Showing papers in "Soil Science Society of America Journal in 1977"

Information Provided on Humic Substances by E4/E6 Ratios

Abstract: The ratio of optical densities or absorbances of dilute, aqueous humic and fulvic acid solutions at 465 and 665 nm (E₄/E₆) is widely used by soil scientists for the characterization of these materials. While it has been suggested that the E₄/E₆ ratio is related to the degree of condensation of the aromatic carbon network, carbon content, and molecular weight of humic substances, little rigorous experimental evidence is available in the literature to confirm these hypotheses. The results of this investigation show that the E₄/E₆ ratio of humic and fulvic acid is: (i) mainly governed by the particle size (or particle or molecular weight); (ii) affected by pH; (iii) correlated with the free radical concentration, contents of O, C, CO₂H and total acidity in as far as these parameters are also functions of the particle size or particle or molecular weight; (iv) apparently not directly related to the relative concentration of condensed aromatic rings; (v) independent of humic acid and fulvic acid concentrations, at least in the 100–500 ppm range. Our data show, in agreement with M. M. Kononova (1966), that E₄/E₆ ratios for humic and fulvic acids should be determined between pH 7 and 8. This can best be done by dissolving the humic material in 0.05N NaHCO₃ solution at concentrations of 200–400 ppm.

The shear resistance of root-permeated homogeneous and stratified soil.

Abstract: Mechanical reinforcement which stabilizes soil on slopes has been attributed to plant roots To measure such reinforcement, direct shear tests were made on 25-cm diameter root-permeated soil columns Roots of alfalfa (Medicago sativa), barley (Hordeum vulgare), and yellow pine (Pinus ponderosa), each increased the shear resistance of homogeneous and compacted layers of silty clay loam at 30-cm depth One-year-old alfalfa had a much greater reinforcing effect than pine trees 16 months after transplanting or barley at its maximum growth Barley had a greater effect in the clay loam than pine, but its effectiveness decreased as depth increased from 15 to 30 to 45 cm Alfalfa roots were more effective than either pine or barley roots in increasing the resistance to shearing between a dense gravel-sand layer (simulating weathered rock) and the overlying soil, increasing shearing resistance to as much as 5 times that of fallow soil A model is presented of soil reinforced by nonrigid roots Calculations are given of slope safety factor increases from root reinforcement

Mass Transfer Studies in Sorbing Porous Media: II. Experimental Evaluation with Tritium (3H2O)1

Abstract: A comparison is made between observed tritium effluent concentration distributions and those calculated with a previously published analytical solution for the movement of chemicals through unsaturated, aggregated sorbing media. In the analytical model the liquid phase of the soil is divided into mobile and immobile regions, with transfer between the two regions diffusion controlled. Effluent data obtained from several displacements of tritium through 30-cm long columns of Glendale clay loam were used to determine the different parameters in the analytical solution by curve fitting. The data indicate some adsorption or isotopic exchange of tritium during its flow through the soil columns. The amount of immobile water increases with decreasing flow velocity and increasing aggregate size, and varies between 6 and 45% of the total volume of water in the columns. The analytical solution provides an excellent description of the experimental effluent data, and shows that tailing can be explained satisfactorily by diffusional exchange of tritium between mobile and immobile regions of the soil. Additional Index Words: intra-aggregate diffusion, immobile water, miscible displacement I PART i OF THIS STUDY (van Genuchten and Wierenga, 1976), a mathematical model was developed to describe the movement of chemicals through unsaturated, aggregated sorbing porous media. The medium was thereby analyzed by dividing the liquid phase into mobile and immobile regions. Transfer between the two liquid regions, presumably a diffusion process, was assumed to be proportional to the concentration difference between the mobile and immobile liquid phases. Sorption processes in both the stagnant and dynamic regions of the medium were further assumed to be instantaneous. The resulting differential equations were solved analytically assuming a linear equilibrium adsorption isotherm. The experiments to be discussed in this study were carried out to evaluate the model of van Genuchten and Wierenga (1976), and to obtain estimates of the different parameters in the adsorption model. The effects of flow velocity, bulk density, and aggregate size on the amount of immobile water (and hence, on the amount of tailing and asymmetry) were also studied. In a subsequent paper (van Genuchten et al., 1977) the theoretical model will be used to describe the movement of 2,4,5-T (2,4,5-trichlorophenoxy acetic acid) through the same soil columns. MATHEMATICAL DESCRIPTION We refer to the original paper of van Genuchten and Wierenga (1976) for a more extensive discussion of the physical system. The following general system of equations describes the mobile (Cm) 'Journal article no. 565, Agric. Exp. Stn., New Mexico State Univ., Las Cruces, NM 88003. The work upon which this report was based was supported in part by funds obtained from the U.S. Dep. of the Interior, Office of Water Resources, as authorized under the Water Resources Act of 1964. Received 7 July 1975. Approved 9 Nov. 1976. Formerly Graduate Student, Dep. of Agronomy, New Mexico State Univ.; presently Research Associate, Dep. of Civil Engineering, Princeton Univ., Princeton, NJ 08540. 'Associate Professor, Dep. of Agronomy, New Mexico State Univ., Las Graces, NM 88003. and immobile (Cim) concentration distributions in a semi-infinite column, resulting from a pulse input of solute. [ dCml f VmC0 0 lim vmCm D -r— = \\ z—»oL \"£ J t ^ t -

Decomposition of Carbon-14 labeled plant material under tropical conditions

Abstract: Ryegrass (Lolium multiflorum) and maize (Zea mays) tissue uniformly labeled with 14C were mixed with soil and allowed to decompose under field conditions in the open or under shade. The incubations were done in the forest zone of Nigeria, using a range of contrasting Nigerian soils. Of the ryegrass Coriginally added to the soil, 20% remained after 1 year, falling to 14% after 2 years. After 1 year the soil retained slightly less maize C than ryegrass C, but the difference was small and the overall pattern of decomposition similar. There was little difference between the rate of decomposition under shade or in the open, even though soil temperatures were considerably greater in the open. A soil containing 6% clay (Apomu series) retained slightly less maize C after 1 year than a soil with 16% clay (Egbeda series), but in general the decomposition rates in the different soils were similar. For ryegrass, the decomposition pattern was very similar under Nigerian conditions to that previously observed for the same plant material in England, except that the whole decomposition process was four times faster in Nigeria.

Effect of pH on Adsorption of Arsenic and Selenium from Landfill Leachate by Clay Minerals

Abstract: Adsorption of As(V), As(III), and Se(IV) anions from municipal landfill leachate solutions by the clay minerals kaolinite and montmorillonite is strongly dependent upon solution pH. The and ions are the apparent primary As(V) and Se(IV) species being adsorbed by the clay minerals. Activation of the anion adsorption sites on the clay minerals by ions is important in determining the amount of ions adsorbed by the clay minerals, but it is apparently not important in determining the amount of ions adsorbed. Montmorillonite adsorbed more As(V), As(III), and Se(IV) anions than kaolinite. Although no definite proof is given, the higher adsorption by montmorillonite is attributed to a higher edge surface area. The results of the study suggest that disposal of As and Se wastes in municipal landfills, especially under alkaline conditions, could produce ground water contamination by As and Se. Additional Index Words: anion adsorption, kaolinite, montmorillonite. Effect of pH on Adsorption of Arsenic and Selenium from Landfill Leachate by Clay Minerals View complete article To view this complete article, insert Disc 4 then click button8

Effects of trace elements on phosphatase activity in soils

Abstract: Studies to evaluate the effects on the activity of acid and alkaline phosphatases of 20 trace elements showed that all trace elements inhibit the activity of phosphatases in soils. Results showed that the relative effectiveness of the trace elements in inhibition of phosphatase activity depends on the soil. When the trace elements were compared by using 25 ..mu..mole/g soil, the average inhibition of acid phosphatase in three soils showed that Hg(II), As(V), W(VI), and Mo(VI) were the most effective (average inhibition >50%) and that Ba(II), Co(II), and As(III) were the least effective (average inhibition <10%) inhibitors. Other elements that inhibited acid phosphatase activity in soils were: Cu(I), Ag(I), Cd(II), Cu(II), Zn(II), Mn(II), Sn(II), Ni(II), Pb(II), Fe(II), Cr(III), Fe(III), B(III), Al(III), V(IV), and Se(IV); their degrees of effectiveness varied in the three soils used. The most effective inhibitors of alkaline phosphatase activity in soils were Ag(I), Cd(II), V(IV), and As(V). All the trace elements that inhibited acid phosphatase activity also inhibited alkaline phosphatase activity. The order of effectiveness of the trace elements in inhibition of acid phosphatase activity was different from that of inhibition of alkaline phosphatase activity. Generally, the inhibitory effect of trace elements was less in the presence ofmore » 2.5 ..mu..mole/g soil than in the presence of 25 ..mu..mole/g soil. In addition to the effect of trace elements, phosphate ion inhibited acid and alkaline phosphatase activities in soils. Related anions such as NO/sub 2//sup -/, NO/sub 3//sup -/, Cl/sup -/, and SO/sup 2 -//sub 4/ were not inhibitory in the presence and absence of buffer.« less

Water Uptake By Plants: II. The Root Contact Model

Abstract: In the standard analysis of root water uptake, it is assumed there is a constant root membrane resistance in series with a soil resistance which is dependent upon the soil moisture diffusivity. The relation of root extraction rate to soil water content and to soil water potential predicted by this standard model was compared to the results of divided root experiments. The extraction rates predicted by the theory were as much as eight times larger than the measured values. A reasonable fit between theory and experiment could only be obtained by assuming in the theoretical calculations that the rooting density was 100 times smaller than that measured in the experiments. An alternative model emphasizing the possibility of a root-soil contact effect was developed. It was assumed that as the soil dried, the surface area of the roots in contact with the soil decreased, which caused an increase in the root membrane resistance. By assuming that the effective root membrane permeability was simply proportional to the relative saturation of the soil, a much better fit to the data was obtained than by using the conventional theory.

A Survey of the Fertility Status of Surface Soils Under “Cerrado” Vegetation in Brazil

Abstract: Agriculture is becoming more intensive in central Brazil, but there is little information available on the fertility status of most of the soils. To determine this status a total of 518 topsoil samples were collected from a 600,000-km² area in central Brazil. Laboratory characterization included pH, exchangeable Al, cation exchange capacity, nutrient levels (including micronutrients), organic matter, texture, and color. The results, presented as frequency distributions, indicate the following general conditions: a pH of 4.8 to 5.2, an organic matter level of 1.5 to 3.0%, extremely low levels of effective CEC and extractable Ca, Mg, P, and Zn, a high level of Fe, a high degree of Al saturation, and a broad range in textural class. Organic matter was the most important soil fraction in relation to effective CEC in these soils. The data suggest that problems associated with fertilizer inputs needed to bring these soils under more intensive agriculture are generally similar, but may vary in degree.

Mass Transfer Studies in Sorbing Porous Media: III. Experimental Evaluation with 2,4,5-T1

Abstract: assuming the presence of a linear equilibrium isotherm. „ . j u . *. ., _, . . . ., «. These solutions were subsequently used in part II (van Comparisons are made between observed and calculated effluent ,-, , , .... .„!_ ,. . . , ., . . concentration distributions for the movement of 2,4,5-T (2,4,5Genuchten and Wierenga, 1977; this issue) to describe tntrichlorophenoxyacetic acid) through 30-cm long unsaturated soil coltlum movement through unsaturated, aggregated Glendale umns. The comparisons are made using both analytical and numerical ^y loam. In this paper, part III of the study, the theoretical solutions of a previously published model, which included the effects of model will be used to describe experimental data on the intra-aggregate diffusion and adsorption. The results in this study inmovement of the herbicide 2,4,5-T (2,4,5-trichlorophenoxdicate that intra-aggregate diffusion and adsorption/desorption are the yacetic acid) through the same Glendale soil. As was menmain mechanisms responsible for effluent tailing. An estimated 60% of tioned in Part I, no analytical solutions exist when either the the adsorption was found to occur in the stagnant region of the soil. adsorption isotherm is nonlinear, or when the adsorptionWhen intra-aggregate diffusion was included in the model, the obdesorption process is not single-valued (hysteretic). For served adsorption/desorption hysteresis phenomenon found to be sig.1. j-.• i » u • L. j TU .„ .. F . . . . . . u . u ui . mese conditions numerical techniques must be used. The mficant in several earlier studies, was shown to be much less important . . . . , . , . , . . in describing the observed concentration distributions. numerical solutions used in this analysis were programmed in IBM S/360 CSMP as described by van Genuchten and Additional Index Words: intra-aggregate diffusion, adsorptionWierenga (1974; 1976b). desorption, hysteresis, miscible displacement, 2,4,5-T. 'Journal article no. 568, Agric. Exp. Stn., New Mexico State Univ., Las _____________________ Cruces, NM 88003. The work upon which this report was based was supported in part by funds obtained from the U.S. Dep. of the Interior, Office IN PART I OF THIS STUDY (van Genurhrpn and Wierenaa of Water Resources Research, as authorized under the Water Resources 7 TM , L • , J V uenucmen ana Wierenga, Ac, of ,%4 Received 8 March 1976 Approved 9 Nov. 1976 iy/oa), a theoretical model was developed to describe the "Formerly Graduate Student, Dep. of Agronomy, New Mexico State movement of chemicals through unsaturated, aggregated Univ.; presently Research Associate, Dep. of Civil Engineering, Princeton sorting porous media. Analytical solutions were derived for ^g£%£^. of Agronomy, New Mexico state Univ., Las both the mobile and immobile liquid regions of the medium, Cruces, NM 88003. VAN GENUCHTEN ET AL.: MASS TRANSFER STUDIES IN SORBING POROUS MEDIA: III 279 Table 1—Soil-physical data for data for various displacements through Glendale clay loam. 6 r-

Determining Cation Exchange Capacity: A New Procedure for Calcareous and Gypsiferous Soils1

Abstract: A new, two-step procedure is presented for determining the cation exchange capacity (CEC) of calcareous and gypsifcrous soils. The method eliminates most of the errors inherent in conventional methods. The cation exchange sites are saturated with sodium by four successive "equilibrations" of the soil (4-5 g) with 33-ml increments of a pH 8.2, 60% ethanol solution of 0.4N NaO Ac 0.1/V NaCI (saturating solution). The saturated sample then is extracted with three 33-ml increments of l.ttV, pH 7 magnesium nitrate. Total sodium (Na,) and chloride (Cl,) are subsequently determined in the extracted solution. Chloride (Cl,) is determined so that the soluble sodium (Na^,), from the excess saturating solution carried over from the saturation step to the extraction step, may be deducted from the total sodium (Na,) to obtain exchangeable sodium (Na,,xc.h), which is equivalent to the CEC. Thus, CEC = Naexch = (Na, Nasol) = Na, [Cl,(Na/Cl)saniraUn« solution] where (Na/Cl) sanirating miMim the ratio of Na to Cl w the saturating solution. Additional Index Words: cation exchange capacity, calcareous soils,

Chemical Reduction of Nitrite and Nitrous Oxide by Ferrous Iron

Abstract: Little is known about the reduction of NO₂⁻ and N₂O by Fe²⁺ in the pH range from 6 to 8, a range of importance in many natural environments. Reduction of 25 mg NO₂⁻-N (approximate concentration=25 ppm) and 5 mg N₂O-N in a distilled-water medium containing 800 mg Fe²⁺ with a helium atmosphere in the presence and absence of Cu²⁺ at pH 6 and 8 was studied at 26°C. Reduction of NO₂⁻ to N₂O (the principal product), N₂ and NH₄⁺ was rapid and quantitive at pH 8 and not greatly affected by Cu²⁺. However, addition of Cu²⁺ decreased the mole ratio of N₂O to N₂ in the evolved gas mixture from 4.2 to 3.1. Reduction of NO₂⁻ at pH 6 was incomplete, but was favored by Cu²⁺ addition. Nitrous oxide was the dominant reduction product at this pH value. Small quantities of NO accumulated at pH 6. Irrespective of Cu²⁺ addition, N₂O was stable in the Fe²⁺ medium at pH 6. In the pH 8-system in the presence of Cu²⁺ 84% of the N₂O was reduced to N₂. Nitrous oxide was relatively stable at pH 8 in the absence of Cu²⁺. Small quantities of H₂ accumulated in the atmospheres of vessels containing the pH 8, plus Cu²⁺ N₂O − N treatment. No H₂ accumulated in the NO₂⁻ series of treatments.

Solute Movement in a Field Soil1

Abstract: Solute and water movement was studied under steady-state flow conditions in a field soil consisting of 70 cm of clay to silty clay over a medium sand. A steady-state water flow condition was maintained by applying irrigation water at a constant flux of 2 cm per day. During the steady-state condition some of the water leached into the plot was labelled with chloride and tritium. The positions of the chloride and tritium as functions of soil depth and time were measured by extracting samples of the soil solution with soil suction probes. Extremes in solute displacement occurred at equal and different depths within the plot. An analysis of these measurements indicated that observations of the pore-water velocity and the apparent diffusion coefficient were log normally distributed. Twenty-four soil suction probes, used to identify the rate at which a solute was displaced in the soil, will yield an estimate of the mean pore-water velocity of this soil within a range of approximately ±25% of its true value providing the effects of potential solute-soil interactions are taken into account.

Influence of pH on the Sorption of Co2+, Zn2+ and Ca2+ by a Hydrous Manganese Oxide1

Abstract: The uptake of Co²⁺, Zn²⁺, and Ca²⁺ by δ-MnO₂ at 24.0 ± 0.5°C as a function of pH and salt concentration was investigated. At pH values < 5, δ-MnO₂ released structural Mn to solution phase during the sorption of Co²⁺ and to a lesser extent during Zn²⁺ sorption. Calcium sorption did not release any Mn to solution phase. In addition Co²⁺, Zn²⁺, and Ca²⁺ were shown to adsorb specifically at the δ-MnO₂ surface by exchange with bound H. The pH values had very little influence on the sorption of these cations. At pH values above 6, abrupt increases in the sorption of Co²⁺ and Zn²⁺ in narrow pH ranges were observed whereas no such marked increase of Ca²⁺ sorption was found up to pH 9.2. This type of sharp increase in sorption is due to specific adsorption and is related to hydroxylation of the metal ions. Measurements of microelectrophoretic mobility of δ-MnO₂ in the presence of Co²⁺, Zn²⁺ and Na⁺ suggested that Co(OH)₂ or Zn(OH)₂ may be nucleated at the δ-MnO₂ surface at pH values lower than those thermodynamically calculated (no supersaturation) which would result in precipitation of the pure hydroxides in bulk solution. At pH values between 5 and 6 any of the above mechanisms may be operative.