Argillic Horizons Research Articles

Characterization and Classification of Some Alluvium Derived Soils under Different Land Uses in Jorhat District of Assam

A study was conducted in the alluvium-derived soils of Jorhat district of Assam in order to characterize the soils as influenced by different land uses. Four representative soil profiles were collected from rice field (P1), vegetable growing area (P2), tea plantation (P3) and natural forest (P4), and analyzed for morphological and physicochemical properties. The soils were dark brown to brownish yellow in colour with dominant hue of 10YR, value ranging from 3 to 6 and chroma ranging from 2 to 8. The soil texture varied from sandy loam to clayey and the soil structure exhibited a trend of development with soil depth. Presence of argillic horizon was observed in rice soils, while development of cambic sub-surface horizon was noticed in soils under other land uses. The soils were acidic (pH 4.5 to 5.5) with appreciable amount of exchangeable H+ [0.19 to 1.15 cmol(p+)kg−1] and Al3+ [0.41 to 2.10 cmol(p+)kg−1]. The process of acidification was not severe in rice soils showing profile weighted mean value of pH 5.3 compared to soils under other land uses (4.6 to 4.7). Surface horizons of all the soils contained higher amount of organic carbon (7.6 to 20.6 g kg−1) and exhibited a regular decreasing pattern with depth. Exch. Ca2+ was the dominant basic cation [1.17 to 3.21 cmol(p+)kg−1] in all the land uses. Rice soils recorded the highest total exchangeable cations, CEC and ECEC [profile weighted means 3.99, 9.6 and 5.25 cmol(p+)kg−1, respectively]. The base saturation was high in the soils under cultivation (weighted mean 41.6 and 34.8% in rice and vegetable growing areas, respectively) followed by forest (30.9%) and tea plantation (30.0%). The soils were classified as Oxyaquic Hapludalf (P1) and Typic Dystrudepts (P2, P3 and P4) at sub-group level.

English

Some soils in Ethiopia reduce plant productivity due to physical and/or chemical limitations. The morphological, physical and chemical properties of problem soils, including sodic, acidic, and saline soils, around southern Ethiopia were characterized and are described in this chapter. The intention is to characterize the soils and better understand the specific nature of the limitations. Sodic soils of Alage, acid soils of Hagereselam, and saline soils of Zeway areas were sampled to represent the problem soils. The soil properties determined included color, electrical conductivity (EC), structure, consistency, bulk density, texture, pH, organic carbon (OC), total nitrogen, available phosphorus (P) and K, exchangeable bases and available micro-nutrients. The soils had considerable heterogeneity in solum and regolith thickness, horizon depth, structural development in surface soils and subsurface horizons, pH, EC and available nutrients. The classification of these soils was made according to Soil Taxonomy and World Reference Base for Soil Resources systems. The sodic soils of Alage had high pH and sodium (Na) content, and low level of OC, available P, copper (Cu) and zinc (Zn). The epipedon was classified as an ochric and the profile also had variation in clay content down the profile to satisfy the requirements of having an argilic horizon and therefore these soils were classified as Typic Natragids. Soils of Hagereselam had very low available P. Available Cu and Zn contents were found to be at the marginal levels for production of most crops. The Hagereselam profile had an argilic horizon with umbric epipedon and was classified as Typic Paleustults. The saline soils in Zeway area had relatively high amounts of calcium and low OC. The profile was found to have ochric epipedon and these soils are classified as Typic Haplocambids. Application of fertilizer including P, Cu and Zn, and removal of sodium and salts from the soil profile may be means of improving the productivity of these soils. Application of organic amendments including manures and crop residues may also be beneficial in increasing the fertility as well as organic carbon content.   Key words: Acidic, saline and sodic soils, reclamation, morphological, physical and chemical soil properties.

Variability of Soil Properties along Selected Transects in Mt. Makiling Forest Reserve, Philippines

A study was conducted to determine the variability of soil properties along selected transects in Mt. Makiling Forest Reserve, Laguna, Philippines. Samples were taken from seven pedons at the northeastern slope, and six pedons at the southern slope. Morphological properties of soils were investigated in situ. Samples were analyzed for their chemical and physical properties. Four landscape positions were identified at the northeastern slope: interfluve, seepage slope, transportational midsope, and colluvial footslope. At the southern slope, three landscape positions were established: interfluve, transportational midslope and colluvial footslope. Soils at the interfluve in both transects had deep soil profiles with low bulk densities and high organic matter. Soils at the seepage slope have argillic horizons, higher soil pH, exchangeable bases, and base saturation. Lithologic discontinuities were observed at the transportational midslope. Soils at the colluvial footslope of the southern slope have higher soil pH, OM content, exchangeable bases, CEC, and base saturation compared to soils at the northeastern slope. Soils at the northeastern slope were classified as Andisols, Inceptisols, Entisols, Ultisols, and Alfisols. On the other hand, soils at the southern slope were classified as Andisols, Alfisols, and Inceptisols.

Impact of rhizome quality on Miscanthus establishment in claypan soil landscapes

Thousands of eroded-soil hectares in the U.S. Midwest have been planted to Miscanthus×giganteus as an industrial or bioenergy crop in recent years, but few studies on factors affecting crop establishment have been performed on these soils. The objective of this study was to quantify how both rhizome quality and depth of soil from the surface to the first argillic horizon (or depth to claypan (DTC11DTC—depth to claypan or argillic horizon.)) affected M.×giganteus establishment. Rhizome quality (i.e., mass, length, diameter, viable buds, score), emergence, growth, and winter survival were measured on rhizomes planted in 2013 at Columbia and 2014 at Centralia, Missouri on clay loam soils with a range of DTC. Rhizome emergence and early tillering slightly increased as DTC increased, but these effects on growth diminished as the season progressed. Rhizome emergence and growth were more influenced by some metrics of rhizome quality; the odds of a rhizome emerging increased by 25 and 40% with each 1cm and 1 bud increase in rhizome length and active bud count, respectively. Furthermore, late tiller counts, basal circumference, and end-of-season biomass increased as rhizome length and mass increased. Winter survival could not be estimated as well as emergence, but the odds of survival across sites increased by 5% with each 1cm increase in rhizome length. When DTC was categorized as soil erosion class or landscape position, only the backslope at Centralia caused greater M.×giganteus growth than other positions. These findings demonstrate the resiliency of M.×giganteus for early growth and establishment on even the most degraded parts of the claypan soil landscape and indicate that propagating larger rhizomes will improve establishment.

Interflow dynamics on a low relief forested hillslope: Lots of fill, little spill

Author(s): Du, E; Rhett Jackson, C; Klaus, J; McDonnell, JJ; Griffiths, NA; Williamson, MF; Greco, JL; Bitew, M | Abstract: We evaluated the occurrence of perching and interflow over and within a sandy clay loam argillic horizon within first-order, low-relief, forested catchments at the Savannah River Site (SRS) in the Upper Coastal Plain of South Carolina. We measured soil hydraulic properties, depths to the argillic layer, soil moisture, shallow groundwater behavior, interflow interception trench flows, and streamflow over a 4-year period to explore the nature and variability of soil hydraulic characteristics, the argillic topography, and their influence on interflow generation. Perching occurred frequently within and above the restricting argillic horizons during our monitoring period, but interflow was infrequent due to microtopographic relief and associated depression storage on the argillic layer surface. High percolation rates through the argillic horizon, particularly through soil anomalies, also reduced the importance of interflow. Interflow generation was highly variable across eleven segments of a 121 m interception trench. Hillslopes were largely disconnected from stream behavior during storms. Hillslope processes were consistent with the fill-and-spill hypothesis and featured a sequence of distinct thresholds: vertical wetting front propagation to the argillic layer; saturation of the argillic followed by local perching; filling of argillic layer depressions; and finally connectivity of depressions leading to interflow generation. Analysis of trench flow data indicated a cumulative rainfall threshold of 60 mm to generate interflow, a value at the high end of the range of thresholds reported elsewhere.

Relict soil evidence for profound quaternary aridification of the Atacama Desert, Chile

A relict soil on a late Tertiary/early Quaternary gravelly alluvial fan in the southern Atacama Desert was studied to determine if evidence of Quaternary climate change was evident in the profile. Stratigraphic relations of soil morphological features indicated that the initial phase of soil development was extensive chemical weathering with the loss of Si, Na, etc. and the formation of secondary clay. This was then followed by a prolonged period of carbonate accumulation that has discontinuously impregnated the earlier formed argillic horizons. The carbonate accumulation occurred both as fine-grained accumulations that engulfed and expanded the gravelly soil matrix, and as carbonate that formed dense and continuous coatings on gravel bottoms. Overlying the carbonate is a Holocene calcareous sand unit. Stable C and O isotope profiles of the disseminated carbonate show that this material appears to have formed under conditions of low to modest plant cover and significant soil water evaporation, very similar to soils presently found at higher latitudes and altitudes in modern Argentina. Seven transects of the carbonate laminations on the gravels at 10-μm scale reveal that all show a nearly 4‰ increase in δ13C values with time over distances of approximately 150mm, and variable δ18O values. The shift in C isotope values, which is unlikely to be due to significant changes in C3 vs. C4 vegetation, reflects a profound and prolonged aridification with a corresponding lowering of plant density and soil respiration. The precise beginning of the aridification is unknown due to a lack of carbonate dating methods amenable to the time frame involved. When all changes are considered, this remarkable soil indicates that local rainfall declined from somewhere between 500 to 1000mm y−1 in the late Tertiary/early Quaternary, to the present climate of about 25mm y−1. Future work will focus on developing more precise geochronological controls, but this initial study reveals the enormous potential that carbonate-bearing relict soils have for understanding climate change.

Characteristics of tea growing soils in relation to soil acidity in upper Brahmaputra Valley of Assam

Six typical pedons collected from Dahingapar (P1), Kotalgorie (P2), Baideha (P3), Neghareting (P4), Chenijan (P5) Tea Estates of Jorhat district and Thurajan Tea Estate (P6) of Golaghat district, representing gently to moderately sloping upland in upper Brahmaputra valley of Assam were studied for their characteristics in relation to soil acidity. The soils were deep to very deep, well to moderately well drained, light yellowish brown to light brownish gray, mostly light in texture (except P6), very strongly to strongly acidic and medium to high in organic matter content. The cation exchange capacity (CEC) of soil ranged from 4.5 to 13.2 cmol(p+) kg−1 and the clay CEC ranged from18 to 41 cmol(p+) kg−1 indicating the heterogeneity of mineralogical composition of clays. The increase in clay content with soil depth and presence of thin patchy clay skins in the sub-surface horizons of P4 and P6 indicated presence of argillic horizons. The soils were classified as Typic Dystrudepts (P1, P3 and P5), Fluventic Dystrudepts (P2), Ultic Hapludalfs (P4) and Typic Hapludults (P6). The exchangeable acidity ranged from 0.79 to 7.61 cmol(p+) kg−1 and was mostly constituted by exchangeable Al3+. Aluminium saturation varied from 11.7 to 82.9% in the soils. Aluminium saturation was more than 50% in the sub-surface horizons of most of the soils except in P2. The total potential acidity of the soil varied from 3.6 to 20.7 cmol(p+) kg−1. The pH dependent acidity of the soils varied from 1.77 to 18.87 cmol(p+) kg−1. The pH dependent acidity contributed 18.8 to 91.2% of the total potential acidity.

Classification and development of shallow soils (< 50 cm) in the USA

In Soil Taxonomy, soils in lithic subgroups have hard rock within 50cm of the surface. An examination of the digital databases of the USDA Natural Resources Conservation Service revealed that soils in lithic subgroups occur in 10 soil orders (excludes Oxisols and Gelisols, for which there were insufficient data) and can be ranked by area: Mollisols>Aridisols, Entisols, Inceptisols>Spodosols, Alfisols>Histosols, Ultisols, Andisols≫Vertisols. This ranking is comparable to the overall ranking of these 10 orders in the USA, except that Aridisols and Mollisols are over-represented while Alfisols and Ultisols are under-represented in lithic subgroups. Seven soil great groups account for nearly two-thirds (65%) of the soils: Calciustolls, Torriorthents, Dystrudepts, Haplargids, Haplocalcids, Haplustolls, and Argixerolls. For mineral soils, the minimum and maximum average depths to bedrock were 23 and 47cm, respectively. Although 47% of the soil series had either a cambic horizon or no diagnostic subsurface horizon, 26% had a calcic horizon, 21% an argillic horizon, and 5% a spodic horizon. Nearly two-thirds (63%) of the soils on an area basis are skeletal, i.e., contain >35% coarse fragments. More than one-quarter (26%) of the soil series have “no competing series,” meaning that they can be considered endemic to the region. Pedogenic processes in lithic soils appear to be analogous to those in soils derived from deeper unconsolidated materials, with the dominant processes being humification, calcification, argilluviation, cambisolization, and podzolization. There is abundant evidence that lithic soils are as developed—if not more developed—than nearby soils derived from deeper unconsolidated sediments. Lithic soils may be important reservoirs of SOC, clays, carbonates, and dust. In that lithic soils are often in upslope or at higher elevations, they play an integral role in landscape evolution.

Genesis of soils with an abrupt textural contrast in the United States

Identified from the USDA Natural Resources Conservation Service databases, soils with an abrupt textural change (ATC)occur in four orders (Alfisols > Mollisols, Ultisols > Aridisols), 14 suborders and 31 great groups and represent nearly 1000 series in the USA. More than three-quarters (79%) of the soils have an argillic horizon and 21% have a natric horizon. An albic horizon is present in 40% of the soils. More than half (58%) of the soils with an ATC occur in Pale- great groups, suggesting older materials. Soil series with an ATC occur primarily in fine (56%) and clayey (14%) particle-size classes; smectitic (49%) and mixed (40%) mineral classes; mesic (39%) and thermic (33%) soil-temperature classes; and ustic (31%), xeric (27%), and aridic (21%) soil-moisture classes. On average, the texture of the eluvial and illuvial horizons varies by two classes on the soil-textural triangle. Of the several theories accounting for the origin of soils with an ATC, clay illuviation from the eluvial to the illuvial horizon must be considered a major process. More than halfof the soils are derived from alluvium and other stratified materials; residual soils may also contain sedimentary layering, which is important in the development of an ATC. Elevenpercent of the soils have a lithologic discontinuity at the eluvial-illuvial junction, which may influence the development of an ATC. The ATC in soils with an aquic soil-moisture regime (13%), such as the Argialbolls, may reflect plugging of the argillic horizon with fine clays which causes lateral subsurface flow that removes clay from the albic horizon.Climate may be important in the development of ATC in the USA, in that many of the soils are subject to extended dry periods, followed by intense thunderstorms. A third (34%) of the soils with an ATC occur in Texas. There are about 271,000 km2 of soils with an ATC in the USA, which accounts for 3.2% of the total land area.

Distribution and origin of glossic horizons in soils of the western Great Lakes Region, USA

The glossic horizon results from degradation of an argillic or other clay-rich diagnostic horizon whereby clay and free iron oxides are redistributed, creating E/B and/or B/E genetic horizons. An examination of the US Department of Agriculture Natural Resources Conservation Service database revealed that 17% of the soil series in the western Great Lakes region contain a glossic horizon. These soil series account for 63% of those in the USA containing a glossic horizon. Seventy-three percent of the soils with a glossic horizon in the Great Lakes region of Minnesota, Wisconsin, and Michigan are classified into five subgroups: Haplic, Aquic, and Oxyaquic Glossudalfs and Alfic and Alfic Oxyaquic Haplorthods. The glossic horizon in 284 soil series averages 31±24cm in thickness and occurs (i) between an ochric or albic horizon and an argillic horizon, (ii) between a spodic horizon and an argillic horizon, or (iii) entirely within the argillic horizon. Glossic soils occupy a belt to the north of the tension zone across the three states, which corresponds approximately with the boundary between the mesic (≥8°C) and frigid (<8°C) soil-temperature regimes, the location of major late Wisconsinan end moraines containing primarily loamy till, and the boundary between non-forest and forest cover. Glossic horizons likely originate from a seasonal high water table accompanied by redox conditions, whereby Fe and clay are removed from the upper argillic horizon.

Hohokam Canal Irrigation and the Formation of Irragric Anthrosols in the Middle Gila River Valley, Arizona, USA

Irragric anthrosols form as a result of prolonged deposition of fine sediments from irrigation water. Ancient irragric soils centuries to millennia old occur in several world regions, especially in arid environments of Asia and the Americas. This article presents evidence for an ancient irragric anthrosol in the North American Southwest, along the Snaketown Canal System in the Middle Gila River Valley, Arizona. This pedostratigraphic unit was formed as a result of a millennium of irrigation by Hohokam farmers from A.D. 450 to 1450. The irragric soil consists of a mantle of silty‐to‐loamy textures with minimal soil formation overlying a natural argillic horizon on a Pleistocene stream terrace. A soil mapped independently by the United States Department of Agriculture‐Natural Resources Conservation Service with these horizons corresponds closely with the canal system. Soil within the canal system tends to be lower in salt, sodium, and pH compared with external soils. This suggests that the irragric process improved soil for crop production through long‐term leaching and additions of fresh sediments with the irrigation water. This anthropogenic process of canal sedimentation has had a long‐lasting impact on the sedimentary record and soils in this arid environment.

Arsenic profile distribution of the wetland argialbolls in the Sanjiang Plain of northeastern China.

The wetland Argialbolls pedon was chosen to investigate the effects of pedogenic processes and anthropogenic activities on the vertical distribution of As concentrations. Two wetland Argialboll cores (90 cm long) were collected from the Sanjiang Plain in northeastern China and analyzed for pH, soil organic matter (SOM), Fe, Mn, and As. The results indicate that SOM accumulated in the upper horizons, while Fe and Mn were reductively leached from the upper horizons and significantly accumulated in the lower argillic horizons. Atmospheric As deposition and As redistribution during the pedogenic process led to the unique vertical distribution of As concentrations in the wetland Argialbolls. Overall, As was leached from upper horizons and then accumulated in the lower argillic horizons. However, continual atmospheric As deposition maintained a slightly elevated As concentration in the top layer. In detail, As concentration in the upper horizons ranged from 1.1 to 5.3 mg kg−1, while it ranged from 18.2 to 65.7 mg kg−1 in the lower argillic horizons. The high As concentration in the argillic horizons might pose a risk to shallow groundwater in the area.

Properties and genesis of Argialbolls in the USA

The presence of an albic horizon below a mollic epipedon has long puzzled pedologists. The purpose of this study was to utilize data from the Natural Resources Conservation Service (NRCS) databases and the published literature to develop a weight-of-evidence assessment of how Albolls form in the USA. There are 69 Albolls soil series in the NRCS database, all of which are Argialbolls. These soils contain horizons with the following mean thickness and standard deviation: mollic epipedon (29±11cm), albic (18±9cm), and argillic (75±37cm). Argialbolls comprise 18,078km2 in the USA (0.02% of total land area), 70% of which occur in North Dakota, South Dakota, Missouri, and Iowa. Argialbolls are somewhat poorly drained, poorly drained, or very poorly drained (78%); and 81% of the Argialbolls are very deep (>1.5m). Silty alluvium and loess comprise 86% of the parent materials. The dominant vegetation is grassland (64%), sedges (16%), or savanna (16%). Ninety percent of the Argialboll soil series are in the fine or fine-silty particle-size classes; 62% have a mesic soil-temperature regime; 36% have an aquic soil-moisture regime, and 64% have a smectitic soil-mineral class. The Argialbolls have formed under modern conditions where development of a smectitic-rich argillic horizon has led to reducing conditions during which Fe and Mn concentrate as concretions in the albic horizon that underlies the mollic epipedon.

Vertical Distribution of Lead and Mercury in the Wetland Argialbolls of the Sanjiang Plain in Northeastern China.

The wetland Argialbolls pedon was chosen to investigate the effects of pedogenic processes and anthropogenic activities on the vertical distribution of lead and mercury concentration and to assess the potential use of soil as an archive of atmospheric Pb and Hg pollution. The soil was sampled from 5 cm from the surface to a depth of 90 cm at two locations in the Sanjiang Plain in northeastern China. The soil was analyzed for pH, soil organic matter (SOM), Fe, Mn, and Al. The results indicate that the SOM concentration gradually decreased with depth, while Fe and Mn were reductively leached from the upper horizons and accumulated significantly in the lower argillic horizons. Atmospheric Pb and Hg deposition and their redistribution during the pedogenic process led to a unique vertical distribution in the wetland Argialbolls. Overall, Pb was leached from the upper horizons and then accumulated in the lower argillic horizons. However, the Hg concentration decreased with depth, following the SOM distribution. The Pb concentration was significantly correlated to the Fe and Mn concentrations in the Argialbolls profiles, while the Hg concentration was significantly correlated with SOM. Post-depositional mobility along the wetland Argialbolls profile is higher for Pb and low for Hg. Therefore, the Argialbolls profile does not provide an accurate reconstruction of atmospheric Pb deposition, but might provide an accurate reconstruction of net atmospheric Hg deposition.

Quaternary subsurface paleosols in Haifa Bay, Israel: A new perspective on stratigraphic correlations in coastal settings

Abstract This study presents the first in-depth analysis of properties, micromorphology, clay mineralogy, magnetic susceptibility, and general properties of Quaternary paleosols from two boreholes in Haifa Bay subsurface, whose chronostratigraphic position was determined previously. The current analytical results allowed us to identify the type, hydrologic regime, polygenesis, mode of pedosedimentary evolution, and some subsequent alterations of paleosols. The analogs of polygenic Haplo- and Rhodo-xeralf soils (Brown, Mollic, and Ortho-Hamra) are associated with marine isotope stages (MIS) 3.1, 5.4, 6 and 8.0 (or 7.4) and likely developed in response to a changing climate. Paleosols prior to MIS 12, yet difficult to attribute to specific MIS, are distinct from Hamra soils by striking mottling, poor preservation of argillic horizons, abundant deformations in microfabric, and overprinting processes of gley and calcium carbonate accumulation. It is assumed that calcification may have affected the earlier formed clay illuviation and amorphous features, and was related with marine/coastal groundwater interface. The older calcic gleyed paleosols of earlier Middle Pleistocene likely indicate that coastal paleogeography was characterized by both high discharge of the ancient Qishon River and a high water table. The thin section micromorphology is particularly valuable for refined paleopedological taxonomy. Identified Pleistocene paleosols including their type, hydrology and mode of evolution, are proposed to serve additional stratigraphic markers in coastal settings.

Characterization and Classification of Soils in Eastern Himalayan Agro-climatic Region: A Case Study in Nongpoh Micro-watershed of Ri-Bhoi District, Meghalaya

Typical pedons representing major landforms of Nongpoh micro-watershed in Ri-Bhoi district of Meghalaya viz., hill top, hill slope and inter hill valley developed from granite–gneiss occurring under varying land use were characterized, classified and assessed. The soils were deep to very deep in depth, brown to greyish brown in colour, slightly acidic to extremely acidic in reaction and high to medium in organic carbon. Further, the soils were low to high in available nitrogen, low to high in available phosphorus and medium to high in available potassium. Pedon P2 and P3 which are located on the hill top and the hill slopes, respectively showed cambic (Bw) sub-surface diagnostic horizon and were classified as Typic Dystrudepts while the hill top pedon P1 and on hill slope pedon P4, sub-surface diagnostic horizon possessed argillic (Bt) horizon which was classified as Typic Paleudalfs. However, pedons located on the inter hill valley (P5 and P6) did not exhibit any diagnostic horizon and hence were classified as Oxyaquic Udifluvents and Aeric Endoaquents, respectively.

Long-term agroecosystem research in the central Mississippi river basin: goodwater creek experimental watershed and regional herbicide water quality data.

Goodwater Creek Experimental Watershed (GCEW) has been the focus area of a long-term effort to document the extent of and to understand the factors controlling herbicide transport. We document the datasets generated in the 20-yr-long research effort to study the transport of herbicides to surface and groundwater in the GCEW. This long-term effort was augmented with a spatially broad effort within the Central Mississippi River Basin encompassing 12 related claypan watersheds in the Salt River Basin, two cave streams on the fringe of the Central Claypan Areas in the Bonne Femme watershed, and 95 streams in northern Missouri and southern Iowa. Details of the analytical methods, periods of record, number of samples, study locations, and means of accessing these data are provided. In addition, a brief overview of significant findings is presented. A key finding was that near-surface restrictive soil layers, such as argillic horizons of smectitic mineralogy, result in greater herbicide transport than soils with high percolation and low clay content. Because of this, streams in the claypan soil watersheds of northeastern Missouri have exceptionally high herbicide concentrations and relative loads compared with other areas of the Corn Belt.

Soil evolution indices in fluvial terrace chronosequences from central Spain (Tagus and Duero fluvial basins)

Soil development in central Spain was studied through analysis of soil chronosequences from Tagus and Duero river valley terrace systems, evolved under similar Mediterranean climatic conditions throughout the Pleistocene. The most complete soil–terrace sequence, representative for our study, developed in the Tagus valley (Aranjuez–Toledo–Talavera sector). We present soil textural data for argillic horizons developed in Late to Early Pleistocene soils from five valleys. We derive soil development indices from clay and silt content and thickness of argillic horizons. Global clay content (GCC), clay content (CC) and clay/silt ratio (C/Si ratio), numerical indices related to clay illuviation, reflect soil evolution, explored by multiple regression of soil–terrace height and soil–age. Relative terrace heights above present river thalwegs (i.e. +100 m) were transformed to ages with a “height–age transfer function” on the basis of preliminary statistical geochronological analyses for central Spain. The height–age transfer function, a 3rd order polynomial (R2 = 0.90), based on 60 published numerical ages and paleomagnetic data for terrace sequences:describes the overall trend of valley downcutting for the last c. 2 Ma in central Spain. It assigns numerical ages to terrace levels at different relative elevations; and gives an estimate of standardized ages for the related soils. The explored age–soil relationships, logarithmic functions (R2 > 0.80), are sensitive to parent material clay content (corresponding to geology of river catchments) for most soil indices. Only the clay/silt ratio minimizes this sensitivity, and can thus be used for regional approaches (Tagus and Duero basins; central Spain) whereas the CC index is applicable only to individual river valleys or different river valleys with similar catchment geology (i.e. Tagus Basin). The C/Si ratio illustrates the logarithmic trend of weathering of soil particles into clay over time, the generation of soil textural balances sustaining illuviation processes, and therefore provides an estimate of “soil ageing”, for the implementation of soil chronofunctions for regional analysis of river valleys evolving in similar climatic conditions and with similar base-level history.

Characteristics of the fifth paleosol complex (S5) in the southernmost part of the Chinese Loess Plateau and its paleo-environmental significance

The most prominent paleosol unit in the Chinese Loess Plateau (CLP) is the fifth paleosol complex (S5) with its well-developed very thick and dark colored pedons. To provide more insight in the formation of S5 and its environmental significance, the pedogenesis and clay mineral transformation in the S5 of the Wugong section (Shaanxi Province) on the southernmost CLP are analyzed. S5 at the Wugong section is essentially composed of three well-developed reddish pedons (i.e., S5-1, S5-2, S5-3) which signify three glacial–interglacial climatic fluctuations during its formation. Complete decalcification in each pedon and a calcic horizon of only 30–50cm in thickness beneath each of the three pedons suggests that after deposition the pedons developed with a relatively stable surface in a sustained warm and humid climate. Clay formation in the S5 includes neogenesis of clay materials by in situ post-depositional weathering and mechanical migration of the fine fraction after complete decalcification.Complete leaching of CaCO3, intensive clay formation (with 60–100% higher clay content than that in the overlying and underlying loess (L5 and L6)) and extremely high magnetic susceptibility in the S5 pedons reflected a warmer, more humid climate and soil environment for pedogenesis than in the ‘optimum’ Holocene. However, the chemical alteration of the phyllosilicate minerals was weak and restrained by the hard calcic horizon, the compact argillic horizon and the flat terrain. The major clay mineral weathering processes during the formation of the S5 pedons at the Wugong section were depotassication, hydrolysis of primary minerals and degradation of chlorite. The pedogenesis in a loess–paleosol sequence and its pedogenic environment can best be deduced from combined data on pedogenic properties, and geochemical and mineralogical characteristics.

Vertisols and Vertic Properties of Soils of the Cherokee Prairies of Kansas

Soils in the Kansas Cherokee Prairies have formed in various parent materials and exhibit differences in the degree of expression of vertic properties. Despite large clay contents, few soils meet the criteria for a Vertisol. In an effort to investigate the cause of these differences, pedogenic processes, primarily clay illuviation and shrink–swell processes, were evaluated in eight pedons. Relatively high coefficient of linear extensibility (COLE) values were found in all soils. The clay mineralogy of four sites was dominated by smectite, but only two of those sites classified as Vertisols. In the other four sites, smectite was the most common clay mineral, yet there were several other clay minerals present in significant quantities. Disruption of illuvial clay features by shrink–swell movement was evident in the thin sections of all soils. Striated b-fabrics dominated except in surface soils that exhibited a small COLE value and occurred above an argillic horizon. Linear planes lined with stress-oriented clay and representing zones of shear failure were observed, along with argillans that had been distorted and embedded within the matrix by swelling pressure and soil movement. In conclusion, when present, a nonexpansive silty surface layer acts as a buffer and may limit shrink–swell by allowing expansive subsoils to dry more slowly.