摘要:

A simple and accurate procedure is given for solving the infiltration equation for nonlinear diffusivity and conductivity. It overcomes some of the numerical difficulties associated with an earlier procedure given by Hogarth et al. (1989), but confirms the accuracy of their results.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Rate coefficients describing sorption of pesticides in soils are commonly obtained under well stirred conditions from batch equilibration studies. Batch techniques involve high solution-to-soil ratios and mechanical agitation, both of which can influence the sorption characteristics of a soil. In addition, rapid equilibration cannot be studied accurately in batch experiments. These disadvantages can, however, be overcome by studying the sorption of pesticides during leaching in saturated columns of soil. Results in this paper show how time-dependent sorption of pesticides in soil can be calculated by applying the peak maxima method from the gas-chromatographic literature to pesticide breakthrough curves of short pulse injections in soil columns.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

An analytical solution (Warrick 1974) for time-dependent infiltration of water under point source was suggested for a narrow soil moisture range at or near saturation. An attempt has been made by the authors to validate numerically Warrick's (1974) theory for a wide range of soil moisture conditions by dividing the hydraulic conductivity-moisture relationship into small segments of 0.02 cc/cc. The outputs were compared with the numerical outputs of Brandt et al. (1971), widely used by various workers, and also with the experimental data generated for four soils of widely different textures under both on-and off-cycles of irrigation. Relative comparison between the theoretical and experimental data also was made quantitatively by Relative Error Percent, defined as the percent difference of the two, in relation to the experimental value. Agreements of both models with the experimental data were entirely satisfactory for Ghatakpukur and Baruipur soils that have lower silt and clay contents. For heavier soils, viz. Canning-A and Canning-B, the agreements for both models with the experimental data were relatively less satisfactory at 15-cm depth and beyond. This may be due to errors in (i) experimental observations (by neutron counts at 10-cm depth intervals) at lower moisture regimes, i.e., at the sub-surface depths, due to the influence of the “sphere of influence,” which is larger at the lower moisture regime and (ii) theoretical outputs in the case of the heavier soils as a result of neglecting the swelling nature of the soil and hysteresis of soil moisture for estimation of the hydraulic conductivity as the input parameter. The theoretical model proposed by the authors thus behaved similarly to that of Brandt et al. (1971). In view of the simple mathematics and much less computer time involved compared with Brandt et al. (1971), our model should be more useful in computing moisture flux for the entire saturated-unsaturated moisture regime.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Solid solutions are homogeneous or regular mixtures of two or more substances in a solid. Examples are glasses, olivine (Mg,Fe)2SiO4, kaolinite (Al(OH)3·AlOOH·2SiO2or Al2Si2O5(OH)4), coprecipitates, isomorphous substitutes, and exchangeable cations. The chemical activity and aqueous solubility of a solid component decrease with its homogeneously- or regularly-mixed concentration in the solid. The decreased solid activity is responsible for the low equilibrium solubility of soil components and for ion retention by soils. This mechanism is also implicitly stated by cation exchange and adsorption equations.Solid solutions are likely to be nonideal mixtures because of the close proximity of ions in solids. The degree of nonideality is expressed by the activity coefficient. Several assumptions permit calculating individual solid ion activity coefficients, which range from 1 to 10 and average about 5, for Al and transition metal ions in soils. Iron(III), carbonate, sulfate, and phosphate ions have higher activity coefficients, representing their tendency to form separate phases (heterogeneous or mechanical mixing) rather than solid solutions. Ion interactions with O2−and OH−ligands on soil surfaces, plus the decrease in chemical activity due to homogeneous mixing of the ions with other ions on soil surfaces, i.e., the formation of solid solutions, can account for the strong retention of so many ions by soils.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Potassium-Ca exchange reactions in soils are often investigated in laboratory-type studies. It is assumed that results from such binary exchange systems can be extrapolated to three-ion (ternary) systems by using appropriate equations. However, such an evaluation is rarely carried out. This study investigated the influence of added ammonium (NH4) on potassium quantity-intensity (Q/I) relationships representing adsorption and desorption by soil samples taken at the 0 to 15-cm and 15 to 30-cm depths for three soils obtained from northern Kentucky. The soils were Eden silty clay (fine, mixed, mesic Typic Hapludalfs), Lowell silty clay loam (fine, mixed, mesic Typic Hapludalfs), and Nicholson silt loam (fine, mixed, mesic Typic Fragiudalfs). These soils have each failed to show K-fertilization response. It was shown that the Q/I plot components, labile K (ExK0), activity ratio of K at equilibrium (ARK0) and linear potential buffering capacity of K (PBCK) were each affected by added NH4. The addition of NH4caused an increase in the quantity of labile K for all three soils at both depths. ARK0was also higher in the presence of NH4, but the PBCKwas lower. The amount of K extracted with 0.1 mol L−1BaCl2was closer to the amount of K estimated from the y-intercept of the Q/I plot of the apparent high-affinity sites than to the amount of K extracted with neutral NH4C2H3O2. Finally, K adsorption was not readily reversible. The quantity of K desorbed was higher in the presence of added NH4than in its absence. The availability of K in these soils should be enhanced by the presence of ammonium in the soil solution.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Soil pore size distribution (PSD) may explain some of the variability in atrazine movement through soil. PSD and atrazine movement were correlated in an in situ study (field applied atrazine) and an in vitro study (laboratory applied atrazine). Undisturbed surface soil cores were taken from sprinkler-irrigated field plots on Crete silt loam (Pachic Argiustoll) under till and no-till corn (Zea maysL.) production. Soil cores were removed in June, July, August, and October 1989 for the in situ study and in November 1989 for the in vitro study. Water release data were collected from undisturbed cores using pressure plate apparatus, and the PSD index, Λ, was calculated. Atrazine concentrations were determined in the subsurface soil at 10 to 25-cm depth for the in situ study and in the water-release effluent for the in vitro study. Relative atrazine concentrations in the in situ July and August sub-surface samples were negatively correlated with Λ for June and July cores, respectively. For the in vitro study, the relative mass of atrazine in the water-release effluent was positively correlated with Λ. Different matric potentials, wetting and drying cycles, and amounts of applied water may explain differences between the in situ and the in vitro studies.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The eutrophic state of adjacent Lake Okeechobee and recently reported increases of phosphorus levels in Everglades National Park have led to efforts to reduce the phosphorus content of agricultural drainage originating in the Everglades Agricultural Area. In order to better understand the movement of phosphorus from agricultural fields to surface water, phosphorus sorption by Histosols was studied by measuring the amount of inorganic phosphorus removed from solution by known amounts of soil (quantity/intensity experiments) for Histosols representing a variety of cultural histories and soil classifications. The experiments were summarized with phosphorus sorption indices and by fitting to sorption models. A sorption index formed by the ratio of the amount phosphorus sorbed and the log of the equilibrium P concentration for a solution initially having 100 mg L−1added phosphorus varied widely even among Histosols with the same classification and similar cultural histories. Among 18 Histosols typical of the Everglades Agricultural Area, the phosphorus sorption index ranged from 37 to 210; the amount of P sorbed at 10 mg L−1equilibrium P, after adjusting for initial labile P, ranged from 20 to 1800 mg kg−1; and the slope of QI plots at 10 mg L−1P concentration ranged from 6 to 840 L kg−1. There was a significant correlation among phosphorus sorption index, ash, pH, total Ca, and free carbonates. When two or three parameter linear models were considered, oxalate extractable Fe and Al, and total Fe became important.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

This paper reports on the formation of soils with salic horizons in some arid regions of Jordan. Quaternary alluvial and eolian sediments were deposited during different episodes, which reflected the influence of different climates. The studied soils were influenced by either eolian or alluvial activities or both. The properties of these soils still resemble the properties of the weathered parent rocks, but display the influence of weak in situ weathering and minimum mixing that occurred before burial beneath other sediments. The presence of salic horizons was observed at different soil depths in some of these soils. The formation of the salic horizon depended on several factors, such as the age of the sediments, stability of the soil surface, climatic changes, type of parent material, and relative position of the zone of the maximum gypsum accumulation in the soil. Soil properties and prevailing climatic conditions suggest that saturation is not attainable in this area. Thus, it is proposed that saturation condition be waived as a requirement for the recognition of the salic horizon in Aridisols. Acceptance of this proposal will permit the classification of some Torripsamments, Gypsiorthids, or Camborthids as Salorthids. Gypsic subgroup is proposed to classify Salorthids with Gypsic horizon. According to the ICOMID proposals, Salorthids and Gypsic Haplosalids are classified as Haplosalids and Gypsic Haplosalids, respectively. A Camble Haplosalids subgroup is proposed to classify Haplosalids with Cambic horizon.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Previous studies suggest that shifts in plant community structure may alter soil nutrient concentrations and availability. A study was conducted to determine the influence of species composition in loblolly pine (Pinus taedaL.) plant communities on surface soil nitrogen (N) and carbon (C) concentrations and potential mineralization after 7 years of growth and development. The study site is located in the Lower Coastal Plain physiographic region near Tallassee, AL. The soil is classified as a Cowarts loamy sand (Fine-loamy, siliceous, thermic Typic Kanhapludults). Utilizing herbicides, distinct plant communities were established in 1984 including herbaceous-hardwood-pine (HHwP), herbaceous-pine (HP), hardwood-pine (HwP) and pine only (P). Organic C and N and potential C and N mineralization were determined on surface soil samples (0 to 5-, 5 to 10-, and 10 to 20-cm depth increments) collected in February 1991. After 7 years, soil organic N differed among the plant communities; soil organic N was lower in the P than in communities including herbaceous species (HHwP and HP). These differences in soil organic N resulted from a net gain under HHwP and HP, rather than net loss under the P community. Gains in soil N may have been due to symbiotic N fixation. In laboratory incubations, respiration and N mineralization were lower in soil from the P than the HHwP and HP communities. Substrate quality as indicated by forest floor C/N and as manifested by relative N mineralization and C/N mineralized were also reduced in the P community compared with others. Our results suggest that silvicultural manipulation of loblolly pine communities may affect soil C and N and, in turn, could influence long-term site quality.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

A procedure was developed for rapidly drying organic soil horizons in a microwave oven. Dry weights consistent with values obtained by conventional drying methods were obtained by drying these materials for 20 to 30 minutes at Medium (490 W) power.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID