摘要:

Soil physics literature contains various expressions for the contracting force of menisci between solid soil particles. To find the most suitable expression, we measured the interaction of water and glass when one or more drops of water resided between two parallel glass plates separated by varying distances within a range of a few millimeters. Several equations were used to quantify the contracting force associated with the surface tension of water. To assess the impact of meniscus length and contact angle, we compared the force exerted by the number of individual menisci with the force exerted by the same volume of water unified in a single meniscus. Results showed that the compression of several menisci into a single meniscus reduced the contracting force. It was observed further that the attraction during the lifting phase (receding contact angle) exceeded the corresponding forces during lowering (advancing contact angle). The difference in contracting force caused by increasing meniscus length was of the same order of magnitude as the difference induced by increasing or decreasing the distance of the plates corresponding to receding or advancing contact angles. The pressure predicted, with the classical capillary equation for curved interfaces multiplied with the contact area and a modified equation, including the contact angle at the three phase line of contact, described the measured data better than the equations of either Kezdi or Bernatzik. However, in most cases, the calculated contracting force exceeded the measured values considerably. Nonlinear regression analysis showed that the closest fits were obtained when the advancing angle of about 20°, measured on smooth glass plates, was adjusted to values of about 40 to 90°. This result supports the hyphothesis that either contact angle hysteresis or the meniscus length have a considerable impact on capillary pressure and attractive forces in partly saturated capillaries.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Soil hydraulic conductivity (K)-water content (&thetas;)-pressure head (h) relationships (K-&thetas;-h) are key parameters for crop growth, irrigation, drainage, and modeling water flow and chemical transport through the soil. Several laboratory methods have been used to determine these parameters and to extrapolate to field conditions. However, it is essential to determine these parameters under field conditions to minimize the effect of spatial variability. The objective of this study was to use the Instantaneous Profile method in combination with capacitance probes, tensiometers, the Guelph permeameter, and the van Genuchten hydraulic functions to determine the hydraulic conductivity-water content-pressure head relationships of a Candler fine sand (hyperthermic, uncoated, Typic Quartzipsamments) soil. The soil was flooded for more than 2 h to achieve a constant water content through the profile and was then covered with a plastic sheet to prevent evaporative loss or addition of rain water. The capacitance probe and tensiometer readings were taken simultaneously at 0.1-, 0.2-, 0.4-, 0.7-, and 1.1-m depths. Both water content and hydraulic conductivity values decreased substantially as water redistributed through the soil profile after saturation. Saturated hydraulic conductivity, measured with the Guelph permeameter for the five soil depths, varied between 6.1 and 10.0 m day−1. More than 50% of the soil water was drained from the soil profile within 8 to 10 h following saturation. The analytical RETC (RETension Curve) model was used to extend soil water release curves beyond the limited range of tensiometer suction measurements. Results of this study demonstrate that the Sentek capacitance probe is a practical tool that can be used in combination with the Guelph permeameter and analytical fitting software (RETC) to determine in-situ soil water characteristic curves, saturated hydraulic conductivity, and k(&thetas;) at difference soil depths.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Organic matter can stabilize soil aggregates against disintegration upon wetting. We examined the tensile strengths of two polymer gels that were chosen to represent features of soil organic matter. The mechanical properties of polyacrylamide and gellan gels (gellan is a natural polysaccharide) are remarkably similar despite large differences in molecular structure. The general shape of the stress strain curves for both gels is similar, although the method and mechanism of gelation differs drastically between the gels. The ultimate breaking stresses for both gels are also similar in magnitude (13 to 1100 kPa for polyacrylamide and 9 to 350 kPa for gellan), even though they differ fundamentally in type of gel cross-links and vary drastically in their ultimate breaking strain relations. Breaking strain tends to decrease with increased polymer and BIS concentration for polyacrylamide gels, whereas the relations tended to be more complex for gellan gels. These measurements allow some constraints to be placed on the effects of polymers on soil aggregate stability. For example, a 1-μm-thick layer of polyacrylamide, on a 1 mm-diameter aggregate, can support a range of maximum internal pressures from 0.046 to 2.5 kPa.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

To obtain a better understanding of the enzymatic properties of soils from tropical and subtropical areas polluted with heavy metals, the effect of zinc on the activities and kinetics of acid phosphatase immobilized on variable-charge soil constituents was investigated. Both free and immobilized enzymes were inhibited significantly in the presence of 0.1 to 4 mM Zn (as ZnCl2), and the inhibition increased with an increase of pH from 5.0 to 6.0. The enzymes immobilized on the Latosol clay and kaolin mineral were as sensitive as free (i.e. soluble) enzymes to Zn toxicity. The inhibitory effect of Zn was reduced when acid phosphatase was immobilized on goethite. Various inhibition patterns by Zn were observed for free and immobilized acid phosphatase at different pH values. Zinc showed uncompetitive inhibition for the goethite-enzyme complex at pH 5.0, 5.5 and 6.0. The inhibition of free enzyme and Latosol clay-enzyme complex by Zn changed from an uncompetitive type at pH 5.0 and 5.5 to a mixed type at pH 6.0. However, the inhibition model of the kaolin-enzyme complex was mixed type at the three pH levels. Variation of inhibition model seems to be related to the degree of deactivation of the enzyme by the Zn. Zinc addition also increased the affinity of free and immobilized enzymes for the substrate (i.e. reduced Kmvalues). The high affinity of goethite-enzyme complex for the substrate induced a substrate inhibition at pH 5.0, which was enhanced by the presence of Zn.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

The role of Fe oxides on the availability and dynamics of phosphorus (P) is poorly understood in calcareous soils. We ascertained its role in laboratory and plant growth experiments on substrates prepared by mixing calcium carbonate (calcite) sand (CCS, 0.2-0.5 mm) and Fe oxide (ferrihydrite)-coated quartz sand (FOCS, 0.2-0.5mm). Distribution of P into the solid and liquid phases was studied after one, three, six, and ten wetting-drying cycles (WDC) after adding P as KH2PO4at the rate of 0, 5, 10, 20, 50, and 100 μg P g−1to substrates containing 0, 10, 30, 50, 70, 80, and 90% FOCS. Different P levels, WDC and substrate composition had a significant (P< 0.05) main and interactive effect on P distribution between solution and solid phases. There was a significant (P< 0.05) 1- to 8-fold increase in the distribution ratio of P between solution and solid phases with increasing FOCS in the substrate at 10 and 20 μg g−1P. The increase in water-soluble P with FOCS was sharper at 10 μg P g−1than at 20 μg P g−1. In the plant growth experiment, durum wheat (Triticum durumcv. Don Pedro) was grown in substrates containing 0, 10, 20, 30, 50, and 90% FOCS, which received P as KH2PO4at the rate of 0, 6, 12, 18, and 72 μg P g−1. Substrates containing higher proportion of CCS tested higher for Olsen P for the same rate of P application. The same trend was observed for Olsen P after cropping. Substrate composition and P application rate had a significant (P< 0.05) main and interactive effect on growth and P uptake by plants. Phosphorus-Zn interaction due to increased P supply in the soil-plant system depressed plant growth with increasing P application rate at higher proportion of FOCS in the substrate. Concentration of P in plants increased significantly with increasing proportion of FOCS in the substrate. The relationship between plant P concentration and concentration of P in the substrate solution could be described by a Michaelis-Menten equation. At a certain level of Olsen P, plant P concentration increased with increasing FOCS proportion in the substrate. This suggests that Olsen P might overestimate plant-available P in Fe oxide-poor relative to Fe oxide-rich calcareous soils.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Radium is a potential Ca analog that may interact with Ca and Al in forest ecosystems. Many hectares of forested land in New England occur on soils with naturally high levels of radionuclides-including radium-but little is known about interactions of Ca, Ra, and Al in forest systems. Our objectives were: (i) to compare soil profile distributions of228Ra,226Ra, Ca, Mg, and Al and (ii) to examine correlations between226Ra,228Ra, Ca, Mg, and Al. We sampled 13 Haplorthods to determine activities of226Ra and228Ra, pedogenic radium deficiency, pH, and NH4Cl-extractable Ca, Mg, and Al. Aluminum concentrations were an order of magnitude higher than concentrations of Ca and Mg and several orders of magnitude higher than estimated concentrations of Ra. Profile distributions indicated that226Ra and228Ra deficiencies-and Ca, Mg, and Al concentrations-were greatest in E horizons and decreased with depth. pH increased with depth. Values of Ca, Mg, and226Ra were highly variable in A horizons, and pH values were highly variable in A and E horizons. Several significant (P< 0.05) correlations were identified between extractable cations and radionuclide activities and deficiencies. In A horizons, deficiencies of both Ra isotopes were closely correlated to Mg concentration. In E horizons,226Ra deficiency was significantly and negatively correlated to Al concentration. In spodic (Bhs and Bs) horizons, both Ca and Al were significantly and negatively correlated to226Ra activity. Although Al clearly dominates this acidic environment and controls the chemistry of the soil system, significant interactions occur between known plant nutrients and radionuclides. This study also suggests that Ra, in addition to Ca and Mg, is being cycled by plants.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Magnetic minerals are important in some Brazilian soils and they are a possible source of trace elements, especially in the highly weathered soils. The purpose of this investigation was thus to determine the trace element content in the magnetically separated sand, silt, and clay fractions from soils of different lithological origins in order to test the hypothesis that higher levels of magnetization are directly proportional to the contents of these elements. To accomplish this objective, the samples were dissolved by concentrated HCl/HF/HClO4acid attack, and the contents of Cd, Cr, Cu, Ni, Pb, Zn, Zr, and V were analyzed by atomic emission spectroscopy with inductively coupled plasma (ICP-AES). The contents of trace elements in the total, magnetic and nonmagnetic sand, silt, and clay fractions were within the normal range presented by whole soils with the exception of Cd, which was higher. Magnetization (σs) of the total, magnetic and nonmagnetic soil fractions was evaluated with a special analytical balance. The σsvalues were higher for sand and lower for silt and clay fractions. The magnetically concentrated fractions gave results with variation in the same fashion but reaching higher values. The relationship between trace elements in soil fractions and magnetization, studied by correlation analysis, gave higher and more significant correlation coefficients when a soil derived from itabirite was excluded from the analysis. Because itabirite and mafic rocks have high magnetization, the correlation result demonstrates that there is a strongly significant association between magnetization and trace element contents for the soils derived from mafic rocks. Analysis of variance showed that there were significant differences between trace element contents of the magnetic compared with the nonmagnetic fractions of the soils derived from mafic rocks, especially in their sand and silt fractions. Tukey's test for comparison of means showed that mafic rocks have a higher content of trace elements compared with the other parent materials. The results obtained confirm the geochemical affinity of trace elements and the components of the magnetic fraction of the soils derived from the mafic rocks, mainly for sand and silt fractions, where higher contents of the investigated trace elements were observed.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID