摘要:

In field-, watershed-, and regional-scale projects, soil water retention is often estimated from soil textural classes shown in soil maps. The textural classes are relatively broad, often only the dominating textural class is shown, cartographers routinely use error-prone field judgments of soil texture, and soil texture is known to vary along slopes and to depend on the land surface shape. We, therefore, hypothesized that including topographic information in water retention estimation would increase accuracy. To test this hypothesis, we extracted data on 216 soil pedons for soils of moderate and large extent from the Natural Resources Conservation Service (NRCS) soil characterization database. Textural classes, genetic horizon numbers, slopes, position on the slope classes, and land surface shape classes were the field descriptors that we used to estimate water retention at −33 and −1500 kPa potentials for each horizon in each pedon. Because our input variables were both categorical and continuous, regression trees were used for subdividing the samples into the smallest number of the most homogeneous groups, which we tentatively called topotextural groups (TTG). The jackknife cross-validation was used to prune the regression trees to prevent overparameterization. Ten or fewer TTGs were defined for both the −33 and the −1500 kPa retention. The TTGs were different for the two matric potential levels. Using topographic variables and soil horizon seemed to be the most accurate way to make up for errors made in field determination of texture. For the A horizon, the topotextural grouping resulted in estimates that were more accurate than those using laboratory textures only. Although most of the topographic variables in this work are categorical, those variables seemed to be useful for improving estimates of water retention.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Solute transport through soil may affect groundwater and surface water quality. In recent years, significant attention has been given to prediction of solute transport in soil and groundwater. Solute transport processes are affected by soil pore water velocity distribution. Pore size distribution estimated from water characteristic curves can be used to infer pore water velocity distribution. The objectives of this study were to develop a simple model relating soil water characteristic curves with soil solute breakthrough curves and to evaluate model predictions through comparisons with measured values. Water characteristic and solute breakthrough data for four soil materials were used to evaluate the model predictions.The simple model developed contains two parameters, one relating to pore size distribution and the other to pore connectivity. The coefficient associated with pore connectivity may be set to a constant value for a variety of soil materials. Therefore, there is only one variable parameter in the model. The results of our comparisons indicate that the simple model is useful for relating characteristic curves and solute breakthrough curves. Predicted values are in good agreement with the measured values. This paper provides a simple, approximate method to predict soil solute transport from soil water characteristic curves.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

The effects of adjuvant on the sorption and desorption of cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpro-panenitrile), atrazine (6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine), and norflurazon (4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone) in several soils were studied for the potential of adjuvant use in remediating herbicide-contaminated soils. The soils evaluated represented a wide range of soil texture, organic carbon, and pH. Sorption of14C-labeled herbicide was determined using batch equilibrium methods (equilibrated in water or adjuvant at 2.5 g L−1). Adjuvants used were classified as nonionic, cationic, and anionic. Desorption at two initial herbicide concentrations was done with water or adjuvant. Adjuvants generally increased the water solubility of these herbicides (10 to 91%). Cyanazine and atrazine sorption (Kd) was reduced in most soils with nonionic adjuvant (ranged 1.18 to 4.50 and 1.59 to 4.28, respectively) compared with water alone (1.36 to 5.59 and 1.75 to 4.59, respectively), whereas norflurazon sorption was increased with nonionic adjuvant (range: 3.88 to 8.76 in water; 4.66 to 9.82 in adjuvant). Similarly, more cyanazine and atrazine were desorbed by solutions containing adjuvant than in water, indicating that adjuvants may be useful in remediating some soils contaminated with certain herbicides. Adjuvant effects on sorption-desorption are, however, herbicide and soil-type specific.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Fragipan formation has frequently been related to the thickness of loess, with the development of gray vertical seams in deeper loess mantles and full fragipan development restricted largely to loess thickness of less than 2 meters. One body of research has suggested that fragipan formation is related to the position of a paleosurface after burial by loess, whereas other research suggests that fragipan formation may be unrelated to loess stratigraphy. Silicon has been implicated in promoting the moisture-dependent consistence of fragipans: consistence is hard when the moisture content is near the permanent wilting point and brittle when the moisture content is near field capacity. The processes responsible for the moisture-dependent consistence remain a matter of active research. Two fragipan containing soils developed in moderately deep loess were selected to determine the role of loess stratigraphy on fragipan genesis and to determine if silicon, possibly in concert with either aluminum or iron, is an effective cementing agent. The clay mineralogy, Al to clay ratio, Fe to clay ratio, and trace element contents suggest that the soils are developed in a single layer of loess overlying intensively weathered residuum. Fragipans formed within the basal mixing zone of these two parent materials. An ammonium oxalate extraction and an aqua-regia extraction did not reveal the presence of appreciable quantities of noncrystalline material in the fragipan or elsewhere in the soil. The Si activity indicates that noncrystalline materials have not affected Si solubility materially. In general, fragipan horizons seem to be associated with the basal mixing zones of the loess and underlying residuum. Silt and clay illuviation into the fragipan seems to be the dominant soil process associated with fragipan formation in the basal mixing zone. No evidence for Si as a binding agent was observed, and we speculate that fragipan consistence is related to the architecture of the soil separates within the fragipan.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

The isotic family was established in U.S. Soil Taxonomy to create a division in the mixed mineralogy class identifying soils exhibiting properties of noncrystalline minerals. Application of the isotic class has resulted in correlation problems of soil series in various regions of the country. This study of 56 pedons in the Blue Mountains physiographic province of eastern Oregon examines isotic and andic properties of soils with the following objectives: (i) review theory and background related to the current criteria for the isotic class; (ii) examine these criteria on selected pedons in the Blue Mountains; and (iii) discuss the current approach to the use of the isotic family classification in this geographic region. The criterion, 1500 kPa water retention/clay percentage ⩾ 0.6, is derived from the rationale of water adsorbed on surfaces of soil materials and is often used as a criterion for appropriate dispersion during particle-size analysis. The reaction of NaF with noncrystalline soil minerals (e.g., allophane) results in the exchange of hydroxyl ions from the mineral surface with fluoride. The nonlinear increase in pH of NaF establishes the semiquantitative nature of the test. For this study, NaF pH has correlation coefficients of 0.80, 0.72, and 0.34 for acid oxalate-extractable Al, Si, and Fe, respectively. The relationship of NaF pH with properties related to andic materials suggests that different OH sites exist in these soils and that they react at varying rates and release differing amounts of acid oxalate-extractable elements. Calculated clay based on 1500 kPa water retention and using criteria in Soil Taxonomy (2.5 rule) always exceeds measured clay for all horizons. A better relationship of calculated and measured clay was achieved with a factor of 1.7 (rather than 2.5). This factor is derived assuming a linear slope of 0.6 (rather than 0.4) between 1500 kPa water retention and clay. In the Blue Mountains survey area, pedons in the isotic versus mixed families do not correlate with specific geographic areas, moisture/temperature regimes, or parent materials. Lack of mappable differences of these two classes results in no discernible differences in properties affecting interpretations as we currently view them.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Paleopedology is important for a proper understanding of the agrochemical and agroenvironmental behavior of surface (nonburied) paleosols. A long agricultural history on the Loess Plateau of China has caused extensive destruction of natural vegetation cover and resulted in profound modification of the original soils. Chinese soil scientists have called them agricultural loess soils and termed them Lutu soil, Zhong Heilutu soil, and Qin Heilutu soil, with variations of profile properties from the southeast to the northwest. The present paleopedological investigation was conducted in three typical profiles at three locations along a south-north transect across the Loess Plateau. New data and evidence of mineralogy, micromorphology, palynology, soil properties, and radiocarbon dating indicate that the so-called agricultural loess soils are Holocene paleopedosedimentary complexes combined with human disturbance, which consist of a paleosol layer developed in the middle Holocene and a cover layer of human disturbed fresh loess developed in the later Holocene. The pedogenetic types of the paleosols varied from Ustalfs to Argiustolls and Haplustepts (in the U.S. system) along the S-N transect, with paleo-bioclimatic pattern of the middle Holocene. The cover layers (Ap horizons) are newly deposited loess of the later Holocene, 10 to 25% of which in the south area was caused by human activities such as application of plaggen manure and irrigation with sediment-rich water. It has also has been affected by modern pedogenesis, and, therefore, it could be classified as a kind of Entisol or anthropogenic soil.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID