摘要:

Bulk diffusion and gaseous permeability coefficients were measured in situ in most morphologic horizons of four soil profiles in central Pennsylvania. Such data are rare in the literature. From the eluvial to the illuvial horizons of individual soil profiles, bulk diffusion coefficients generally decrease by nearly an order of magnitude, and gaseous permeability coefficients decrease by about two orders of magnitude. In all four profiles, the diffusion and permeability coefficients are higher in the upper, coarsely textured horizons than the lower horizons and, at corresponding depths, in more coarsely textured than finely textured pedons.The accuracy of the diffusion coefficients is confirmed by the similarity of an observed radon-222 (Rn) concentration profile to that estimated using the measured diffusion coefficients and a two-dimensional finite difference model.Several published methods of estimating bulk diffusion coefficient from air-filled porosity are statistically compared with the data following log transformation. Although all the methods tested were highly correlated to the in situ data, the estimates of Millington (1959, Science, 130:100–102) and Sallam et al. (1984, Soil Science Society of America Journal, 48:3–6) produced values most similar to those measured.Logarithmically transformed values of the bulk diffusion and permeability coefficients are highly correlated with each other for both our data and previously published data. This relationship holds true for eight different soils measured by separate researchers using different methodologies and appears to be generally applicable. Since the permeability values encompass a larger relative range than the diffusion values, measured gaseous permeability coefficients can be used to estimate bulk diffusion coefficients. This empirical approach to estimating diffusion coefficients is useful in that, in addition to air-filled porosity, permeability reflects the continuity and tortuosity of the air-filled pore system. Since the continuity and tortuosity of air-filled pores affect the diffusion coefficient, and neither of these properties are directly reflected in estimates based on air-filled porosity, estimation of diffusion coefficient using permeability accounts for important soil properties not directly accounted for with methods based on air-filled porosity alone.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

In this work, we investigated the change in the hydraulic conductivity of a sandyloam soil as a function of SAR and electrolyte concentration of the percolating solutions. High sodium adsorption ratios and low electrolyte concentrations caused a progressive reduction in the percolating rate, resulting in a non-linear relationship between effluent volume and time. In all these cases (nonsteady-state drainage) Darey's Law, which characterizes steady or stationary flow processes, does not apply. The relationships between effluent volume and time were then analyzed to find an empirical equation for predicting drainage from soil columns under saturated conditions. Characteristic parameters of the proposed model are the saturated hydraulic conductivity at timet= 0 (Ksat) and a rate constant that takes into account the progressive reduction in the initial hydraulic conductivity of the flow process. Validation of the model was found by applying it to montmorillonite-sand systems.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

We designed a three-piece constant head well permeameter (DCHWP) consisting of a burette reservoir, a detachable water transfer tube, and an outlet tube. It measuredin situfield-saturated hydraulic conductivity (K0), flux potential (m), and sorptivity above the water table accurately in a series of field tests carried out in sandy, loamy sand, and sandy loam soils. The sensitivity of the designed permeameter is 0.25 mm. It is cheap, and its components are easily available. Its working range over the K0measurement (1.1 x 10-8ms-1≤ K0≤ 7.7 x 10-5ms-1) and over the depth profiling (0.05m ≤ D ≤ 4.0m) is quite large so that it can be used in different soils of widely varying K0. Its water requirement (2.0–2.25 liters) and time requirement (20–30 min) are low, in practical range. It presents minimum operational problems. The DCHWP method consistently gave results similar to the double-ring infiltrometer method. Averaging for three soils, the inverse auger hole method overestimated K0by 50%, and the soil core permeameter method underestimated it by 48%. The K0and m values were not significantly affected by height of water in the well or radius of the well.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The use of isoelectric focusing (IEF) to characterize soil humic substances is a technique that appears to be of widespread interest. The use of carrier ampholytes permits the electrophoretie separation in the presence of a pH gradient, but until now the problem of interferences that could occur between the soil organic matter and the carrier ampholytes during the IEF separations have not been recognized. The commercial carrier ampholytes have different chemical structures; thus, interferences with soil humic substances could be different.In this study, two soil extracts were fractionated using the IEF technique in the presence of six commercial carrier ampholytes with a wide pH range (about 3–10). The main results can be summarized as follows:(i) Each ampholyte used had a marked influence on the patterns obtained, and good resolution of the focused bands was obtained only in the presence of some carrier ampholytes (Ampholine and Preblended Ampholine from Pharmacia-LK B and Bio-Lyte from Bio-Rad).(ii) The carrier ampholytes influenced both the number and the apparent isoelectric point of the focused bands.The achievement of a pH gradient that allows electrophoretie runs without formation of complexes and of possible interferences between carrier ampholytes and humic substances is still to be shown. However, the IEF technique is highly reproducible and useful for the characterization of soil humic substances once the experimental conditions and the carrier ampholytes are precisely defined.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The Inland Bays watershed in southern Delaware is dominated by a large and growing poultry industry that uses land application as the primary means for disposal of manure. Runoff and drainage waters from an extensive ditch drainage system in the area eventually enter the Inland Bays. Phosphorus in these waters has been suggested as a contributing factor to eutrophication of the Inland Bays.Soil test P (STP, Mehlich 1, 0.05NHCI + 0.025NH2SO4) levels in cultivated and field border areas and P sorption maxima (PSM) were determined for selected profiles of agricultural fields and non-cultivated field borders. Relationships between PSM and soil properties and a rapidly determined p sorption index (PSI) were also investigated. STP in cultivated topsoils from 48 sites ranged from 43 to 632 mg/kg and from 42 to 568 mg/kg at 0–5 and 5–20 cm, respectively, relative to a high value for STP for agronomic crops of 35 mg/kg. STP levels were consistently greater in cultivated profiles than in field borders. Downward movement of P in some cultivated profiles occurred to a depth of 40 cm. PSM ranged from 95 to 2564 mg/kg in cultivated soils and from 200 to 2000 mg/kg in field borders. PSM was highly correlated (r= 90) with clay content and was consistently higher in subsoil horizons. When clay contents were similar, P sorption was usually greater in field borders than in cultivated fields. PSI was highly correlated with PSM (r†= 0.94) except when PSI exceeded 1400 mg/kg.This study suggests that past and current P management practices in the Inland Bays have increased soil P levels in this watershed to levels that may require much more intensive management of manure and fertilizer P in the future. Although subsoil horizons may prevent direct P leaching into groundwater, the high P values noted at the 0 to 5-cm depth and in some subsoils point to the need to assess the potential for soluble P losses in runoff and drainage waters.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The objective of this study was to determine the extent and location of eroded phases of an Alfisol at a cultivated site. The area was located on a backslope, with 5 to 10% slopes, which was to be used for tillage research. The extent of soil loss from cultural erosion was calculated using comparisons of soil properties with a forested site having similar soil, slope, and landscape characteristics. It was not possible to find a virgin site with similar characteristics. Most land in Southern Illinois has been cleared during the 190 years since initial European settlement. The interpretation of direct comparisons of A (and E) horizon thicknesses of the forested site with A (and E) material thicknesses in the Ap horizon of the cultivated area was complicated by the tillage equipment mixing Bt materials into the Ap over time. This resulted in pockets of Bt material in the Ap, a lighter matrix Ap color, thinning of the underlying Bt horizon, and an increase in the clay content of the Ap horizon. The depths to gray mottles and Bx horizon were also reduced at the cultivated site under a crop rotation that included forages for extended periods of time. The cultivated site was dominated by the moderately eroded phase of the Grantsburg (finesilty, mixed, mesic Typic Fragiudalf) soil. Based on paired transect comparisons with a forested site, changes in the clay content of the Ap, thickness of the Bt, and depth to gray mottles and Bx horizon, approximately 7.5 cm of soil (38% of the upper 20 cm of original soil) has been eroded from the cultivated site during the past 80 years.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID