Search

The pedogenic development of volcanic ash mantles within the interior U.S. Pacific Northwest is thought to be a function of the interaction between climatic regimes, plant communities, eolian reworking, and colluvial mixing with subsurface soil parent materials. Little research, however, quantitatively characterizes the extent of these interactions on soil property expression and nutrient retention. This study examined soil physical and chemical properties associated with volcanic ash mantles overlying extrusive and metasedimentary geologic parent materials across an array of precipitation zones. The results were then correlated with NO 3 , SO 4 , and BO 3 sorption isotherms to assess parent material influence and degree of mineral weathering on nutrient retention. Analyses indicated that ash-mantled extrusive soils had ∼25 to 50% more plant-available water, two to three times greater volcanic glass, two to five times less quartz, and up to five times more short-range-order Fe in the Bw1 and 2Bw2 horizons than ash-mantled metasedimentary soils. Mean annual precipitation zones showed a strong positive correlation with the development of short-range-order Al minerals. Anion sorption isotherms were in the order of SO 4 > BO 3 ≥ NO 3 for surficial Bw1 horizons, and SO 4 ≥ NO 3 > BO 3 for subsurface Bw2 and 2Bw2 horizons. Anion retention mechanisms were positively correlated with the presence of short-range-order AI and Al-substituted Fe oxides. Soil property characterization indicated that surficial ash soils often reflect the physical and mineral characteristics of subsurface soils, but climatic conditions control their chemical expression through the degree of mineral weathering.

Despite the broad consensus that humic substances (HS) can act as electron shuttles in bioreduction processes occurring in aquatic and terrestrial environments, no published studies have presented direct evidence that electron shuttling by HS plays a role in the mechanisms of Fe(III) bioreduction in soils. We conducted experiments in which two HS with very different chemical properties, a soil humic acid and a mixture of peat humic acid with aquatic organic matter, were added to an Ultisol and incubated under anoxic conditions to evaluate their effect on Fe(III) bioreduction by the indigenous microorganisms in the soil. Our experiments were designed to have the same initial reducing capacity available from the added HS but to have very different initial total carboxyl concentrations to examine the role of Fe(II) complexation by HS in the bioreduction process. The addition ofHS significantly increased soluble Fe(II) production relative to that observed in control experiments without added HS. Moreover, after an initial period of incubation, soluble Fe(II) production was accelerated by the presence of a higher initial carboxyl content. These trends support the hypothesis that, at the initial stages of in Fe(III) bioreduction, electron shuttling dominates, whereas later Fe(II) complexation dominates. Our results apparently are the first to demonstrate directly that added HS can indeed enhance the bioreduction of Fe(III) minerals in soil by at least two different mechanisms.

Changes in soil water retention of the surface soil brought about by tillage can significantly alter the amount of rain water that infiltrates into the root zone and is available for plant growth. Soil tillage generally increases porosity and changes the pore-size distribution, leading to changes in the soil water retention curve and hydraulic conductivities. The objective of this study was to investigate some simple ways of estimating the soil water retention curve of a tilled soil from that of an untilled soil, knowing the change in soil porosity or bulk density due to tillage. The study of literature and empirical analysis of the available data indicated: (i) under field conditions the tillage did not significantly change the air-entry value of the soil; (ii) tillage increased the absolute value of the slope of the log-log relationship below the air-entry value; and (iii) the changes due to tillage in the retention curve occurred only in the larger pore-size range, approximately between the air-entry pressure head value and 10 times the air-entry value. Assuming these observations hold in general, two simple methods of estimating the water retention curve of a tilled soil from that of its untilled condition are proposed. The first method is a simple imposition of the Brooks and Corey function between the air-entry value and 10 times this value. The second method assumes that the change in soil water content at a given pressure head in the above range of pressure heads was inversely proportional to the value of the pressure head. The tests on four pairs of measured water retention curves on three different soils showed that these methods provided good approximations.

Recently developed methods for the accurate measurement of surface charge components and the points of zero charge in soils containing both permanent- and variable-charge constituents were applied to a Haplorthod that is typical of podzolized soils in North America, Scandinavia, and Russia. Independently measured values of the density of structural surface charge, net proton charge, and net adsorbed ion charge were obtained for soil samples from the E2 and Bhs horizons suspended in LiCl or Li 2 SO 4 solutions of ionic strength 1 or 10 mmol kg -1 . Data taken across the pH range 2 to 6 tested successfully for conformity to the law of surface charge balance. Relationships between the point of zero net proton charge and the point of zero net charge were consistent with the fact that, at any pH, increasing ionic strength favored an increase in the net adsorbed ion charge, whereas changing from Cl - to SO 4 2- favored a decrease in the net adsorbed ion charge. The methodology and charge balance analysis are applicable to any podzolized soil.

The ability to predict the effect of various factors on soil flocculation depends on a knowledge of the mechanisms by which those factors operate. The purpose of this study was to investigate the mechanisms that control the effects of organic matter content, pH, and bivalent cation type on soil flocculation. The electrophoretic mobility of an illitic soil before and after treatment with H2O2 was measured at a soluble Ca or Mg charge fraction of 0.0 to 0.9 at pH 6, and at pH 4 to 9 and a soluble Ca charge fraction of 0.0 or 0.5. The change in the net particle charge as pH was increased from 6 to 8 at a soluble Ca concentration of 0.65 and 2.26 mol m(-3) was determined from proton and Ca ion titrations. Electrophoretic mobility was independent of both Ca vs. Mg and organic matter content, which suggests that the influence of these factors on flocculation was caused by steric effects. The effect of pH on soil flocculation and electrophoretic mobility in the presence or absence of organic matter, on the other hand, was consistent with an electrostatic mechanism. The change in net particle charge from pH 6 to 8 became more positive as the soluble Ca concentration was increased, and this result was modeled in terms of competitive binding of H+ and Ca(2+) by soil functional groups.

Measurements of the separate contributions of structural and pH-dependent charge are essential to understanding and predicting surface-chemical reactions in soils and other heterogeneous adsorbents that contain a mixture of surface functional groups. An ion-adsorption method was developed to measure the accessible structural charge density (σ₀) in adsorbents that comprise both structural-charge and variable-charge surface functional groups. The method is based on the preference of siloxane surface sites for Cs⁺ over Li⁺ and on the much lower selectivity of ionizable surface groups for Cs⁺. To estimate σ₀, CsCl-saturated adsorbents were reacted with 0.05 mol CsCl kg⁻¹ at pH 7, washed with ethanol to reduce the entrained CsCl concentration, dried at 65 °C to promote the formation of inner-sphere surface complexes between Cs⁺ and permanent-charge sites, and then extracted with 0.01 mol LiCl kg⁻¹ to displace Cs⁺ from variable-charge adsorption sites, which exhibit little or no preference for Cs⁺ over Li⁺. Finally, Cs⁺ was displaced from permanent-charge sites by repeated NH₄OAc extraction until no Cs was present in the equilibrium supernatant solution. According to this method, σ₀ represented about 85 and 70% of the accessible cation-adsorption sites at pH 7 on

Recent efforts to characterize soil water properties in terms of porosity and particle-size distribution have turned to the possibility that a fractal representation of soil structure may be especially apt. In this paper, we develop a fully self-consistent fractal model of aggregate and pore-space properties for structured soils. The concept underlying the model is the representation of a soil as a fragmented fractal porous medium. This concept involves four essential components: the mathematical partitioning of a bulk soil volume into self-similar pore- and aggregate-size classes, each of which is identified with a successive fragmentation step; the definition of a uniform probability for incomplete fragmentation in each size class; the definition of fractal dimensions for both completely and incompletely fragmented porous media; and the definition of a domain of length scales across which fractal behavior occurs. Model results include a number of equations that can be tested experimentally: (i) a fractal dimension ≤3; (ii) a decrease in aggregate bulk density (or an increase in porosity) with increasing aggregate size; (iii) a power-law aggregate-size-distribution function; (iv) a water potential that scales as an integer power of a similarity ratio; (v) a power-law expression for the water-retention curve; and (vi) an expression for hydraulic conductivity in terms of the conductivities of single-size arrangements of fractures embedded in a regular fractal network. Future research should provide experimental data with which to evaluate these predictions in detail.

The theoretical chemical basis of the sodium adsorption ratio (SAR) was examined in order to clarify two important issues that arise in connection with the problem of estimating potential sodium hazard: (i) the relation between SAR and the theory of cation exchange, and (ii) the implications of the universal practice of replacing SAR by what may be termed the practical SAR, which employs total concentrations of Na, Ca, and Mg instead of their free cationic concentrations. In regard to (i), it was confirmed that SAR can be given a chemical basis in terms of the Gapon cation exchange equilibrium constant which is provided in this paper with a rigorous derivation for the first time. The Gapon constant is shown to differ from the Vanselow exchange constant only because of the (arbitrary) convention adopted as to what entity is termed the ligand in the cation exchange reaction. In regard to (ii), it was shown through a direct calculation of SAR and the practical SAR for 161 different soil solutions or water extracts, utilizing a comprehensive chemical equilibrium computer program, that an exact functional relationship between the two parameters does not exist. Only a statistical correlation between SAR and practical SAR could be established because of the random variation in the composition of metalcomplexing anions in the soil solutions or water extracts. These anions reduce the free cationic concentrations of Na, Ca, and Mg, but their amounts are not chemically related to the presence of the metals and, therefore, to the value of the practical SAR. It follows that the practical SAR has no exact chemical relation to the theory of cation exchange, although its use in strictly empirical studies of Na hazard may be justified, in a statistical sense, by its acknowledged practical value. Additional Index words: cation exchange in soils, cation exchange theory, computer studies of soil solutions, exchangeable Na, Gapon equation, Na hazard. View complete article To view this complete article, insert Disc 4 then click button8

Differential equations which describe the equilibrium vertical distribution of moisture in swelling soil have been derived previously using both thermodynamic and fluid-mechanical methods. A fundamental disagreement exists between these expressions which is resolved in this paper through a detailed thermodynamic analysis of the equilibrium state of water in a swelling soil. The correct differential equation for the moisture profile is found and the discrepancy between previous results is shown to have been caused either by introduction of extra thermodynamic variables or by an incorrect handling of the effect of overburden pressure on the soil water. /Author/

The energy concept of soil water introduced by Buckingham and developed by Richards is employed to derive a differential balance equation for the partial specific internal energy of water in an unsaturated soil. This equation is shown to comprise both heat flux and chemical work terms, in correspondence with the first law of thermodynamics. For the case of isothermal water flow through a rigid, homogeneous soil, the balance law for internal energy reduces to the Richards Equation with the soil water matric potential as the dependent variable. This result, which does not require the conventional direct appeal to the law of mass balance, demonstrates that the energy concept of soil water provides a self-consistent method for obtaining the Richards Equation. Either the energy balance concept or the traditional mass-balance approach can be used to establish the macroscopic physical foundations of the Richards equation.