Acid Ammonium Oxalate Research Articles

Atomic force microscopy of soil inorganic colloids

Atomic force microscopy (AFM) is one of the major techniques for investigating mineralsurface heterogeneity and heterogeneous systems in situ. To date, little attention has been paid to the use of AFM to analyze on a nanometer scale the in situ surface features of naturally occurring soil clays under ambient conditions. In this study, the in situ surface features of clay particles of an Ultisol from Hengshan, China were investigated on a nanometer scale under ambient conditions by using AFM coupled with X-ray diffraction (XRD) before and after sequential selective-dissolution treatments. The results showed that the soil clay after different selective-dissolution treatments displayed distinct AFM surface features. The noncrystalline materials present as surface coatings on phyllo-shaped (predominantly kaolinite) and spheroidal particles (predominantly goethite and hematite) were clearly observed by AFM. The treatment with acid ammonium oxalate in the dark (AOD) virtually removed all of the film-shaped noncrystalline materials and some small spheroid-like particles. The dithionite-citrate-bicarbonate (DCB) treatment removed spheroid-like particles present outside pores but not those inside pores. The AOD and DCB treatments also extracted poorly crystalline aluminosilicates. Citrate extraction following the DCB treatment removed not only the interlayer materials of vermiculite but also the spheroid-like particles present inside pores and abraded the corners and edges of kaolinite particles. The results showed that AFM can be used as a major technique to investigate the in situ surface features of naturally occurring soil mineral colloids under ambient conditions, especially noncrystalline materials which cannot be easily studied by conventional microscopic methods such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) due to the aggregation caused by high vacuum.

Iron (Hydr)Oxide Crystallinity Effects on Soil Aggregation

Differences in crystallinity may explain why total Fe (hydr)oxide content has a variable effect on aggregate stability. Therefore, surface soil samples with a range of poorly crystalline Fe (hydr)oxide contents were characterized for water-stable aggregates >0.25 mm (WSA), mean-weighted diameter (MWD), soil organic C (OC), particle-size distribution, pH, exchangeable cations, citrate/bicarbonate/dithionite (subscript d), and acid ammonium oxalate (subscript o) extractable Fe, Al, and Si. The WSA and MWD range from 23 to 95%, and 0.3 to 5.1 mm, respectively. The effects of Feo (1.1–6.8 g kg−1), Fed (3.2–19.6 g kg−1), OC (2.4–24.0 g kg−1) and clay (141–467 g kg−1) contents on WSA and MWD of both A and B horizons of these soils was studied using linear regression. The poorly crystalline Fe (hydr)oxide (Feo) and OC contents are significantly correlated with WSA in the A horizons (r2 = 0.95, n = 6, p = 0.001, and r2 = 0.93, n = 6, p = 0.002, respectively) and in the B horizons (r2 = 0.73, n = 6, p = 0.029, and r2 = 0.76, n = 6, p = 0.024, respectively). When regressed against MWD, Feo has an r2 of 0.89 (n = 6, p = 0.004) in the A, and 0.97 (n = 6, p = 0.000) in the B horizons. The coefficient of determination of MWD vs. OC contents is 0.98 (n = 6, p = 0.000) in the A and 0.79 (n = 6, p = 0.018) in the B horizons. Clay and Fed contents are not significantly correlated to WSA or MWD. Apparently, the Feo component (poorly crystalline) is more effective than Fed at stabilizing soil aggregates, even though it is present in lower concentrations. The Feo component appears more important than OC in terms of WSA and MWD for soils with relatively low soil organic matter contents.

Evaluation of an Acid Ammonium Oxalate Extraction to Determine Fluoride Resident Concentrations in Soils

Evaluation of an Acid Ammonium Oxalate Extraction to Determine Fluoride Resident Concentrations in Soils

The influence of parent material and livestock intensification on plant‐available phosphorus values by the Olsen method in Northern Ireland

Abstract. Leaching of phosphorus (P) from agricultural land is the major cause of eutrophication of surface waters in Northern Ireland. However, soil testing using the Olsen method has shown that while soil P in some catchment areas of the Province is low, surface waters within these catchments are, nonetheless, every bit as eutrophic as other local catchments where soil P is high. Soil P measurements on over 6000 samples from Northern Ireland soils (A horizon only) have indicated that Olsen‐P values of improved grassland on most parent materials are linearly related to animal intensification. Exceptions are soils derived from peat, marl and basalt. For each of the latter soils, the measured Olsen‐P was shown to be around 10 mg L–1 lower than expected for farms with similar intensification on other parent materials. In particular, the mean Olsen‐P values of samples from basaltic soils under grass with total Fe above 62 g kg–1 and total Mg above 16 g kg–1 were significantly lower than those from basaltic soils with low total Fe (<37 g kg–1) and total Mg (<8 g kg–1). As a result of the depressed Olsen‐P value, excessive quantities of P may be applied to these soils to maintain a recommended soil P index thereby enhancing the potential for nutrient enrichment of adjacent surface waters. In such cases, coworkers have shown that acid ammonium oxalate may be a better extractant than bicarbonate as an indicator of plant‐available P.

Phosphorus reserves and apparent phosphorus saturation in four weakly developed cultivated pedons

Phosphorus (P) reserves and related characteristics of soil horizons were studied in two clay (Inceptisols) and two coarse-textured (Inceptisol, Spodosol) cultivated soils in Finland. The soils were young and they all had been formed of multiple parent materials, the deposition of which were linked to the evolutionary stages of the Baltic Sea. In all C horizons, the content of total P and measured Chang and Jackson P fractions were very similar with 80–90% of the fractionable P in H 2SO 4-soluble form. The dominance of this fraction from the approximate depth of 70 cm down demonstrated the low degree of weathering of the soils. In the B horizons, soil formation had decreased total P and the proportion of H 2SO 4-soluble P while fertilizer P had accumulated in the Ap horizons, leading to an anthropic epipedon in one of the four soils. Accumulation of secondary P was studied by measuring the degree of phosphorus saturation (DPS) of the horizons as a molar ratio of P to the sum of Al and Fe in acid ammonium oxalate extracts. Oxalate extracted from 1.2 to 8.5 times more P than the sum of NH 4Cl–, NH 4F– and NaOH–soluble P fractions, suggesting that a substantial amount of the oxalate-extractable P was primary acid-soluble P. The method thus led to erroneously high DPS values in the poorly weathered soils. Phosphorus saturation calculated as a molar ratio of the sum of NH 4Cl–, NH 4F– and NaOH–soluble P fractions to oxalate-extractable Al and Fe did not indicate a high P saturation in any of the subsurface horizons studied. The result was in accordance with the very low amounts of water-extractable P in the respective horizons.

Relações entre a substituição isomórfica de Fe por Al e as características químicas e mineralógicas de hematitas sintéticas

Foram sintetizadas hematitas com diferentes graus de substituição isomórfica (SI) de Fe por Al até um máximo de 16Almol%. Os minerais foram caracterizados por difratometria de raios-X, análise química total, extração de ferro e alumínio solúveis em oxalato ácido de amônio (Fe o+Al o), determinação da capacidade de troca aniônica (CTA) e área superficial específica (ASE). Os resultados das análises mostraram que os métodos de síntese resultaram em uma quantidade mínima de impurezas e que, de maneira geral, os valores de substituição isomórfica previstos foram alcançados. Os valores de ASE foram relativamente baixos para as hematitas com até 10% de Al e elevados para as hematitas com 13 e 16% de Al de substituição isomórfica. Foi observada estreita correlação entre a ASE e a CTA com os teores de Fe e Al solúveis em oxalato ácido de amônio (Fe o+Al o).

Holocene Soil Development on Till and Outwash Inferred from Lake-Sediment Geochemistry in Michigan and Wisconsin

Two geochemical techniques from soil mineralogy were used with lake sediment to reconstruct soil development in the catchments of two lakes—one on outwash and the other on till—in the Great Lakes region. A sodium pyrophosphate extraction provides information about leaching of exchangeable and organically complexed cations from terrestrial sources, while an acidified ammonium oxalate extraction removes mineral materials without well-developed crystalline structure that are indicative of podzolization. More evidence of pedogenesis is preserved in sediment of the lake on outwash—likely a result of greater retention of material within the soil profile in the catchment on till. This difference between the records suggests that excessively well-drained (leaky) catchments may provide better records of soil development than those with higher water-holding capacity. Evidence of acidification and podzolization appeared first at the lake on till and 4500 years later at the lake on outwash. Both records reveal a late Holocene intensification of weathering that coincided with a regional climatic change to greater moisture availability. The variability in weathering rates observed presents a more complex picture of weathering than the simple exponential functions often assumed in chronosequence studies. Use of lake sediment records provides better temporal resolution of significant events in soil formation than could be achieved using a chronosequence approach.

Interrelations between iron extractability and phosphate sorption in reflooded air-dried sediments

To understand the effects of drying on phosphorus sorption properties of wetland sediment, the acid ammonium oxalate and citrate-dithionite-bicarbonate (CDB) extractions were applied to sediments from seven shallow lakes near Perth, Western Australia. Air-drying generally led to a marked increase in oxalate- and CDB-extractable Fe, but not in extractable-Al which varied less than 2%. Air-drying also induced changes in equilibrium concentration, labile potential and sorption capacity of phosphorus in reflooded sediments. These were accompanied by changes in pH, turbidity, E440 and soluble Fe3+ in the water. The FeOx/FeCDB ratios of wet and air-dried sediments suggested an increase in iron crystallinity upon drying, correlated with the decrease in P-sorption capacity (R2= 0.72, p < 0.05).

The crystal structure of the decomposition product NH4HC2O4 from powder diffraction data

Abstract The crystal structure of anhydrous ammonium acid oxalate, NH4HC2O4, obtained from the thermal decomposition of the related hemihydrate has been determined from powder diffraction data collected with monochromatic laboratory X-rays, using the capillary technique. The crystal structure is monoclinic, P21/c, Z = 4, a = 4.3629(1) Å, b = 13.5657(1) Å, c = 7.7100(1) Å, β = 102.602(1)° and V = 445.3(1) Å3. The structure has been solved with the direct methods and refined with the Rietveld method, from which the agreement factors R wp = 8.19% and R B = 5.18% have been obtained. The crystal structure consists of corrugated layers built from seven-fold ammonium coordination polyhedra and connected by oxalate groups. The hydrogen atom positions have been located and the hydrogen bond network is extensively discussed.

Estimating available molybdenum in biosolids-amended soil using iron-oxide impregnated filter paper*1

Various extraction procedures have been used to characterize molybdenum (Mo) in soils, and attempts have been made to correlate the results with plant response. However, few of these studies attempted to characterize available Mo in biosolids-amended soils, and none used Fe-oxide impregnated filter papers as an availability index. The purpose of this study was to determine the potential of Fe-oxide impregnated filter papers to assess the availability of Mo in biosolids-amended soils, and compare the assessments with those offered by a popular chemical extraction method (acid ammonium oxalate) and total soil Mo (EPA Method 3050A). Soil and plant samples were obtained from a field study with bahiagrass (Paspalum notatum Flugge) as the test crop and analyzed for Mo. The grass was grown in a sandy Spodosol, amended with two Class A biosolids at three rates. Though the effect of biosolids addition on total soil Mo was visible in Year 1, it gradually disappeared with time. However, tissue Mo contents varied significantly with biosolids application rates each year, suggesting that a part of the Mo in the biosolids was readily available for plant uptake. The changes in pH of amended soils did not significantly affect Mo extracted by any of the methods. The best correlation between tissue Mo and soil Mo was obtained using the Fe-oxide strips, followed by acid ammonium oxalate; total soil Mo was generally not well correlated with Mo uptake. Results suggest that the Fe-oxide strips can serve as an analytically satisfactory, theoretically reasonable, and practically acceptable procedure for assessing available Mo, even in soils amended with biosolids. *Contribution of the Agric. Exp. Stn. Journal Series No. R-07612.

PHYSICAL AND CHEMICAL PROPERTIES OF FRAGIPAN HORIZON MATERIALS AMENDED WITH FLUIDIZED BED COMBUSTION ASH

Fragipan horizon strength, which has been attributed to Si cementation, limits soil productivity by restricting the depth of root growth and water movement. Fluidized bed combustion (FBC) ash, a high pH material, was evaluated as a potential amendment to dissolve the cementing agent and decrease the strength of the horizon. Fragipan horizon samples (<2 mm) weighing 1.5 kg were mixed with FBC ash (<0.5 mm) at rates equivalent to 0, 11.2, 22.4, and 44.8 Mg ha -1 , placed in 1650 cm 3 cylinders, and wetted to 40% of the sample weight with distilled water. Five replicates were prepared for each treatment. The cylinders were sealed and equilibrated for periods of 30, 90, 180, and 365 days. At each sampling period, soil materials were characterized for strength by modulus of rupture (MR), pH, and acid ammonium oxalate (AAO) and citrate-bicarbonate-dithionite (CBD) extractable Fe, Al, Si, Ca, and Mg. Soil solutions extracted from the samples were analyzed for the water soluble component of the same elements. Modulus of rupture averaged over the 365 days equaled 0.43, 0.94, 0.83, and 0.42 MPa in increasing order of amendment rate. Water soluble Si and Mg in the soil extracts exhibited the most consistent response to treatment. Silica concentrations for the four sampling periods averaged 30.1, 7.3, 4.6, and 3.2 mg kg -1 , whereas, Mg averaged 0.20, 174.0, 82.5, and 6.0 mg kg -1 , respectively. Linear regression models and correlation coefficients determined for MR versus soil and water chemistry indicated that water soluble Mg was the most highly correlated property and explained 79% of the variability as a single variable model. The results suggest that at relatively low Ca/Mg ratios, Mg acts as a dispersing agent and contributes to the development of greater soil strengths (MR) at the lower amendment rates following Si cementation. At the higher amendment rates, the dispersive effects of Mg are eliminated by its precipitation with Si as a poorly crystalline mineral and by the large increase in Ca concentrations.

Selective dissolution of magnetic iron oxides in the acid-ammonium oxalate/ferrous iron extraction method-I. Synthetic samples

SUMMARY In soil magnetism, the magnetic parameters alone are not always sufficient to distinguish the lithogenic from the pedogenic magnetic fractions. Sequential extraction techniques have therefore been incorporated into magnetic studies to constrain the environmental interpretation. Here we report on the dissolution behaviour of magnetite and maghemite in the acid‐ammonium oxalate method to see whether the method is suitable for specific dissolution of magnetic minerals from soils and sediments. To prevent changes in the extraction mechanism during the experiments (see Appendix A), we used an adapted version of the acid‐ammonium oxalate (AAO) method, in which Fe 2+ is added to the extraction solution prior to the experiment [the AAO-Fe(II) method]. The procedure was divided into several 30 min extraction steps to check the dissolution progress. Synthetic samples containing a quartz matrix with 0.1 wt per cent of iron oxides were extracted with the AAO-Fe(II) method. The iron oxides consisted of either magnetite or maghemite with grain sizes of <0.5 mm (fine grained or SD/PSD) and <5 mm (coarse grained or MD/PSD), or a 1 : 1 mixture of both minerals. Because only magnetite and maghemite were studied, the changes in magnetic characteristics could be monitored after each extraction step by analysis of the bulk susceptibility and hysteresis parameters measured at room temperature. The AAO-Fe(II) method preferentially dissolved the smaller iron oxides from the samples. For samples containing iron oxides with coarse grain size there is a preference for dissolving maghemite rather than magnetite. Extractions of the samples containing mixtures of two different grain sizes or with different mineralogy show that the method preferentially dissolves the smaller grains before attacking the coarse grains in the sample.

Relationships between biosolids treatment process and soil phosphorus availability.

Laws mandating phosphorus (P)-based nutrient management plans have been passed in several U.S. Mid-Atlantic states. Biosolids (sewage sludge) are frequently applied to agricultural land and in this study we evaluated how biosolids treatment processes and biosolids P tests were related to P behavior in biosolids-amended soils. Eight biosolids generated by different treatment processes, with respect to digestion and iron (Fe), aluminum (Al), and lime addition, and a poultry litter (PL), were incubated with an Elkton silt loam (fine-silty, mixed, active, mesic Typic Endoaquult) and a Suffolk sandy loam (fine-loamy, siliceous, semiactive, thermic Typic Hapludult) for 51 d. The amended soils were analyzed at 1 and 51 d for water-soluble phosphorus (WSP), iron-oxide strip--extractable phosphorus (FeO-P), Mehlich-1 P and pH. The biosolids and PL were analyzed for P, Fe, and Al by USEPA 3050 acid-peroxide digestion and acid ammonium oxalate, Mehlich-1, and Mehlich-3 extractions. Biosolids and PL amendments increased extractable P in the Suffolk sandy loam to a greater extent than in the Elkton silt loam throughout the 51 d of the incubation. The trend of extractable WSP, FeO-P, and Mehlich-1 P generally followed the pattern: [soils amended with biosolids produced without the use of Fe or Al] > [PL and biosolids produced using Fe or Al and lime] > [biosolids produced using only Fe and Al salts]. Mehlich-3 P and the molar ratio of P to [Al + Fe] by either the USEPA 3050 digestion or oxalate extraction of the biosolids were good predictors of changes in soil-extractable P following biosolids but not PL amendment. Therefore, the testing of biosolids for P availability, rather than total P, is a more appropriate tool for predicting extractable P from the biosolids-amended soils used in this study.

Relating Soil Phosphorus Indices to Potential Phosphorus Release to Water

AbstractRelationships between soil test phosphorus (STP) and release of P in surface and subsurface runoff are needed to help identify source areas for implementing management strategies to limit P loss to water. To determine whether soil P release could be predicted either by STP values, sorption‐desorption indices, or the degree of soil saturation with phosphorus (DSSP), 11 sites with contrasting chemical properties and management histories were sampled from long‐term field trials in the UK. Each site offered up to three treatments, resulting in a total of 29 soil samples. The results showed that the amount of P desorbed using a successive dilution procedure had no relationship with either total soil P content or P sorption capacity. The most significant property was the extent of P saturation. There was little desorption for DSSP values below 10%; above this point, the amount of P desorbed increased linearly with the DSSP. Five STP methods (Olsen, Mehlich‐3, acidified ammonium oxalate‐oxalic acid, Fe2O3‐coated paper strip, and distilled water) were compared to predict their effectiveness in predicting potential P release to water. While STP values obtained using acidified ammonium oxalate proved to be least effective, those extracted with water correlated best with the amount of P desorbed, accounting for 96% of the variability in differential P release from the soils.

Mineralogia, química e micromorfologia de solos de uma microbacia nos tabuleiros costeiros do Espírito Santo

Solos dos tabuleiros costeiros do Estado do Espírito Santo foram estudados, com o objetivo de investigar e inter-relacionar suas características mineralógicas, químicas e micromorfológicas e fornecer subsídios para a reconstrução do ambiente pedogenético. Onze perfis de solos foram descritos, coletados e caracterizados analiticamente. Em amostras de perfis selecionados, determinou-se o Fe extraído pelo ditionito-citrato-bicarbonato de sódio. Determinou-se também o Fe menos cristalino pelo oxalato ácido de amônio e o Si amorfo pelo NaOH 0,5 mol L-1. Nas amostras de mosqueados e nódulos determinou-se a proporção de hematita e goethita e a substituição em Al na goethita. Análises mineralógicas foram realizadas por métodos óticos, difratometria de raios X e análise térmica diferencial. De amostras indeformadas dos horizontes subsuperficiais foram confeccionadas lâminas delgadas para análise micromorfológica. Constatou-se que o ambiente pedogenético atual está propiciando a estabilização da caulinita e formação de goethita, removendo a hematita e possivelmente sendo responsável pelo amarelecimento (xantização) dos horizontes superficiais. O processo de segregação de ferro é evidenciado pelo seu acúmulo nos nódulos e mosqueados em relação à matriz do solo, provavelmente por difusão, sendo a fonte a matriz. Os nódulos e mosqueados vermelhos estão em processo de destruição e não de formação. As gotículas de ferro, que com freqüência ocorrem no interior dos nódulos e concreções, constituem uma etapa do processo de formação ou destruição dessas estruturas.

Estimation of the Total Amorphous Content of Hawai'i Soils by the Rietveld Method

The crystalline and amorphous phases of the clay fraction of 10 agriculturally important Hawaiian surface soils were analyzed by x‐ray diffraction (XRD) and the Rietveld method. The clay fraction of these soils was also analyzed by x‐ray fluorescence (XRF). A balance sheet method was used to assign the oxide content of the clays to the crystalline phases. The remaining oxides after the crystalline phases were accounted for served as an estimate of the chemical composition of the amorphous fraction. Amorphous phase mass fractions ranged from 0.28 to 0.77. Loss of weight on ignition (LOI) was found to be an important constituent of the amorphous fraction. Acid ammonium oxalate extractions were compared with the results of the Rietveld method. Because of the differences between the results of the two methods, it was apparent that they were determining different fractions of the amorphous material. The Rietveld method determines the total amorphous content of a sample, whereas oxalate extraction determines what we refer to as the chemically active portion of the amorphous fraction. The influence of the amorphous fraction on a soil's chemical and physical properties has remained speculative because the determination of the actual percentage of the total noncrystalline component was not possible by past analytical procedures. Furthermore, because the amorphous component is not considered in the Rietveld computation of the mass fraction breakout of the crystalline phases, estimates of the actual crystalline phases' mass fractions cannot be made without a reliable estimate of the total amorphous content.

Surface reactivity of poorly-ordered minerals in podzol B horizons

The surface reactivity of mineral soil horizons from three podzolised forest soils in Scandinavia was examined. The amount of accumulated C was low, between 1.8 and 2.3% in the top of the B horizons. Selective extractions in combination with infrared (IR) spectroscopy and transmission electron microscopy (TEM) indicated a predominance of poorly-ordered imogolite-type materials (ITM) in the oxalate extractable fraction in all examined B horizons. The presence of well-ordered imogolite was only indicated in the Nyänget B3 horizon. A large proportion of free Fe was removed by ammonium oxalate. Comparisons of Mössbauer spectra (both at room temperature and at 4.2 K) before and after treatment with ammonium oxalate showed that the oxalate treatment resulted in a removal of a (super)paramagnetic Fe 3+ phase, probably ferrihydrite. A comparison of the Mössbauer Fe 3+ parameters at room temperature and 4.2 K indicated a close intergrowth of a ferrihydrite-like oxide with a magnetically neutral matrix, e.g., allophane. The specific surface area (SSA) was determined by N 2 adsorption before and after treatment of the samples with acid ammonium oxalate. The loss of SSA after oxalate treatment was considerable in the B horizon where only between 3.8 to 13.3% of the original SSA remained after treatment. The point of zero charge salt effect (PZSE) increased with depth in the B horizon from between 4.4 and 5.1 in the upper horizons to between 5.7 and 7.7 in the lower part of the B horizon. The increased PZSE with depth paralleled a decrease in the ratio of pyrophosphate soluble C to oxalate soluble Fe+Al. The affinity for SO 4 2−, a good indicator of the presence of active surface hydroxyls, was measured by comparing the H + buffering capacity of a sample titrated in 2.5 mM Na 2SO 4 with a sample titrated in 5 mM NaNO 3. The buffering capacity of the soil in the Na 2SO 4 electrolyte was well correlated with the amount of oxalate minus pyrophosphate soluble Fe+Al ( r 2=0.88). The sulphate exchange capacity was considerably higher than CEC, especially in lower parts of the B horizon. The calculated surface area of the oxalate soluble material (OSM) ranged between 74 and 289 m 2 g −1 and the calculated surface site density of the same material ranged between 0.6 to 3.3 site nm −2. It was concluded that the surface reactivity in the B horizons is dominated by the poorly-ordered variable-charge oxides resulting in a low capacity to retain cations but a high capacity for adsorption of weak acid anions like SO 4 2− and organic acids.

A Petrographic Investigation of two Sequential Extraction Techniques Applied to Anaerobic Canal Bed Mud

The application of sequential extraction procedures to determine metal speciation in sediments is fraught with uncertainty regarding what is actually dissolving or re-precipitating at each stage. In order to choose an appropriate scheme for the investigation of contaminated anaerobic mud two different sequential extraction procedures (Kersten and Forstner, 1986; Quevauviller, 1998) were investigated using a Cryogenic SEM (CryoSEM) technique coupled with energy dispersive X-ray analysis (EDXA). This enabled assessment of the degree of reagent selectivity and any re-precipitation associated with the respective methods. Analysis of the non-leached sediment revealed the most abundant authigenic minerals in order of decreasing abundance to be Fe2+-phosphate vivianite (Fe3(PO4)2·8H2O), mixed Fe, Zn, Cu sulphides, pyrite and calcite. After each stage of the sequential extraction the sediment residue was examined using CryoSEM. After extraction of the exchangeable fraction no obvious evidence of mineral dissolution was observed. Calcite was not completely dissolved during the carbonate extraction stage of either procedure. Vivianite began to dissolve in the carbonate extraction stage of both procedures and was completely dissolved by oxide extraction stage of both procedures. The sediment leached by acidified ammonium oxalate, contained abundant Fe oxalate crystals, suggesting that a large proportion of the Fe released from the vivianite has been re-precipitated. The Fe oxalate was then dissolved with the subsequent sulphide fraction. The technique used to extract the sulphide and organic fraction is the same in both schemes and no sulphide or metal rich organic matter was found in either residue.

Electrophoretic mobility of some tropical soil clays: effect of iron oxides and organic matter

The effect of organic matter and iron oxides on the electrophoretic mobility (EM) behaviour of some colloidal clay fractions of some tropical soils was studied. The clays from the A-horizon of the soils were treated to remove organic matter (oxidation by Na hypochlorite treatment), ferrihydrite (by acid ammonium oxalate) and better crystalline iron oxides (reductive dissolution by dithionite–citrate–bicarbonate treatment). Organic matter and iron oxides particularly better crystalline ones contributed to the pH-dependent charge. The removal of organic matter resulted in an increase in the EM and isoelectric point (iep). Deferration decreased both the EM and iep of the soil clays. Increase in pH resulted in negative EM while salt concentration did not have consistent effect on the EM and iep values.

Identification of throughflow using the distribution of secondary iron oxides in soils

The hypothesis that the spatial distribution of secondary iron (Fe) oxides and hydroxides in soils can be used an indicator for subsurface flow (throughflow) is tested on a podzolised hillslope at Bicknoller Combe, Somerset, UK. Acid ammonium oxalate solution and citrate-bicarbonate-dithionite solution are used to estimate the amount of Fe oxides (Fe o and Fe d respectively) in the soils collected at five depths from 64 soil profiles across the hillslope. The spatial distribution of Fe oxides matches the pattern of throughflow over the slope which had been previously identified by mapping of soil water potential and soil morphological analysis. The distribution of Fe d on the hillslope correlates well with the relative intensity of lateral throughflow in topsoils; the upper convex shoulder slope shows the highest intensity of throughflow, gradually decreasing towards the base of the slope. On convergent slopes, the intensity of throughflow greatly increases along the line of convergence. The spatial distribution of Fe o and Fe o/Fe d reflects a more dynamic aspect of the removal and deposition processes of Fe associated with podzolisation and gleying processes than that of Fe d on the study slope. Significant amounts of Fe removed from upslope soils are precipitated in oxidised subsurface soil horizons at lower slope positions. In the comparison with the ratio Mn d/Fe d which has been previously proposed as an indicator of lateral throughflow in an acid hillslope, the distribution of Fe d, Fe o, and Fe o/Fe d better reflects spatial patterns of throughflow and its relative intensity.