high-charge smectite or vermiculite is interstratified with illite. A low-charge montmorillonite is also present both in Tirs and in Dehs soils. The high-charge beidellitic phase is believed to be a transformation product of mica, whilst the low charge montmorillonite is thought to be inherited from the parent material.     

QUANTIFICATION OF WEATHERING, SOIL GEOCHEMISTRY AND SOIL FERTILITY

Continental chemical weathering is discussed with reference to a diagram, in which the ratio (CaO + Na₂O + K₂O)/(Al₂O₃+ CaO + Na₂O + K₂O) is plotted against the ratio (SiO₂+ CaO + Na₂O + K₂O)/(Al₂O₃+ SiO₂+ Na₂O + K₂O). The former ratio is a measure of the degree of feldspar breakdown, which is accompanied by the formation of secondary minerals (illite, smectite, etc.). The second ratio is a measure of the enrichment during weathering of Al, Si oxide phases such as kaolinite, quartz and gibbsite. The application of the diagram to a series of global examples leads to the statement: 1) Chemical weathering is the principal process by which continental surfaces are modified. 2) The extent of chemical weathering is correlated with the age of continental surfaces. 3) Global agricultural productivity is correlated with geologically recent additions of fresh rock debris by processes of volcanism, glaciation or alluviation.     

中国土壤胶体研究——ⅥⅡ.五种主要土壤的粘粒矿物组成

粘粒矿物对土壤特性和肥力的意义是众所周知的。土壤胶体的特性主要决定于粘粒矿物的类型及其数量。随着土壤科学研究的进展,粘粒矿物组成的分析工作,已由定性分析逐渐进入到定量分析。在粘粒矿物的鉴定中,X射线分析是一个很重要的手段,也有用来进行粘粒矿物的定量分析,但由于不同土壤中的矿物结晶程度差异很大,所以侧定的准确性较差。因此,在土壤粘粒矿物分析方面,X射线一般只能作为定性和半定量之用,其分析结果往往用“主导”“伴随”“较多~较少”等术语来表示。目前这种表示粘粒矿物组成的方法已不能满足研究工作的需要,何况不同学者的判读标准又很难一致。     

Relationship between mineral composition or soil texture and available silicon in alluvial paddy soils on the Shounai Plain,Japan

Abstract

To evaluate the relationship between the amount of available Silicon (Si) in paddy soils and their mineral properties on the Shounai Plain in Japan, which is formed from several parent materials, we evaluated the amount of available Si, the particle size distribution, the oxide composition of crystalline minerals and the amount of oxalate-extractable Si (Si_o), iron (Fe_o) and aluminum (Al_o) in the soil. The amount of available Si in the soil and the oxide content of the crystalline minerals differed among four soil groups that were distinguished by their clay mineral composition. There was no difference in the particle size distribution among the soil groups. The amount of available Si was positively related to the SiO₂/Al₂O₃ ratio of clay, the CaO concentration of silt and fine sand, and the amounts of Si_o, Fe_o and Al_o in the soil. The amount of available Si in the soils was negative correlated with the Na₂O and K₂O concentrations of silt, the K₂O concentration of fine sand, and the coarse sand content. These results suggest that the amount of available Si in soils is affected by the weathering resistivity of their minerals and that the particle size distribution and mineral composition are related to the available Si of the soils. Mineralogical properties, including the particle size distribution and mineral composition such as the SiO₂/Al₂O₃ ratio × clay fraction content and the amounts of CaO and MgO in silt-sized particles, were positively correlated with the amount of available Si in the soil, but these correlations were not found for fine sand-sized particles. The Si_o, Fe_o and SiO₂/Al₂O₃ ratio × clay fraction contents contributed approximately 50% to the amount of available Si in the soils. The amount of available Si in the soil was divided into two groups according to the location of the paddy field. The amount of soil-available Si in the alluvial plain was affected by the geology upstream through the mineral composition.     

SOLUBILITY OF ALUMINIUM IN SOILS OF THE HUMID TROPICS

Soil samples obtained from genetic horizons of an upland and acid sulphate soil of Sierra Leone, were equilibrated with 0.01 M CaCI using a 1:2 soil to solution ratio. Al³⁺ activity was estimated from total Al measured in the equilibrium extract, by accounting for hydrolysis and the formation of other complex species. The Al³⁺ activity was pH-dependent, but the Al(OH)³ ion product was I'airly constant throughout the upland soil profile; Al³⁺ activity was near that expected for equilibrium with kaolinite and quartz. In the acid sulphate soil alunite appeared to control the activity of Al³⁺.     

Inhibition of acidification of kaolinite and an Alfisol by aluminum oxides through electrical double‐layer interaction and coating

Soil chemical properties are affected significantly by surface charge characteristics of the soil. Interaction between oppositely charged particles in variable‐charge soils plays an important role in variation of soil electrochemical properties. In this study, the effects of Al oxides on surface charge and acidity properties of kaolinite and an Alfisol during electrodialysis were investigated. The results indicated that Al oxides, when mixed into kaolinite or the Alfisol, decreased the effective cation exchange capacity (ECEC) and exchangeable acidity and inhibited the decrease in pH. Gibbsite had less effect than γ‐Al₂O₃ and amorphous Al(OH)₃ in reducing the ECEC and acidity of kaolinite and the Alfisol; γ‐Al₂O₃ and amorphous Al(OH)₃ displayed comparable effects. However, this effect is inconsistent with the order of the surface positive charge per unit mass that the Al oxides carried. Their effect on the ECEC of kaolinite and Alfisol varied irreversibly with ionic strength of the bathing solutions. X‐ray diffraction spectra indicated that amorphous Al(OH)₃ and γ‐Al₂O₃ were more effective than gibbsite in decreasing peak intensity of electro‐dialyzed kaolinite when mixed with these Al oxides at the same rate. The results demonstrated that Al oxides could decrease the effective negative charge and inhibit acidification of kaolinite and an Alfisol through diffuse‐double‐layer overlapping between oppositely charged particles and coating of Al oxides on these materials. Both mechanisms intensified with increasing rate of added Al oxides, which can therefore act as anti‐acidification agents in variable‐charge soils.     

Red soil chemistry and mineralogy reflect uniform weathering environments in fluvial sediments, Taiwan

Purpose

Information on the physicochemical properties, mineral species and micromorphology of lateritic soils and gravel soil layers in paleo-environmental soil profile is severely lacking. Red soil profile of the Taoyuan terrace was employed to demonstrate its different extents of lateritic weathering. The objectives of this study were to compare the physicochemical properties of lateritic soils and gravel soil layers and identify using conventional and synchrotron X-ray diffraction (XRD) analyses mineral species in nanoparticles separated by automated ultrafiltration device (AUD) apparatus.

Materials and methods

Soil samples were collected from paleo-environmental lateritic soils. Soil samples were examined using elemental analysis, conventional and synchrotron XRD analyses, high gradient magnetic separation, separation and collection of nanoparticles by AUD apparatus, and transmission electron microscopy (TEM).

Results and discussion

The soil pH, redness index, quantities of free Al- and Fe-oxides (Al_d and Fe_d), and clay content of lateritic soils are higher than those of gravel soil layers. Illite, kaolinite, gibbsite, quartz, goethite, and hematite were identified in clay fractions and nanoparticles by conventional and synchrotron XRD analyses. TEM images show presence of hematite nanoparticles on the surface coating of kaolinite nanoparticles and aggregated hematite nanoparticles overlapping the edge of a kaolinite flake in a size range of 4?C7?nm. Synchrotron XRD techniques are more straightforward and powerful than conventional XRD with random powder methods for identifying nanoparticles in red soils, particularly for illite, kaolinite, goethite, and hematite nanoparticles. According to chemical compositions of clay fractions and red soil features in the Taoyuan terrace, these red soils can be taken as lateritic red earths or red earths.

Conclusions

This work suggests that physicochemical properties, mineral species, and micromorphology of red soil at all depths can shed light on the extent of paleo-environmental lateritic weathering.     

中国土壤胶体研究 Ⅵ.西藏高原几种主要土壤的粘土矿物组成和演变

西藏高原突起于我国西南,绝大部分地面的海拔高度在4000米以上,为世界上最高的大高原。它大致在第三纪开始形成,后来曾受第四纪冰川的深刻作用,高山顶部至今仍是冰川的活动场所^[1,2]。高原为昆仑山、唐古拉山、喜马拉雅山和横断山等大山脉所盘踞。     

胡敏酸对铵钾在粘土矿物上交互作用的影响

Interaction of ammonium (NH+4) and potassium (K+) is typical in field soils. However, the effects of organic matter on interaction of NH+4 and K+have not been thoroughly investigated. In this study, we examined the changes in major physicochemical properties of three clay minerals (kaolinite, illite, and montmorillonite) after humic acid (HA) coating and evaluated the influences of these changes on the interaction of NH+4 and K+on clay minerals using batch experiments. After HA coating, the cation exchange capacity (CEC) and specific surface area (SSA) of montmorillonite decreased significantly, while little decrease in CEC and SSA occurred in illite and only a slight increase in CEC was found in kaolinite. Humic acid coating significantly increased cation adsorption and preference for NH+4, and this effect was more obvious on clay minerals with a lower CEC. Results of Fourier transform infrared spectrometry analysis showed that HA coating promoted the formation of H-bonds between the adsorbed NH+4 and the organo-mineral complexes. HA coating increased cation fixation capacity on montmorillonite and kaolinite, but the opposite occurred on illite. In addition, HA coating increased the competitiveness of NH+4 on fixation sites. These results showed that HA coating affected both the nature of clay mineral surfaces and the reactions of NH+4 and K+with clay minerals, which might influence the availability of nutrient cations to plants in field soils amended with organic matter.     

Characterization of kaolinite in the hardsetting clay fraction using atomic force microscopy,X-ray diffraction,and the Rietveld method

Purpose

Brazilian soils that present extremely hard sub-superficial horizons when dry and friable when humid are similar to the Australian and South African hardsetting horizons whose hardness can be mainly related to low crystallinity. Studies involving refinement by the Rietveld method with X-ray diffraction (RM-XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and their relation have not been carried out in hardsetting horizon soils. Thus, the objective of this study is to obtain information about the kaolinite in the hardsetting horizon of a Yellow Argisol clay fraction, taking into consideration the results of isomorphic substitution, crystallite average size, and microstrains, relating them to particle image analysis regarding their morphology and size.

Materials and methods

Soil samples were collected in the hardsetting horizon of a Yellow Argisol in the Coastal Tablelands region, which covers the whole Brazilian Northeast coast and part of the Southeast region. The sample was powdered, sieved, and submitted to dispersion and physical fractioning process by sedimentation. The clay fraction was analyzed by RM-XRD, AFM, and SEM techniques.

Results and discussion

The RM-XRD provided improvement of indices with isomorphic substitutions in the goethite [Fe_0.70Al_0.30O(OH)], kaolinite [Al_1.44Fe_0.56Si₂O₅(OH)₄], and halloysite [Al_1.42Fe_0.58Si₂O₅(OH)₄]; 29 nm crystallite average size; 5 × 10^?3 microstrain; and 49.5% kaolinite. AFM analyses indicated particle average size from 80 to 250 nm and average height from 60 to 80 nm. By relating this data, it was possible to estimate that the particles under analysis are kaolinite composed of 3 to 9 crystallites and stacking of 88 to 112 layers.

Conclusions

The process, analyses, and comparisons such as crystallographic and morphologic information about the kaolinite mineral particles contribute to the comprehension of the hardsetting horizon soil nature as well as other soils that present minerals with a high degree of isomorphic substitution.

     

Aluminium and sulphate in acidic soils and groundwaters on the Swedish west coast

Acidic groundwaters and soils in Halland County (Hailands län), southwest Sweden, have been investigated with respect to conditions of soluble aluminium (Al) and sulphate (SO₄ ^2?. Basic Al-sulphate, Fe-oxide, Al-oxide, Al-hydroxide and clay minerals, are discussed and evaluated in their roles for governing Al and SO₄ ^2? in the groundwaters. Based on this investigation, it is suggested that Al³⁺ solubility is controlled by amorphous Al-hydroxide. The SO₄ ^2? in the groundwaters will depend primarily on the H₂SO₄ input. The H₂SO₄ load enhances soil mineral weathering which enhances the production of Fe-oxides, i.e. anion exchange surface sites, to which groundwater SO₄ ^2? attain adsorption equilibra. The factors that control solubility of Al and SO₄ ^2? are both influenced by the acidity in the soil catenas which in the area largely depend on the H₂SO₄ input. Clay minerals such as illite, smectites, halloysite, and variable composition Al-silicates do not exert strong control on Al in the groundwaters investigated.     

Verhalten von Dünger-NH4 in Böden unterschiedlicher tonmineralischer Zusammensetzung im Inkubationsversuch

Fate of fertilizer ammonium in soils with different composition of clay minerals in an incubation experiment In an incubation experiment with three different soils (gray brown podsolic soil from loess, alluvial gley, and brown earth, derived from basalt) the specific adsorption (fixation) and release of fertilizer NH₄⁺ was investigated. In one treatment 120 mg NH₄–N/kg soil was added, while the other treatment (control) received no nitrogen. Soils samples were taken every ten days and analyzed for nonexchangeable and exchangeable NH₄⁺ and NO₃^?. The experimental results are showing that the specific adsorption of applied NH₄⁺ was related to the type of clay minerals. While the loess soil, rich in illite, and the alluvial soil, rich in expansible clay minerals, bound about 40% of the added NH₄⁺ specifically, the soil derived from basalt with mainly kaolinite bound only about 10 %. From the recently “fixed” fertilizer NH₄⁺ about a half was nitrified during the incubation period of about 9 weeks. In the control there was no significant release of specifically bound NH₄⁺. Obviously this NH₄⁺ is located more deeply in the interlayers of the clay minerals and not available to microorganisms.     

Kinetic behaviour of alkaline phosphatase desorbed from different clay minerals

The kinetic (K_m, V_max) of alkaline phosphatase (AP) desorbed from different Ca-homoionic clay minerals (montmorillonite, illite, and kaolinite) by extraction with Tris-Malate-Citrate buffer solution (pH 9.6) was studied in model experiments. After extraction (shaking for 15 min.) the K_m and V_max were measured in the extract, the remaining sediment and in the whole set-up. With kaolinite and illite, V_max of the desorbed AP was lower than that of the sediment. However, with montmorillonite, V_max of AP in the extract and whole system increased if compared to the control, but decreased in the sediment. The K_m of desorbed AP increased from 4.3 × 10^?3 (control) to 5.0 × 10^?3 M (illite), 5.4 × 10^?3M (kaolinite), and 5.5 × 10^?3M (montmorillonite). These values were lower than those obtained with the various sediments and whole experimental systems. An aberrant behaviour was recorded with the illite sorbed AP which showed an increase in affinity towards the substrate. Generally speaking, AP desorbed from clays may be reduced in its affinity towards the substrate p-nitrophenylphosphate by residual inhibitor and/or conformational change of the enzyme.     

Influence of Aluminum Hydroxide on Potassium Release from Two Colombian Soils

Abstract

The effect of sesquioxides on the mechanisms of chemical reactions that govern the transformation between exchangeable potassium (Kex) and non‐exchangeable K (Knex) was studied on acid tropical soils from Colombia: Caribia with predominantly 2∶1 clay minerals and High Terrace with predominantly 1∶1 clay minerals and sesquioxides. Illite and vermiculite are the main clay minerals in Caribia followed by kaolinite, gibbsite, and plagioclase, and kaolinite is the major clay mineral in High Terrace followed by hydroxyl‐Al interlayered vermiculite, quartz, and pyrophyllite. The soils have 1.8 and 0.5% of K₂O, respectively. They were used either untreated or prepared by adding AlCl₃ and NaOH, which produced aluminum hydroxide. The soils were percolated continuously with 10 mM NH₄OAc at pH 7.0 and 10 mM CaCl₂ at pH 5.8 for 120 h at 6 mL h^?1 to examine the release of Kex and Knex. In the untreated soils, NH₄ ⁺ and Ca²⁺ released the same amounts of Kex from Caribia, whereas NH₄ ⁺ released about twice as much Kex as Ca²⁺ from High Terrace. This study proposes that the small ionic size of NH₄ ⁺ (0.54 nm) enables it to enter more easily into the K sites at the broken edges of the kaolinite where Ca²⁺ (0.96 nm) cannot have access. As expected for a soil dominated by 2∶1 clay minerals, Ca²⁺ caused Knex to be released from Caribia with no release by NH₄ ⁺. No Knex was released by either ion from High Terrace. After treatment with aluminum hydroxide, K release from the exchangeable fraction was reduced in Caribia due to the blocking of the exchange sites but release of Knex was not affected. The treatment increased the amount of Kex released from the High Terrace soil and the release of Knex remained negligible although with Ca²⁺ the distinction between Kex and Knex was unclear. The increase in Kex was attributed to the initially acidic conditions produced by adding AlCl₃ which may have dissolved interlayered aluminum hydroxide from the vermiculite present, thus exposing trapped K as exchangeable K. The subsequent precipitation of aluminum hydroxide when NaOH was added did not interfere with the release of this K, and so was probably formed mostly on the surface of the dominant kaolinite. Measurement of availability of K by standard methods using NH₄ salts could result in overestimates in High Terrace and this may be a more general shortcoming of the methods in kaolinitic soils.     

THE EQUILIBRIA OF K:Al EXCHANGE IN CLAY MINERALS AND ACID SOILS1

Conventional K: Al exchange isotherms for montmorillonite showed that Al³⁺ was strongly preferred to K⁺ in o-oin solutions. The exchange coefficient, K', calculated using the isotopically exchangeable K, was greater than unity and did not vary with the Al-saturation or with the initial pH of the AlCl₃ solutions. Isotherms for vermiculite, illite, and soils in o·oin solutions also showed Al³⁺-preference but unlike those for montmorillonite were not asymptotic to q_Al/q_o= 1, q_Al being the amount of adsorbed Al and q_o the total adsorbed (Al + K), indicating that some of the isotopically exchangeable K could not easily be exchanged by Al³⁺ ions; this difficultly exchangeable K (DEK) was estimated for each exchanger. K' values for vermiculite, illite, and soils were less than unity and did not vary with Al-saturation or initial pH if the isotopically exchangeable K was corrected for DEK. This showed that K⁺ was adsorbed more strongly than Al³⁺. Strengths of K⁺ adsorption referred to Al³⁺ as the counter-cation were in order: soils > vermiculite, illite > montmorillonite.     

The forms and availability to plants of soil potassium as related to mineralogy for upland Oxisols and Ultisols from Thailand

Potassium (K) deficiency is widespread in crops on highly weathered upland soils under a tropical monsoonal climate. Critical assessment of the forms of K in soils and of the ability of soils to release K for plant uptake is important for the proper management of K in crop production. The relationships between different pools of K were investigated as a function of silt and clay mineralogy for 14 upland Oxisols and 26 upland Ultisols soils from Thailand. Most soils contained no K-minerals in the silt fraction. XRD showed that kaolinite is the dominant clay mineral with variously minor or moderate amounts of illite, hydroxy-Al interlayered vermiculite and smectite present in some soils. For some soils, both conventional and synchrotron XRD were unable to detect illite. Analytical TEM including EFTEM of individual clay crystals showed that clay in the apparently illite-free samples contained very small amounts of illite. Many kaolinite particles appear to contain K which may be present in illite interleaved with kaolinite crystals. A glasshouse K-depletion experiment was conducted to assess the K supply capacity and changes in chemical forms of K and K-minerals using exhaustive K depletion by Guinea grass (Panicum maximum). Potassium deficiency symptoms and mortality of plants occurred on light textured soils, whereas plants survived for six harvests for Oxisols with clay texture, relatively high CEC and higher NH₄OAc-K (exchangeable K plus water-soluble K). There is a strong linear relationship of unit slope between NH₄OAc-K and cumulative K uptake by plants indicating that NH₄OAc-K is a major form of K available to plants. Thus K-bearing minerals contributed little K to plants over the time scale of the experiment and XRD patterns of whole soil samples, silt and clay from soils after cropping mostly showed no change from those for the initial soil. An exception was for a single surface soil clay where a minor amount of smectite was formed from illite by K release to plants.