scispace - formally typeset
Search or ask a question

Showing papers in "Soil Science Society of America Journal in 1980"


Journal ArticleDOI
TL;DR: Van Genuchten et al. as mentioned in this paper proposed a closed-form analytical expression for predicting the hydraulic conductivity of unsaturated soils based on the Mualem theory, which can be used to predict the unsaturated hydraulic flow and mass transport in unsaturated zone.
Abstract: A new and relatively simple equation for the soil-water content-pressure head curve, 8(h), is described in this paper. The particular form of the equation enables one to derive closedform analytical expressions for the relative hydraulic conductivity, Kr, when substituted in the predictive conductivity models of N.T. Burdine or Y. Mualem. The resulting expressions for Kr(h) contain three independent parameters which may be obtained by fitting the proposed soil-water retention model to experimental data. Results obtained with the closed-form analytical expressions based on the Mualem theory are compared with observed hydraulic conductivity data for five soils with a wide range of hydraulic properties. The unsaturated hydraulic conductivity is predicted well in four out of five cases. It is found that a reasonable description of the soil-water retention curve at low water contents is important for an accurate prediction of the unsaturated hydraulic conductivity. Additional Index Words: soil-water diffusivity, soil-water retention curve. van Genuchten, M. Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892-898. T USE OF NUMERICAL MODELS for simulating fluid flow and mass transport in the unsaturated zone has become increasingly popular the last few years. Recent literature indeed demonstrates that much effort is put into the development of such models (Reeves and Duguid, 1975; Segol, 1976; Vauclin et al., 1979). Unfortunately, it appears that the ability to fully characterize the simulated system has not kept pace with the numerical and modeling expertise. Probably the single most important factor limiting the successful application of unsaturated flow theory to actual field problems is the lack of information regarding the parameters entering the governing transfer equations. Reliable estimates of the unsaturated hydraulic conductivity are especially difficult to obtain, partly because of its extensive variability in the field, and partly because measuring this parameter is time-consuming and expensive. Several investigators have, for these reasons, used models for calculating the unsaturated conductivity from the more easily measured soil-water retention curve. Very popular among these models has been the Millington-Quirk method (Millington and Quirk, 1961), various forms of which have been applied with some success in a number of studies (cf. Jackson et al., 1965; Jackson, 1972; Green and Corey, 1971; Bruce, 1972). Unfortunately, this method has the disadvantage of producing tabular results which, for example when applied to nonhomogeneous soils in multidimensional unsaturated flow models, are quite tedious to use. Closed-form analytical expressions for predicting 1 Contribution from the U. S. Salinity Laboratory, AR-SEA, USDA, Riverside, CA 92501. Received 29 June 1979. Approved 19 May I960. 'Soil Scientist, Dep. of Soil and Environmental Sciences, University of California, Riverside, CA 92521. The author is located at the U. S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92502. the unsaturated hydraulic conductivity have also been developed. For example, Brooks and Corey (1964) and Jeppson (1974) each used an analytical expression for the conductivity based on the Burdine theory (Burdine, 1953). Brooks and Corey (1964, 1966) obtained fairly accurate predictions with their equations, even though a discontinuity is present in the slope of both the soil-water retention curve and the unsaturated hydraulic conductivity curve at some negative value of the pressure head (this point is often referred to as the bubbling pressure). Such a discontinuity sometimes prevents rapid convergence in numerical saturated-unsaturated flow problems. It also appears that predictions based on the Brooks and Corey equations are somewhat less accurate than those obtained with various forms of the (modified) Millington-Quirk method. Recently Mualem (1976a) derived a new model for predicting the hydraulic conductivity from knowledge of the soil-water retention curve and the conductivity at saturation. Mualem's derivation leads to a simple integral formula for the unsaturated hydraulic conductivity which enables one to derive closed-form analytical expressions, provided suitable equations for the soil-water retention curves are available. It is the purpose of this paper to derive such expressions using an equation for the soil-water retention curve which is both continuous and has a continuous slope. The resulting conductivity models generally contain three independent parameters which may be obtained by matching the proposed soil-water retention curve to experimental data. Results obtained with the closedform equations based on the Mualem theory will be compared with observed data for a few soils having widely varying hydraulic properties. THEORETICAL Equations Based on Mualem's Model The following equation was derived by Mualem (1976a) for predicting the relative hydraulic conductivity (Kr) from knowledge of the soil-water retention curve

22,781 citations




Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate that 0.14M sodium pyrophosphate at pH 7.1 extracts phosphatase, urease, casein, and benzoylarginamide hydrolyzing proteases.
Abstract: Extractions of three soils at different incubation times demonstrate that 0.14M sodium pyrophosphate at pH 7.1 extracts phosphatase, urease, casein, and benzoylarginamide hydrolyzing proteases. Extraction yields of phosphatase and casein-hydrolyzing proteases are remarkably high. The most efficient length of extraction depends both on the type of soil and on the selected enzyme. Correlation analyses of extracted enzymes and extracted C and organic N content generally show a significant correlation (p = 0.05 and 0.01).

436 citations








Journal ArticleDOI
TL;DR: In this paper, a 14-day anaerobic incubation at −0°C was evaluated as an index of forest soil fertility and site productivity, and the results showed that the relative variability in mineralizable soil N decreases with soil depth and increases with the magnitude of the soil test mean.
Abstract: Soil N mineralized during 14-day anaerobic incubation at ll0°C is evaluated as an index of forest soil fertility and site pro­ ductivity. Mineralizable soil N determined under standard con­ ditions correlates significantly with N mineralized anaerobically for 6 months in the field, with site index and yield potential of Pinus ponderosa L. growing on volcanic, metavolcanic, and metasedimentary soils, and with foliar concentrations of N. Soils testing less than 12 ppm of mineralizable N are judged clearly deficient, but stands of pine and fir on soils testing as high as 16 ppm of N still may respond well to fertilization. Absolute variability in mineralizable soil N decreases with soil depth and increases with the magnitude of the soil test mean, but relative variability is' constant. Comparing ammonium pro­ duced during laboratory incubation with that mineralized over 6 months in the field shows that field soil temperature has a strong bearing on the interpretation of the laboratory test. Additional Index Words: soil testing, soil variability, site productivity, foliar analysis, ponderosa pine, fertilization.

Journal ArticleDOI
TL;DR: In this article, the morphological properties of iron coatings on rice roots at different growth stages were analyzed using scanning electron microscopy and X-ray microanalysis, and it was found that the outermost cell wall of the root decomposed, a mixture of FeOOH and soil particles began to fill the rectangular spaces that had once been occupied by epidermal cells.
Abstract: Scanning electron microscopy and X-ray microanalysis were employed to characterize the morphological properties of iron coatings on rice roots at different growth stages. This information is needed for further understanding of the influence of Fe coatings on rice plant development. Rice root coatings are visible about 1 week after flooding as a brownish discoloration which thickens with age of the root. No coating was found on younger parts of major roots near their tips or on young secondary roots which are critical regions of nutrient uptake. Roots of ‘Brazos’ cultivar rice (Oryza sativa L.) plants grown in Beaumont clay soil had a relatively thin coating of FeOOH mixed with soil particles before panicle differentiation. As a rice plant approached maturity and the outermost cell wall of the root decomposed, a mixture of FeOOH and soil particles began to fill the rectangular spaces that had once been occupied by epidermal cells. Casts in open cell cavities are porous and rough on the exterior side of the root. There were many shapes of casts and they generally matched the varied shapes of the outer layer of epidermal cells of rice roots. Roots of Brazos cultivar rice grown to maturity in Katy fine sandy loam soil have completely developed polyhedral casts. Precipitation of relatively pure FeOOH on cell walls formed hollow casts with the shapes of the original cells. The models presented describe hypothetical steps in the formation of the two types of casts observed by the oxidation of Fe²⁺ by O₂ and precipitating of iron on the walls of closed and open cell cavities.




Journal ArticleDOI
TL;DR: In this article, the authors compared the hydraulic conductivity of three fallow field soils covered with plastic to prevent evaporation and sampled to depths of nearly 2 m. They concluded that a greater number of observations made possible by simplifying assumptions, and hence, less instrumentation and cost, is preferable to fewer observations with more exact methods that are not as amenable to statistical analyses over larger land areas.
Abstract: Soil water content profiles monitored as a function of time following steady-state infiltration conditions in a field soil provide the only data necessary to estimate the soil hydraulic conductivity as a function of soil water content. For two proposed methods, it is assumed that water redistributes only in response to the gravitational field and that the hydraulic conductivity is an exponential function of soil water content. Experiments were conducted on three fallow field soils covered with plastic to prevent evaporation and sampled to depths of nearly 2 m. Values of hydraulic conductivity calculated by both the two proposed methods as well as by a third recently reported method were compared with those calculated by integrating Richards' equation without simplifying assumptions. Although the above comparisons at given sites within a field revealed that the three methods may sometimes yield poor estimates of the hydraulic conductivity, distributions of observed values averaged over the entire field were statistically comparable for 20 observations. It is concluded that a greater number of observations made possible by simplifying assumptions, and hence, less instrumentation and cost, is preferable to fewer observations with more exact methods that are not as amenable to statistical analyses over larger land areas. Additional Index Words: soil water redistribution, spatial variability, soil water content. View complete article To view this complete article, insert Disc 4 then click button8

Journal ArticleDOI
TL;DR: In this paper, the adsorption and precipitation of cadmium ions in the presence of calcium carbonate was studied in an effort to understand the processes of Cd/sup 2 +/ retention in calcareous soils.
Abstract: The adsorption and precipitation of cadmium ions in the presence of calcium carbonate was studied in an effort to understand the processes of Cd/sup 2 +/ retention in calcareous soils. Measurement of cadmium removal from solution by pure CaCO/sub 3/ using a selective ion electrode indicated that the quantity of cadmium adsorbed depended upon the CaCO/sub 3/ particle size. High levels of added cadmium apparently caused nucleation of CdCO/sub 3/ on CaCO/sub 3/ as the solutions approached saturation with respect to CdCO/sub 3/. The initial chemisorption of Cd/sup 2 +/ on calcium carbonate was very rapid, while CdCO/sub 3/ precipitation at higher cadmium concentrations was slow. Chemisorption may control Cd/sup 2 +/ activity in some calcareous soils, producing solubilities much lower than predicted by the solubility product of CdCO/sub 3/. The surface area of soil carbonates is likely to be important in determining the levels of cadmium that can be immobilized by chemisorption. The affinity of calcite for metal ions appears to be a function of ionic radius, with divalent metal ions of the same size as Ca/sup 2 +/ readily substitution for Ca/sup 2 +/ in accessible surface sites.

Journal ArticleDOI
TL;DR: Several factors examined in this paper which control opal dissolution are: specific surface area, Al content, hydration state, age, and rate of organic matter biodegradation of the encasing vegetative tissues.
Abstract: Several factors examined herein which control opal dissolution are: specific surface area, Al content, hydration state, age, and rate of organic matter biodegradation of the encasing vegetative tissues. These factors are covariable with opal of different origin. Recent (6 months old) opal phytoliths of deciduous origin are most hydrated (11%), have lower A1 content (2%) and highest dissolution (9 mg Si/liter, cold water; 50 mg Si/liter, hot water; and 3 mg Si/liter under natural environments). In contrast, opal of coniferous origin is older (30 months), more rigid, has higher Al content (3 to 4%), is encased within litter that is more slowly biodegradable and yields lower dissolution (2 to 3 mg Si/liter, cold water; 20 mg Si/liter, hot water; and 0.5 mg Si/liter under natural environments). Gramineous phytoliths associated with understory forest vegetation generally are intermediate in the above properties and dissolution. Biogenic opal has solubilities that approximate geologic opal-A. It is relatively stable and not sufficiently labile under most soil environments to support observed soluble Si levels.







Journal ArticleDOI
TL;DR: In a follow-up paper as discussed by the authors, the same authors pointed out that their reference to the Olsen and Watanabe (1957) procedure was misleading since their procedure did correct for surface adsorbed P by exchange.
Abstract: I. C. R. Holford has suggested that the Langmiur adsorption isotherm data of Green et al. (1978) and McCallister and Logan (1978) is misinterpreted because no correction was made for native adsorbed phosphate. Holford suggests that what was measured was residual adsorption capacity and that the \"checkmark\" phenomenon found in our Langmuir isotherms is eliminated when correction is made for native adsorbed P. It was our intention to measure only residual adsorption capacity, and therefore, our reference to the Olsen and Watanabe (1957) procedure was misleading since their procedure did correct for surface adsorbed P by \"P exchange. We were using the Langmuir isotherm to examine the relative sorption of P by soils and sediments, a process determined by the sorption characteristics of the soil or sediment, but also by the degree to which the adsorption capacity had already been saturated. I would concur with Holford that the term residual adsorption capacity be used in instances where no correction is made for \"native\", or more correctly, \"previously\" adsorbed P. Holford has also suggested that, if Fig. 1 of McCallister and Logan (1978) is corrected by adding 50 /ig P/g (Holford's estimate of native adsorbed P, and one he considers conservative), then the data is better fit to a two-phase Langmuir model than to a single line. Using the 50 /ig P/g estimate which we feel should be closer to 25 /ig P/g (based on desorption and Bray PI extractable P data for the Hoytville soil in Fig. 1 of McCallister and Logan), we find that either a singleor two-phase model is satisfactory in describing the data. The single-phase model has an r* of 0.976, while the r values for the two-phase Langmuir isotherms were 0.978 and 0.987. The two-phase model has greatest utility at low equilibrium phosphate concentrations, but at this range, the native adsorbed P has its most significant effect on net adsorption. Holford has correctly recognized the failure of Green et al. (1978) and McCallister and Logan (1978) to correct Langmuir adsorption data for native adsorbed P. We contend that our omission was not in clearly indicating our intention to measure residual adsorption capacity, and we laud the effort of Holford in bringing this to the readers' attention.

Journal ArticleDOI
TL;DR: In soils with saturation extracts (ECe) of 1, 3, 4, 6, and 12 mmho/cm, mycorrhizal plants consistently outweighed the controls and plants inoculated with Gl.
Abstract: The effect of 5 levels of salinity on the growth of onions (Allium cepa L. cv. ‘Early Yellow Globe’) and bell peppers (Capsicum annuum L. cv. ‘California Wonder’) inoculated with the mycorrhizal fungi, Glomus fasciculatus or Gigaspora margarita, was compared to uninoculated nonmycorrhizal controls. In soils with saturation extracts (ECe) of 1, 3, 4, 6, and 12 mmho/cm, mycorrhizal plants consistently outweighed the controls. Plants inoculated with Gl. fasciculatus, also, grew larger than those inoculated with Gi. margarita. In both onion and bell pepper, the effect of salinity on decreasing percent fresh weight was greatest in the nonmycorrhizal controls. Also, in bell pepper, decreases in percent fresh weights were less with plants infected by Gl. fasciculatus than with those infected by Gi. margarita. Thus, plants infected with mycorrhizal fungi may be more salt tolerant than uninfected nonmycorrhizal plants.