摘要:

A simple analytical expression is derived to describe soil water profiles satisfying the Bruce and Klute equation. Two parameters have to be determined from integral conditions. The simplicity and accuracy of the technique is checked when the soil-water diffusivity obeys an exponential or a power law function. For those two examples no numerical work is necessary as the results are expressed in terms of tabulated functions only. The method extends to the profile determination the optimal technique used earlier for the determination of the sorptivity, with the same precision.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Numerically simple models for soil transport processes based on the use of the slopes of soil parameter curves as dynamic parameters have proved to be a stable and accurate supplement to other models. In this study, the recently presented moving mean slope (MMS) model for vertical, unsaturated water flow is evaluated, using series and numerical analyses. Results show that the MMS model takes into account additional parameters in the estimation of the soil hydraulic conductivity (K) between the calculation depths compared with the arithmetic or geometric mean value considerations used normally. This suggests that the MMS modeling approach implicitly provides a good estimation of the nonlinear change ofKwith soil depth and explains the observation that it is often possible to use larger depth and time increments in the MMS model than in many of the traditionally used numerical models. The vertical MMS model is modified for horizontal flow, and the corresponding criterion for avoiding numerical errors in the calculations is derived. The agreement between horizontal flow calculations using the simple MMS model and the Crank-Nicolson finite difference model is excellent. A new series expression for the horizontal flow velocity is obtained by using a reformulated, integrated version of the horizontal Darcy flow equation. After a short infiltration time, only the first few terms of the series expression are quantitatively important. This seems convenient for analyzing and mathematically describing infiltration into soils.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Seal formation is affected by clay dispersion, and both parameters depend on the same soil and soil solution properties. Since clay dispersion can be determined easily in the laboratory, its use for predicting soil susceptibility to seal formation was evaluated. Laboratory clay dispersion tests were conducted on soil samples from semi-arid (northern Cameroon and Israel) and humid (Georgia, USA) regions. Clay dispersion (presented as percent of total clay) in distilled water (to simulate rain-water) was significantly correlated with silt and clay content. Clay dispersion was severe (>40%) in soils with silt and clay content >15%, which were the kaolinitic soils from the humid region and the predominantly smectitic soils from semi-arid regions. Kaolinitic and illitic soils from the semi-arid regions, whose silt and clay content was <15%, were less dispersive. Deflocculation of kaolinitic clay by decomposed organic matter present in humid conditions was suggested as an additional cause for the dispersivity of the kaolinitic soils from Georgia. In stable soils, clay dispersion was affected by exchangeable sodium percentage (ESP) at all levels. In dispersive soils, only high ESP levels increased clay dispersion. Clay dispersion decreased in electrolyte solution (EC = 0.8 dS m−1) and was significant mainly in the kaolinitic soils from both northern Cameroon and Georgia. The degree of clay dispersion depended also on the energy applied; different soils needed different amounts of energy for reaching their utmost dispersion. The correlation obtained between clay dispersion and the final infiltration rate or percent runoff explained only a relatively small proportion of the variance measured and, hence, prevented an accurate prediction of these two variables from percent dispersed clay.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

A single-probe gamma density depth gauge and soil cores were used to obtain wet and dry bulk density information to a depth of 1.0 m in poorly drained soils. The bulk density data were needed in order to determine volumetric soil water content for neutron depth gauge calibration and for modeling purposes. The factory equation for the gamma gauge was evaluated with access tube soil core data, which was obtained with a hydraulic coring device, and with soil water content data from small 15-mm-diameter soil samples obtained about 1.0 m from the access tube. Soil core bulk density estimates were highly correlated to the gamma gauge results (r2of 0.85 to 0.92), and there was no significant difference between using the factory equation and an equation calibrated with field data. The small remote soil samples provided useful information for converting gamma gauge estimates of wet bulk density into dry bulk density data.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Volatilization can reduce soil selenium. A method was developed to measure volatile Se from soils and plants using a chamber in which temperature and light conditions were similar to the ambient. Inserting the chamber into the soil isolated a soil column and collected volatile Se for a known period of time. In a laboratory test with a known quantity of dimethyl selenide solution, the Se recovery was 98%. In soil systems with and without plants, the collection of volatile Se was reproducible. The soil and air temperatures inside and outside the chamber were similar because of the chamber design.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

The solubility of gypsum in different salt solutions containing Ca, Mg, Na, Cl, and SO4was determined experimentally at 298° K. Sixteen subprograms were developed by using five methods of calculating the single ion activity coefficient and four combinations of ion pairs. Statistical analysis of the difference between the experimentally determined solubility of gypsum (average of seven data sets) and the solubilities predicted with the sixteen subprograms was used to determine the subprogram that best predicts the experimental data.Results showed that acceptable predicted values within the experimental error of measurement can be obtained by considering only CaSO4and MgSO4ion pairs. The values for the dissociation constant at 298° K for CaSO4and MgSO4ion pairs of the best subprogram were 4.9 * 10−3and 5.9 * 10−3.5, respectively. The best agreement between experimental and predicted solubility was found when a constant “1.3” was used instead of the value of the effective diameter of the hydrated ions “αi” multiplied by the coefficient “β” which involves universal constants, dielectric constant, and temperature in the Debye-Huckel theory for calculating the single ion activity coefficient.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

A field experiment was conducted to monitor decomposition of corn stalk residue and the concomitant development of the microbial population inhabiting the residue. The quantitative and qualitative involvement of bacteria, actinomycetes, and fungi were examined during the course of decay. The soil in which the study was conducted had a mean annual microbial biomass of 71 g*m−2to a depth of 10 cm. The maximum biomass associated with the corn residues was 14 g*m−2of which 85% was fungal. During the active period of the first year, approximately 70% of the residue was lost. By relating residue (substrate) utilization to population estimates over time, the mean first year generation time for microbial biomass was calculated as 10 days, with the peak activity demonstrating a generation time <1 day. The peak fungal growth rate was 0.008 g biomass (dry)*g−1of residue*day−1. The initial predominant invading fungal genus wasMyrothecium.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

We studied chemical kinetics of weathering in the humus horizons of young Andisols from northeastern Japan. Since there is no idealized chronosequence of Andisols, we selected eight pedons that have many similarities in the properties of parent material, soil moisture regime, vegetation, drainage, etc., but show significant differences in soil age and soil temperature. We introduced “soil age normalized to 10°C,” which was defined as the number of years a soil has been subjected to pedogenesis under mean monthly soil temperature (MMST) of 10°C throughout the year. For this calculation, temperature dependency coefficient (Q10) of Al release from volcanic glass was determined to be 1.5 by the laboratory dissolution experiment using separated glass samples. Mean monthly air temperature (MMAT) plus 1°C was employed as MMST based both on the comparison between MMST and MMAT and the determination of diurnal variations of soil temperature. Of the various elements contained in volcanic ash, Al was found to be most reliable for evaluating chemical weathering of the bulk soils, and released Al was determined by the acid oxalate method. The correlation between the soil age normalized to 10°C and acid oxalate extractable Al (Alo) showed that the chemical weathering is expressed as zero-order kinetics.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

The objective of this study was to evaluate the relative effects of Ca, Mg, and K on the adsorption of Zn and Cu by several Florida soils under citrus production and to differentiate the specific cation effects vs. effects of ionic strength on adsorption of Zn and Cu. A batch-equilibration technique was employed to evaluate the adsorption of Cu and Zn (added at 10−4M) in the presence of varying concentrations of Ca or Mg (2.5–15 mmol) or K (5.0–30 mmol). Adsorption of both Cu and Zn decreased with an increase in concentrations of either Ca or Mg. The inhibitory effects of added cations on the metal adsorption was much greater for Zn than for Cu. Addition of K slightly decreased the adsorption of Zn but had little effect on Cu. The inhibitory effects of Ca and Mg on the adsorption of Cu or Zn appear mainly due to competition for the exchange sites on soil colloids.

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