摘要:

Hydraulic properties of unsaturated soil profiles are needed for quantitative analysis of water movement. A recently proposed method of estimating some of these properties from water content profiles after drainage was examined for a uniform soil satisfying certain constraints that allowed a new exact analytical solution to be obtained. A new analytical and numerical examination of the method, assuming correct property functional forms and boundary conditions, established that it can give good estimates of unsaturated hydraulic conductivities. If a matric potential value is available at one depth in the profile, matric potentials at other depths can also be estimated accurately. The method seemed to work reasonably well even when assumed property functions and boundary conditions had an incorrect form, although hydraulic conductivities were then in error up to 25 percent. This seemingly large error reflects the difficulty in measuring a highly variable dynamic property, like conductivity, in soils.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Analytical solutions to Richards' equation are valuable in testing numerical models of water movement in soils, but few such solutions are available, especially for general soil hydraulic properties. We present such a solution, valid for restricted boundary conditions during infiltration and drainage, and illustrate its use by comparing numerical and analytic solutions. In the case of infiltration the solution is based on an earlier result which is extended here to describe drainage from soil with a uniform initial water content. Accurate numerical solutions could be obtained with a coarse spatial grid of eleven points.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Six different hysteresis models for the moisture retention characteristic (MRC) are described and their performance evaluated using laboratory measurements and literature data. The choice of these models is based on reported accuracy in literature and their limited data requirement for calibration. Several wetting and drying curves were measured for two sand samples in three replicates. In addition, eight data sets from literature were included in the analysis. Three statistical criteria and three statistical hypotheses were used to evaluate the models' performance. This study confirms that neglecting hysteresis may result in significant errors when describing wetting and drying data. Models using one branch of the MRC for calibration purposes have an inferior performance relative to those employing two branches. The best models were the conceptual models needing two branches for calibration. Using more than the main loop for calibration did not significantly increase the accuracy of the best performing models in describing the data. It was found that only uneven order wetting and even order drying scanning curves need to be considered.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Thermodynamics of potassium exchange were compared using equilibrium and kinetic approaches in Ca-saturated samples of a Tulewal loamy sand soil profile from Punjab. The classical Argersinger theory was employed for the equilibrium approach, while Arrhenius and Von't Hoff theories were used for the kinetic approach.The curves of Vanselow selectivity coefficients (kv) against equivalent fractions of K-adsorbed (XK), Inkv vs. XK, showed perference for K over Ca. The values of kv varied for each corresponding value of XK, which suggests non-ideal behavior of the exchanger phase. The isotherms of equivalent fraction of K on the exchanger phase (qK/qO, where qK is the amount of K adsorbed in equivalents per unit weight of the soil and qO is the total amount of K and Ca adsorbed in equivalents per unit weight of the soil) and in the equilibrium solution (cK/cO) also exhibited preference of K over Ca when compared with nonpreferential curves. The values of standard free energy, Δ0, for K+-Ca2+exchange ranged from −3.96 to −3.10 kJmol-1at 298K and from −3.62 to −2.86 kJmol-1at 313K. This suggested the exothermic nature of K+-Ca2+exchange. The negative values (−12.02 to −6.79 kJmol-1) of standard enthalpy change (ΔH0) further confirmed the exothermic nature of K+-Ca2+ exchange.The thermodynamic parameters in perchlorate anionic media were higher than those observed in chloride media, suggesting and increase in the spontaneity of the K+-Ca2+excgabge process in perchlorate media.As with the equilibrium approach, the kinetic approach also exhibited negative values of ΔG0that were higher in ClO4- (−2.32 to 2.71 kJmol-1) anionic background compared with Cl-(−2.21 to −2.60 kJmol-1) as the background anion. These negative values also decreased with an increase in temperature. Although the magnitude of thermodynamic parameters for the two approaches compared poorly, they compared very well in trend and showed similar inference of ion behavior in the soil. It seems reasonable, therefore, to use kinetic approaches to describe adsorption-desorption wherever possible. The kinetic approach has the added advantage that, in addition to the thermodynamic parameters, one can study the mechanisms and rates of exchange in soil and clay minerals.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Organic matter from different soil size fractions (<2 μm; 2–20 μm; 20–50 μm; 50–150 μm, and > 150 μ) was obtained, using a combination of physical separation and light chemical treatment, in an Argentine Mollisol. Quantification of semiquinone free radicals was done using electron spin resonance spectroscopy, taking into account the organic carbon content of each sample. Values obtained for fractions less than 50 μm in size were significantly higher than values for fractions greater than 50μm. Comparison of this data with C/N ratios suggests that there is an association between fractions where more humified organic matter occurs and a higher level of semiquinone is detected. A 2 to 20-μm size fraction was identified as that with the highest humification degree and one which undergoes only minor changes as a result of agricultural practices. Alterations associated with soil compaction, principally in an A12horizon in an area cultivated for 80 years, were accompanied by a reduction in the level of semiquinone free radicals compared with a native grassland area. Otherwise, in an area with only 10 years of soybean monoculture and where soil compaction effects are not yet extensive, a strong increase of free radicals was observed, principally in the Aphorizon. This increase of free radicals is associated with higher oxidative conditions of a relatively new cultivation area and also with probable rapid decomposition of soybean residues and incorporation as humified organic matter.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The adoption of the kandic horizon for use in recognizing the predominance of low activity clay has resulted in reclassification of several major soil series on the Atlantic Coastal Plain of the Southeastern U.S. To provide a more definitive basis for predicting the geographic distribution of the kandic feature, 41 sites correlated to successive geomorphic surfaces in North Carolina were sampled and characterized. The locations represented five defined Coastal Plain surfaces encompassing the Upper, Middle, and Lower Coastal Plain. Well to poorly drained upland soils were included. Results indicate that both the cation exchange capacity 7 and the effective cation exchange capacity requirement for the kandic horizon were consistently satisfied only in the well drained soils on the Upper Coastal Plain surface. Soil with greater wetness and soils on lower, younger geomorphic surfaces (Middle and Lower Coastal Plain) did not consistently satisfy requirements for the kandic horizon.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Field experiments were carried out in three Sahelian agroecosystems (Kala Pate, N'Dounga, and Chikal) to compare the rooting systems and water and nutrient extraction of millet and sorghum under different levels of water, fertilization, and plant density. Initial soil moisture content was so low (<0.04 m3m-3) that crop rooting was limited by the maximum depth of the wetting front. Soil acidity (pH 4.5) and aluminum and manganese toxicities reduced the effective crop rooting depth to less than 45 cm at Kala Pate. Millet utilized water more efficiently under less favorable soil conditions, but in most cases it extracted less nutrients and utilized rainfall less efficiently than sorghum. Results from this study suggest that root penetration of both crops was controlled by the seasonal wetting front and soil pH. These factors were most limiting for efficient nutrient and water use. Inputs of water and fertilizer significantly increased rainfall use efficiency and nutrient uptake by both crops.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

A greenhouse study was conducted to determine the effects of bleached, primary papermill sludge on the yield and elemental concentrations of common bermudagrass (Cyanodon dactylonL.) grown on a mine soil. Pots containing soil treated with lime, fertilizer, and sludge (at the rate of 56, 112, and 224 Mg/ha added alone or in combination with fertilizer) were planted to bermudagrass from April through September 1988, following subterranean clover (Trifolium subterranean, L. W. woogenelux).Fertilizer application was found necessary to revegetate the mine soil. Addition of sludge and fertilizer gave significantly higher yields in two of the last three harvests. Maximum production was obtained in the second harvest with 112 Mg/ha sludge and fertilizer. Addition of N, P, and K resulted in concentrations lower than reported sufficient levels for N and K and similar levels for P in the forage. Generally, high tissue concentrations of Mg, Fe, Zn, Mn, and Cu and low levels of S were observed in all three harvests. Calcium was low in the fertilizer-only treatment. Aluminum decreased as the sludge increased because of its liming ability, and Na increased as the sludge increased as a result of the high levels of exchangeable Na in the sludge. Neither concentration was high enough to cause determintal effects.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID