MINERALOGY AND GENESIS OF CLAYS IN RED-BLACK SOIL TOPOSEQUENCES ON BASIC IGNEOUS ROCKS IN KENYA

The clay mineralogy of two red-black soil toposequences on basic igneous rocks in Kenya has been investigated. Besides some illite and amorphous material kaolinites predominate in the slope soils (ultisols) whereas smectites are the main clay minerals in the soils of the depressions (vertisols). The kaolinites are poorly crystalline and have high surface area. According to chemical analysis, IR spectroscopy, and D.T.A. the smectites appear to be a ferriferous member of the montmorillonite-beidellite series having approximately 0·5 Fe³⁺ in octahedral and 0·2 Al per O₁₀(OH)₂ in the tetrahedral position. The profile and slope distribution of kaolinites and smectites led to the conclusion that the smectites are the first weathering product and, depending on hydrological conditions governing the soil solution composition (Si, Mg, pH), either persist (depression) or are decomposed (slope) and followed by kaolinites.     

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.     

Tonmineral- und K-Ca-Austauscheigenschaften von Oberböden des Nährstoffpotentialversuches Hallertau

Clay mineralogy and K-Ca-exchange properties of surface soils from the nutrient potential trial Hallertau (Bavaria) In soils of four locations of the Hallertau nutrient potential trial, with a soil texture consisting of sand, silty sand, silty loam and sandy clayey loam, clay mineral properties were measured with the standardized glycerol expansion method and with n-alkylammonium (Rn_c-NH₃⁺-clay). The expandable minerals of the sandy soils consist exclusively of smectites s.s., (s.s. = sensu stricto) with 0.42 to 0.28 charge equivalents per formula unit (p.f.u.). The expandable minerals of the loams are an assemblage of smectites s.s. and vermiculites. The total layer charge of the smectites s.s. extend from 0.54 to 0.28 charge eq. p.f.u. The fine clay fractions (< 0.1 μm) do not contain vermiculites. The layer charge density of vermiculites with homogeneous charge in the coarse fractions varies between 0.60 and 0.95 charge eq. p.f.u. The immediate K-Ca-exchange was extended with the values of the continued K exchange versus Ca at low K intensity. The Q/I isotherms of sandy soils have a more pronounced curvature than the isotherms of the loams; in all cases, however, the exchange curves have a continuous form. This phenomen is discussed in terms of the clay mineralogy of the soils. After 8 years without K fertilizing, samples gave values between 168 and 497 kg smectite-K/ha for the surface soils. The constant rates of K-desorption vary between 12.8 and 28.7 kg K/ha (surface soil). The rates are better differentiated between unfertilized and fertilized soils for the loams than for the sandy soils. The constant rates of K release were found to be controlled at an AR-level between 1.6 · 10^?4 M^1/2 (unfertilized sandy soil) and 5.2 · 10^?4 M^1/2 (fertilized sandy clayey loam soil).     

Umwandlungen von Dreischichtsilikaten unter K-Abgabe und K-Aufnahme

Transformation of three-layer silicates through K-release and K-uptake (A review). In soils formed from unconsilidated sediments with illite-vermiculite-smectite mixtures in their clay fractions the clay mineral transformation from illite to vermiculite and smectite between pH 5 and 7 cannot be considered so far as proven on the grounds of available analytical data. Studies of soil profiles showed that the formation of illite from expandable three layer silicates predominates over the formation of smectite from illite. Practically all samples of loess and calcareous tills of the areas discussed contain smectites. K loesses between pH 5 and 7 are probably very small, because K released during the formation of vermiculites from micas is immediately fixed by smectitic layers which are converted to illite. In those soils potential K fixation of the clay fraction (« 2 μ) is lower than in the corresponding parent sediments despite of vermiculite formation in the soils. Further analytical data (X-ray, interlattice-K, K-selectivity, specific surface) support the hypothesis of illitization of smectites during soil formation.     

Critical potassium potentials for crops. I. The effect of soil type on the growth of ryegrass and creeping red fescue

The quantity: potential relationships for Ca→K exchange in six soils were evaluated, where potential is defined by ΔG_K,Ca+Mg. Using the percentage K saturation of the CEC as the index of quantity, the Worcester Series soil, rich in hydrous micas, was shown to have the highest concentration of K selective sites, and Newport Series soil, with mainly kaolinitic clay, the lowest. The other soils, containing mainly smectites, had intermediate K selectivities. An algebraic transformation of this relationship to separate the effects of exchangeable K and CEC showed that 0.01 m C_aCl₂ released more K than m ammonium acetate. From the exchangeable K : ΔG relationship, two regions of K buffering were observed for all but the Newport soil, the transition occurring at a mean ΔG_K,Ca+Mg value of ?20.7 kJ mol^?1, signifying the K concentration below which K from ‘perlpheral’ regions of micaceous minerals is released. This may explain why the percentage K saturation of the CEC of a soil cropped exhaustively (without K manuring) in the field does not drop below a minimum value. Based on pot experiments, exhaustion and optimum K potentials (ΔG_exh and ΔG_opt) were derived from second-degree polynomials fitted to the response curves of plant dry matter yield against ΔG_K,Ca+Mg for five soils, the Worcester soil showing little response. ΔG_exh was inversely related to the 2 : 1 layer silicate content of the soil (r²=0.98 and 0.94 for ryegrass and fescue respectively), and similarly, ΔG_opt, to their CEC values (r²=0.74 and 0.77). Potassium uptake was more closely correlated with exchangeable K than with ΔG_opt.     

Solubility and Stability of Calcium Arsenates at 25∘C

The solubility and stability of calcium arsenates at 25 ^°C was determined by both precipitation and dissolution experiments. Ca₃(AsO₄)₂? 3H₂O(c), Ca₃(AsO₄)₂? 2¹/₄H₂O(c), Ca₅(AsO₄)₃(OH)(c) and Ca₄(OH)₂(AsO₄)₂? 4H₂O(c) were identified in our experiment over a wide range of pH and for Ca/As molar ratios between 1.25 and 4.0. The solids precipitated at pH = 3 ～ 7 and Ca/As = 1.5 were phase-pure and well-crystallized Ca₃(AsO₄)₂? xH₂O(c) and had relatively larger grain size than those formed at pH > 7. Based on the analytical results and using the computer program PHREEQC, the solubility products for Ca₃(AsO₄)₂? 3H₂O(c), Ca₃(AsO₄)₂? 2¹/₄H₂O(c), Ca₅(AsO₄)₃(OH)(c) and Ca₄(OH)₂(AsO₄)₂? 4H₂O(c) were calculated as K _sp of 10^{? 21.14}(10^{? 20.01} ～ 10^{? 22.02}), 10^{? 21.40}(10^{? 20.08} ～ 10^{? 21.98}), 10^{? 40.12}(10^{? 37.53} ～ 10^{? 42.72}) and 10^{? 27.49}(10^{? 26.10} ～ 10^{? 28.91}), respectively. Correspondingly, the free energies of forming (Δ G _f ^o ) of these calcium arsenates were calculated to be ?3787.87 kJ/mol, ?3611.50 kJ/mol, ?5096.47 kJ/mol and ?4928.86 kJ/mol.     

K-fixing phyllosilicates in soils: the role of inherited components

K-fixing potential of 29 illite/smectite mixed-layer minerals, covering a broad range in layer ratios and stacking arrangements, has been investigated utilizing X-ray diffraction as the principal investigative tool. Samples were derived from a borehole core transecting Permian sedimentary rocks. In 93% of all samples, K⁺ was retained against replacement with Mg²⁺ from a 1 M MgCI₂ solution in an average of 33% of the smectitic interlayers, defined via expansion to 1.7 nm upon ethylene glycol solvation after Mg-saturation. This non-ideal smectite behaviour points to a vermiculite-type layer charge. Ordered structures fixed on average 20% more K than did random ones, but results were widely scattered. The Greene-Kelly test revealed predominantly tetrahedral substitution, intercalation with dodecylammonium chloride molecules a high interlayer charge density in samples with a high K-fixing capacity. The findings have implications in two fields: in pedology, soils developed from thermally altered sedimentary rocks may inherit a considerable potential for K–fixation; in mineral identification, interlayers in illite/smectite interstratifications fully expanded to 1.7 nm upon ethylene glycol solvation despite their vermiculite–type layer charge. Glycerol solvation provided equally inconsistent results, while dodecylammonium spacings seemed to correlate reasonably well with interlayer charge density characteristics.     

Heavy Metal–Mineral Associations in Coeur d’Alene River Sediments: A Synchrotron-Based Analysis

Nearly a century of mining activities upstream have contaminated Lake Coeur d’Alene and its tributaries with Pb, Zn, and other heavy metals. Heavy metal concentrations in sediments of the Coeur d’Alene watershed have been shown to be inversely proportional to the sediment size fraction; thus, analysis on a very small scale is essential to determine the mobility and stability of heavy metals in this environment. Micron-scale synchrotron-based methods were used to determine the association of heavy metals with solid phases in sediments of the Coeur d’Alene River. Bulk X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy, and synchrotron-based microfocused XRD combined with microfocused X-ray fluorescence mapping indicate the presence of crystalline Pb- and Zn-bearing mineral phases of dundasite [Pb₂Al₄(CO₃)₄(OH)₈·3H₂O], coronadite [PbMn₈O₁₆], stolzite [PbWO₄], mattheddleite [Pb₁₀(SiO₄)_3.5(SO₄)₂Cl₂], bindheimite [Pb₂Sb₂O₇], and smithsonite [ZnCO₃]. Likely phases for Zn and Pb adsorption were ferrihydrite, diaspore [AlO(OH)], manganite [Mn^(III)O(OH)], muscovite [KAl₂(Si₃Al)O₁₀(OH,F)₂], biotite [K(Fe,Mg)₃AlSi₃O₁₀(F,OH)₂], and montmorillonite [Na_0.3(Al,Mg)₂Si₄O₁₀(OH)₂·8H₂O]. The large predominance of Fe and Mn (hydr)oxides over other sorbent minerals suggests that the metal sorption behavior is dominated by these (hydr)oxide phases.     

Potassium fixation of some calcareous soils after short term extraction with different solutions

This investigation was done to determine the release of potassium (K) from five calcareous soils of southern Iran using 0.025 M CaCl₂, HCl and citric acid during six successive extractions and to study the K fixation capacity of the soils after K release experiment. Mineralogical study indicated that Vertisols and Mollisols were dominated with smectites; while other soils had illite, chlorite, palygorskite and smectite. Results indicated that citric acid extracted more K than CaCl₂ and HCl (137 vs. 111 and 113 mg kg^?1, respectively). The analysis of calcium (Ca), magnesium (Mg) and K concentrations in the solutions suggests that the exchange of K with soluble Ca and Mg (originated from dissolution of carbonates by acidic solutions) is the main mechanism of K release, but citrate is able to dissolve K-bearing minerals and release K in slightly calcareous soils. Soils with more illite released more K. Potassium fixation capacity of soils increased after extractions of soils with different extractants from 324 to 471 mg kg^?1, with no significant difference. It is suggested to apply more K fertilizers in K-depleted calcareous soils and use of different solutions for extracting K from soil minerals may be a temporary and short term solution.     

Adsorption mechanisms of thiazafluron in mineral soil clay components

The adsorption of the herbicide thiazafluron, 1,3-dimethyl-1-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)urea, by three smectites, illite, kaolinite, ferrihydrite and the clay fraction of an illitic soil (54.9% illite, 17.0% montmorillonite and 24.9% kaolinite) and a montmorillonitic soil (33.9% illite, 55.0% montmorillonite and 11.1% kaolinite) has been determined. Thiazafluron adsorbed on neither kaolinite nor iron oxide. The adsorption isotherms on smectites and illite conformed to the Freundlich equation. Values of K_f-obtained for smectites were larger than for the illite and increased as the layer charge of the smectite decreased. Desorption of thiazafluron on smectites was shown to be highly irreversible. Adsorption isotherms of thiazafluron on different homoionic montmorillonite samples suggest an important role of the exchangeable cations in the adsorption. Infrared spectra and X-ray diffraction analysis of the complexes of thiazafluron with homoionic montmorillonites indicated that thiazafluron adsorbs in the interlamellar space of the smectites, mainly by substitution of water molecules associated with the exchangeable cations through the carbonyl-amide group and formation of H-bonds or waterbridge between the NH group of the amide and the basal oxygens of the montmorillonite. The illitic soil clay adsorbed more of the herbicide than the montmorillonitic one did, suggesting that illite and montmorillonite may be present in soils in altered forms giving rise to different adsorption capacities from those of the pure minerals.     

Mobile and readily available C and N fractions and their relationship to microbial biomass and selected enzyme activities in a sandy soil under different management systems

Within different land‐use systems such as agriculture, forestry, and fallow, the different morphology and physiology of the plants, together with their specific management, lead to a system‐typical set of ecological conditions in the soil. The response of total, mobile, and easily available C and N fractions, microbial biomass, and enzyme activities involved in C and N cycling to different soil management was investigated in a sandy soil at a field study at Riesa, Northeastern Germany. The management systems included agricultural management (AM), succession fallow (SF), and forest management (FM). Samples of the mineral soil (0—5, 5—10, and 10—30 cm) were taken in spring 1999 and analyzed for their contents on organic C, total N, NH₄⁺‐N and NO₃^—‐N, KCl‐extractable organic C and N fractions (Corg_(KCl) and Norg_(KCl)), microbial biomass C and N, and activities of β‐glucosidase and L‐asparaginase. With the exception of Norg_(KCl), all investigated C and N pools showed a clear relationship to the land‐use system that was most pronounced in the 0—5 cm profile increment. SF resulted in greater contents of readily available C (Corg_(KCl)), NH₄⁺‐N, microbial biomass C and N, and enzyme activities in the uppermost 5 cm of the soil compared to all other systems studied. These differences were significant at P ≤ 0.05 to P ≤ 0.001. Comparably high C_mic:C_org ratios of 2.4 to 3.9 % in the SF plot imply a faster C and N turnover than in AM and FM plots. Forest management led to 1.5‐ to 2‐fold larger organic C contents compared to SF and AM plots, respectively. High organic C contents were coupled with low microbial biomass C (78 μg g^—1) and N contents (10.7 μg g^—1), extremely low C_mic : C_org ratios (0.2—0.6 %) and low β‐glucosidase (81 μg PN g^—1 h^—1) and L‐asparaginase (7.3 μg NH₄‐N g^—1 2 h^—1) activities. These results indicate a severe inhibition of mineralization processes in soils under locust stands. Under agricultural management, chemical and biological parameters expressed medium values with exception for NO₃^—‐N contents which were significantly higher than in SF and FM plots (P ≤ 0.005) and increased with increasing soil depth. Nevertheless, the depth gradient found for all studied parameters was most pronounced in soils under SF. Microbial biomass C and N were correlated to β‐glucosidase and L‐asparaginase activity (r ≥ 0.63; P ≤ 0.001). Furthermore, microbial biomass and enzyme activities were related to the amounts of readily mineralizable organic C (i.e. Corg_(KCl)) with r ≥ 0.41 (P ≤ 0.01), suggesting that (1) KCl‐extractable organic C compounds from field‐fresh prepared soils represent an important C source for soil microbial populations, and (2) that microbial biomass is an important source for enzymes in soil. The Norg_(KCl) pool is not necessarily related to the size of microbial biomass C and N and enzyme activities in soil.<?show $6#>     

A nano‐scale study in a soil chronosequence from southern Spain

A soil chronosequence from the River Guadalquivir (Spain) was studied at a nanometre scale (10^?9 m), this being a poorly investigated aspect of soil genesis. The fine mineral fractions (< 50 µm) were studied by X‐ray diffraction (XRD) and analytical electron microscopy (TEM‐AEM). At a nano‐scale, the dominant materials in the chronosequence were dioctahedral 2:1 phyllosilicates, such as potassium mica (mainly muscovites and illites), mixed‐layer smectite/illite and smectite (beidellite). Sodium mica brammallite was described for the first time in soils. These dioctahedral phases are related genetically to illite by inheritance of muscovite and posterior transformation of illite into beidellite. These processes involve (i) loss of regularity in the layer stacking sequence in high‐resolution TEM images, even to the point of losing the contrast between layers; (ii) loss of three‐dimensional order in selected area (electron) diffraction (SAED) patterns, with increased diffusivity and weak spots on hkl levels; (iii) smaller layer packets producing superimposed SAED patterns (high‐angle boundaries in 00l); (iv) changing from two‐layer ordered polytypes (2M₁) to one‐layer random polytypes (1M_d); and (v) chemically, loss of charge and ions in the interlayer. The final result is beidellites with almost tactoid morphologies. As these processes are related to the soil‐forming factor of time it was possible to establish logarithmic chrono‐functions tending to steady state for nano‐scale properties such as the percentage of 2M₁ polytypes, the layer charge, interlayer charge and the interlayer ion content in 2:1 phyllosilicates (more smectite‐like micas). This shows that time affects soil‐forming processes at the nano‐scale to the same extent as described for other scales such as ultra‐microfabric (10^?6 m), horizon (10^?1 m) or pedon (10⁰ m).     

Differential response of mineral-associated organic matter in tropical soils formed in volcanic ashes and marine Tertiary sediment to treatment with HCl,NaOCl, and Na4P2O7

Quantitative knowledge of the amount and stability of soil organic matter (SOM) is necessary to understand and predict the role of soils in the global carbon cycle. At present little is known about the influence of soil type on the storage and stability of SOM, especially in the tropics. We compared the amount of mineral-associated SOM resistant to different chemical treatments in soils of different parent material and mineralogical composition (volcanic ashes – dominated by short-range-order aluminosilicates and marine Tertiary sediments – dominated by smectite) in the humid tropics of Northwest Ecuador. Using ¹³C isotope analyses we traced the origin of soil organic carbon (SOC) in mineral-associated soil fractions resistant to treatment with HCl, NaOCl, and Na₄P₂O₇ under pasture (C₄) and secondary forest (C₃). Prior to chemical treatments, particulate organic matter was removed by density fractionation (cut-off: 1.6 g cm^?3). Our results show that: (1) independent of soil mineralogical composition, about 45% of mineral-associated SOC was resistant to acid hydrolysis, suggesting a comparable SOM composition for the investigated soils; (2) oxidation by NaOCl isolated a SOM fraction with enhanced stability of mineral-bound SOM in soils developed from volcanic ashes; while Na₄P₂O₇ extracted more SOC, indicating the importance of Al-humus complexes in these soils; and (3) recently incorporated SOM was not stabilized after land use change in soils developed from volcanic ashes but was partly stabilized in soils rich in smectites. Together these results show that the employed methods were not able to isolate a SOM fraction which is protected against microbial decay under field conditions and that the outcome of these methods is sensitive to soil type which makes interpretation challenging and generalisations to other soils types or climates impossible.     

The nature and stability of aluminum hydroxide precipitated on wyoming montmorillonite

A freeze—fracture technique was used to prepare replicas of interlayer surfaces of a hydroxy-aluminum montmorillonite (OH-Al-SWy-1), synthesized by titrating an Al-clay with NaOH. Aqueous suspensions containing 0.01 M NaClO₄ and 4% OH-Al-SWy-1 were frozen rapidly with Freon 22 to vitrify the solution phase and stored in liquid N₂. The frozen samples were fractured at 10^?6 Torr and 158K and C-Pt replicas were made of the exposed surface. Transmission electron microscopy (TEM) of the replicas showed a randomly distributed surface precipitate. The precipitate was flat and angular, resembling gibbsite morphologically. Gibbsite crystals were observed outside the interlayer when the suspension was air-dried on microscope grids, underscoring the value of the freeze—fracture technique in minimizing alteration of the OH-Al-SWy-1 complex. Using the mass of Al(OH)₃ fixed by the clay and the average particle perimeter observed by TEM (49.3 nm), we estimated the maximum Al(OH)₃ surface charge density and site density available for phosphate adsorption. The range of surface charge density for the Al(OH)₃, estimated from crystallographic data, was very close to the range of surface charge density for montmorillonite, suggesting that the latter may influence the maximum particle size of the precipitate. The total charge and edge [-Al(OH)(H₂O)] site density were less than the observed CEC reduction on SWy-1 and maximum phosphate retention on OH-Al-SWy-1 at pH 5.3, respectively. Interlayer Al(OH)₃ is known to be unstable with respect to gibbsite and it is possible that a relatively high specific surface area was responsible for the high solubility.     

Aspects of the adsorption of pirimicarb by smectites and soils

A study was conducted to examine the adsorption and interaction mechanism of pirimicarb with smectites and soils. Studies were carried out in pesticide-smectite or soil-water systems and in pesticide-smectite-organic solvent systems, using homoionic samples of montmorillonite, smectites of different layer charge and soils with smectite in their clay fraction. The adsorption isotherms obtained at 30° and 45°C follow the empirical equation of Freundlich. In the adsorption of the pesticide by soils a highly significant correlation was seen between adsorption (distribution coefficient) and the content of smectite in them. In the studies in organic medium the formation of a smectite-pirimicarb bilayer complex with a basal d₀₀₁ spacing of 18.55 Å was observed; the formation of this is also related to the nature of the interlayer cation and the layer charge of the smectite. According to the I.R. results, the interlayer cations of the silicate and the oxygen atom of the C–O group of the pesticide molecule are involved in the interaction mechanism.     

Chemical and colloidal stability of sols in the Al2O3-Fe2O3-SiO2-H2O system: their role in podzolization

Concentrations of dialysable silica in equilibrium with Al₂O₃-SiO₂-H₂O sols at pH 4.5–5.0 confirm the formation of a poorly ordered non-dialysable proto-imogolite species with an Al : Si ratio near 2, close to that of imogolite. Sols with Al : Si>2 give nearly constant levels of free silica in solution in the range 2–6 μg/cm³, indicating equilibrium between proto-imogolite and aluminium hydroxide species. These findings indicate that imogolite-like precipitates in acid soils will buffer silica in solution to within this range during leaching episodes. Imogolite is more stable than a previous estimate suggested, and a revised value for its free energy of formation is proposed: ΔG⁰_f(298.15) = -2929.7 kJ/mol. In Fe₂O₃-SiO₂-H₂O sols, the Fe : Si ratio of the non-dialysable species varies smoothly from 11 to 3 as free silica in solution ranges from 4 to 35 μg/cm³. Such sols are much less colloidally stable than hydroxyaluminium silicate sols, but mixed Al₂O₃—Fe₂O₃—SiO₂—H₂O sols are almost as stable as iron-free sols up to a Fe : Al ratio of 1.5. Thus migration of Al and Fe as mixed hydroxide sols can account for the almost constant ratio of Al to Fe with depth in oxalate extracts from Bs horizons of podzols.     

Soil solution aluminium activity related to theoretical A1 mineral solubilities in four Australian soils

Four soils were treated with HNO₃, CaCO₃ and K₂SO₄ to enable observation of the response of the soil solution composition and the solution A1 ion activity (Al³⁺) to the treatments and to time. The clay fraction of three of the soils was dominated by illite, kaolinite and quartz. The fourth was minated by kaolinite and iron oxides. The initial pH in 0.01 M CaCl₂ varied between 4.0 and 5.0 and the organic carbon content from 0.7 to 1.1%. The soil solutions from soils dominated by kaolinite, illite and quartz were generally supersaturated with respect to quartz and well ordered kaolinite, and unsaturated with respect to illite. The soil solutions from the soil dominated by kaolin and iron oxide were generally unsaturated with respect to quartz but still saturated with respect to ell crystallized kaolin. Within mineral groups such as Al₂SiO₅ compounds, A1₂Si₂O₅(OH)₄ (kaolinite group), and Al(OH)₃ (A1 oxide) minerals, the more soluble forms became less supersaturated or unsaturated with time for many treatments. Lime treatment usually increased the ion activity product of AI(OH)₃ in all soils, and of minerals with the composition, Al₂SiO₅, in the illite/kaolinite soils. Acid treatment reduced the apparent solubility of Al(OH)₃, and the A1 silicates in the Al₂SiO₅, and Al₂, Si₂, O₅,(OH)₄, mineral groups on all soils. These results are interpreted to indicate that lime treatment led to the formation of trace quantities of more soluble A1 minerals that subsequently controlled (Al³⁺), whereas acid treatment dissolved trace quantities of such minerals leaving less soluble minerals to control (Al³⁺). The results suggest that, in mineral soils such as these, (Al³⁺) is under the control of inorganic dissolution and precipitation processes. These processes conform to expectations given the free energy of various inorganic aluminium compounds. Furthermore the sequence of dissolution and formation processes appears to be governed by the Gay-Lussac—Ostwald step rule.     

Veränderungen von Dreischicht-Tonmineralen durch natives K in holozänen Lößböden Mitteldeutschlands und Niederbayerns

Changes of three-layer clay minerals by native K in holocene loess derived soils of Central Germany and Lower Bavaria . Transformations of the clay minerals illite, vermiculite and smectite were determined in Central German (31 profiles) and Lower Bavarian loess districts (5 profiles) using 31–36 soil properties. To get significant differences between solum and sediment, the multivariate diskriminant analysis was applied. In the Central German loess district the following properties were crucial, to receive significant separation between solum-horizons and sediments: smectite content, several values of potassium fixation, interlattice potassium. Important were also some parameter of the K-Ca-exchange curve: Activity ratio \documentclass{article}\pagestyle{empty}\begin{document}$ \left({\frac{{{\rm aK}}}{{\sqrt {{\rm aCa}}}}} \right) $\end{document} of a soil solution in equilibrium with a soil (AR_o), buffering capacity of the soil for K at equilibrium (BCKE), labile K(K₁), exchange capacity of the exchange sites with a specific affinity for K minus labile K(EC_x—K₁). The differences of these values can be explained by assuming that (at pH 6–7) the beidellitic layers of smectites of the parent loess were transformed to illitic layers in the solum-horizons. In the Lower Bavarian loess landscape potassium fixation, total K in clay fraction (< 2 μm) and the parameters of the K-Ca-exchange curve indicate the same clay mineral transformation as in Central Germany. Smectite values were not introduced to the diskriminant function, because they were highly significantly correlated with potassium fixation. In this case potassium fixation has caused a better separation between the solum-horizons and the parent loess than X-ray data. From these results, from total -K of the fraction 2–20μm (solum-horizons smaller than sediments) and the vermiculite content fo the clay fraction, it can be concluded, that the rate of potassium release from micas and the rate of potassium fixation by beidellitic sheets during the Holocene period were nearly equal.     

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.     

Effect of long‐term fertilization and manuring on potassium release properties in a Typic Ustochrept

A long‐term fertilizer experiment, over 27 years, studied the effect of mineral fertilizers and organic manures on potassium (K) balances and K release properties in maize‐wheat‐cowpea (fodder) cropping system on a Typic Ustochrept. The treatments consisted of control, 100% nitrogen (100% N), 100% nitrogen and phosphorus (100% NP), 50% nitrogen, phosphorus, and potassium (50% NPK), 100% nitrogen, phosphorus, and potassium (100% NPK), 150% nitrogen, phosphorus, and potassium (150% NPK), and 100% NPK+farmyard manure (100% NPK+FYM). Nutrients N, P, and K in 100% NPK treatment were applied at N: 120 kg ha^—1, P: 26 kg ha^—1, and K: 33 kg ha^—1 each to maize and wheat crops and N: 20 kg ha^—1, P: 17 kg ha^—1, and K: 17 kg ha^—1 to cowpea (fodder). In all the fertilizer and manure treatments removal of K in the crop exceeded K additions and the total soil K balance was negative. The neutral 1 N ammonium acetate‐extractable K in the surface soil (0—15 cm) ranged from 0.19 to 0.39 cmol kg^—1 in various treatments after 27 crop cycles. The highest and lowest values were obtained in 100% NPK+FYM and 100% NP treatments, respectively. Non‐exchangeable K was also depleted more in the treatments without K fertilization (control, 100% N, and 100% NP). Parabolic diffusion equation could describe the reaction rates in CaCl₂ solutions. Release rate constants (b) of non‐exchangeable K for different depth of soil profile showed the variations among the treatments indicating that long‐term cropping with different rates of fertilizers and manures influenced the rate of K release from non‐exchangeable fraction of soil. The b values were lowest in 100% NP and highest in 100% NPK+FYM treatment in the surface soil. In the sub‐surface soil layers (15—30 and 30—45 cm) also the higher release rates were obtained in the treatments supplied with K than without K fertilization indicating that the sub‐soils were also stressed for K in these treatments.