摘要:

Oxygen concentration measurements in clay soils that are representative of a soil layer are difficult to obtain. Distribution of oxygen is strongly related to the soil structure, and, therefore, reliable oxygen concentration measurements in clay soils require a description of the soil structure. A procedure for obtaining combined measurements of soil structure and oxygen concentrations is presented in this paper. Oxygen micro-electrodes are used to measure the oxygen concentrations directly in the soil, while the soil structure is concurrently determined with the aid of computer assisted tomography scanning. Good agreement was found between the soil structure and oxygen concentration distributions.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Macropores are important in preferential flow; therefore, function and continuity of macropores are often more important than visual observation. The objective of this research was to compare several indirect methods of determining macropore volume. Ponded and tension infiltration measurements were made in situ at the soil surface and 0.25 m deep in a Waukegan silt loam (fine-silty over sandy, mixed, mesic Typic Hapludoll), and air-filled porosity at each pressure head was calculated from conductivity determinations. Undisturbed, unconfined samples were taken from the 0.25− to 0.35-m depth. Air-filled porosity as a function of pressure head was determined on these samples using wetting and draining equilibrium curves in rotated cores (to even out the gravitational gradient), measured/calculated after steady-state lab conductivity, and calculated from ped size distribution (assuming a ped shape factor and ped size/crack size relation, with known bulk density and ped density). Water adsorption/desorption in rotated cores in only four out of 20 samples displayed an “air-entry” pressure head, and no samples displayed a “water-entry” pressure head (that is the head at which no more water enters pores during equilibrium soil water adsorption). All methods tested estimated air-filled porosity in the same range. Air-filled porosity determined from ped-size distribution and from ponded and tension infiltration measurements was much less tedious than from rotated core measurements.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Moisture changes in swelling clays are associated with corresponding volume changes, causing these soils to swell upon wetting and shrink upon drying. These volume changes cause subsidence, heave, and crack formation. It is becoming increasingly clear that the engineering and agronomic use of such soils requires special care.Present methods of determining soil volume changes have many constraints. They require the use of hazardous chemicals, take a long time to complete, are not useful near the wet end of the moisture range, and have limited accuracy. A new method of soil volumetric shrinkage measurements using a balloon apparatus is presented. In this method, the soil bulk volume is determined by measuring the displacement of the soil specimen in water with the soil specimen wrapped in a flexible rubber membrane, an ordinary rubber balloon. The method is very simple and doesn't require use of any chemicals. Therefore, it is environmentally safer than other methods. The volumetric shrinkage measurements can be completed for the full moisture range in a short time. The proposed method compares very favorably with the results of the resin coating method.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Comparisons (of the first harmonic) between exact and two approximate analytical solutions to the one-dimensional heat conduction equation for an inhomogeneous soil show that the approximate analytical solutions are potentially more useful for profiles of soil thermal properties that exhibit positive or zero concavity than for those that exhibit negative concavity. Comparisons between the two approximate analytical solutions also suggest that one solution provides a much easier method for estimating profiles of soil thermal properties from soil temperature profiles than does the other. A brief summary of three analytical solutions to the one-dimensional heat conduction equation is also given. Furthermore, some of these extent analytical solutions are unique to the present study and employ relatively simple and easily implemented algorithms for their evaluation. For many applications involving periodic variations in soil temperature, these algorithms are likely to provide more realistic results than can be obtained by assuming homogeneous soil properties with their associated “exponentially decaying” solution for soil temperatures. It is further suggested that the solutions presented in this work could be used to verify more complex numerical models of soil heat flow.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Information on soil properties that influence herbicide sorption throughout the soil profile is needed to identify soil-herbicide combinations with varying degrees of risk for groundwater contamination. In this study we investigated the effects of soil properties on the sorption of five herbicides commonly used in grain production in the Atlantic Coastal Plain (atrazine, cyanazine, dicamba, 2,4-D, and metolachlor), using subsoil and surface horizons from six Delaware soils. Our objective was to develop simple screening models, based on soil properties available from routine soil tests or regional soil databases, to identify soils that may have an increased risk of groundwater pollution. Each soil was sampled by horizon to a depth of approximately 2 m to include major variations present in the soil profile above the surface water table. Herbicide distribution coefficients (Kdm) were determined for each horizon and herbicide by sorption studies using a batch equilibrium technique and14C-labeled herbicides. The ranking of herbicide sorption, over all soil horizons, was metolachlor > cyanazine > atrazine > 2,4-D > dicamba (Kdm= 1.03, 0.94, 0.85, 0.65, and 0.25 L kg-1, respectively). Multiple regression analysis between Kdmand soil properties identified two rapid soil tests (KCl exchangeable acidity and organic matter content) as important predictive variables for atrazine, cyanazine, dicamba, and 2,4-D sorption (R2= 0.81***, 0.56***, 0.26*, 0.68***); metolachlor retention was best described by oganic matter content and effective cation exchange capacity (R2= 0.78***), another easily measured or estimated soil property.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Subsoil K availability may be an important factor limiting the precision of K fertilizer recommendations, but it is too expensive to routinely sample and analyze subsoils. Our objective was to develop and validate simple empirical equations for predicting indexes of K supply capability of subsoils based on physical and chemical properties commonly reported by Soil Survey. Ten Minnesota soils were sampled by genetic horizons to a depth of 1 m and were analyzed for exchangeable K from air-dried (EKD) and moist (EKW) samples, nonexchangeable K (NK), potential K buffering capacity (PBCK), water-soluble K (SK), and 39 physical and chemical characteristics. The best regression equations for each K index had the indicated coefficients of determination (in parentheses): EKD (2= 0.85), EKW (r2= 0.77), NK (r2= 0.86), and PBCK(r2= 0.88). No equation could be developed for SK. Validation of the EKD equation with a total of 88 independent soil samples and Soil Survey data from Minnesota had a standard error of prediction of $49 mg K kg-1. The EKD equation had a larger error of prediction ($63 mg K kg-1) when data from seven midwestern states (n= 512) were included. This equation tended to predict greater amounts of exchangeable K than were measured, especially when EKD < 200 mg K kg-1. EKW could be estimated from EKD (r2= 0.94). Equations for NK and PBCKcould not be validated as thoroughly as that for EKD because the database is not available. Standard error of prediction for NK (n= 6) and PBCK(n= 7) equations were $515 mg K kg-1and $11 cmol kg-1mol-0.5, respectively. Depending on the degree of precision desired, these equations could be used for estimating subsoil K supply capability in the presence of adequate Soil Survey data, but the tendency toward overprediction should be recognized.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID