摘要:

We carried out experiments to measure the rate of adsorption of cadmium for static and dynamic flow conditions. The water content of samples was maintained at —12.6 kPa, corresponding to 87 percent of saturation. Adsorption rates at apparent pore water velocities of 1.3, 2.3, and 2.7 cm/h were measured. Diffusive conductance,Kd, and forward (k1) and backward (k2) rate constants were calculated using the Nelder-Mead Simplex method based on a nonlinear least-square procedure.Values ofKdand of the ratiok1/k2increased with increasing pore water velocity. The rate of adsorption was higher in a sample with solution flowing through it than in a sample with the same solution under static conditions. The magnitude ofKdranged from 0.07 X 10−3] at static conditions to 0.4 x 10−3] s−1] with &OV0622; = 2.7 cm/h.Results imply that the thickness of the subsurface layer decreases with &OV0622;, thus increasing the diffusion rate of ions across this layer. When the adsorption process is far from equilibrium, diffusion across the subsurface layer became a rate-limiting step in the adsorption process. Lateral diffusion cannot be ignored in studies of the kinetics of adsorption.The increase in values ofk1/k2with increasing &OV0622; indicates that the surface reaction proceeded at a more rapid rate in the presence of a continuous supply of solution. Flowing solution serves as a continuous source of the introduced ions, as well as a continuous sink for the desorbed ions of other species originally present.The accelerated rate of adsorption induced by flow of solution through the soil may be attributed both to an increase in the rate of diffusion as the thickness of the subsurface layer decreases and to an increase in the rate of the kinetic reactions at the surface as the exchange process proceeds in the presence of the continuous sink.

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID

摘要:

We compared Hesperia and Columbia soil experimental infiltration data to calculated profiles obtained by solutions of Richards' equation requiring soil water diffusivityDand hydraulic conductivityKas functions of the water content &thetas;. The calculations were made using a finite-difference, iterative method (FINDIT). Predicting wetting profiles by this method is somewhat more rigorous than some other solutions of Richards' equation. The method reduces calculations for infiltration to a two-term algebraic equation, partitions infiltration into matric and gravitational components, and gives an asymptotic relationship between the infiltration rate and the saturated conductivity as time approaches infinity. An array of solutions for the two indicated soils, obtained with and without the diffusion lip procedure, raises questions of the need for its use. A new method for estimating the Λ - &thetas; area in horizontal infiltration analysis, using only diffusivity data, gives estimates to less than 3 percent of the final value. Integrated mean values ofDandKwere required for accurately predicting soil moisture wetting profiles over a range of time periods and &thetas; divisions. Arithmetic and geometric means and nonaveraged values, particularly ofD, produced unreliable wetting profiles.

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID

摘要:

Using Warburg respirometers, we studied the oxidative power of various soils, as well as the clays and silts isolated from them. We also examined the humic substances formed through the catalytic polymerization of phenolic compounds by a soil derived from loess deposits and the silt of a Mollisol. The catalytic oxidative power of the silt fraction (2 to 50 micrometers) of the soils ranges from 23 to 67 percent of that of the corresponding clays. The oxidative power of the oxides removed by the citrate-dithio-nite-bicarbonate treatment accounts for 1 to 45 percent and 7 to 25 percent of that of the nontreated clays and silts, respectively. The percentages of the conversion of the total phenolic acids and other compounds added to the loess-derived soil and Mollisol silt into phenolic polymers are 38.7 and 27.4 percent respectively, indicating that the formation of many other yet unidentified products as a result of side reactions during the polymerization of phenolic compounds is substantial. Furthermore, the rate of the polymerization varies with the nature of the phenolic compounds. The resultant humic acids formed outweigh the fulvic acids. The nature of the humic substances formed is characterized by elemental analysis, functional groups, and infrared and electron spin resonance spectra.

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID

摘要:

We investigated the effect of the fungicide mancozeb (ethylene-bis-dithiocarbamate) on the microorganisms of the soil of four Bordeaux vineyards. Populations of bacteria, fungi, and actinomycetes were reduced by application of the fungicide at 10 mg/kg soil. Nitrogen mineralization was altered and nitrification decreased for 3 months: Mancozeb degradation in soil is due to both chemical and biological factors. SpecificBacillusbacteria were isolated from soil during in vitro degradation. The fungicide was completely degraded in 3 months, after which the microbial populations returned to normal levels.

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID

摘要:

We conducted laboratory and field experiments to study the influence of the depth of cultivation on (1) the loss of water from and below the disturbed layer, (2) the cumulative evaporation (CE) and time (t) relationship, and (3) the advance of isohydric fronts into soil, under various evaporativity (E0) conditions. PVC cylinders, 10 cm ID and 95 cm long, were filled with air-dry (&thetas; = 1 ± 0.25 g/100 g) sandy loam soil to a bulk density of 1.5 g/cm3]. The columns were wetted from the top with 19 cm of water, allowed to equilibrate for a week, cultivated to depths of 0, 6, and 12 cm, and then subjected to drying under evaporativities of 16 (high), 8 (medium), and 4 (low) mm/ day. Also, we studied the drying of 3 X 3 m field plots cultivated to 0, 5, 10, and 15 cm depths from 8 November to 16 December, 1976, whenE0ranged from 1.8 to 4.3 mm/ day (average 2.8 mm/day).Cultivation reduced evaporation under all theEoS, with the highest reduction under the lowE0. The maximum evaporation reduction was obtained after 29 days under highE0and after 55 days under medium and lowEos. The differences in water loss between the 6− and 12-cm cultivation depths were greater under higherEo. The relationshipCE=Kt0.5] gave an excellent fit for both the 6− and 12-cm cultivation treatments under allEos. The regression coefficientKincreased linearly withE0, and at a givenE0was lower for 12-cm than for 6-cm cultivation.The cultivated layers dried more rapidly than the corresponding depths of uncultivated soil, but the layers below the loosened soil always had more water in cultivated than in uncultivated and in 12-cm than in 6-cm cultivation. The magnitude of differences in soil water across the junction of cultivated layers and the undisturbed soil below was influenced both by cultivation depth andE0. The position of 0.18 and 0.15 cm3]/cm3] isohydric fronts was always deeper in uncultivated than in cultivated soil, except within the loosened zone.

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID

摘要:

Using greenhouse cropping and chemical procedures, I evaluated the status of available copper and manganese in 55 representative soils of the seasonally dry Nigerian savanna. Both metals were extracted with 0.1MEDTA in 1NNH4OAc pH 7.0, and in addition, Cu was extracted with 0.1NHCl and Mn with 0.05NHCl + 0.025NH2SO4. Thirty of the soils were cropped to cotton (Gossypium hirsutum L.) in two experiments (one for Cu and the other for Mn). Each crop was harvested after 42 days, oven-dried, and analyzed for the appropriate metal.Relationships among extractable Cu and Mn, uptake of the micronutrients under greenhouse conditions, and selected soil variables were evaluated through simple and multiple correlation and regression analysis. Extractable Cu levels appeared to be strongly related to organic matter levels and to a lesser extent, to the clay fraction. Extractable Mn was strongly related to pH and to a lesser extent, to soil organic matter. Plant uptake of the two micronutrients was best described by multiple regressions that included the extractable metal, percentage of organic matter, and percentage of clay as independent variables.The extractable Mn levels increased from the slightly leached Inceptisols in the far north of the country to the heavily leached, more acid Ultisols and Oxisols in the south, and the extractable Cu levels increased in the opposite direction. Double-acid and EDTA-extractable Mn ranged from 12.8 to 53.9 ppm (mean of 31.6 ppm) and 1.40 to 47.1 ppm (mean of 24.0 ppm), respectively. Dilute HCl- and EDTA-extractable Cu ranged from 0.54 to 1.69 ppm (mean of 1.03 ppm) and 0.49 to 1.50 ppm (mean 0.87 ppm), respectively. The Nigerian savanna soils appear to be adequately supplied with available Cu and Mn at present, and deficiency problems are unlikely in the immediate future.

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID

摘要:

We conducted an experiment to measure the evaporation rate from a soil surface as affected by transient shading. The soil surface temperature showed abrupt changes when the shading began and when it ended. Because temperature is a major factor regulating vapor pressure, the evaporation rate (latent heat flux) from the soil surface was strongly affected by shading. The experimental results were compared with the predictions of a previously published mathematical model. The validity of the simulation model improved when the effect of advection was taken into account.

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1983

数据来源: OVID