Natural Undisturbed Soil Research Articles

Impact of remolded soil fabric on the volumetric deformation behavior of clayey soils

The volumetric deformation of clayey soils, leading to a reduction in the bearing capacity and serviceability of pavements and building structures, is a major concern during their design, construction, and maintenance. Several approaches are often followed to mitigate the volume expansion and concomitant damage, including removal and replacement, moisture treatment with appropriate compaction protocols, and chemical treatment. During these treatment processes, the in-situ fabric is altered as the natural undisturbed soils are remolded and compacted. Hence, it is crucial to understand the effect of remolding on the volumetric characteristics of clayey soils. To investigate this effect, coefficient of linear extensibility (COLE) tests were conducted on both natural and remolded soil samples. The objective was to evaluate the impact of soil fabric modification on volumetric characteristics such as suction compressibility index (γh) and soil water-retention characteristics, i.e., the soil–water characteristic curve (SWCC) of clayey soils. Our findings indicated that remolded soils had approximately 10% to 30 % higher γh-values than those of unaltered soils, which can be attributed to changes in porosity. Two distinct mechanistic models were developed using the packing theory concept to link the γh-value and SWCC of remolded and natural soils. Finally, an analysis was conducted to compare the potential vertical movement (PVM) of natural and remolded clay soils. This analysis revealed that the remolded soil fabric substantially increased the PVM values, particularly for high-plasticity clay soils. This effect should be considered when assessing the impact of treatment that requires remolding, which substantially alters the soil structure and fabric.

Analysis of influence of air-entry values to unsaturated soil properties

Soil slope instability is a grave concern in regions with high annual precipitation, especially in tropical countries. The impact of soil instability on the economies of these countries is severe since it usually causes fatalities and property loss. The research and development of the systems to reduce the impact of soil slope failures by predicting slope failure, monitoring the slope, and early failure warning has made it possible to implement safety measures. Despite the considerable effort to implement these measures, the factors contributing to soil slope instability are complex and have not been explored extensively. The air-entry value (AEV), which is the matric suction, is a critical factor in predicting the risk of slope instability. This paper will analyze the interaction of the basic soil properties and the soil hydraulic properties, also known as AEV. The researchers collected natural undisturbed soil samples from several locations to determine their AEVs which obtained by plate pressure extractor apparatus. The basic soil properties, namely the natural moisture content, particle size distribution and bulk density were determined in the laboratory, and the values were used to determine the correlation between the basic soil properties and the soil AEV. The results show that the soil with higher coefficient of curvature and percentage of sand have lower air-entry values, which mean they could experience disastrous slope failure during heavy rainfalls. This strong correlation underscores the significance of soil hydraulic properties as the critical factor in soil slope instability.

Mechanism and modeling of different plant root effects on soil detachment rate

Plant roots significantly affect soil erosion, although the mechanism of how roots affect the soil detachment rate (SDR) by overland flow on the Loess Plateau is not fully understood. This study was conducted to investigate the mechanism of how roots affect SDR using natural undisturbed soil samples taken from five sampling plots with the dominant species of Agropyron cristatum, Caragana korshinskii, Medicago sativa, Caragana korshinskii, and Robinia pseudoacacia. Bare land was used as the control plot. The samples were subjected to flow scouring through hydraulic flume experiments under six different shear stresses. The results showed that the roots could significantly reduce the SDR by >83.48% compared with the bare soil. The herbs with fibrous roots (A. cristatum) had a strong root-reducing effect on the SDR. Due to the weaker root-reducing effect of herbs with taproots, the comprehensive reducing effect of herb roots was lower than that of shrubs (C. korshinskii). Because of the higher soil organic matter, the tree roots (R. pseudoacacia) still had a higher reduction effect on the SDR even with a smaller root content, and its SDR was between that of herbs and shrubs. The SDR could be divided into 3 zones based on root mass density (RMD): zone Ⅰ (SDR is sensitive to root action), zone Ⅱ (the effect of soil properties on SDR is prominent), and zone III (SDR is relatively stable). The critical conditions between different zones and the main controlling factors affecting the SDR in different zones were analyzed. The prediction accuracy of the SDR model could be improved by constructing the model using the zoning method (NSE > 0.95). A prediction model of SDR suitable for root-soil complexes of different plant species was developed (NSE = 0.90).

Establishment of regional background for heavy metals in the soils of the Lower Don and the Taganrog Bay coast

Data on the regional geochemical background and threshold values of heavy metals are required to establish anomalies and assess soil pollution. As a rule, the background values are the average contents of elements in natural undisturbed soils, or the threshold values for the study area, obtained by statistical methods. The aim of the study is to obtain geochemical threshold values of heavy metals in the soils of the Lower Don and the Taganrog Bay coast using different statistical approaches. A total of 86 topsoil samples were collected from the study area. The concentrations of Cr, Mn, Ni, Сu, Zn, As, Cd, and Pb were analyzed by X-ray fluorescence. The median element concentrations in the soils of the study area were consistent with world soil average and metal concentrations in background soils of protected area. Using a ‘geochemical’ approach is not suitable for this dataset because it does not take into account the natural variability of concentrations in different soil types. The Tukey inner fence method delivers estimates that do not detect outliers for Ni, As, Cd, and Pb. The ‘median + 2 median absolute deviations’ method was the most appropriate, as it consistently provided the most conservative background values.

Soil detachment by overland flow on hillslopes with permanent gullies in the Granite area of southeast China

Permanent gullies are often initiated from rill erosion, followed by the formation of abrupt, deep and wide gullies when topsoil and subsoil are carried away by fast-flowing surface water. Few studies have been performed to evaluate soil detachment processes on hillslopes with permanent gullies, which may significantly affect the further development of gully erosion. Soil detachment was investigated by using natural undisturbed soil samples collected in different spatial locations (upper catchment, middle slope, lower slope, and scour channel) of hillslopes with permanent gullies. The relationships of soil detachment capacity with soil property, root system, and hydraulic parameters were quantified. Selected samples were tested in a hydraulic flume (3.8 m length × 0.2 m width) under five different shear stresses (3.67, 7.81, 10.50, 13.97 and 17.28 Pa). Soil detachment capacity was found to be significantly greater in the scour channel than in the other three hillslope positions. For the initial and stable gullies, the soil detachment capacity was notably reduced in the middle slope relative to the upper catchment and lower slope, while a different trend was observed in the active gully. The variability of soil detachment capacity at different spatial locations showed a negative correlation to bulk density, soil cohesion, clay, organic matter, and root density (P < 0.05), and a positive correlation to total porosity (P < 0.01). The rill erodibility (Kr) showed a power function decrease with soil bulk density, organic matter, and root density (P < 0.01). Critical shear stress (τc) had a significant and positive correlation with soil cohesion. Based on the WEPP model, detachment capacity of soil on hillslopes with permanent gullies could be achieved by using measurable parameters such of shear stress, bulk density, organic matter, root density, and soil cohesion.

An experimental study on the microstructure and triaxial shear of structured clay in contact with landfill leachate

City solid waste stock has exceeded 8 billion tons in China, and the output and coverage area of solid waste are increasing daily. Landfill leachate can be generated during biodegradation of disposed and stored solid waste. Leachate contains high concentrations of organic mixtures and heavy metal ions. Natural undisturbed soil has high anti-seepage capacity and its permeability coefficient is less than 1 × 10−7 cm/s in underlying stratum, which can delay leachate contamination of the environment surrounding a landfill site. However, undisturbed soil exhibits certain structural characteristics. After long-term leachate corrosion, leachate leakage will inevitably influence the stability of stratum underlying a landfill. In this study, the mechanical characteristics of undisturbed clay under the actions of landfill leachate and stress were analyzed via a consolidated-undrained triaxial shear test. Comprehensive microstructure laboratory tests, including X-ray diffraction, scanning electron microscopy, and laser particle size tests, were performed to determine at different times the microstructure of undisturbed clay due to corrosion by landfill leachate. Experimental results indicate the strength and microstructural evolution of structured clay under the actions of landfill leachate and stress. The research results are helpful for evaluating the safety and predicting the stability of a landfill’s underlying stratum.

The Diversity of Bacterial Communities in Urban Soils

Urbanozems (Urbic Technosols) contaminated by heavy metals and polychlorbiphenyls (Urbic Technosols Toxic) and intruzems (Urbic Technosols Toxic) were studied in Moscow; additionally, we studied recreazems (Urbic Technosols Thaptohumic) and culturozems (Urbic Technosols Pantohumic) on the territory of the Botanical Garden of Moscow State University (Aptekarskii Ogorod, the Apothecaries’ Garden). In the soils contaminated with heavy metals and oil products, the number of viable cells of bacteria decreased, whereas the content of filterable forms of bacteria increased. The taxonomic structure of saprotrophic bacterial complexes in contaminated urban soils was transformed towards an increase in the diversity of bacterial taxa atypical of natural undisturbed soils. Rhodococci (Rhodococcus genus) predominated in the soils contaminated with oil and polychlorbiphenyls, enterobacteria (Escherichia, Enterobacter, and Klebsiella genera) predominated in the soils contaminated with municipal wastes, and Arthrobacter genus was dominant in the soils contaminated with cement dust. Soils of both Botanical Gardens of Moscow State University were characterized by the high population density and specific distribution of bacteria in the profile; the structure of their saprotrophic bacterial complex had some similarity with that in the soils of more southern regions. The obtained data on the bacterial diversity of urban soils attest to considerable transformation of bacterial communities both in the contaminated urban soils and in the soils of botanical gardens.

Characteristic of soils of Odesa City

In the article, the basic morphological features, physical and chemical properties of soils of Odesa, analyzed indicators nutrient content, humus pHN2O, soil particle size distribution. The composition of salts and absorbed bases in soils is established. The purpose of this study is to characterize the soil cover of Odesa City. 47 full-profile sections were laid in 9 key sections in the city area. Key areas are characterized by the different anthropogenic impact on the soil upper layer and intensity of its use. According to the result of the studies of soils in Odesa City, the following groups of soils are distinguished in the territory of the city: natural undisturbed soils, anthropogenically externally disturbed natural soils, anthropogenically deeply disturbed soils (urban-soils). For natural undisturbed and anthropogenically externally disturbed natural soils, heavy loamy and medium loam granulometric composition is characteristic in most cases. The upper horizons of the soils of the firth overspill have a lighter composition – from light loamy to sandy. The granulometric composition of urban soils is heavy loamy, during a tendency toward its alleviation due to the anthropogenic increase of the content of sand fractions and skeletal material. A characteristic feature of undisturbed soils of the Odesa City is fairly high variability in the nitrogen content of nitrification, mobile phosphorus and exchange potassium, and organic substance. The sum of the absorbed bases in natural undisturbed and anthropogenically externally disturbed natural soils, as in urban areas, is low. The ratio between calcium and magnesium ranges from 2:1 to 6:1. Automorphic soils are predominantly weakly solonetzic. Marsh soils are characterized by a certain increase in the proportion of sodium from the sum of the absorbed bases - up to 36.47 %. Natural humus soils are characterized by a lack of salinity. Meadow soils from the surface have a weak degree of chloride type of salinity, and an average and a strong degree of chloride type of salinity is noted down to the profile. Urban soils have a weak degree of chloride-sulfate salinization, associated with the use of mixtures against ice in winter. Marsh soils have a strong degree of soda-chloride type of salinity from the surface. Key words: Odesa, urban soils, anthropogenic transformation of urban soils.

Organic amendments and mulches modify soil porosity and infiltration in semiarid mine soils

AbstractThe restoration of soils affected by mining activities depends on the improvement of soil physical quality. This study aimed to determine the medium‐term effects of organic amendments and mulches on soil physical properties and soil hydrological response. The experimental restoration was conducted in a homogeneous slope using a factorial design (organic amendments × mulches). The organic amendments consisted of compost from domestic organic residues, sewage sludge from an urban water treatment facility, and a control without amendment. The mulches consisted of gravel, woodchips, and a control without mulch. Modifications of the soil structure induced by the combinations of techniques were quantified by micromorphometric analysis of 2D porosity. Undisturbed soil cores were collected to determine the soil bulk density. Soil hydrological behaviour was determined by rainfall simulations. Samples from natural undisturbed soils surrounding the quarry were also taken as a reference. Restored soils with organic amendments have a higher total porosity than did nonamended substrates. Each restoration treatment combination resulted in a different pore system with very diverse hydrological responses. Organic amendments resulted in infiltration improvement and water erosion reduction. The woodchip mulch was more effective at trapping runoff and sediment, although this type of mulch did not favour vertical water movement towards deeper horizons. Therefore, the use of organic amendments should be encouraged in mine soil restoration and should be strongly recommended for its proven environmental benefits (enhancement of soil structure and infiltration and reduction of soil losses). However, the mulches did not produce the expected results in the tested arid–semiarid area.

Quantifying the surface covering, binding and bonding effects of biological soil crusts on soil detachment by overland flow

AbstractBiological soil crusts (BSCs) have impacts on soil detachment process through surface covering, and binding and bonding (B&amp;B) mechanisms, which might vary with successional stages of BSCs. This study was conducted to quantify the effects of surface covering, binding and bonding of BSCs on soil detachment capacity by overland flow in a 4 m long hydraulic flume with fixed bed. Two dominant BSC types, developed well in the Loess Plateau (the early successional cyanobacteria and the later successional moss), were tested using natural undisturbed soil samples collected from the abandoned farmlands. Two treatments of undisturbed crusts and one treatment of removing the above‐ground tissue of BSCs were designed for each BSC type. For comparison, bare loess soil was used as the baseline. The collected soil samples were subjected to flow scouring under six different shear stresses, ranging from 6.7 to 21.2 Pa. The results showed that soil detachment capacity (Dc) and rill erodibility (Kr) decrease with BSC succession, and the presence of BSCs obviously increased the critical shear stress, especially for the later successional moss crust. For the early successional cyanobacteria crust, Dc was reduced by 69.2% compared to the bare loess soil, where 37.7% and 31.5% are attributed to the surface covering and B&amp;B, respectively. For the later successional moss crust, Dc decreased by 89.8% compared to the bare loess soil, where 68.9% and 20.9% contributed to the surface covering and B&amp;B, respectively. These results are helpful in understanding the influencing mechanism of BSCs on soil erosion and in developing the process‐based erosion models for grassland and forestland. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Сравнительная оценка методов определения педогенного органического углерода в углесодержащих почвах

The research considered determination of carbon reserves of soil organic substances in coal-bearing soils as an important problem of the study of technogenic landscape soils of coal deposits. Basing on the ability of soils of technogenic landscapes to perform the functions of natural undisturbed soils, the total fund of soil organic substance in such soils is said to be formed not only by humic compounds, but also by oxidized carbonaceous particles. Therefore, only developing the methods for differentiating soil organic substance performing the function of humus, and non-transformed lithogenic organic substance can solve the problem of evaluating the state of coal-bearing soils. The aim of the work is a comparative evaluation of the possibility of using traditional approaches and two those which we propose to determine the content of pedogenic organic carbon in soils of coal deposit dumps. Embryosems formed on the surface of coal and brown coal deposit dumps in Kemerovo oblast (N 53 o 39'; E 86 o 53') and Krasnoyarsk Krai (N 55 o 58'; E 90 o 23') were objects of the research. Methodically, along with traditional methods based on the content of total and organic carbon determination, we use approaches that rely on functional characteristics of soil organic substance. The first approach is based on the ability of soil organic substances to deposit nitrogen in soils. This approach involves calculation of carbon content of pedogenic organic substances using the parameters of soil nitrogen reserves and its ratio with carbon (C / N) in zonal undisturbed soils. The second approach is based on the ability of soil organic substance to form organic and mineral complexes with clay particles. At the same time, the content of pedogenic carbon is determined by the value of lithogenic potential of humus accumulation (LPHA), which depends on the number of clay particles in soils. Our research has shown that for carbon-containing soils, methods based on determination of total and organic carbon give incorrect results and cannot be used in complex ecological studies of techogenic soils and landscapes. Approbation of the proposed approaches has shown that, the definition of LPHA makes it possible to obtain reliable results for embryozems formed on a dense, stony substrate of coal deposits (See Table 2), since the accumulation of clay particles in such soils is the result of biochemical weathering and their amount is proportional to the reserves of organic substance. For embriozems formed on a loose substrate of brown coal deposit dumps, the results obtained with the help of C / N ratio and nitrogen content in the investigated soils are the most representative (See Table 2). Thus, we can conclude that, even in this approximation, the proposed approaches allow carrying out large-scale studies on evaluating the ecological state and resource potential of coal-bearing soils of technogenic landscapes. The article contains 2 Tables, 39 References.

Late Holocene soil and vegetational changes in the foothills of the SE Carpathians (Ukraine), based on the study of pedochronocatenas

A study of pedochronocatenas which include soils of archaeological sites is a reliable approach to reconstruct environmental changes since the time of the settlements existence. The studied pedochronocatenas include: 1) soils buried under ancient earth ramparts, 2) soils formed on the surface of these ramparts, and 3) natural undisturbed soils located close to the ramparts. The studied sites are located on the plateau between the Prut and Siret rivers (in the Glyboka district of the Chernivtsi region, South-Eastern part of the Ukrainian Carpathians). At present, this area is covered mainly by Haplic Greyzems and Dystric Cambisols under broad-leaved forests. Pedochronocatenas at two sites have been studied: 1) the Glyboka defensive rampart built during the IX-Xth centuries AD (1100-1000 yr BP, 14 C 1040±190, the Early Slavic time) and 2) the Grushivka rampart built as a defence from the Scythians during the Vth century BC (2500-2400 BP), but also with the new material deposited on the old rampart during the Early Slavic times. According to archaeologists [3–6] the ramparts were built in order to defend those sides of the settlements which were unprotected by natural barriers. Morphological and morphometrical soil analysis, as well as pollen study have been carried out in order to trace pedogenic, vegetational and climatic changes since the time of the rampart’s construction.

Land use impacts on soil detachment capacity by overland flow in the Loess Plateau, China

Land use and its adjustment may greatly affect soil detachment process by overland flow via altering soil properties, root systems, and tillage operations, but few studies were performed to quantify their effects on soil detachment in the Loess Plateau. This study was conducted to investigate the potential effects of land use on soil detachment capacity by overland flow (Dc, kgm−2s−1) using natural undisturbed soil samples taken from four different land uses on the red loess soil and six different land uses on the yellow loess soil, and to quantify the relationships between soil detachment capacity and hydraulic parameters, soil properties, and root systems in the Loess Plateau. The collected samples were tested in a 4.0m long, 0.35m wide hydraulic flume under six different shear stresses (5.51–16.59Pa). The result showed that both soil type and land use had significant effects on Dc. For two tested soils, the mean Dc of the yellow loess soil was 1.49 times greater than that of the red loess soil. For the red loess soil, Dc of cropland was the maximum, which was 5.57, 5.85, and 34.08 times greater than those of shrub land, orchard, and grassland, respectively. For the yellow loess soil, cropland was much more erodible than other five land uses. On average, the ratios of the cropland Dc to those of orchard, shrub land, woodland, grassland, and wasteland were 7.14, 12.29, 25.78, 28.45, and 46.43, respectively. The variability of Dc under different land uses was closely related to soil properties, root systems, and tillage operations. Soil detachment capacity was positively related to silt content, and inversely related to sand content, cohesion, water stable aggregate, aggregate median diameter, organic matter, and root density. The measured detachment capacity could be well estimated by measurable parameters of stream power, slope gradient, soil bulk density, median diameter, silt content, cohesion, and root density (Nash–Sutcliffe efficiency=0.89).

Determination of dose rate from Chernobyl-derived radiocaesium in Estonian soil

The deposition and vertical depth distribution of 134Cs and 137Cs in the natural undisturbed soil profiles down to 20–25 cm were studied at locations in the North Eastern Estonia, which were most strongly affected by the Chernobyl fallout in 1986. The total depositions were estimated based on summing the sampled and measured activities of 134Cs and 137Cs in all sections of soil profiles. The Chernobyl 137Cs deposition values varied considerably from site to site and the range was from 3.9 kBq m−2 to 50.2 kBq m−2, with the average of 22.8 kBq m−2 for the region (reference data May 1, 1986). The ratio of total activities, A(134Cs)/A(137Cs), varied in the range from 0.47 to 0.55. Using a compartment model and the observed data on the 134Cs and 137Cs activity concentrations (Bq m−2) in four compartments, 0–1 cm, 1–5 cm, 5–15 cm and 15–30 cm, of soil collected in 1991–2003, the approximate residence half-times of radiocaesium in soil were determined. The latter increased from 3.7 y in the top-most compartment to 8.6 y and 36.4 y in the deeper compartments, respectively. The time dependence of the external gamma-dose rate at the height of 1 m above a flat ground area arising from the deposited and migrating radiocaesium was calculated using the modeled data on the 134Cs and 137Cs activity in soil compartments. Considering the total depositions of radiocaesium in soil from the Chernobyl 1986 accident, the 50 year effective doses caused by external gamma exposure varied in the range from 0.13 mSv to 1.74 mSv, with the mean of 0.79 mSv in the region.

Soil contamination by organic and inorganic pollutants at the regional scale: the case of Piedmont, Italy

Background, aim, and scope Diffuse soil contamination has often been neglected in scientific literature, as most studies focus on contaminants from point-sources (either of industrial or agricultural origin). However, soil pollution from diffuse sources is recognized as one of the major soil threats by the EU Soil Thematic Strategy. In fact, some pollutants are nowadays ubiquitarious in the soil system, and they have to be considered for the implementation of environmental legislation, the definition of clean-up values in remediation activities and, more generally, for a sustainable management of rural areas. In the literature large scale studies on diffuse contamination are few and scattered and often do not consider a wide range of contaminants, the effect of land use, the vertical variability and the potential natural contribution. Aim of this work was to provide an overview of the diffuse soil contamination on a regional scale for a large set of contaminants. Soil inorganic (Cd, Cr, Ni, Pb, Cu, Zn) and organic contaminants (polychlorinated dibenzo-p-dioxins(PCDD), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs) as well as other soil general parameters were investigated on a 18 × 18 km grid covering a whole Italian region heavily industrialized and intensively cultivated. Soils were sampled at different depths both for natural–undisturbed soil and for agricultural–plow soil. Differences in the trends of investigated contaminants, as a consequence of land use, vertical variability, natural or geogenic origin, relationships among contaminants, and with main soil properties, were explored by means of enrichment factors, bi- and multi-variate statistics.

Detachment of Undisturbed Soil by Shallow Flow

Quantification of soil detachment rates is necessary to establish a basic understanding of soil erosion processes and to develop fundamental-based erosion models. Many studies have been conducted on the detachment rates of disturbed soils, but very little has been done to quantify the rates of detachment for natural soil conditions. This study was conducted to evaluate the influence of flow discharge, slope gradient, flow velocity, shear stress, stream power, and unit stream power on detachment rates of natural, undisturbed, mixed mesic typical Udorthent soil. Flow rates ranged from 0.25 to 2.0 L s -1 and slope gradient ranged from 8.8 to 46.6%. This study was compared with a previous study that used disturbed soil prepared by static compression. The results indicated that the detachment rates of disturbed soil were 1 to 23 times greater than the ones of natural undisturbed soil. It was necessary to use natural undisturbed soil samples to simulate the detachment process and to evaluate the influence of hydraulic parameter on detachment rate. Along with flow rate increasing, detachment rate increased as a linear function. Detachment rate also increased with slope gradient, but the functional relationship between the two variables depended on flow rate. Stepwise regression analysis indicated that detachment rate could be well predicted by a power function of flow rate and slope gradient (R 2 = 0.96). Mean flow velocity was closely correlated to detachment rate (r 2 = 0.91). Flow detachment rate was better correlated to a power function of stream power (r 2 = 0.95) than to functions of either shear stress or unit stream power.

Detachment of Undisturbed Soil by Shallow Flow

Quantification of soil detachment rates is necessary to establish a basic understanding of soil erosion processes and to develop fundamental-based erosion models. Many studies have been conducted on the detachment rates of disturbed soils, but very little has been done to quantify the rates of detachment for natural soil conditions. This study was conducted to evaluate the influence of flow discharge, slope gradient, flow velocity, shear stress, stream power, and unit stream power on detachment rates of natural, undisturbed, mixed mesic typical Udorthent soil. Flow rates ranged from 0.25 to 2.0 L s−1 and slope gradient ranged from 8.8 to 46.6%. This study was compared with a previous study that used disturbed soil prepared by static compression. The results indicated that the detachment rates of disturbed soil were 1 to 23 times greater than the ones of natural undisturbed soil. It was necessary to use natural undisturbed soil samples to simulate the detachment process and to evaluate the influence of hydraulic parameter on detachment rate. Along with flow rate increasing, detachment rate increased as a linear function. Detachment rate also increased with slope gradient, but the functional relationship between the two variables depended on flow rate. Stepwise regression analysis indicated that detachment rate could be well predicted by a power function of flow rate and slope gradient (R2 = 0.96). Mean flow velocity was closely correlated to detachment rate (r2 = 0.91). Flow detachment rate was better correlated to a power function of stream power (r2 = 0.95) than to functions of either shear stress or unit stream power.

SCANNING ELECTRON MICROSCOPE OBSERVATIONS ON SOIL CRUSTS AND THEIR FORMATION

Scanning electron micrographs (SEM) of crusts of loessial soils are presented. SEM observations were performed on crusts formed by raindrop impact at various stages of their formation. The crust structure was compared to the natural undisturbed soil. During the crust formation, a middle-term stage developed at which coarse particles, stripped of the fine ones, composed the surface layer of the soil. At the final stage of the crust formation, the coarse particles were washed away, and a thin seal skin, about 0.1 millimeter thick, formed the uppermost layer of the soil. A depositional crust, which was formed mainly by the translocation of fine particles, was marked by the presence of a thin skin also about 0.1 millimeter thick, suggesting involvement of similar secondary mechanisms of formation. This work illustrates the use of SEM for the study of soil crust formation and structure.