ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

A theoretical method has been proposed for the construction of phase diagrams for silicates in contact with water. If ΔGcdenotes the free energy change due to the crystallization of a mineral from a solution of a defined composition, then it is envisaged that different mineral phases have defined regions of stability which are determined by the relative values of ΔGc's for all the phases that can exist. The phase diagram is, thus, constructed by computation of the limiting conditions for the ΔGcof each mineral species to attain the most negative value. By utilizing this method, it is possible to distinguish between stable, metastable, and unstable phases. It is concluded that all nonstoichiometric clay minerals are metastable with respect to their end-members.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

A computer model for simulating the kinetics of dissolution of CaCO3, CARKIN-1, is presented and partially validated with experimental data. Carbonate reactions in the aqueous phase are described in the model by chemical equilibria and equations defining ionic strength, ionic activity, carbon mass balance, charge balance, and calcium mass balance. Physico-chemical processes at the interface between the gaseous and liquid phases and the interface between the liquid and solid phases are described in the model by time-dependent reactions. The rate of CaCO3dissolution is defined by a partial differential equation similar to the Reddy-Nancollas model for precipitation. The rate of exchange of CO2gas at the gas-liquid interface is defined by a second partial differential equation that incorporates Fick's law of diffusion, Henry's law, and the rate of change of total aqueous carbon.Data to validate the model are from a “free-drift” laboratory dissolution study with reagent grade powdered CaCO3and silt-sized Iceland spar at undersaturated to saturated equilibrium concentration levels and several ranges of CO2pressure at 25°C. CARKIN-1 demonstrates the potential to simulate dynamic changes in pH and Ca ion activity. Sensitivity of several model parameters is illustrated. The surface area of CaCO3, which is not currently considered explicitly in the model, is clearly an important variable.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Forage production in the Appalachian Region is limited by soil acidity and nutrient deficiencies including phosphorus. North Carolina phosphate rock (PR), a carbonate substituted fluorapatite, should be reactive in these soils and may be appropriate as a low cost P fertilizer. An experiment was conducted to evaluate the extent of PR dissolution, extractability of P after dissolution, and soil factors controlling dissolution of PR and subsequent levels of extractable P in 32 acidic soil horizons. PR was mixed with each soil at a rate of 500 mg P/kg and incubated at a moisture content corresponding to 33 kPa tension. In an initial set of 15 soil horizons, samples were taken at 0.1, 3, 7, 14, 30, and 60 days and analyzed for PR dissolution by the NaOH (ΔP) and BaCl2-TEA (ΔCa) methods and for extractable P by the Bray and Olsen methods. A second set of 17 soil horizons was incubated for 30 days and analyzed for extent of PR dissolution and extractable P. North Carolina PR exhibited substantial solubility (1.3 to 71.5% after 30 days of incubation) in a majority of the soil horizons. Soil factors controlling the H+supplying power of the soil (pH, pH-buffer capacity) and the concentrations of P and Ca in soil solution (exchangeable Al, P-sorption index, P-retention, soil moisture, organic C, and exchangeable Ca) were significantly correlated with extent of PR dissolution. Multiple regression equations indicated that no single soil property was responsible for promoting the dissolution of PR over a wide range of soils. The levels of extractable P in the soil were significantly related (P< 0.01) to the extent of PR dissolution but were modified by the characteristics of the individual soils. Soils with the greatest extent of PR dissolution did not necessarily have the highest levels of extractable P. Many of the soil properties controlling dissolution of PR were also significantly correlated with extractable P. Organic C apparently played an important role in maintaining the availability of P after PR dissolution. Results from this study suggest that North Carolina PR should have sufficient solubility and subsequent availability after dissolution to serve as a direct application P source in many pasture areas in the Appalachian Region.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

We designed a two piece, constant head stalagmometer consisting of a capillary tube of 10-cm length connected with a 10-ml mariotte burrette. It measures volume of 10 drops of given and reference liquid to estimate surface tension from the drop-volume method. This paper reports the results of the laboratory tests in water (reference liquid), acetic acid, benzene, and ethyl alcohol and its mixture with water at 5, 10, 20, 40, and 60 per cent concentration for different numbers of drops. Fifteen stalagmometer runs were conducted to compare drop weight method with drop volume method at 50 drops for designed constant head (DCH) stalagmometer. Error percentage (E%) and coefficient of variation (CV) were higher for the drop weight method than the volume weight method in the DCH stalagmometer. Mean surface tensions obtained with this stalagmometer are not significantly different from those with a conventional variable head (CVH) stalagmometer for 50 drops for all liquids. Measurements with a DCH stalagmometer are more reproducible and accurate. The error percentage and coefficient of variation in surface tension remained <1.7 and <1.0 per cent, respectively. The corresponding values of the CVH stalagmometer are <6.6 and <3.0 per cent, respectively. The DCH stalagmometer does not present liquid-filling problems. The time required to carry out a single run is reduced from 384 to 21 seconds. Its drop-falling rate can be adjusted by adjusting the height of the bubble tube, making it usable for all types of liquids. Its liquid requirement is low (1 ml/run) and, therefore, it is more practical, especially for soil extracts whose supply is often limited. Error percentage remains more or less constant in the DCH stalagmometer with increasing numbers of drops, whereas it decreases sharply in the CVH stalagmometer. The maximum error percentage for 10 drops among all liquids was 5 per cent in the DCH stalagmometer. The corresponding value for CVH stalagmometer was 45 per cent.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Cation exchange processes in hard pans of paddy fields were studied. Ca2+and Sr2+transport in them were simulated assuming the equilibrium between the concentrations of Ca2+and Sr2+in solution and the amounts adsorbed in soil during the cation exchange process. Cation transport in four kinds of hard pans having vertical tubular pores made by rice roots was explained by the coaxial cylindrical model. Cation transport in the hard pan of the sand was approximated by the one-dimensional convective dispersive equation including ion exchange. The simulation was performed using the measured values and parameters which have been given in the calculation of nonsorbed Br-. The results explained the measured breakthrough curves well.The amounts of adsorbed Ca2+obtained by the batch experiments (well disturbed soils) and the breakthrough experiments (undisturbed soils) were compared. The results showed that cations could exchange well even in the compact hard pans, provided that the cations are supplied sufficiently and sufficient time is expended for the experiment.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

We have performed series of batch experiments at different temperatures and times to study the kinetics of FeEDDHA reaction in a calcareous soil. Several kinetic equations (zero-order, first-order, two constant, parabolic, and Elovich equations) were applied to obtain the correspondent rate constants. First-order, two-constant, parabolic, and Elovich equations described better FeEDDHA behavior. In all cases high activation energies (between 38.5 and 61.8 kJ/mol) were observed, indicating sorption to be a nondiffusional process. Calculated value of ΔH° was 58.7 kJ/mol and ΔS° was positive (171.7 J/mol × K). Our calculations indicate that the reaction of FeEDDHA is a chemically controlled sorption on the external surfaces of soil particles accompanied by simultaneous desorption of other molecules from the soil.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Soil enzymes mediate biochemical transformations involving organic residue decomposition and nutrient cycling in soil. A field study was conducted to determine the activity and persistence of soil enzymes with repeated additions of different organic residues. The activities of 10 soil enzymes involved in carbon, nitrogen, phosphorus, and sulfur cycling were assayed in an Arlington coarse-loamy soil which had received 100 Mg ha-1of either poultry manure, sewage sludge, barley straw, (Hordeum vulgare) or fresh alfalfa (Medicago sativa) over a 31-month time period (25 Mg ha-1× 4 additions). The enzyme activity assayed in the amended soil was increased by an average of 2− to 4-fold by incorporation of the four organic amendments when compared with the unamended soil (tillage alone) during the 31-month study. In general, the addition of organic amendments greatly increased the enzyme activity during the first year of the experiment, but subsequent additions failed to sustain high enzyme activity in the amended soil. The straw amendment was the most effective amendment enhancing the soil enzyme activity for all the enzymes assayed except for urease. In contrast to other reported studies, soil enzyme activities were not inhibited by sewage sludge addition in this investigation. The increased level of enzyme activity in the organic-amended soil may be a reflection of the increased protective sites within the soil as a result of enhanced humus content.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The Castro County, Texas study area has 666 playa lakes which serve as catchment basins for surface water runoff. These ephemeral lakes are restricted to depressional areas in which the soils have been classified as the Randall series (fine, montmorillonitic, thermic Udic Pellustert). Castro County has two draws, Running Water Draw and Frio Draw, that transect the county while two draws, Middle Tule Draw and the North Fork of Running Water Draw, protrude into the county. The study area was characterized with respect to size, depth, elevation, and distribution of its playa lakes. Playa lakes were additionally characterized as to their linear orientation within 1, 5, and 10 km of the draws. Mean playa lake size was approximately 17 ha with a watershed area of 382 ha per playa lake. For Castro County as a whole, the strongest linear spatial distribution was along an axis of approximately west northwest through east southeast.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The objective of this study was to evaluate alternative methods of estimating the postsettlement soil erosion from hillslopes in a small watershed in Bureau County, northwestern Illinois. Methods used include: (i) estimation of the soil erosion using total organic C balance or enrichment ratio, (ii) selected soil property comparisons between the cultivated, sloping, and eroded soils of the watershed and uncultivated, sloping, and uneroded reference soils of the same taxa occurring in the area cemeteries, and (iii) Universal Soil Loss Equation (USLE). Previously measured sediment remaining in the sediment basin and calculated sediment delivery were used as the bases for evaluating the effectiveness of proposed methods to estimate soil erosion from the surrounding hill-slopes. Differences in thickness of the A horizon consistently underestimated soil erosion by 64% relative to the estimates made by the total sediment remaining in basin and delivered to the stream. This suggests erosion classes which are based on erosion of topsoil underestimate the extent of erosion from both the topsoil and subsoil, particularly where soils have been severely truncated. Organic C redistribution was highly correlated to sediment delivery method and appear to be related directly to the quantity of soil eroded and could provide an alternative to A horizon thickness differences as a basis for quantifying soil erosion. The soil erosion estimated by the USLE was 80% of the soil erosion estimated by the sediment delivery method. This difference can be accounted for by the fact that the sediment delivery method assumed all the sediment came from the sideslopes and USLE is not designed to include erosion resulting from concentrated overland flow (gully erosion). The comparison between cultivated and uncultivated sites of the depth to carbonates and the lowest part of the Bt horizon containing 27% clay between eroded and uneroded phases of Tama soils each produced an estimate of soil erosion within 17% of the soil erosion estimated by sediment delivery method and could be used to estimate soil erosion.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID