Spodic Materials Research Articles

Using pXRF and Vis-NIR for characterizing diagnostic horizons of fine-textured podzolic soils in subtropical forests

Proximal soil sensing is a promising tool to quantify soil properties through spectral data and machine learning algorithms for application in pedological investigations. However, the relationship between pedological characteristics and sensor signals is difficult to be elucidated with machine learning models. This study aimed to establish direct linkages between pedological features and data from proximal sensors, namely portable X-ray fluorescence spectrometry (pXRF) and visible and near-infrared spectroscopy (Vis-NIR). Ten podzolic soil profiles, including seven Spodosols, two Ultisols, and one Inceptisol, were described and sampled from Central Taiwan. Principal component analysis (PCA) and k-means clustering method revealed that the major variances of the pXRF were affected by the amounts of spodic materials and organic carbon derived from plant debris. For Vis-NIR, a remarkable absorption of Fe-oxides was observed in the B horizons, and the major variations of Vis-NIR spectra extracted by PCA represented the influence of metal-complexed organic carbon and clay. The principal components (PC) of pXRF and Vis-NIR can be used to separate soil horizons, especially the E horizons were clearly separated as indicated by its high user’s and producer’s accuracies. However, the B and C horizons were poorly separated by the PCs and k-means clustering method. Moreover, the PCs can be used directly to predict the nanocrystalline Fe and Al and the optical density of the oxalate extract. Ten soil profiles were classified based on the predicted values, and nine out of ten soil profiles were in agreement with their classification using chemical extractions. The proposed pXRF and Vis-NIR approaches predicted the chemical criteria of diagnostic horizons and facilitated fast determination of Spodosols.

Soil texture analysis by laser diffraction and sedimentation and sieving–method and instrument comparison with a focus on Nordic and Baltic forest soils

PurposeLaser diffraction (LD) for determination of particle size distribution (PSD) of the fine earth fraction appeared in the 1990s, partly substituting the Sieving and Sedimentation Method (SSM). Whereas previous comparison between the two methods predominantly encompasses agricultural soils, less attention has been given to forest soils, including pre-treatment requirements related to their highly variable contents of carbon and Alox+ Feox. In this small collaborative learning study we compared (1) national SSM results with one type/protocol of LD analysis (Coulter), (2) LD measurements performed on three different LD instruments / laboratories, and (3) the replication error for LD Coulter analysis of predominantly sandy and loamy forest soils.MethodsWe used forest soil samples from Denmark, Norway and Lithuania and their respective national SSM protocols / results. LD analyses were performed on Malvern Mastersizer 2000, Sympatec HELOS version 1999, and Coulter LS230, located at University of Copenhagen, Aarhus University and Helsinki University, respectively. The protocols differed between laboratories, including the use of external ultrasonication prior to LD analysis.ResultsThe clay and silt fractions content (<20 μm) from the LD analysis were not comparable with SSM results, with differences ranging from −0.5 to 22.3 percentage points (pp) for clay. Preliminary results from loamy samples with spodic material suggested inconsistent effects of external ultrasonication to disperse aggregates. The comparison between the three LD instruments showed a range in the clay and silt fractions content of 1.9–5.3 and 6.2–8.1 pp, respectively. Differences may be related to the instruments, protocols, and content of a given particle size fraction. The replication error of the Coulter LD protocol was found to be <3 pp in sandy soils, but up to 10 pp in loamy soils.ConclusionDifferences in the clay fraction results partly affected the classification of soil types. The fast replication of the LD analysis enables more quality control of results. The pedological evaluation of non-silicate constituents and optional pre-treatment steps (e.g., soil organic matter or sesquioxides) remains the same for LD and SSM. For comparison of results, detailed descriptions of the analytical protocol including pre-treatments are needed irrespective of instrument and theoretical approach.

Differentiation of fine-textured podzolic soils controlled by climate and landscape in Taiwan

Podzolization is considered to be facilitated by factors such as cool and humid climate, sufficient supply of certain types of organic acids, and coarse-textured parent materials. However, podzolic soils in subalpine areas in Taiwan are characterized by clay content higher than 30%, which differed from that in typical podzolic soils in temperate and boreal regions. This study examined the importance of factors controlling the differentiation of fine-textured podzolic soils in Taiwan. Samples from 31 pedons were examined in this study, and the pedons were grouped into four types, namely Spodosols, Ultisols, Inceptisols, and Inceptisols with placic horizons. Soil colors with 7.5YR 5/6 (the accumulation of spodic materials) and micromorphological observation of oriented clay coatings in the Spodosols confirmed the occurrence of podzolization and clay illuviation, and the selective extraction of Fe and Al confirmed that the Spodosols (Type I) had a higher accumulation of organometallic complexes than the other podzolic soils. A two-step mechanism, namely (1) clay illuviation at an early stage of pedogenesis and (2) podzolization after the canopy vegetation formed, was proposed for the formation of fine-textured Spodosols. Photomicrographs of the Ultisols (Type II) displayed clay hypocoatings masked by spodic materials, and argillic horizons were characterized by a high amount of nanocrystalline iron and high optical density of the oxalate extract. However, the accumulated nanocrystalline iron oxides in the Ultisols were due to the degradation of organic matter under high summer temperatures. Moreover, the leaching of iron in the albic horizons of the Ultisols was attributed to episaturation caused by high clay contents. The Inceptisols (Type III) were transitional soils before forming the Spodosols or the Ultisols, and high clay contents further induced episaturation and formed placic horizons in the Inceptisols (Type IV). Principal component analysis revealed that soil texture and precipitation were the main factors controlling the differentiation of podzolic soils in Taiwan. These factors determined the penetrability and leaching potential of soils, and only the coarse-textured soils with higher leaching potential had the chance of developing Spodosols. However, the developments of Ultisols and Inceptisols with placic horizons were controlled by air temperature and landscape, namely elevation and slope. The bleached E horizons in podzolic Ultisols were attributed to their flat landscape, which favored episaturation and caused the reduction of iron. Low elevation and high temperature in these regions also facilitated the degradation of organic matter and the accumulation of nanominerals.

Morphology and pedogenesis of placic horizons in podzolic Ultisols with high clay content in humid subtropical forests

Placic (Bsm) horizons are commonly found in different types of soils worldwide, but are very rare in Ultisols. The formation of Bsm horizon and podzolization are different processes in pedogenesis. This study collected five podzolic Ultisols containing Bsm horizons from the subalpine forests in Taiwan to illustrate the morphological characteristics and genesis of the Bsm horizons in these soils with both clay illuviation and podzolization. The distinctive Bsm horizons were 3 to 10 mm thick and had abrupt and wavy boundaries with overlying albic (E) and underlying argillic (Bt) horizons. All Bsm horizons contained over 35% of clay fraction which significantly increased from E horizon to B horizons in the pedons. The Bsm horizons showed much higher bulk density than adjacent horizons above or below them, elucidating that such iron pans were relatively impermeable to water and plant roots. The maximum contents of pedogenic Fe occurred in the Bsm horizons mainly cemented by poorly crystalline Fe oxides associated with high amounts of organic matter and epiaquic conditions. Pedogenic Al appeared to be accumulated in the Bt horizons where spodic materials were identified according to the oxalate-extractable amount of Fe and Al. The polygenetic processes of these podzolic Ultisols with Bsm horizons were proposed as three stages. Firstly, clay illuviation predominantly occurred to form Bt horizon in the interglacial periods during the mid to late Pleistocene. Then, podzolization gradually became a major process to accumulate spodic materials in the Bt horizon because of the coniferous forest formation under the cool and humid climates close to the late Pleistocene; however, water was perched at the soil surface due to poor drainage and abrupt textural change between E and Bt horizons after the Holocene. Eventually, the reduced iron from E horizon was gradually re-oxidized and precipitated on the top of Bt horizon where exhibited oxidizing condition to form Bsm horizon until today. Because of the occurrence of Bsm horizon in these Ultisols, a new subgroup Placic Epiaquults, was therefore proposed to the keys to Soil Taxonomy.

Metal sorption to Spodosol Bs horizons: Organic matter complexes predominate

While metal sorption mechanisms have been studied extensively for soil surface horizons, little information exists for subsoils, for example Spodosol Bs horizons. Here the sorption of cadmium(II), copper(II) and lead(II) to seven Bs horizons from five sites was studied. Extended X-ray absorption fine structure (EXAFS) spectroscopy showed that cadmium(II) and lead(II) were bound as inner-sphere complexes to organic matter. Addition of o-phosphate (to 1 μmol l−1) did not result in any significant enhancement of metal sorption, nor did it influence EXAFS speciation. An assemblage model using the SHM and CD-MUSIC models overestimated metal sorption for six out of seven soil samples. To agree with experimental results, substantial decreases (up to 8-fold) had to be made for the fraction ‘active organic matter’, fHS, while the point-of-zero charge (PZC) of ferrihydrite had to be increased. The largest decreases of fHS were found for the soils with the lowest ratio of pyrophosphate-to oxalate-extractable Al (Alpyp/Alox), suggesting that in these soils, humic and fulvic acids were to a large extent inaccessible for metal sorption. The low reactivity of ferrihydrite towards lead(II) can be explained by potential spillover effects from co-existing allophane, but other factors such as ferrihydrite crystallisation could not be ruled out. In conclusion, organic matter was the predominant sorbent for cadmium(II), copper(II) and lead(II). However, for lead(II) the optimised model suggests additional, but minor, contributions from Fe (hydr)oxide surface complexes. These results will be important to correctly model metal sorption in spodic materials.

Mapping pedomemory of spodic morphology using a species distribution model

Red spruce (Picea rubens) ecosystems in the high elevations of the central Appalachians of the eastern United States are the focus of ongoing restoration efforts due to the valuable ecosystem services these forests provide. Recent research has shown that spodic materials still present in the soil represent the pedomemory of the historic extent of red spruce forests in the region. A dataset containing 221 points with varying spodic intensities and 29 environmental variables collected from the Monongahela National Forest in West Virginia, USA, was used to evaluate the utility of a species distribution model, Maximum Entropy (MaxEnt), for predicting the presence of spodic properties. MaxEnt was selected for evaluation because, as a presence-only model, it inherently omits absence locations and thereby reduces the risk of including false absences (i.e., herein, locations that have undergone some level of depodzolization) unlike other models previously used to predict pedomemory. Model outputs that employed three different spodic intensity class inputs—very weak to strong expression, weak to strong expression, and strong expression—resulted in similar spodic probability predictions, though there was less area mapped as transitional probabilities in the strong expression model than the two models that included weaker spodic intensity input data. Permutation importance indicated that no single or small subset of environmental variables controlled the three model outputs, perhaps because the environmental covariates may have been too coarse or not strongly enough associated with podzolization processes to be very important. When the output from the MaxEnt model using the full range of spodic intensities (very weak to strong) was compared to an output produced using a presence-absence model (random forests), there was approximately 62% agreement (where both models predicted presence or both predicted absence) for the cells in the top 40% of the predicted probabilities.

Lateral water flux in the unsaturated zone: A mechanism for the formation of spatial soil heterogeneity in a headwater catchment

AbstractMeasurements of soil water potential and water table fluctuations suggest that morphologically distinct soils in a headwater catchment at the Hubbard Brook Experimental Forest in New Hampshire formed as a result of variations in saturated and unsaturated hydrologic fluxes in the mineral soil. Previous work showed that each group of these soils had distinct water table fluctuations in response to precipitation; however, observed variations in soil morphology also occurred above the maximum height of observed saturation. Variations in unsaturated fluxes have been hypothesized to explain differences in soil horizon thickness and presence/absence of specific horizons but have not been explicitly investigated. We examined tensiometer and shallow groundwater well records to identify differences in unsaturated water fluxes among podzols that show distinct morphological and chemical differences. The lack of vertical hydraulic gradients at the study sites suggests that lateral unsaturated flow occurs in several of the soil units. We propose that the variations in soil horizon thickness and presence/absence observed at the site are due in part to slope‐parallel water flux in the unsaturated portion of the solum. In addition, unsaturated flow may be involved in the translocation of spodic material that primes those areas to contribute water with distinct chemistry to the stream network and represents a potential source/sink of organometallic compounds in the landscape.

Revisiting the definitions of the sombric horizon in soil taxonomy and world reference base for soil resources

A review of the literature suggests that the sombric horizon (from French sombre, dark) was established in Soil Taxonomy (ST) and the World Reference Base for Soil Resources (WRB) from limited data and without a clear understanding of how this horizon forms. This paper reviews data on sombric horizons, evaluates four hypotheses regarding their origin, and offers suggestions for improving the identification of sombric horizons. Of the 30 pedons recognized in the literature as having sombric or sombric-like horizons, 12 fully satisfied the existing criteria in ST and the WRB. Soils with a true sombric horizon may be restricted to the highlands of central Africa (Burundi, Rwanda, Congo) on relatively cool (mean annual air temperature 16–20°C), moist (mean annual precipitation 1450–2000mm) plateaus and mountains at elevations ranging from 1450 to 2000m. Soils with a sombric horizon occur primarily on highly weathered materials from a variety of crystalline rocks. The surface of the sombric horizon occurs at depths of 40 to 110cm from the surface (average=76cm) and ranges from 27 to 100cm in thickness (average=63cm). The sombric horizon commonly is dark reddish brown (5YR 3/3), acidic (average pH=4.7), low in exchangeable bases (average base saturation=4%), high in organic C (average=1.3%), and despite abundant clay (average=56%) has a low cation-exchange capacity (average=12cmol(+)/kg soil). Based on existing data, the sombric horizon contains humus that has migrated downward in the soil, possibly in response to climate and vegetation change. Sombric horizons are not to be confused with sombric-like horizons which may contain andic soil properties or spodic materials. In Soil Taxonomy, soils with sombric horizons are classified primarily as Sombriudoxes (8 pedons) and Sombrihumults (4 pedons). In the World Reference Base for Soil Resources, sombric horizons occur primarily in Umbric Ferralsols (Sombric).

Genesis of Spodic Material underneath Peat Bogs in a Danish Wetland

The chemistry of wet Spodosols (Aquods) differs from well‐drained Spodosols. Two different hypotheses have been suggested to explain the contrasting genesis of wet spodic horizons. This study attempted to determine whether Aquods at a Danish peat‐bog wetland site are a result of (i) in situ illuviation under a fluctuating water table, or (ii) degradation of a former Fe‐rich, well‐drained spodic horizon. Methods included soil surveys, wet chemical analyses, micromorphology, pollen analysis, and radiocarbon dating of soil organic matter (OM) fractions. Aquods, and soil material with spodic features, were exclusively found in sandy material at the margins of or underneath sphagnum peat bogs, whereas Inceptisols were found on well‐drained sandy deposits only. Aluminum content was very high and Fe low in spodic materials with ortstein properties. Solid‐state 13C nuclear magnetic resonance (NMR) spectra of wet sandy 2Bs horizons were dominated by alkyl C (70%) from water‐insoluble OM and were clearly distinct from NMR spectra of overlying 2Bhsm material. Pollen analysis revealed that an open forest with a thick mor layer dominated until shortly after 5000 yr BP, when sphagnum was first recorded. Radiocarbon ages of bulk soil C in the spodic horizons had mean residence times of 4500 to 4400 yr. Accordingly, the spodic B horizon was probably formed by strong in situ illuviation of Al–OM complexes before the sphagnum peat bog formation. This suggests that spodic material formation and thus strong C accumulation underneath this Danish peat bog took place for a limited period, only in susceptible parent material, and at the margins of the expanding peat bog.

Genesis of the sombric horizon in ultisols (red argisols) in southern Santa Catarina, Brazil

Dark subsurface horizons, with properties similar to the sombric horizon characterized by the USA Soil Taxonomy, are frequent in Southern Brazil. The genesis of this horizon is controversial and poorly understood. This study aimed to describe the occurrence of sombric-like horizons in Ultisols in the South of Santa Catarina State, at low altitudes, and suggest possible processes of humus transference, accumulation and persistence in these horizons. Physical, chemical and mineralogical properties of four Ultisols were evaluated; three were sampled in a toposequence, and another representative one in an isolated profile (RSP). The dark subsurface horizons coincide with the AB and BA transitional genetic horizons; they are acid, low in base saturation, and have a similar clay mineralogy in all horizons. Very high amounts of Fe and Al extracted by ammonium oxalate and sodium pyrophosphate solution as well as maximum Al extracted by CuCl2 solution were observed in these dark subsurface horizons, indicating a possible migration of these elements in the form of organometallic complexes. The contents of Al plus ½ Fe extracted from the RSP soil horizons with ammonium oxalate indicated spodic materials in the sombric-like horizon, although the soil morphology was not compatible with Spodosols. Maximum contents of fine clay were also found in the sombric-like horizon, suggesting Fe and Al migration as clay-humic substances. However, the hypothesis that sombric-like horizons in these soils are a relict feature of a grass paleovegetation, different from the current dense seasonal forest, should not be discarded but investigated in further studies.

SOIL-LANDSCAPE RELATIONSHIPS IN THE TAIGA OF NORTHWESTERN RUSSIA HIGHLIGHT THE DIFFERENCES IN THE U.S. AND RUSSIAN SOIL CLASSIFICATION SYSTEMS

Relatively little work has been published about soils in northwestern Russia, and soil-parent material-vegetation relationships are not well established for this region. Seven pedons developed on late-Pleistocence glacial deposits in the northern, middle, and southern taiga zones of northwestern Russia were described and classified according to the official classification systems of both the United States and Russia. Morphological descriptions and laboratory data that included pH, particle size distribution, selective dissolution analysis, and carbonate equivalent were used for classification. Psamments (Podzols) with Bs and Bhs horizons are most common in the northern taiga, where parent materials are dominated by felsic till and glacial outwash, and vegetation is mostly evergreen conifers. Parent material heterogeneity in the middle taiga promotes soil diversity. Soils on thin calcareous till over limestone are Rendolls (Soddy-calcareous soil); soils on thick calcareous till are Cryalfs (Podzolic soil), and those on slightly calcareous glacial-lacustrine deposits are Psamments (Podzol), with much weaker spodic character than the soils of the northern taiga. Vegetation in the middle taiga varies from broadleaf, deciduous trees on highly calcareous soils, to evergreen conifers on the weakly calcareous soils. The southern taiga region soil formed on calcareous two-layer till is a Cryept (Soddy-podzolic soil) under evergreen conifer forest. Soil properties are clearly linked to the composition of the glacial deposits and to the forest vegetation. We also show that strongly podzolized soils, as indicated by albic horizons and significant subsoil accumulations of carbon and oxalate-extractable Al and Fe, nonetheless fail U.S. taxonomic criteria for spodic material and Spodosols, primarily on the basis of soil color.

TRANSITIONAL SOIL CHARACTERISTICS OF ULTISOLS AND SPODOSOLS IN THE SUBALPINE FOREST OF TAIWAN

This study examined the illuviation of clay and spodic materials in three Ultic Haplorthods derived from fine-textured materials from the Alishan Mountain of central Taiwan. Soil analysis, including field description, physical and chemical analyses, and micromorphological studies, was used to study the soils. The soil colors are close to 7.5YR 5/6 in all subsurface horizons, with different degrees of spodic material accumulation. Various clay coatings and pellet-like aggregates were found in all subsurface horizons, which satisfied the morphological and chemical criteria for a spodic horizon (Bhs) as defined in Soil Taxonomy. The illuvial clay coatings have relatively weak birefringence but are masked by organo-metallic complexes. Spodosols were found locally to have a thick organic layer, which may intensify the podzolization process. We hypothesized that Fe and Al were chelated by organic matter in the surface portion of the pedon to form organo-metallic complexes. The translocation of clay occurred before the illuviation of organo-metallic complexes in the subsurface horizons during soil development. The slow accumulation of organic-metallic complexes in the Bhs horizons within the 50-cm depth is attributed to fine texture and less porosity. The soils derived from two pedogenic processes to show the transitional status between Ul-tisols and Spodosols that the argillic horizon is overlain by the spodic horizon.

SOIL CHARACTERISTICS AND CLAY MINERALOGY OF TWO SUBALPINE FOREST SPODOSOLS WITH CLAY ACCUMULATION IN TAIWAN

Two Spodosols with significant spodic morphological characteristics and clay accumulation were selected from a subalpine forest region where the elevation ranges from 2400 to 2700 meters, with high precipitation and low temperatures, and cool and humid weather conditions prevail. The objectives of this study were to investigate the soil characteristics, weathering patterns of clay mineralogy, and genesis of the two Spodosols with clay accumulation in the Chunta Mountain region of central Taiwan. We have also proposed modification of the description of Spodosols in Soil Taxonomy. Soil characteristics, clay distribution, low soil pH, and very low base saturation percentages caused by strongly leaching processes indicate that podzolization and clay illuviation are the pedogenic processes in the study area. The subsurface horizons of the two Spodosols meet the chemical criteria of the spodic horizon defined in Soil Taxonomy, which suggests that translocation of spodic materials has occurred. The value of organic carbon and Fep/Feo or Alp/Alo ratio reaches a profile maximum in the spodic horizon, which suggests that the spodic horizon was formed predominantly by the illuviation of organo-metallic complexes. Soluble organic matter leached with soil solutions along the soil pores from the upper horizons to the spodic horizon. The spodic horizon is characterized by many black and other dark pellets of organic matter. The clay mineralogy of the A and albic horizons is dominanted by illite, vermiculite and vermiculite-illite interstatified minerals (VI), but the Bhs horizons contain mainly vermiculite and hydroxy-interlayer vermiculite (HIV). The clay mineralogy of the eluvial horizons differs somewhat from that in the illuvial horizons as a result of moderate weathering. The weathering sequence of clay minerals in these two loamy Spodosols is: illite → vermiculite (or VI) → HIV. According to the USDA Soil Taxonomy, the two pedons with clay accumulation can be classified as a Typic Haplohumod and a Typic Haplorthod.

Clay Mineralogy of Spodosols with High Clay Contents in the Subalpine Forests of Taiwan

AbstractThree Ultic Haplorthods with significant illuviation of clay and spodic materials in the subalpine forests located in the Alishan area of central Taiwan were selected to identify the clay mineral compositions by X-ray diffraction (XRD) and to explain the transformation of the clay minerals. The three pedons are dominated by vermiculite and vermiculite-illite interstratified minerals, and they have minor kaolinite, quartz and gibbsite. No hydroxy interlayered vermiculite (HIV) was found in the E horizon of the three pedons because the forest soils are very acidic and have very low Al contents. The presence of HIV in the spodic (Bhs) and argillic horizons (Bt) of the three pedons was associated with greater free Fe and Al contents (Fed and Ald), more favorable pH ranges, and coatings of organo-metallic complexes which prevented continuous weathering. The specific pedogenic process, clay illuviation and podzolization occurred sequentially in the Alishan area, and induced the unusual clay mineral distribution and transformation. The largest amounts of illite are in the C horizon and the amounts of vermiculite increased with decreasing soil depths. A reverse distribution between illite and vermiculite through the soil profile was observed. Illite was transformed to vermiculite due to the strong weathering environment associated with extremely low exchangeable K contents. The weathering sequence of clay minerals of Spodosols with fine textures in the study area is proposed as: illite → vermiculite (or interstratified vermiculite-illite minerals) → HIV and vermiculite.

Comparison of the composition of iron pans in Danish burial mounds with bog iron and spodic material

Excavations of Danish burial mounds dated to the South Scandinavian Early Bronze Age (1700–1000 BC) occasionally show thin iron pans encapsulating the interior of the mounds. When strongly cemented these pans can be classified as placic horizons. The encapsulated core was often wet and anaerobic and in Jutland well-preserved oaken log coffins with remnants of organic artefacts and human beings were discovered. Various theories for the development of these pans have been proposed. The chemical composition of the cement of two iron pans in burial mounds is compared with that from a bog iron deposit and a spodic horizon. This shows that the cement from the iron pans is similar to that in bog iron and not to the spodic material.

Subalpine Loamy Spodosols in Taiwan: Characteristics, Micromorphology, and Genesis

AbstractFour subalpine loamy Spodosols with illuvial clay in the spodic horizons were selected for study to understand their characteristics, micromorphology, and genesis. These Spodosols, located in central Taiwan, are covered by coniferous vegetation, receive high levels of precipitation, and occupy gentle slope positions at elevations >2400 m. Soil textures, which range from loam to clay, are finer than those of Spodosols formed in temperate regions. The pedogenic products in the soils of the study area are mainly organo‐metallic complexes or illuvial clay mixed with organo‐metallic complexes in the spodic horizons. Typical micromorphological features in the spodic horizons of these Spodosols revealed infillings of organo‐Fe complexes, argillans, or argiferrans along the irregular voids. The four selected pedons have loose spodic horizons where illuvial clay also has accumulated. The large amounts of spodic materials and illuvial clays in the spodic horizons indicate that podzolization and clay accumulation were the two dominant pedogenic processes. Some of the illuvial clay translocated into the spodic horizons is mixed with organo‐Fe (or ‐Al) complexes along the irregular voids of the spodic horizons. This may also have resulted in a lack of well‐oriented clay in these horizons despite the occurrence of large amounts of illuviated clays.

Spodic Material for In Situ Treatment of Arsenic in Ground Water

AbstractThe leaching of chromium‐copper‐arsenic salts from old wood preservation sites is a threat to ground water at many places in Sweden. The installation of in situ reactive barriers is an attractive “passive’ technique to prevent the further spreading of contaminants. The use of peat as a reactive barrier material has been suggested for heavy metals, but this material was expected to be unsatisfactory for arsenic (As). Therefore, the feasibility of using spodic B horizon material for the retention of arsenic was tested in laboratory column experiments. Contaminated soil was taken from an old preservation site and leached under conditions designed to imitate the field conditions. The arsenic load during the three‐month duration of the test corresponded to a load at the field site during three years. The B horizon material proved to be efficient for retention of arsenic, despite the observation that As(III) dominated the As speciation. The As(III) concentration was reduced from 1 to 3 mg dm−3 to < 0.02 mg dm−3. Pure peat was, as expected, not suited as a reactive barrier for As, and a mixed B horizon/peat reactive barrier also proved unsatisfactory for the removal of As. It is therefore important to separate the B horizon material from any peat that is used to sorb heavy metals. Before applying the B horizon reactive barrier technique in the field, the effect of the naturally occurring variability of the reactive compounds should be tested. The inclusion of oxidizing agents in the barrier could possibly improve the lifetime considerably. Furthermore, the influence of the flow rate should be evaluated since the kinetics of the arsenic adsorption is relatively slow.

Properties, some criteria of classification and genesis of upland forest Podzols in Rwanda

In southwest Rwanda, upland forest soils developed on quartzites and micaceous sandstones along steep slopes have sometimes the macromorphological look of ‘true’ Podzols. An investigation of the micromorphological, mineralogical and chemical properties, however, reveals only weak indications of illuviation of amorphous organic complexes. This process of cheluviation seems to be secondary relative to biological activity, Fe precipitation and weathering. The concept that cheluviation is the dominant process in the formation of spodic horizons and spodic materials would exclude these soils for the class of Podzols. Definitions of spodic horizon and spodic materials proposed in international soil classification systems, should give preference to macro and micromorphological properties over chemical properties, because the former are directly related to different genetic processes (biological activity, oxido-reduction) which also could be responsible for their formation.

Properties of Some Norwegian Spodosols Classified According to the Definitions of Soil Taxonomy 1975 and 1992

Abstract Ninety-three Spodosols selected from a soil database were used to study the properties and classification of some Norwegian Spodosols. The evaluation was kept on an order and suborder level and was based on the upper spodic subhorizon. Of the soils selected, 72 were classified as Orthods, 12 as Humods and 9 as Aquods according to the 1975 classification. A tentative classification according to the 1992 criteria suggested that 70 of the 93 upper spodic subhorizons would qualify for spodic material, 16 did not fulfil the colour requirements, 6 had too high pH and one had too low carbon content. Of the 70 profiles, 5 were classified as Aquods, 14 as Humods and 51 as Orthods according to the 1992 classification. Only 2 of the 1992 version Humods were Humods according to the 1975 classification.

PODZOLIZATION IN ULTRAMAFIC MATERIALS IN SOUTHEAST ALASKA

Ultramafic materials weather rapidly to maintain relatively high saturation with exchangeable Mg, and they have very high Fe and high Mn contents. These features retard the translocation of humus compounds, which is essential for podzolization. Three soils in a range of ultramafic materials were sampled to relate their morphology and spodic properties to parent material differences. Pedon 1 in residuum from weathered dunite is a coarseloamy, oxidic Dystric Cryochrept with no E horizon. Pedon 2 in ultramafic till with a cemented substratum is a loamy, serpentinitic, frigid, shallow Aquic Durochrept. It has E and Bs horizons, but the pH in the Bs horizon is too high (pH > 6) for spodic materials. Pedon 3 in mixed (partially ultramafic) till with compact substratum is a loamy, mixed, shallow Typic Cryaquod. It has an E and a spodic B horizon. The B horizons of all three pedons have low acid oxalate extractable Al (Alo) and very high iron (Feo) contents. They have low pyrophosphate-extractable to dithionite-extractable Fe ratios (Fep/Fed < 0.5) as a result of very high Fed contents. The highest Feo and Fed in each pedon are 92 and 284 g/kg in the Bw horizon of Pedon 1, 98 and 84 g/kg in the Bs horizon of Pedon 2, and 34 and 38 g/kg in the Bhs horizon of Pedon 3. Feo/Fed ratios > 1, as in the Bs horizon of Pedon 2 and in the C horizons of all three pedons, are due primarily to high magnetite contents in the soils. Weathering and leaching in the perhumid climate of southeast Alaska have not been sufficient to form a Podzol or a Spodosol in the soil on dunite. The free Mn and exchangeable Mg contents are lowered sufficiently to form a Podzol in the ultramafic till, but the pH remains too high for a Spodosol. A soil with Podzol morphology and Spodosol chemistry has developed in the mixed till.