THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOIL

If an exchangeable ion in soil diffuses along a liquid and solid pathway, its diffusion coefficient may be expressed as where D, v, f, C are diffusion coefficient, volume fraction, impedance factor, and concentration terms and the suffixes _l,_S refer to liquid and solid. The self-diffusion coefficient of the ion is then where D′, D′_t, and D′_s, are self-diffusion coefficients. D and D′ will vary with concentration. In diffusion out of the soil to a zero sink, the appropriate average diffusion coefficient is, approximately, the self-diffusion coefficient in the undisturbed soil. Diffusion of one ion species is influenced by other ions diffusing in the system through the diffusion potential set up. When ions are diffusing to plant roots, the diffusion potential is likely to be small. A more likely, though more complicated, expression for D than the first equation above is derived by assuming the ion to follow solid and liquid pathways in series as well as in parallel.     

SATURATION PERCENTAGE AS A MEASURE OF SOIL TEXTURE IN THE LOWER INDUS BASIN

The saturation percentage is related to the mechanical constituents of a soil: and is therefore a quantitative expression of soil texture. Profiles may be described in terms of the S.P.; thus Maps may be drawn showing quantitative changes in texture, and associated statements of the reliability of an S.P. estimate and the probable error of the position of a textural contour be made. Decomposition of the calcium carbonate component should be avoided in routine soil survey work.     

A comparison of the convection-dispersion equation and transfer function model for predicting chloride leaching through an undisturbed, structured clay soil

The transfer function mode) (TFM) and convection-dispersion equation (CDE) were compared for predicting Cl ^? transport through a calcareous pelosol during steady, nearsaturated water flow. Large, undisturbed soil cores were used at constant irrigation intensities (q₀) between 0.3 and 3 cm h^?1, with a step-change in Cl^? concentration. The assumption of a lognormal distribution of travel times–characterized by the mean (μ) and variance (σ²)–permitted the flux-averaged breakthrough curves (BTCs) to be modelled very accurately by the TFM. The BTCs could be modelled equally well by the CDE when both the mean pore water velocity (v) and dispersion coefficient (D) were optimized simultaneously by the method of least squares, but not when v was put equal to q₀/_v, where _V was the mean volumetric water content. The best estimate of v was consistently > q₀/_v, which suggested that not all the pore water was effective in chloride transport. An operationally defined transport volume (θ_st) was calculated from the mean () or median (τ_m) travel times derived from the TFM. Chloride exclusion was not solely responsible for θ_st() being <_V: immobile water also contributed. The positive skewness of the travel time distributions meant that θ_st(τ_m) < θ_st(), indicating the effectiveness of macropore flow in solute transport. Dαv^1.42 (from the CDE), and σ²αv (from the TFM), confirmed that Cl^? dispersion increased as flow velocity increased. Flux-averaged concentrations were used to calculate the volume-averaged resident concentrations. They matched the measured Cl^? concentrations most closely when there was a gradual decrease in measured Cl ^? concentration with depth, but not when Cl ^? decreased sharply below c. 10 cm. Calculations assuming that all the water was effective in chloride transport gave less accurate results. Comparison of the measured and predicted concentrations of solute demonstrated that this must be a critical part of the evaluation of any model of solute transport.     

A new method for the estimation of total dissolved salts in saturation extracts of soils from electrical conductivity

We have determined electrical conductivity (E_e) and total dissolved salts (S) in saturation extracts from 39 soil samples from the Baza basin (Province of Granada, south-east Spain). E_e ranged from 2.8 to 110dS m^?3, and S from 2 to 444 8 dm^?3. The relationship between S and E_e was not linear. When the saturation extracts were diluted with progressively larger quantities of distilled water and their electrical conductivity calculated (E_ec) with the equation where E_d and E_w are the conductivity of the diluted extract and the distilled water and f is the dilution factor, the relationship between S and E_ec tended to become linear. The highest linear correlation coefficient relating S (mg dm^?3) and E_ec (dS m^?1) was reached when E_ec, values were calculated for dilutions with a conductivity (E_d) between 0.1 and 0.3 dS m^?1 (E*_ec). The regression equation was S= 490 E*_ec with r²= 0.999. This relationship can be used in all saturation extracts, regardless of the concentration and type of ions present.     

KINETICS OF ISOTOPIC EXCHANGE OF PHOSPHATE ADSORBED ON GIBBSITE

An Elovich-type equation has been used to describe the kinetics of isotopic exchange of phosphate adsorbed on the surface of gibbsite. The equation is where A and B are parameters; θ=bF/[b+a(I-F]; a and b are the molar concentrations of the phosphate on the crystal surface and in solution respectively and F is the fraction exchange of the radio-isotope at time t. First-order rate constants were obtained from the equation. The reference state for the first-order rate constants, and the distribution of activation energies, for exchange can be related to the Elovich equation parameters. The kinetic results are consistent with S_N1 dissociation or S_N2 bimolecular solvolysis for the phosphate ligand. The eschange reaction is subject to an acid-base catalysis the exact nature of which could not be determined from the available data.     

QUALITY CONTROL IN SOIL SURVEY

The paper examines 66 Australian soil surveys in a variety of terrains (but not close forest), by several survey procedures, and published at a range of map scales. It relates the Survey Effort (E) of professional staff (in man-days per km²) to (I) survey procedure, (2) the kind of mapping unit, and (3) the intricacy of the soil pattern mapped. Intricacy (I), the average number of mapped soil boundaries crossed by 1 km of random linear traverse, is related to the total length of mapped boundary (km per km²). When the surveys are grouped according to survey procedure and mapping unit, the survey effort for each group may be described by a regression of the form . B could not be shown to differ significantly between groups. D varied in the ratio 0.5: I, according to whether or not surveys used air photograph interpretation, and in the ratio 0.3:0.7:1.o, according to whether they mapped land systems, other compound mapping units, or simple mapping units. Since the choice of survey procedure and mapping unit is usually governed by the intricacy of the soil pattern the effect of these factors can be summarized in a single regression for all 66 surveys: There is a significant (P≤ 0.001) log-log regression between I and map scale.     

Relationship between equivalent salt solution series of different soils

Equivalent salt solution series have been previously defined as solutions with combinations of sodium absorption ratio (SAR) and electrolyte concentration (E_c) producing the same extent of clay swelling in a given soil. The present study shows that there is a high (r²>0.96) positive correlation between log E_c and log SAR of equivalent salt solutions series, in the equation: where a₁ and b₁ are constants for each equivalent salt solution series for a given soil. Log a₁ could also be represented as a linear function of b₁ resulting in the equation: where a₂ and b₂ are constants for a given soil. Solving this equation using any given value of b₁ yields the combinations of SAR and E_c which make up each equivalent salt solution series for a given soil. The relationship between log a₁ and b₂ for three soils from western United States, namely Waukena, Pachappa and Grangeville, was similar, with their combined data having a r² value of 0.96. This indicated that a single set of equivalent salt solution series values could be used for these three soils which have different clay contents and clay mineralogy. Prediction of hydraulic conductivity decreases with E_c reduction at given values of SAR in red-brown and alluvial soils from southern Tasmania, using the equivalent salt solution series values for Waukena soil, showed similar patterns to measured values and also to those predicted using the equivalent salt solution values applicable to the respective Tasmanian soils. Thus, available data indicate that the same set of equivalent salt series could be applied to the five soils studied. If further testing shows that a single set of equivalent salt solutions values could be applied to all or large groups of soils, this would facilitate the application of the equivalent salt solution concept to predict salt solution flow in the field.     

The Burns leaching equation

The simplicity and utility of Burns' leaching equation make it worthy of study. The equation may be written as where X is the fraction of initially surface-resident fertilizer leached below depth z by net rainfall I, in soil with a volumetric water content at ‘field capacity’ of θ. The equation is analysed using transfer functions. The analysis shows that Burns' equation is consistent with an ‘independent flow tube’ soil leaching model, rather than the soil solution being well-mixed at each soil depth as Burns suggested. The flux and resident soil solution soil concentration profiles are shown to be quite different. An alternative definition of θ is suggested. The behaviour of ‘a Burns soil’ for different initial and boundary conditions is discussed.     

WATER MOVEMENT IN DRY SOILS

Semi-infinite columns of dry soil closed at one end had the other exposed to a turbulent atmosphere at constant relative humidity (0.98) and a range of constant temperatures. After varied times the water content of the columns was measured gravimetrically, in 1 cm layers, from which the total quantity taken up Q and the distribution of water content Ø with time t and distance x were found. Assuming that the Boltzmann transform, λ=xt^?t can be applied to the standard diffusion equation, two soil parameters are derived. A test of the assumption is that Ø should be uniquely dependent on λ, and then the diffusivity is calculable from where Ø_t is the initial uniform water content. The second parameter—the sorptivity, S= Qt^?½—and is not independent of Ø_t or the value maintained at x= 0. Results show that in a clay soil, between pF 5.8 and pF 4.2, water moves predominantly as vapour: in a non-swelling silicate mineral (sepiolite) there is significant liquid movement between pF 5.1 and pF 4.2. Long-continued use of organic manure has little effect on either D or S; aggregate size has some effect, in opposite senses for a sand and a clay; a mulch or a still atmosphere affects S but not D; evaporation suppressants decrease S; ignition decreases S and greatly alters D; and degradation of structure causes small changes in D.     

A LANGMUIR TWO-SURFACE EQUATION AS A MODEL FOR PHOSPHATE ADSORPTION BY SOILS

For forty-one soils (pH > 5.0) from southern England and eastern Australia, the Langmuir equation was an excellent model for describing P adsorption from solutions < 10^-3M P, if it was assumed that adsorption occurs on two types of surface of contrasting bonding energies. For most of these soils, which were relatively undersaturated with P, this equation may be written as: where x = adsorption, k = adsorption/desorption equilibrium constant, x_m= monolayer adsorption capacity, and c = equilibrium solution concentration. The relative magnitude of the parameters for each surface were approximately: x_m= 0.3 x^″_m=0.3 and k′= 100 k. More than 90 per cent of the native adsorbed P occurs on the high-energy surface in most soils.     

THE RESPONSE OF UNSATURATED SOILS TO ISOTROPIC STRESS

Soil compression is caused in agriculture by tillage implements, plant roots, treading by animals, and by wheels and tracks of vehicles. Increases in soil density resulting from compression usually reduce crop growth and yield. Compression and expansion of samples of five remoulded soils, each at several moisture contents, were investigated. Soil samples were subjected to isotropic stress of up to 3.5 MN m^?2 in a pressure cell. Volume changes were measured by the volume of pore fluid effused or infused through one of the sample surfaces. Particle packing densities, D, were well described by the equation where D₀ is the maximum limiting value of D, P is the applied isotropic stress, and B, C, K, L are adjustable parameters. One of the exponential terms in this equation describes deformation of soil crumbs and the other describes rearrangement of individual particles. Two sample sizes gave similar values for the equation parameters. A small increase in moisture content results in a large increase in soil compressibility. It is hypothesized that resistance to compression may be one of the principal influences in the mechanical restriction of root growth.     

THE VOLUME AND POROSITY OF SOIL CRUMBS

A simple and inexpensive apparatus (a test-tube, burette, and pin) is described for measuring volumes by liquid displacement to an accuracy of greater than 0.5 per cent. This has been adapted to measure soil crumb porosities, ε_c, by saturating 3–4 g samples of crumbs with kerosene, measuring the weight of kerosene retained internally, then measuring their volume by displacement. Three estimates of crumb porosity from these measurements are compared. Experimental values range from ε_c= 0.205 for the headland of an arable field to ε_c= 0.351 for a permanent pasture. Crumb porosity is proposed as a measure of structural status for soils because it assesses the degree to which soil management has succeeded in holding the constituent primary particles apart from the positions of inherent closest packing that they would ultimately assume in an unstable soil. By comparison, the inter-crumb porosity, ε_v, can be used as a measure of cultivation status. In the form expressed, these two porosities are related to the more frequently encountered total porosity ε_t by the relation      

THE EFFECT OF ORGANIC MATTER ON THE BULK AND TRUE DENSITIES OF SOME UNCULTIVATED PODZOLIC SOILS

Organic matter content was found to have a dominant effect on both the bulk and true densities of soil in the organic and eluvial horizons of the podzolic soils examined. The soils were stone-free, structureless, and of similar texture. The effects of organic matter on bulk density were described by the equation: A similar equation was devised for true density. Use of these equations indicated that, in the soils examined, total pore space could be predicted from organic matter content measured as per cent loss on ignition.     

Determination of organic carbon and nitrogen in particulate organic matter and particle size fractions of Brookston clay loam soil using infrared spectroscopy

The objective of this study was to determine whether models developed from infrared spectroscopy could be used to estimate organic carbon (C) content, total nitrogen (N) content and the C:N ratio in the particulate organic matter (POM) and particle size fraction samples of Brookston clay loam. The POM model was developed with 165 samples, and the particle size fraction models were developed using 221 samples. Soil organic C and total N contents in the POM and particle size fractions (sand, 2000–53 µm; silt, 53–2 µm; clay, <2 µm) were determined by using dry combustion techniques. The bulk soil samples were scanned from 4000 to 400 cm^?1 for mid‐infrared (MIR) spectra and from 8000 to 4000 cm^?1 for near‐infrared (NIR) spectra. Partial least squares regression (PLSR) analysis and the ‘leave‐one‐out' cross‐validation procedure were used for the model calibration and validation. Organic C and N content and C:N ratio in the POM were well predicted with both MIR‐ and NIR‐PLSR models ( = 0.84–0.92; = 0.78–0.87). The predictions of organic C content in soil particle size fractions were also very good for the model calibration ( = 0.84–0.94 for MIR and = 0.86–0.92 for NIR) and model validation ( = 0.79–0.94 for MIR and = 0.84–0.91 for NIR). The prediction of MIR‐ and NIR‐PLSR models for the N content and the C:N ratio in the sand and clay fractions was also satisfactory ( = 0.73–0.88; = 0.67–0.85). However, the predictions for the N content and C:N ratio in the silt fraction were poor ( = 0.23–0.55; = 0.20–0.40). The results indicate that both MIR and NIR methods can be used as alternative methods for estimating organic C and total N in the POM and particle size fractions of soil samples. However, the NIR model is better for estimating organic C and N in POM and sand fractions than the MIR model, whereas the MIR model is superior to the NIR model for estimating organic C in silt and clay fractions and N in clay fractions.     

Determination of total soil organic C and hot water‐extractable C from VIS‐NIR soil reflectance with partial least squares regression and spectral feature selection techniques

The calibration of soil organic C (SOC) and hot water‐extractable C (HWE‐C) from visible and near‐infrared soil reflectance spectra is hindered by the complex spectral interaction of soil chromophores that usually varies from one soil or soil type to another. The exploitation of spectral variables from spectroradiometer data is further affected by multicollinearity and noise. In this study, a set of soil samples (Fluvisols, Podzols, Cambisols and Chernozems; n = 48) representing a wide range of properties was analysed. Spectral readings with a fibre‐optics visible to near‐infrared instrument were used to estimate SOC and HWE‐C contents by partial least squares regression (PLS). In addition to full‐spectrum PLS, spectral feature selection techniques were applied with PLS (uninformative variable elimination, UVE‐PLS, and a genetic algorithm, GA‐PLS). On the basis of normalized spectra (mean centring + vector normalization), the order of prediction accuracy was GA‐PLS ? UVE‐PLS > PLS for SOC; for HWE‐C, it was GA‐PLS > UVE‐PLS, PLS. With GA‐PLS, acceptable cross‐validated (cv) prediction accuracies were obtained for the complete dataset (SOC, , RPD_cv = 2.42; HWE‐C_cv, , RPD_cv = 2.13). Splitting the soil data into two groups with different basic properties (Podzols compared with Fluvisols/Cambisols; n = 21 and n = 23, respectively) improved SOC predictions with GA‐PLS distinctly (Podzols, , RPD_cv = 3.14; Fluvisols/Cambisols, , RPD_cv = 3.64). This demonstrates the importance of using stratified models for successful quantitative approaches after an initial rough screening. GA selection frequencies suggest that the spectral region over 1900 nm, and in particular the hydroxyl band at 2200 nm are of great importance for the spectral prediction of both SOC and HWE‐C.     

A simple equation to describe sorption of anions by goethite

A simple equation to describe sorption of anions by goethite would be useful as a means of characterizing batches of goethite and in studies of plant uptake of anions from the sorbed form. A suitable relationship between solution concentration (c) of phosphate or citrate and their sorption (S) by goethite at a constant pH or at different pH values is where b is a parameter, SMax is the maximum sorption, and a is a parameter at constant pH. In the middle range of sorption (from about 30% to about 70% of maximum sorption) this equation approximated to a Tempkin equation, but the full equation is more useful as it applied over the whole sorption range. The values of a varied with pH. This variation could be explained by changes in the electric potential of the adsorbing surfaces and in the degree of dissociation of the anions. The parameter a could therefore be replaced by a function of pH. The effects were consistent with formation of bidentate phosphate complexes and tridentate citrate complexes with the goethite surface.     

EFFECTS OF CATION EXCHANGE MATERIAL ON ZINC ADSORPTION BY SOILS

The adsorption of Zn by soils which are different in their major cation-exchange materials was measured at equilibrium Zn concentrations up to 10^?2 M in 10^?2 to 10^?3 M CaCl₂. The results are interpreted on K^Zn_Ca[Zn]_soil plots, where K^Zn_Ca is the selectivity coefficient defined by the equation All natural samples except those containing halloysite exhibited no or very small specific Zn adsorption. All Ca-saturated samples exhibited specific Zn adsorption dependent on cation-exchange materials. The cation-exchange sites with high selectivities for Zn (K^Zn_Ca > 10) constitute more than 40 per cent of the total exchange sites in soils containing allophane, imogolite, and halloysite, whereas those with moderate to low selectivities for Zn (K^Za_Ca < 10) predominate in montmorillonitic, vermiculitic, and humic soils. Differences in the contribution of the respective cation-exchange materials to specific Zn adsorption are discussed relating to differences in the origin of their negative charge.     

Lead sorption to selected Portuguese soils

We studied the interaction of lead with seven Portuguese soils with different physical and chemical properties in order to elucidate more fully the behaviour of Pb in soil. We studied these adsorption phenomena by voltammetric titrations with differential pulse polarography (DPP) at different pH (6.0–7.2) and ionic strengths, I (0.010–0.50 m ) in order to clarify some of the factors that might control soil sorption capacity for Pb. From the voltammetric data, average formation constants, , and binding capacity, C_c, have been estimated according to a surface complexation model based on Scatchard and van den Berg–R?zic methods. Linear Scatchard and van den Berg–R?zic plots (r≥ 0.99) indicated that the results can be interpreted according to the existence of just one predominant active site for Pb(II) adsorption. The values from both procedures () agreed in all cases (r= 0.938, n= 66, P < 0.001). The same happened with C_c values that were statistically equivalent (r= 0.9998; n= 66; P < 0.001). The C_c values were found to depend on the pH and I, as well as on the soil properties. Either Langmuir or Freundlich isotherms fitted the experimental data well (r > 0.90, P < 0.05). The lead binding capacities were strongly and significantly correlated (P < 0.05) with pH, cation exchange capacity, organic carbon, loss‐on‐ignition, total Al₂O₃ content, extractable forms of Al and pyrophosphate extractable Fe, [Fe_p]. From a forward, stepwise regression model we concluded that [Al₂O₃], [Pb′] (concentration of labile lead in solution), [Fe_p], pH and I are able to explain more than 99.7% of the variation in lead sorption in our soils. The soils’ surface groups with special affinity to Pb(II) are in the inorganic fraction associated with aluminium.     

Soil wetting‐drying and water‐retention properties in a mine‐soil treated with composted and thermally‐dried sludges

The main objective of this study was to analyse how different sewage sludges influence soil wetting and drying dynamics. Three composted and three thermally‐dried municipal sludges from different wastewater plants located in Catalonia (NE Spain) were mixed with a mine‐soil obtained from a limestone quarry. Measurements of the time required to reach zero contact angle () and water holding time (WHT) provided information on the time required for a mine‐soil to reach its complete wettability and the residence time of water stored between ?0.75 and ?25 MPa of soil suction, respectively. One month after sludge amendments, one composted and one thermally‐dried sludge significantly increased . WHT was increased in the mine‐soil treated by composted sludges (50.6% by Blanes' sludge, 65.5% by Manresa's sludge and 52.5% by Vilaseca's sludge) one month after sludge amendments. The amount of water retained in the mine‐soil was increased by all composted sludges and one thermally‐dried sludge after one month (by 42.3% with Blanes' sludge, 42.3% with Manresa's sludge, 65.7% with Vilaseca's sludge and 23.9% with Mataró's sludge) and one year after sludge amendments and at a small suction. Increments in WHT corresponded with the amount of water retained so the time‐scale of soil water availability should also be considered. The value was modified mainly by increments in carbon stock and microbial biomass, while the WHT was modified mainly by increments in pH and electrical conductivity. Under similar air‐drying conditions, mine‐soil treated with composted sludges retained more water for longer compared with thermally‐dried sludges.