The effect of seasonal flooding on the clay mineralogy of a soil series in the Volta Lake drawdown area,Ghana

Four pedons of Arenic/Grossarenic Paleustalf (Denteso Series), in the Volta Lake drawdown area in Northern Ghana, were described and sampled just before seasonal floodings commenced in the area. After 5 years of periodic flooding the soils were re-examined and sampled. Soil properties required for soil classification, including the clay mineralogy, of both the pre-flooding and the post-flooding samples were determined. One of the main objectives was to identify changes in soil properties which result from the periodic flooding. X-ray diffraction (XRD) indicated that before flooding the main clay minerals of the Denteso were kaolinite and smectite, and there were also some mica and quartz in the total clay fraction. Comparison of the pre-flooding with the post-flooding data revealed that practically all the smectite disappeared from all the three sampled pedons that were flooded for 5 to 20 weeks during each flood cycle, while the smectite persisted in the non-flooded pedon. With the disappearance of the 2 : 1 lattice clays there occurred a considerable decrease in cation exchange capacity (CEC) and in base saturation of the flooded pedons ranging from 0.04 to 3.63 cmol kg ⁻¹. Also, there was an increase in pH by 0.4 to 1 unit in most horizons of the flooded pedons in spite of the general decrease in base saturation. These changes in CEC, base saturation and pH support the XRD evidence that the seasonal floodings caused pedochemical weathering of the smectite in this loamy sand soil at a very fast rate during the 5 year period and this had resulted in the lowering of the buffering capacity and a general impoverishment of the soil series.     

Geomorphological and paleoclimatic implications of soil development from siliceous materials on the coral-reef terraces of Liuchiu Island in southern Taiwan

Liuchiu Island is an uplifted coral-reef island located off southwestern Taiwan. A total of four soil pedons, labeled as LC-1 and LC-2 from the Holocene terraces and LC-3 and LC-4 from the Pleistocene terraces, were sampled on the island for this work. These soils were siliceous, and were characterized by enrichment of clay and free iron (Fe_d). According to Soil Taxonomy, pedons LC-3 and LC-4 were classified as Paleudults and pedons LC-1 and LC-2 were Dystrudepts. The soil properties showed progressive changes from pedon LC-1 to pedon LC-4 in morphology, physical and chemical properties, and clay mineralogy. The contents of total Fe and dithionite-citrate-bicarbonate extractable Fe were significantly higher in pedons LC-3 and LC-4 with high weathering degree than in pedons of LC-1 and LC-2 with less weathering degree. Enrichment of kaolinite and gibbsite in pedons LC-3 and LC-4 also suggested high chemical weathering degree of the soils. The estimated soil ages for all studied pedons were consistent with their degrees in pedogenesis, where pedons LC-3 and LC-4 were located at older terraces and pedons LC-1 and LC-2 were located at younger terraces. Namely, it complied with the geologic interpretation of the continuous and simultaneous uplift and tilt of the island over time. Instead of the in situ weathering from the underlying coral reef limestone, all soils developed from siliceous parent materials deposited onto the surfaces. The SiO₂/Al₂O₃ ratios of soils indicated a component of loess may have been incorporated from continental China as part of the parent material, which confirmed a climate change of strong monsoons or severe dust storms occurred before the Holocene. However, soil development increased by the subsequent warm and humid climates of the interglacial stage over time.     

Clay mineral evolution along a soil chronosequence in an Alpine proglacial area

As a consequence of global warming, additional areas will become ice-free and subject to weathering and soil formation. The most evident soil changes in the Alps will occur in proglacial areas where young soils will continuously develop due to glacier retreat. Little is known about the initial stages of weathering and soil formation, i.e. during the first decades of soil genesis. In this study, we investigated clay minerals formation during a time span 0-150 years in the proglacial area of Morteratsch (Swiss Alps). The soils developed on granitic till and were Lithic Leptosols.Mineralogical measurements of the clay (< 2 μm) and fine silt fraction (2-32 μm) were carried out using XRD (X-ray Diffraction) and DRIFT (Diffuse Reflectance Infrared Fourier Transform). Fast formation and transformation mechanisms were measured in the clay fraction. The decreasing proportion of trioctahedral phases with time confirmed active chemical weathering. Since the start of soil formation, smectite was actively formed. Some smectite (low charge) and vermiculite (high charge) was however already present in the parent material. Main source of smectite formation was biotite, hornblende and probably plagioclase. Furthermore, irregularly and regularly interstratified clay minerals (mica-HIV or mica-vermiculite) were formed immediately after the start of moraine exposure to weathering. In addition, hydroxy-interlayered smectite (HIS) as a transitory weathering product from mica to smectite was detected. Furthermore, since the start of soil evolution, kaolinite was progressively formed. In the silt fraction, only little changes could be detected; i.e. some formation of an interstratified mica-HIV or mica-vermiculite phase.The detected clay mineral formation and transformation mechanisms within this short time span confirmed the high reactivity of freshly exposed sediments, even in a cryic environment.     

Soil genesis along a chronosequence on marine terraces in eastern Taiwan

Soil chronosequences developed on elevated marine terraces are ideal for studying changes in soil-forming processes with time. The coastal range of eastern Taiwan is a product of active arc–continent collision. Vertisols, Mollisols and Entisols are generally found on the different levels of marine terraces herein, but no detailed investigations of soil chronosequence have been conducted by integrating field morphology, physio-chemical characterization, micromorphology and mass-balance interpretations. Five soil pedons were selected on the three marine terraces including Tt-1 and Tt-2 pedons (Typic Hapluderts) on the first higher level with the oldest soil age (9–10 ka), Tt-3 (Vertic Hapludolls) and Tt-4 pedons (Typic Hapludolls) on the second intermediate level (5–6 ka), and Tt-5 pedon (Typic Udipsamments) on the third lower level with the youngest soil age (≤ 3.5 ka). The morphological characteristics showed that strongly developed angular blocky structures, pressure faces and slickensides are more common in higher terrace soils than in lower terrace soils. In this study, depth to C horizon, solum thickness, and thickness of the clay-enriched zone increase with relative terrace age. Although only one to two profiles per terrace were characterized, the following soil analytical characterizations increase with time: the degree of sand grains weathering, pH (H₂O), organic carbon, CEC, contents of Fe_d, Fe_o and Mn_d. Based on X-ray diffraction analysis of the clay-size fraction, soils on all terraces have a mixed mineralogy. Mica, smectite, and kaolinite have slightly increased with increasing terrace age. Furthermore, the dominant processes identified with mass-balance analysis include loss of bases (Ca and Mg), iron, and clay with time. The soil properties, including analytical and mineralogical characterizations, which do not have notable changes with time are primarily due to relatively young soil age (< 10 ka).     

Microtopography as a factor in the degradation of Vertisols in central India

Earlier studies on soil degradation in Vertisols of the Purna Valley of central India indicated that the semiarid climate characterized by a mean annual rainfall (MAR) of 875 mm and a tropustic moisture regime is responsible for the development of calcareous sodic soils. Recent observations, however, indicate that in the adjacent east upland of the Purna Valley, namely in the Pedhi Watershed, Vertisols have drainage problems, although the area receives a higher MAR than the Purna Valley, the total MAR being 975 mm. The Pedhi Watershed covers an area of 44 321 ha, and is characterized by a tropustic moisture regime and a hyperthermic temperature regime. Vertisols of the Pedhi Watershed are deep, calcareous, clayey and very dark greyish‐brown to dark yellowish‐brown in colour. Vertisols occur on both microhigh (MH) and microlow (ML) positions. The distance between the MH and ML positions is approximately 6 km and the elevation difference is 0.5–5 m. Cracks > 1 cm wide extend down to the slickenside zones in soils of ML whereas they cut these zones in some soils in MH positions. The soils of the MH positions are strongly alkaline and those of ML are mildly alkaline. The present study attempts to relate the distinctly different morphological and chemical properties of Vertisols in the MH and ML positions to pinpoint the prime factor responsible for the impairment of drainage on the basis of physical, chemical, mineralogical and micromorphological data. These data were obtained from 13 Vertisol pedons of methodically selected sites in the Pedhi Watershed. Despite their similar coefficient of linear extensibility (COLE), volumetric shrinkage potential (VSP), clay contents and amounts of fine smectite clay, the plasmic fabric of the slickenside horizons in soils of ML is porostriated, whereas in soils of MH it is stipple speckled to mosaic speckled, indicating weak plasma separation. The soils have both pedogenic and non‐pedogenic calcium carbonates (CaCO₃). The semiarid climate induces the precipitation of CaCO₃ with a concomitant development of subsoil sodicity. The degree of development of sodicity (Exchangeable sodium percentage (ESP) ≥ 5) is more in soils of MH as evidenced by the higher amount of pedogenic CaCO₃ (PC). The lack of water in soils of MH position is the reason for weak swelling of smectite, for larger amounts of PC, for higher alkalinity and sodicity, and for cracks cutting through the slickenside zones. Formation of sodic Vertisols in MH alongside non‐sodic Vertisols in ML positions is a unique phenomenon. It develops because of microtopographic differences which modify distribution of water across the landscape and facilitate greater penetration of rainwater in ML positions. The development of sodicity due to microtopographic differences assumes a great importance when a future land resource management programme on Vertisols in the higher MAR zone of overall a semiarid climate is considered. Copyright © 2002 John Wiley & Sons, Ltd.     

Genesis and transformation of minerals in the formation of red (Alfisols) and black (Inceptisols and Vertisols) soils on Deccan basalt in the Western Ghats, India

Six noncalcareous pedons from the basaltic terrain of the Western Ghats in Maharashtra, India, were identified for the present study. Of these, two red-soil pedons (Typic Haplustalfs) and one black-soil pedon (Vertic Ustropept) are from the Bhimashankar plateau at an elevation of 1000 m above mean sea level, experiencing a humid (>5000 mm rainfall) tropical climate. The other three pedons of black soils (Typic Chromusterts and Typic Ustropept) are from the semi-arid zone (500–1000 mm rainfall), at an elevation of 825–893 m. Clay mineralogical investigation indicated that interstratified smectite-kaolin (Sm/K) is dominant in red soils whereas smectite is dominant in black soils. The Sm/K is formed by the transformation of montmorillonite, the first weathering product of Deccan basalts in a humid tropical climate. We suggest that the interstratification of kaolin with chloritized smectite may also be an important ephemeral stage during the transformation of smectite to kaolinite. The presence of zeolites provided sufficient bases to prevent the complete transformation of Sm/K to kaolinite. The presence of smectites and zeolites made the formation of black soils possible in microdepressions even in a tropical humid climate. The genesis of Sm/K and smectite in red and black soils, respectively, suggests that these soils formed through a progressive landscape reduction process. The presence of both Sm/K and smectite in black soil clays of semi-arid climate suggests that the smectite of these soils was formed in an earlier humid climate.     

Clay mineralogy in agrochernozems of western Ukraine

The mineralogy of clay fractions separated from deep low-humus deep-gleyic loamy typical agrochernozems on loess-like loams of the Upper Bug and Dniester uplands in the Central Russian loess province of Ukraine consists of complex disordered interstratifications with the segregation of mica- and smectite-type layers (hereafter, smectite phase), tri- and dioctahedral hydromicas, kaolinite, and chlorite. The distribution of the clay fraction is uniform. The proportions of the layered silicates vary significantly within the profile: a decrease in the content of the smectite phase and a relative increase in the content of hydromicas up the soil profile are recorded. In the upper horizons, the contents of kaolinite and chlorite increase, and some amounts of fine quartz, potassium feldspars, and plagioclases are observed. This tendency is observed in agrochernozems developed on the both Upper Bug and Dniester uplands. The differences include the larger amounts of quartz, potassium feldspars, and plagioclases in the clay material of the Upper Bug Upland, while the contents of the smectite phase in the soil profiles of the areas considered are similar. An analogous mineral association is noted in podzolized agrochernozems on loess-like deposits in the Cis-Carpathian region of the Southern Russian loess province developed on the Prut–Dniester and Syan–Dniester uplands. The distribution of particle-size fractions and the mineralogy of the clay fraction indicate the lithogenic heterogeneity of the soil-forming substrate. When the drifts change, the mineral association of the soils developed within the loess-like deposits gives place to minerals dominated by individual smectite with some mica–smectite inter stratifications, hydromicas, and chlorite.     

Chemisch-mineralogische und mikromorphologische untersuchung einer eutrophen (Ramann-) braunerde des Budapester Hügellandes

In spite of the fact that Ramann's concept of Braunerde was extensively used throughout the world, some pedogenic processes and soil properties which are important for the formation and existence of Brown Forest Soils (eutrophic brown earths) are still not well known. Hence, a soil was investigated which was classified as a “Braunerde” by Ramann in 1909. The main question was, whether there might be soil constituents such as inorganic amorphous substances (allophanes) which are able to stabilize the brown-earth fabric, or to inhibit clay migration. From chemical data as well as from the magnitude of the pH-dependent CEC, it was deduced that there are no appreciable amounts of allophanes within the soil. The fabric, however, seems to be stabilized by colloid-chemical flocculation of the clay particles caused by large amounts of carbonates and silicates deposited with the parent material (loess). The decalcified solum is still rich in silicates, especially sand and coarse-silt-sized micas and feldspars (60–200 and 20–60 μm fractions, respectively). Since - in comparison - loesses and loess-derived soils of the Central German mountain region contain less silicates, mainly in fine and medium silt fractions (2–6 and 6–20 μm diameter, respectively), the coarser-grained silicates were assumed to be important for maintaining the Ca²⁺ and Mg²⁺ saturation of the soil by continuous weathering and thus stabilizing the brown-earth fabric.Although the soil is saturated predominantly with Ca²⁺ and Mg²⁺ ions, clay migration proceeds within the upper horizons. This was shown by calculation of the amounts of clay formed by breakdown of micas originally present in the parent material (“clay formation balance”), as well as by micromorphological studies. Furthermore, micromorphological studies and x-ray diffraction data gave some evidence for the migration of preferably finest grained montmorillonitic clays (smectites) penetrated by organic substances (humus). This kind of “selective clay migration” was assumed to be caused by high Ca²⁺ (or Mg²⁺) concentrations in soil solution required for flocculation of humus-penetrated (humus-coated) smectites. From the occurrence of these “humus-smectites” the possibility was assumed that the soil studied has been developed from a former Chernozem type.Quantitatively, clay migration does not reach the amounts of clay formation (breakdown of micas) accompanied by precipitation of iron oxides on mineral surfaces (“Verbraunung”). Thus, the soil profile visually and macromorphologically clearly exhibits the features of brown earths. On the other hand, however, some clay migration was observed. Hence, the soil was classified as a “brown earth with some clay migration” (“schwach durchschlämmte Braunerde”).Judging from the results of the “clay formation balance” an appreciable pedogenic (autigenic) clay formation from weathering products of feldspars was excluded for the soil studied.     

Depositional seals in polyacrylamide-amended soils of varying clay mineralogy and texture

Purpose  

Depositional seals, formed when turbid waters infiltrate into soils, lead to a reduction in soil hydraulic conductivity (HC) and enhance runoff and soil erosion. Since clay size particles constitute a dominant proportion of depositional seals, soil texture and clay mineralogy play a significant role in determining the seal’s hydraulic characteristics. Presence of high molecular weight anionic polyacrylamide (PAM) in suspension flocculates fine sediments, and therefore, its application to the soil surface may modify the characteristics of the depositional seal. The impact of PAM on the latter is expected to be influenced by soil properties. The aim of this study was to elucidate the effects of PAM application on clay flocculation and the HC of depositional seals formed in four soils varying in texture (ranging from loamy sand to clay loam), and diverse proportions of clay mineral constituents (kaolinite, smectite, and vermiculite).     

Mineral Composition and Weathering of Soils Derived from Xiashu Loess

Mineralogical, physical and chemical analyses of the soils derived from Xiashu loess were carried out. The primary minerals of these soils were found to be mainly composed of light minerals, such as quartz, feldspar and mica, with traces of heavy minerals. Clay minerals, more complicate in composition, were dominated by hydromica, accompanied by smectite, vermiculite, chlorite, kaolinite, 2:1/1:1 randomly interstratified minerals and small amounts of quartz, goethite, lepidocrocite and hematite, Clay minerals were characterized by low crystallinity and fine particle size. In light of the quartz/feldspars ratio of the 0.01-0.05mm silt fraction, and the clay mineral composition, the freeness of iron oxide, and the silica/sesquioxide and silica/alumina ratios in < 0.002mm clay fraction, it is concluded that the weathering intensity of these soils was lower than those of red soil and yellow earth, but higher than that of brown earth, and that the soil allitization, depotassication and hydroxylation of cl     

Genesis and characteristics of brown forest soils derived from serpentine in Kyushu,Japan

Recently, micropedological investigations have become an important means to explain some pedogenetical processes. The micromorphological features of Brown Forest soils and Mountain Brown Forest soils have been reported by several investigators(12, 14–15), Braunerde can be distinguished from Braunlehm by the presence of a spongy structure, immobility of peptizable iron hydroxides, or by the absence of any fluidal structure in its (B) horizon. In his review on micropedology, osmond(16) has stated that Brown Earths do not show any evidence of illuviation of ferruginous compounds or clay formed by oxidative weathering. As a result of micromorphological investigations on soils derived from serpentine in Greece, Krause(12) has described that the dominant soil of the hillsides is rubefied Braunlehm (Rotlehm), whereas the soil developed on valley terraces is Braunerde which appears as a transformation product of Rotlehm and has a spongy fabric in the A horizon.     

辽西褐土的细粒矿物组成

辽宁省西部是低山丘陵区,年平均温度7.1℃,年平均降雨量400—500毫米,降雨集中在夏季,具有明显的大陆性气候特征,干燥度小于1,属半干旱类型.植被为油松柞木和草原灌木丛林.土壤属棕色森林土向栗钙土过渡的褐土地带.母质主要为花岗片麻岩风化物.山麓缓坡和河谷两岸为黄土丘陵.在黄土沉积物覆盖层下,常见红色风化壳露头,即红色粘土层.此外,并有松软易风化岩层,如砂岩、页岩和变质岩等.本区近百年来,由于自然植被受到严重破坏,大量水土流失,土壤侵蚀严重,土壤有机质含量低,成为辽宁省的低产区.因此,鉴定本区不同母质的土壤矿物胶体组成及其特征,不仅可以研究这一特定自然条件下土壤矿物的转化和形成,并且有助于了解土壤特性和肥力特征.     

Mineralogical and geo-chemical characterization of a diapiric formation in the North of Spain

We have selected seven profiles located in a diapiric formation in the North of Spain. The profiles have been analyzed for the mineralogy and the chemical composition of original materials, soils developed above them and clay fractions. Three soils formed on basic rock of volcanic origin (ophite) and rich in alterable minerals, three others formed on clay marl and one soil formed on gypsiferous marl. Plagioclases, pyroxenes, vermiculites, and biotites are the main minerals found in the soil samples and ophitic rocks. Biotite, smectite, chlorite and interstratified chlorite–vermiculite make up the predominant mineralogical association in the clay fraction of the soils. Calcite, biotite and on top of all chlorite are the main minerals in the marls and the soils developed on them, with gypsum predominant in the gypsiferous marl. The mineralogy of its clay fraction is comprised mainly of chlorite and biotite. The variations in content of Al₂O₃, TiO₂ and Na₂O in the ophites are considered to be associated with the differences in the evolution of the pyroxenes. The variability of the chemical composition of the Keuper sediments and the soils is attributed more to the chaotic disposition of the Triassic materials in the formation of the diapir than to intense chemical weathering. The low concentrations of silica, iron, and aluminum extractable with ammonium oxalate indicate the low proportion of non-crystalline products. Fundamentally, it is the semiarid conditions in the study zone, together with the processes of extrusion and hydrothermal activity affecting the formation of the diapir, that are responsible for the genesis of the minerals.     

南极半岛海洋气候区的土壤Ⅶ. 矿物学特性

南极海洋气候区岩石风化和土壤形成过程中有明显的原生矿物蚀变作用和自生矿物成矿作用。本文以粗骨寒冻灰化土和石灰性扰动冻土两种有代表性的土壤类型为例,阐述了本区土壤矿物学特征。指出铝氧化物、绿泥石、碳酸盐是本区玄武岩类风化物质上发育土壤中的主要自生矿物类型,蒙脱石、特别是绿泥-蒙脱石混层矿物是南极海洋气候区土壤粘粒部分的特征矿物。不同土壤由于成土环境、成土过程、成土历史的差异,其土壤物质的矿物学组成、含量、形态、分布具有明显不同。土壤发生性铁氧化物与成土作用和土壤过程密切相关,其矿物类型、含量、形态特征、分布模式在不同的土壤中明显不同,是表征土壤发育程度与剖面形态表达的有效指标。     

中国东南部某些水稻土的矿物学

Limited information is available concerning the mineralogy of paddy soils in the southeastern China. Using chemical methods in conjunction with X-ray diffractometry, we studied the mineral composition of three paddy soils: Jinghua (paddy soil on Quaternary red clay), Fuyang (Hapl-percogenic loamy paddy soil), and Shaoxing (gleyic clayey paddy soil). All the soils contained quartz, mica, vermiculite, chlorite and kaolinite, and the distribution of these minerals varied with soil particle size fractions. The clay fraction of the Fuyang and Shaoxing soils also contained smectite. Although X-ray data did not show the presence of smectite in the Jinghua soil, this mineral was identified by the chemical method, suggesting a transitional property of the mineral in the soil. Hydroxy-Al interlayered minerals were also present in the clay fraction. The amount of smectite in the soils was 31.6 (Shaoxing), 16.5 (Fuyang), and 21.4 (Jinghua) g kg^-1; for vermiculite it was 33.3 (Shaoxing), 16.5 (Fuyang), and 8.5 (Jinghua) g kg^-1. Smectite was only found in the clay fraction. In contrast, amounts of vermiculite in soil particle size fractions were 3.0~11.4 (sand), 2.1~6.0 (coarse silt), 4.6~18.9 (medium silt), 0.9~40.0 (fine silt), and 17.0~108 (clay) g kg^-1. The amount of noncrystalline aluminosilicates in the soils in g kg^-1 decreased in the order: Shaoxing (2.4) > Jinghua (1.9) > Fuyang (1.7). This study has provided useful mineralogical information that is fundamental in future development of management strategies of the soils.     

海南砖红壤的微形态特征以及南方网纹红土与砖红壤环境意义的差异

主要运用土壤微形态学、粘土矿物学和土壤化学方法,对海南岛三个不同母质的砖红壤剖面的微形态特征和成壤过程进行研究,并将网纹红土和砖红壤进行比较.结果表明,砖红壤典型的微形态特征为易风化矿物颗粒和粘粒胶膜的缺失、均质状的b-垒结以及风化成因铁锰质结核的出现;典型的成壤过程为强烈的化学风化、红化、均质化、淋溶以及显著的脱硅富铝化过程.随着成土母质和土壤排水条件的变化,砖红壤的微形态特征亦有所差异.而我国南方的网纹红土的发育程度低于砖红壤,不宜被划分为砖红壤或氧化土,而更趋向归入老成土,在我国土壤发生学分类中大致相当于红壤.     

Soil genesis along a chronosequence on marine terraces in eastern Taiwan

Soil chronosequences developed on elevated marine terraces are ideal for studying changes in soil-forming processes with time. The coastal range of eastern Taiwan is a product of active arc–continent collision. Vertisols, Mollisols and Entisols are generally found on the different levels of marine terraces herein, but no detailed investigations of soil chronosequence have been conducted by integrating field morphology, physio-chemical characterization, micromorphology and mass-balance interpretations. Five soil pedons were selected on the three marine terraces including Tt-1 and Tt-2 pedons (Typic Hapluderts) on the first higher level with the oldest soil age (9–10 ka), Tt-3 (Vertic Hapludolls) and Tt-4 pedons (Typic Hapludolls) on the second intermediate level (5–6 ka), and Tt-5 pedon (Typic Udipsamments) on the third lower level with the youngest soil age (≤ 3.5 ka). The morphological characteristics showed that strongly developed angular blocky structures, pressure faces and slickensides are more common in higher terrace soils than in lower terrace soils. In this study, depth to C horizon, solum thickness, and thickness of the clay-enriched zone increase with relative terrace age. Although only one to two profiles per terrace were characterized, the following soil analytical characterizations increase with time: the degree of sand grains weathering, pH (H₂O), organic carbon, CEC, contents of Fe_d, Fe_o and Mn_d. Based on X-ray diffraction analysis of the clay-size fraction, soils on all terraces have a mixed mineralogy. Mica, smectite, and kaolinite have slightly increased with increasing terrace age. Furthermore, the dominant processes identified with mass-balance analysis include loss of bases (Ca and Mg), iron, and clay with time. The soil properties, including analytical and mineralogical characterizations, which do not have notable changes with time are primarily due to relatively young soil age (< 10 ka).     

Polygenetic Vertisols of the Purna Valley of Central India

The Purna Valley, a part of the Payanghat Plain, is an oval basin covering an area of about 1.9 Mha in central India. The soils are deep, calcareous, clayey and very dark greyish brown to dark brown in colour, and meet criteria for the Vertisols order of Soil Taxonomy. Cracks extend down to the slickenside zones in soils of the northeastern part of the valley, but cut through the slickenside zones in the soils of the southwestern part. Soils of the southwestern part are strongly alkaline with exchangeable sodium percentages (ESPs) of 5–26, whereas those of the northeastern part are moderately alkaline and have ESP values <5. Despite their similar coefficients of linear extensibility, volumetric shrinkage potentials, clay contents and amounts of fine clay smectite, the plasmic fabric of the slickenside horizons in soils of the northeastern part is porostriated, whereas in soils of the southwestern part, it is stipple-speckled to mosaic-speckled, indicating weak plasma separation. The soils have both pedogenic and nonpedogenic calcium carbonate, but those of the southwestern part have more pedogenic carbonate than those in the northeastern part. The semiarid climate removes Ca²⁺ ions from the soil solution by precipitating carbonate, and also causes the ESP and sodium absorption ratio to increase with depth. The lack of soil water in soils of the southwestern area is thought to be the reason for weak swelling of smectite, for larger amounts of pedogenic carbonate and for cracks cutting through the slickenside zones. The Vertisols of the southwestern part of the of the Purna Valley are, therefore, polygenetic because they exhibit distinctly different soil properties resulting from increasing aridity in the valley during the late Holocene.     

b-Fabric of some benchmark Vertisols of India in relation to their mineralogy

At present precise information on the b-fabric of Vertisols is inadequate for gaining a comprehensive knowledge about such soils formed in alluvium of basic igneous and metamorphic rocks and spread in varying agro-climatic zones of peninsular India. The aim of the present study, on five benchmark Vertisol series (Sarol, Aroli, Nimone, Bellary and Kovilpatti), was to assess the possible reasons for differences in the b-fabric of the subsoils despite their common shrink-swell properties, as evidenced by the presence of sphenoids and/or slickensides, and similar clay contents and amounts of fine clay smectite. In Sarol and Aroli soils from the sub-humid region, the b-fabric is porostriated, whereas in Nimone and Kovilpatti soils of the semi-arid and Bellary soils of the arid regions the fabric is either mosaic-speckled or granostriated, indicating weak plasma separation, a result of restricted swelling of clays. The related distribution pattern of the s-matrix in all these soils was open porphyric. Recent literature has suggested that weak plasma separation is an effect of dissolution and crystallization of calcite. However, generally low concentrations of soluble calcium (c. 1 mmol dm^?3) in soil solutions of Vertisols in arid and semi-arid agroclimates suggest that the presence of calcite does not guarantee the presence of sufficient soluble calcium to impair swelling of clays by contracting the diffuse double layer. Weak plasma separation in Bellary and Kovilpatti soils compared with Sarol and Aroli soils is explained in terms of a decrease in the internal surface area of fine smectite, as evident from a very high degree of chloritization in smectite interlayers, and smaller proportions of coarse smectite. In Nimone soils the weak plasma separation is primarily due only to a decreased internal surface area of fine smectite.