bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction of methyl nitrite with the inorganic and organic fractions of soils
Authors: MAGALHÃES A. M. T. , CHALK P. M.
Published: July 2006

The reactions of methyl nitrite (CH3ONO), a gaseous product of NO−2decomposition in soils, were studied by exposure of soils in closed vessels to the gas. The N transformations occurring in soils at different soil‐water states were assessed by measuring CH3ONO and other gaseous forms of N in the gas space, soil inorganic N (NH4+, NO−2, NO3−) and incorporation of CH3O15NO into the soil organic N fraction.

The initial rate of uptake of CH3ONO increased with decreasing soil‐water content, but the rate of hydrolysis decreased as soil‐water content decreased below – 33kPa matric potential. Uptake was not affected by y‐irradiation of soils. Adsorption isotherms conformed to the Langmuir equation in each of 22 oven‐dry soils studied. Langmuir adsorption maxima were positively correlated with the clay contents of the soils, and adsorption was reversible to some extent at all soil‐water states.

Small amounts of added CH3ONO were recovered as N2and N2O and as15NH4+in γ‐irradiated soils. From 60 to 72% of added CH3O15NO was recovered by Kjeldahl digestion; this was indicative of a chemical reaction with soil organic matter.

The results suggest that the physical process of adsorption of CH3ONO by clay minerals and the chemical fixation of CH3ONO by soil organic matter are key factors controlling the atmospheric concentration of CH3ONO, and that the combined effect of these processes, together with hydrolysis in the soil solution, will inhibit the emission of CH3ONO formed in N‐fertilized soils.
34S natural abundance variations in prairie and boreal forest soils
Authors: SCHOENAU J. J. , BETTANY J. R.
Published: July 2006

The systematic nature of the 34S natural abundance variations (δ34S) in a prairie and boreal forest ecosystem enabled construction of hypotheses regarding the origin and cycling of S in the two soil‐plant systems. By considering the 34S abundance variations in relation to soil S transformations, a better understanding of S isotope fractionation in soils was also achieved.

The δ34S values suggest that atmospheric S becomes increasingly more important as a S input as pedogenesis proceeds in these soils. The origin and movement of sulphate salt in a saline seep was evaluated using δ34S values, demonstrating the usefulness of the δ34S technique in soil salinity studies. The 34S‐enrichments and depletions found in soil organic S fractions were consistent with postulated differences in lability, mobility, and turnover rate. Wheat plants growing on the saline, sulphate‐saturated prairie soil were found to be enriched in 34S relative to surrounding S sources, providing indirect evidence for release of 34S‐depleted H2S by the plants as a S‐stress relief mechanism.
Effects of lime applications to parts of an upland catchment on soil properties and the chemistry of drainage waters
Authors: ADAMS W. A. , EVANS G. M.
Published: July 2006

The liming of soils in the lower part of an upland catchment was found to have a major effect on both soil properties and the chemistry of drainage waters. Exchangeable Al was closely correlated with soil pH and showed a very steep rise from 2.6‐4.8 meq 1−1 over the pH range 5.5‐4.5. As streams flowed from unimproved through improved land there was an increase in pH and the concentration of all major anions and basic cations. The greatest increase was in Ca (approximately 3.5‐fold). The concentrations of all dissolved Al species decreased, with inorganic monomeric Al falling to near zero. Leachates were examined from soils representative of the most acidic and the least acidic. Calcium concentrations differed by almost tenfold. Aluminium was present in leachates from the limed soil, but most was unreactive and none was inorganic monomeric. Most of the Al leached from the acid soil was monomeric.

A model of soil acidification is proposed in which soil Ca is depleted at a rate of 8% of the exchangeable Ca per annum. The model predicts that liming a soil to neutrality would be likely to influence drainage water chemistry for 30‐40 years and that the most acidic soils of the catchment show no net loss of Ca to drainage.
Potassium and magnesium in some Kenyan soils: their mineral sources and release to Ca‐resin
Authors: ONCHERE J. , GOULDING K. W. T. , WOOD I. G. , CATT J. A.
Published: July 2006

Five soil types from Kenya were examined for their ability to release K and Mg to crops by extracting them with a calcium‐saturated cation exchange resin, and for the mineral sources of K and Mg. Amounts and rates of release of exchangeable, short‐ and long‐term reserves of K and Mg were determined. This was done for K before and after one, two and 10 wetting and drying (WD) cycles, designed to mimic the effect of the local climate on K release. The order of decreasing K and Mg contents of the soils was Muguga > > Katumani > Kampi‐ya‐Mawe (KYM) ∼ Ithookwe ∼ Kwale. In the case of K, wetting and drying increased the exchangeable fraction in all of the soils. The Muguga, Ithookwe and KYM soils have the capacity to release a great deal of K in the long‐term, but at very different rates; the Kwale soil has few reserves and fixes K on WD.

The K and Mg contents of the soils were closely related to the mica contents of the clay + silt fractions. Exchangeable and non‐exchangeable K was released from soil micas primarily by ion exchange, with some contribution to non‐exchangeable K from dissolution. Release of exchangeable Mg was primarily by ion exchange, whilst that of non‐exchangeable matrix Mg was by dissolution of trioctahedral mica in the clay + silt and the sand fractions of the soils. Response would be expected only to K fertilizers and only with the Kwale soil. However, all the other soils should, in the interests of good husbandry, receive maintenance dressings of K and Mg at convenient times in the crop rotation.
Evidence for aliphatic mixed mode coordination in copper(II)–fulvic acid complexes
Authors: GREGOR J. E. , POWELL H. K. J. , TOWN R. M.
Published: July 2006

A comparative approach based on ion–selective electrode measurements is presented for studying Cu(II) complexing by fulvic acids. Metal binding curves (% free metal vs pH) and complexation capacity curves (% free metal vs total metal at fixed pH) for copper(II) fulvic acid systems are compared with those for appropriate model ligands.

It is inferred that citrate, malonate and amino acid moieties could dominate complexing to Cu(II) at pH 3 to 7. The involvement of amino acid moieties is supported by fluorescence quenching and by anodic stripping voltammetry (ASV) studies. The proportion of ASV–non labile complexes is reported for Cu(II) and Pb(II) fulvic acid systems at pH 4.8.

A mixed mode of coordination is proposed, with the dominant binding sites varying with pH and metal:ligand ratio.
A rapid method for predicting the lime requirement of acidic temperate soils with widely varying organic matter contents. I. Development of the lime requirement model
Authors: BAILEY J. S. , STEVENS R. J. , KILPATRICK D. J.
Published: July 2006

A model to describe the shapes of soil pH‐CaCO3 neutralization curves is presented. The model can be used to predict lime requirement in soils with widely varying organic matter contents. Parameters of the model are predicted from the easily measured soil properties of sample density and initial pH. The results suggest that soil titratable acidity is primarily related to soil organic matter content, and that soil organic matter content may be predicted from measurements of soil sample density. The proposed method for predicting lime requirement should be superior to other established quick‐test methods for routine soil testing both in terms of speed and versatility.
Roughness of soil fracture surfaces as a measure of soil microstructure
Authors: GRANT C. D. , DEXTER A. R. , HUANG C.
Published: July 2006

The examination of soil fracture surfaces that are created under tensile stress may reveal a great deal about the internal structural condition of the soil. A simple technique for quantifying the roughness of soil fracture surfaces from a measurement of their topography in cross‐section is described. The technique involves calculating the standard deviation of the differences between the measured elevations of soil fracture surfaces and their corresponding running‐mean values. The standard deviation, σR, is used as a measure of the fracture surface roughness. Advantages of this technique over others are discussed.

Two methods for measuring the topography of soil fracture surfaces are presented: a bisection (single transect) method, and a laser scanning (multiple transects) method. The laser scanning method is to be preferred because it requires no sample preparation and enables greater and more rapid replication. Also, fracture surfaces created by applying direct tension in the hands produced values of σRthat were statistically indistinguishable from those created using indirect tension in a loading frame. This result makes the technique of fracture surface analysis usable for both laboratory as well as field investigations of soil structure.

An example of the technique is presented to illustrate the role that air‐filled pores play in the brittle fracture of unsaturated soil (air‐filled pores are closer together in drier soils and further apart in wetter soils.) A strong positive linear correlation was found between the gravimetric water content,wand σRof natural soil clods, which supports the contention that brittle fracture of unsaturated soils under tensile stress occurs at least partly because of the propagation of air‐filled pores.
The power‐function model for the soil moisture characteristic
Authors: BUCHAN G. D. , GREWAL K. S.
Published: July 2006

The minimum number of parameters required to model the unsaturated soil moisture characteristic, relating volumetric water content (θ) and matric suction (χ), is shown to be two. A third parameter, θ=θs at saturation, is required to define its saturation limit. The popular power‐function χ/χe, = (θ/θs)b is the most general three‐parameter model, with χ normalized by a notional air‐entry potential, χe. When log‐transformed, e.g. as In χ= a + bln(θ/θs), it gives a good fit to observations over varying ranges of χ. We show, using US, Australian, UK and NZ data, that a,b and θs, in this formulation are uncorrelated across a wide range of textures, thus providing a ‘basis set’ of independent parameters. Gregson et al. (1987) used the alternative formulation In χ= a″+ bln(100θ), with χ rescaled to a percentage. Their reported correlation between a″ and b, which led to their ‘one‐parameter model’ of the characteristic, is shown to be a mathematical artefact, arising from absorption of the term – b[ln(100)+ ln θs] into a″.
Forest soil chemical changes between 1949/50 and 1987
Authors: BILLETT M. F. , PARKER‐JERVIS F. , FITZPATRICK E. A. , CRESSER M. S.
Published: July 2006

Soil profiles from the Alltcailleach Forest in north‐east Scotland originally sampled in 1949/50 were resampled in 1987. Soil pH, exchangeable Ca, Mg, K and Na, extractable Al and cation exchange capacity were measured on the original stored and resampled soils. Chemical changes were characterized by decreases in pH, base cations, base saturation and cation exchange capacity. Extractable amounts of Al increased. Sequential leaching experiments showed a significant increase in the amount of extractable sulphate in mineral soil horizons.

Changes in soil chemistry were interpreted to result from a combination of nutrient depletion caused by tree growth, natural pedogenic processes and atmospheric pollution effects.
Effect of liming and added phosphate on charge characteristics of acid soils
Authors: NAIDU R. , SYERS J. K. , TILLMAN R. W. , KIRKMAN J. H.
Published: July 2006

The effects of liming‐induced pH changes on surface charge were studied with four acid Fijian soils. All the soils possessed both positive and negative charge. Much of this charge was pH‐dependent, but some negative charge was present at pH values as low as 4.0 and small quantities of positive charge were detected at pH values as high as 8.2.

Adsorption of phosphate (P) reduced positive charge at low pH and increased negative charge at high pH. The net amount of surface charge per unit of P adsorbed varied with pH and appeared to depend on the mineralogy of the soil.
Calibration and validation of a model of non‐interactive solute leaching in a clay‐loam arable soil
Authors: VINTEN A. J. A. , REDMAN M. H.
Published: July 2006

A capacity‐type approximate leaching model (Addiscott et al., 1986) with a simple treatment of soil matrix permeability was tested, using field tracer experiments with CaBr2, on hydrologically isolated plots. The model predictions are sensitive to the value of the soil matrix permeability factor, a, and four methods of estimating this parameter were evaluated: (1) using a calibration based on soil texture; (2) least‐squares fitting of the model to successive neutron probe measurements of the water content profile; (3) least‐squares fitting to daily drainage outflow; (4) least‐squares fitting to cumulative drainage outflow. The best independent method (method 3) led to slight (20–30%) under‐prediction of leaching losses for two of the four experiments, but in one experiment leaching was much less than predicted. As a management model the approach seems promising but more attention needs to be paid to estimation of the value and variability of the permeability parameter, a.

The convective‐dispersion equation, using steady‐state assumptions and a fitted dispersion length, gave as good a prediction of cumulative leaching losses as the approximate model studied here.
The use of air‐filled porosity and intrinsic permeability to air to characterize structure of macropore space and saturated hydraulic conductivity of clay soils
Authors: BLACKWELL P. S. , RINGROSE‐VOASE A. J. , JAYAWARDANE N. S. , OLSSON K. A. , MCKENZIE D. C. , MASON W. K.
Published: July 2006

Intrinsic permeability to air of macropore space (ka) is related to macroporosity (ɛ) and organization of macropore space (O). Organization is defined as ka/ɛ. The use of ka for estimating saturated hydraulic conductivity (Ka) is also considered. The relationship between Log (O) and ɛ (Oɛ characteristic) can be used to describe changes to the macropore space of clay soils by amelioration and compaction. The effects of dominant macropore shape can also be identified and calculated as an empirical index of the efficiency of the pore organization E (E=log (O)/ɛ). Intrinsic permeability can then be related to E in a E:ka characteristic. Intrinsic permeability is the parameter most sensitive to structural change and E is mainly influenced by the dominant shapes of the macropores. Thus, the E:ka characteristic is suggested as a basis for studying differences in macropore space as may occur in response to external and internal stresses upon the soil and different systems of soil management, for example increases of packing pores by cultivation or of fissures by gypsum application and loss of packing pores by compaction. Empirical data indicate that Ks of the B horizons of Australian red‐brown earths can be estimated from ka of macropore space at a standard potential.
Nutrient changes in decomposing beech leaf litter assessed using a solution flux approach
Authors: JOERGENSEN R. G. , MEYER B.
Published: July 2006

The decomposition of beech (Fagus sylvatiea L.) leaf litter was examined in lysimeters. The experiment allowed comparison of data from mass changes of bioelements in leaf litter and the solution flux balance of through fall input and lysimeter output over a two‐year period (1983 and 1984). The annual C loss from the leaf litter was 19%. Na and K concentrations in this leaf litter decreased in the first year and remained constant in the second, while those of Ca and Mg showed no significant changes. N and S amounts per ha increased during the first year by about 7 kg N ha−1 and 0.5 kg S ha−1. Ash, Fe, Mn and Al amounts per ha increased to 470, 130 and 240% of their initial levels. These net increases in the first year of decomposition are discussed.

The mean annual water and element input by through fall during the experiment corresponded approximately to the long‐term average. Thirty to 40 mm more water was evaporated from the litter layer in the drier year (1983) than in 1984. Total N, NO, Na and C1 flux rates of through fall inputs and lysimeter outputs were equal. NH, input rates were greater and organic N smaller than the output rates of the lysimeters. Water balance data indicated that the lysimeter output of K, Mg, Ca and SO, exceeded through fall input. Probable reasons for these differences are discussed.
Mechanism of sulphate adsorption by two tropical SoilS
Authors: CURTIN D. , SYERS J. K.
Published: July 2006

The mechanism of sulphate (SO4) adsorption was investigated using two highly‐weathered soils with contrasting surface‐charge characteristics (one with a net negative charge and the other with a net positive charge). The addition of SO4caused a release of OH and increased the adsorption of cations, in keeping with the widely‐held view that SO4is chemisorbed by replacing ‐OH and ‐OH2ligands. On the other hand, adsorbed SO4was completely recovered by washing with an indifferent electrolyte solution (1 M KC1), an extractant which is usually taken to be specific for ‘exchangeable’ ions. The amounts of SO, adsorbed were several times higher than those of non‐specifically adsorbed Cl. It is suggested that the experimental observations can be interpreted in terms of a model in which SO4adsorbs in a plane closer to the surface than monovalent anions (e.g. in the Stern layer). The consequence of higher counter‐ion valence and proximity to the surface is that additional positive charge is induced onto the surface by OH release. Charge reversal in the diffuse layer allows more cations to be adsorbed. This type of adsorption may involve forces other than the purely electrostatic but, in contrast to chemisorption, the adsorbed anion does not become chemically coordinated to the surface metal atoms.
Evaluation of porous ceramic cups for monitoring soil‐water aluminium in acid soils: comment on a paper by Raulund‐Rasmussen (1989)
Authors: HUGHES S. , REYNOLDS B.
Published: July 2006

Following recent observations by Raulund‐Rasmussen (1989) implicating A1 contamination of soil solutions isolated by suction‐cup samplers, A1 release from porous ceramic cups in acid solutions was investigated. In our studies a flush of Al, followed by a gradual decrease in leaching over successive extractions was observed. The amount of Al released was retarded by the presence of 37 μmol dm−3 of A1 in solution. Gibbsite solubility controls were not observed; all solutions isolated by the cups were undersaturated with respect to amorphous gibbsite. The cups evaluated in this study are appropriate for sampling acidic soil solution, provided they are suitably pretreated and then equilibrated in the field before use.
Humic substances in acid organic soils: modelling their release to the soil solution in terms of humic charge
Authors: TIPPING E. , WOOF C.
Published: July 2006

Batch titration experiments were carried out with organic soil samples in order to investigate the release to the solution phase of humic substances (HS). Measurements were made of pH, dissolved organic carbon (DOC) concentration, and the concentration of mono‐meric (inorganic + organic) aluminium, as functions of added acid or base. DOC was taken to be entirely due to HS. The results can be interpreted in terms of a model in which the soil is considered to contain two types of HS–mobile or potentially mobile (HSM), and immobile (HSI). The binding of inorganic ions by the HS is calculated using humic ion‐binding model IV, previously developed in this laboratory. Model IV allows the charges on the HS (ZHSM, ZHST) to be calculated; these are determined mainly by the binding of H+ and A13+. Concentrations of HS in solution, [HSaq], are given by the equation:

image

where |ZHSM| is the modulus of ZHSM, nHSM is the carboxyl group content of HSM, cHSM is the soil content of HSM, β is a fitting parameter, and square brackets, [ ], indicate concentrations. For most of the soils a value for β of 3 gives acceptable agreements between measured and calculated values of [HSaq], indicating a major influence of charge on release. The optimized value of cHSM differs considerably among soils, whereas cHIS varies by only a factor of about two. Total humic contents (cHSM+ cHSI estimated by model optimization are in approximate agreement with values estimated by extraction of the soils with NaOH.
Phosphate sorption in allophanic soils and release of sulphate, silicate and hydroxyl
Authors: PARDO M. T. , GUADALIX M. E.
Published: July 2006

Phosphate (P) sorption and the concomitant release of sulphate (SO,), silicate (Si) and hydroxyl ion (OH) were determined on three allophanic soils from Spain, at different P concentrations. P effectively replaced SO4, Si and OH. However, at every stage of P sorption, the molar ratios of the total amounts of anions released (SO4+ Si + OH) to that of the sorbed P were low.

The amount of added P affected the relative proportions of SO4, Si and OH exchanged. At low concentrations of P, phosphate sorption was accompanied mostly by release of the adsorbed SO4 with some Si. As more P was sorbed an increasing displacement of OH was also observed.
A comparison of the surface impedance factors of Ca, Na, Rb and Cs derived from their self‐diffusion coefficients in various soils
Authors: STAUNTON S.
Published: July 2006

Self‐diffusion coefficients of calcium, sodium, rubidium and caesium on the topsoils and subsoils of contrasting soil series are presented. The distribution ratios of each cation on each soil, at the moisture content of the diffusion experiments, have been calculated by extrapolation from series of suspensions.

Diffusion coefficients were found in each case to be greater than expected by considering the liquid phase alone. Surface‐phase impedance factors, fs, were calculated. Although impedance factors are considerably smaller on the surface than in solution, the surface flux may exceed the flux in the liquid phase. Differences of an order of magnitude separated average values of fs for each cation. For a given cation, variations between soils were considerably less. No correlation was found between fs and soil mineralogy or composition.

Values of fs were found to decrease with increasing distribution ratio. Reasons for this observation are discussed, in particular the energy of adsorption of the cations. The possibility of extrapolating these findings to other soils, to potassium and to other cations is discussed.
13C NMR characterization of humic acids from soils of a development sequence
Authors: NEWMAN R. H. , TATE K. R.
Published: July 2006

Humic acids were isolated from nine topsoils in New Zealand tussock grasslands. Cross‐polarization 13C NMR spectra of solid samples were used to estimate fractions of carbon contained in different types of chemical functional groups. The degree of oxygen substitution of aromatic rings showed a strong negative correlation with soil development. Aromaticities greater than 0.25 were found in humic acids from only the two least‐developed soils.
Use of force‐deformation curves to estimate Young's modulus and its applications to soil aggregate breakdown
Authors: OKUNLOLA A. , PAYNE D.
Published: July 2006

Soil aggregates from a clay and a sandy loam were subjected to uniaxial compression and their force‐deformation relationships determined. From these relationships the elastic deformation was identified, and estimates were made of Young's modulus, elastic strain energy and work of fracture. Also estimated were the increases in surface area produced on breakdown and hence surface energies. The results were interpreted in terms of Griffith's crack theory, and apparent crack lengths were calculated. The calculated values were high, but the variability of the data prevented a full assessment of this approach to the study of brittle failure of aggregates.
Comparison of soil solution chemistry in zero tension and ceramic‐cup tension lysimeters
Authors: HENDERSHOT W. H. , COURCHESNE F.
Published: July 2006

The chemistry of soil solutions collected from plastic 0‐tension and ceramic‐cup tension lysimeters was compared. Except for NO−3 there was no systematic difference in the chemistry of major cations, anions, pH or Al speciation of the solutions collected by the two types of lysimeter. although there were some differences at both 25 and 75 cm depths, on such occasions the difference at the other depth was either in the apposite direction or non‐significant. The concentration of NO−3 was significantly lower in the 0‐tension lysi meters and, although the mechanism is not clearly understood, this may be due to uptake by micro‐organisms feeding on fine organic matter particles in these solutions. The choice of lysimeter can be based on the objectives and constraints of the research project, rather than on the fact that the soil solution is more representative with one type of lysimeter than another.
The use of cation activity ratios to estimate the intensity of soil acidity
Authors: CONYERS M. K. , MUNNS D. N. , HELYAR K. R. , POILE G. J.
Published: July 2006

Soil solution extracts can be relatively difficult to obtain and to handle. The aim of this study was to investigate whether a simple 0.01 M CaCI, extraction could be used to estimate the intensity factor of soil acidity by employing the Schofield Ratio Law. Activity ratios of H, Ca and A1 were compared in soil solutions and in 0.01 M CaCI, extracts on four soils adjusted to a range of pH values with acid and lime. The ratios pH‐1/2pCa (0.01 M CaCl2) and pH‐1/2p(Ca + Mg) (soil solution) were not statistically significantly different. The ratio pH‐1/3pAl was less strongly correlated, but again was not significantly different between the soil solution and extractant. The Schofield Ratio Law did not hold over the entire range of data for pCa‐+2/3pAl, but the values in the two solutions correlated closely. Cation activity ratios in 0.01 M CaCl2 can be used to estimate the intensity factors of acidity in a soil, even though the extractant has a higher ionic strength than many soil solutions in variable charge soils. Therefore pH and A1 as measured in 0.01 Cacl2 extracts may be useful tests for routine diagnostic purposes and soil surveys.
Organic matter available for denitrification in different soil fractions: effect of freeze/thaw cycles and straw disposal
Authors: CHRISTENSEN S. , CHRISTENSEN B. T.
Published: July 2006

The 0 to 20‐cm surface layer of a sandy loam soil was sampled in early autumn from plots where straw had either been removed or incorporated annually for 22 years. Denitrification in whole soils, 1–2‐mm wet‐stable aggregates, clay and silt size fractions was determined by acetylene blocking during anaerobic incubation with excess nitrate. Thus available organic matter was the limiting factor. Samples were exposed to one or two freeze/thaw cycles, or used unfrozen. K2SO4‐extractable carbon (C) was determined before and after CHCI, fumigation.

Freeze/thaw increased denitrification in whole soils and in aggregates. In aggregates and in whole soil without straw the increase in denitrification was similar following two freeze/thaw cycles, and well above the amount that could be fed by extractable soil C. In whole soils with straw addition, an extra denitrification increase occurred at first thaw only. This straw‐induced denitrification surplus was matched by a decline in soil microbial biomass. For other samples and treatments, the freeze/thaw released C from additional organic matter sources.

The availability of C in clay for denitrification was twice that of silt‐associated C. Straw disposal generally had no effect on the bioavailability of particle‐bound C. In contrast to whole soils and aggregates, the availability of organic matter in clay and silt after one freeze/thaw cycle was only half that observed from unfrozen samples.

The effect of freeze/thaw on whole soils and aggregates may be to release organic matter available for denitrification by killing the microbial biomass and by disintegrating aggregates. However, the impact of freeze/thaw on completely dispersed samples such as clay and silt may be to promote the formation of granular structures (micro‐aggregation) in which organic matter may become less accessible to denitrifiers.
Potassium release and fixation in Ferralsols (Oxisols) from Southern Togo
Authors: POSS R. , FARDEAU J. C. , SARAGONIT H. , QUANTIN P.
Published: July 2006

Potassium release and fixation were studied on Ferralsols (typic Eutrustox) of Southern Togo both by isotopic exchange with 42K+ ions and by exchange with 1 M ammonium acetate. Experiments were conducted on soil samples taken from surface (0–30 cm) and subsoil (80–100 cm) layers of four plots. These plats, with the same soil type, were subjected to various kinds of use: forest and crops, with or without fertilizers. A single wetting and drying of the soil samples increased the amounts of exchangeable K. This exchangeable K release could stem from clay protonation during drying. However, the K release is insufficient to allow permanent intensive agriculture without K fertilizer addition. Soil samples were therefore treated with KCI in order to study K fixation. About 20% of the applied K was fixed and the percentage increased with wetting and drying. The fixation can be explained by the presence of about 2% of interstratified clay minerals in these kaolinitic soils. Comparison of the kinetic characteristics of the isotopically exchangeable K with the amounts of ammonium exchangeable K revealed the existence of several kinds of exchange site for K ions.
Assessing the fixation and availability of sorbed phosphate in soil using an isotopic exchange method
Authors: HE Z. L. , YUAN K. N. , ZHU Z. X. , ZHANG Q. Z.
Published: July 2006

An isotopic exchange method was used to characterize quantitatively the fixation and plant availability of phosphate previously sorbed by soils. In general, the exchangeability of the sorbed phosphate was much higher than its desorbability for both soils and clay minerals. Isotopic exchangeability of the sorbed phosphate increased with sorption saturation during the initial stage (15–60% saturation), but the increase was less with increasing saturation from 60–90% for all soils tested. Therefore a sorption saturation of 60% was recommended as the upper limit of P fertilization in terms of economical efficiency. For clay minerals, with increasing sorption saturation, the isotopic exchangeability of the sorbed P increased significantly for kaolinite and sesquioxides, but decreased for montmorillonite. Most of the phosphate sorbed by montmorillonite and kaolinite was found to be isotopically exchangeable, but only a small amount of the P sorbed by goethite could be exchanged. The P sorbed by Al oxide exhibited isotopic exchangeability between that of kaolinite and Fe oxide. The isotopically exchangeable phosphate pool could readily account for the P uptake of plants and the available P determined by some commonly used chemical methods, such as Olsen‐P and Bray‐P.
Development of a technique for the measurement of root geometry in the soil using resin‐impregnated blocks and image analysis
Authors: COMMINS P. J. , McBRATNEY A. B. , KOPPI A. J.
Published: July 2006

A method is described that permits the measurement of roots in situ in the soil using computerized image analysis to process images obtained from planar faces cut from a resin‐fixed pot. Soil sample pots are initially impregnated with rapid‐curing epoxy resin. Blocks are then cut from the impregnated pot with a diamond saw to expose a planar face. Modification of the present resin‐impregnation procedure is necessary as impregnation results were mostly unsatisfactory. Roots are contrasted from the soil matrix by enhancing autofluorescence using longwave ultraviolet light and appropriate filters. After image capture and segmentation to show regions of root material, measurements of volume fraction of roots, volumetric surface area, horizontal root length and horizontal root separation, were made with a larger resolution with soil depth than previously.
Validation of a model of the transport of zinc to an artificial root
Authors: BAR‐TAL A. , BAR‐YOSEF B. , CHEN Y.
Published: July 2006

Zinc movement in soils is influenced by soil pH, the presence of organic chelators, the total quantity of Zn in the soil and by root exudates. The effect of these factors on Zn uptake by a single plant root was simulated mathematically by Bar‐Yosef et al. (1980). The objective of this paper was to validate this simulation model and evaluate the relative importance of soil chemical properties, the stability of Zn complexes and the soil pH in determining Zn transport in soil and absorption by plant roots. To avoid problems of soil and plant variability, an artificial root, namely a ceramic tube through which solutions of known composition flowed, was used. The soil was a montmorillonite‐sand mixture of known Zn adsorption characteristics. The model satisfactorily described the decline in net Zn influx into the root (Fzn as soil pH increased or as total Zn in soil (Qzn) decreased, and also described Fzn, as a function of time. Addition of fulvic acid (FA) to the soil decreased Fzn. The decreased uptake rate stemmed from the lower permeability of the root to the large Zn‐FA molecule. When the pH of the solution that flowed inside the tube was decreased from 5.0 to 2.0, thus generating a greater efflux of H+ by the roots, Fzn, increased four‐to six fold, depending on the time. The model satisfactorily described the decline in Fzn, as the clay fraction in the soil increased, while maintaining the same quantity of Zn per volume of soil.
Properties of sub soils in relation to various measures of surface area and water content
Authors: CHURCHMAN G. J. , BURKE C. M.
Published: July 2006

Neither the specific surface area values (from N2 sorption) nor the ethylene glycol monoethyl ether (EGME) retention values of 21 soil samples from New Zealand and Fiji could all be accounted for by the sum of the contributions from their component minerals. Much EGME is probably retained by internal surfaces of inter layered and interstratified clay minerals. EGME retention correlated well with cation exchange capacity (CEC) and a number of measures of water content of these soils. The water contents of air‐dried soils (measured as ‘moisture factors’ or the ratios of air‐dry and oven‐dry weights) showed almost as close a relationship to CEC as EGME retention for this set of 21 subsoils. Moisture factors and CEC were closely related, within groups defined by dominant clay mineralogies, for a much wider selection of 1318 New Zealand soil horizons with low carbon contents. The relationships between surface area and a number of other soil properties including dispersibility of soils were also examined.
A brief discussion on the effect of temperature on the reaction of inorganic ions with soil
Authors: BARROW N. J.
Published: July 2006

Published reports on the effect of temperature on the sorption of ions by soil, or soil constituents, are in conflict with regard to the direction of the effect and its interpretation. It is argued that, in many cases, increased sorption with increased temperature is due to an increased rate of the reaction that follows adsorption. The effects of temperature on the adsorption reaction itself are complex because of separate effects on the ions in solution, on the charge on the surface, and on the affinity of the ions for the surface. On balance, adsorption of anions should decrease with increasing temperature, and adsorption of cations should increase. However, decreased adsorption of cations is possible, particularly at high pH. With one exception, these conclusions are consistent for all published reports seen.
Interactions between peat and sodium acetate, ammonium sulphate urea or wheat straw during incubation studied by 13C and 15N NMR spectroscopy
Authors: Benzing‐Purdie L. , Cheshire M. V. , Williams B. L. , Ratcliffe C. I. , Ripmeester J. A. , Goodman B. A.
Published: July 2006

Transformations of sodium acetate, ammonium sulphate, urea and wheat straw in peat have been studied by determining the distribution of 15N‐labelled material, and by 13C and 15N nuclear magnetic resonance spectroscopy (NMR) using cross polarization (CP) and magic‐angle spinning (MAS). Samples of an oligotrophic blanket peat were incubated for 6 months at 15°C with 15N ammonium sulphate, 15N urea, 13C15N urea, 15N‐labelled wheat straw or 13C sodium acetate. The incubated samples were separated into fractions of >1 mm, 1–0.5 mm, 0.5–0.25 mm, 0.25–0.15 mm, 0.15–0.05 mm, 0.05–0.005 mm and a water‐soluble fraction by wet sieving, and were then freeze‐dried. The distribution of 15N between the fractions was obtained after isotope‐ratio analysis by mass spectrometry, and the 0.5–0.25 mm, 0.05–0.005 mm and water‐soluble fractions from the incubations were examined by 13C and 15N NMR.

13C‐labelled acetate increased carbohydrate resonances in the 0.05–0.005 mm and soluble material, but an organic acid derived from the substrate was still present 6 months later. Incorporation of 15N from ammonium sulphate into the peat was low, and more than 50% of the added N was detected in the soluble fraction still present as 15NH+4. As carbohydrate and soluble organic matter were detected in the peat, it was concluded that microbial activity and N immobilization were restricted by poor aeration and low pH. Urea, in contrast, interacted with all the fractions examined, with some 15N being incorporated into a range of compounds that included protein, peptides, amides, amino acids and carbamates or lactam derivatives. A small proportion of labelled 15N from wheat straw, orginally present in the > 1 mm and 14.5 mm fractions, had moved into the 0.05–0.005 fraction during incubation and sieving. The 13C spectra suggested that the presence of the straw may have stimulated decomposition of the peat components.
Acoustic determination of air‐filled porosity and relative air permeability of soils
Authors: MOORE H. M. , ATTENBOROUGH K.
Published: July 2006

Two acoustic techniques for determining the air‐filled porosity and air permeability of soil surfaces, and the near‐surface variation of these properties with depth, have been applied to several soils in situ. The techniques depend upon an inversion process using data on sound propagation near to the soil surface and through the soil surface. The measured difference in acoustic spectra received by two vertically separated microphones above the ground surface and by probe microphones beneath the surface were matched theoretically to deduce the porosity of air‐filled connected pores and an effective air permeability which depends on pore shape and size distribution. This modified permeability parameter and the acoustically deduced porosity varied realistically with surface sealing and compaction. The acoustically deduced porosities were within 10% of those obtained conventionally. Changes in acoustically deduced parameters occurred with changes in moisture content both in soil bin and field experiments. The acoustic techniques were successful in monitoring surface sealing and near‐surface layering on a fine scale.
A physically based model for surface sealing of soil
Authors: BONSU M.
Published: July 2006

A physically based model describing the mechanism of surface seal formation of soil from a suspension of soil in flowing water was formulated on the basis of the principle of conservation of mass and Darcy's equation for the flow of water through a layered soil column. The model shows that, for sufficiently large times, the flux density of the filtrate is proportional to the inverse square root of time. The proportionality constant is a function of the saturated hydraulic conductivity and bulk density of the seal, and the concentration of the suspension. The components of the proportionality constant suggest that the constant could be used as an index of structural stability of soil. For the three suspensions used in testing the validity of the model, log‐log plots of the flux densities of the filtrates and time showed linear relationships with slopes approximately equal to –0.5, as predicted by theory. However, using calculated components of the model to predict flux densities of the filtrates failed to show agreement for two out of the three suspensions used. This was attributed to some over‐simplified assumptions used in the model, such as constant concentration and no sedimentation effects during the filtering process. It is suggested that incorporation of a time‐dependent concentration and sedimentation factors could perhaps improve the efficacy of the model.
Gypsum applications to aggregated saline—sodic clay topsoils
Authors: ARMSTRONG A. S. B. , TANTON T. W.
Published: July 2006

Gypsum application was highly efficient as a treatment for reducing amounts of exchangeable sodium and inhibiting clay dispersion in highly structured saline‐sodic clay topsoils.

Phosphogypsum, applied to the surfaces of aggregates in soil columns treated with simulated rainfall, was dissolved efficiently by frequent intermittent ‘rainfall’. The calcium released by dissolution displaced sodium on the exchange complex within soil aggregates ranging from 7.5 mm to 45 mm in diameter. Of the applied calcium, 64–74% was transferred to ion exchange sites; however, almost one‐third of this adsorbed calcium displaced exchangeable magnesium.

Comparison of three gypsum treatments, phosphogypsum, rock gypsum, and a saturated gypsum solution (representing a top dressing of highly soluble gypsum), showed that the effectiveness of calcium uptake on the exchange complex followed the order: rock gypsum < saturated solution < phosphogypsum, but differences were small.

Application of phosphogypsum caused a 90% reduction in the total amount of dispersed clay released in column leachates, and decreased the maximum clay concentration in the effluents by at least 80%, when compared to soils leached without gypsum treatment.
The effects of pH and chloride concentration on mercury sorption. I. By goethite
Authors: BARROW N. J. , COX V. C.
Published: July 2006

The effects of mercury concentration, pH, and chloride concentration on mercury adsorption by goethite were measured. With increasing pH, mercury adsorption increased rapidly at first, until a maximum was reached, and then decreased slowly. In the absence of chloride, the maximum was just below pH 4. With increasing chloride concentration, the maximum moved to higher pH values. This behaviour contrasts with that of the other heavy metals for which, beyond a certain pH, adsorption increases rapidly with further increase in pH.

The results were described by a model in which HgOH+ and HgCl+ ions were adsorbed and the electric potential of the reacting surfaces decreased with increasing pH. The maxima in the adsorption corresponded to maxima in the concentration of HgOH+. Although a good description of the results was obtained, there were consistent discrepancies. These suggested that the charge component of the model was unable to produce the correct relationship between electric potential and pH.

The results indicated that mercury adsorption was controlled by the same principles as adsorption of the other heavy metals. However, mercury's lower pK for dissociation and its higher affinity for chloride mean that the effects of pH on the ions present in solution are different, and this is the cause of the different behaviour.
Chemical extractability of heavy metals during and after long‐term applications of sewage sludge to soil
Authors: McGRATH S. P. , CEGARRA J.
Published: July 2006

Sequential extractions were used on soils from a long‐term experiment treated with either metal‐contaminated sewage sludge or inorganic fertilizers between 1942 and 1961. The four extracts employed were CaCl2, NaOH, EDTA and aqua regia. These showed that large increases in the proportions of Pb, Cu, Zn, Ni and Cd in at least one of the first three fractions occurred during the first 10 years of sewage sludge additions. Cr always remained predominantly in the aqua regia‐soluble fraction. For 30 years after this, including a period of more than 20 years after application of sludges to the field had ceased, there was very little change in the percentage of each metal extracted by each reagent.

Although the ‘residual’ (aqua regia‐soluble) and EDTA fractions usually contained the largest amounts of metals in either sludge‐ or fertilizer‐treated soils, there were clear differences between the metals: Pb represented the largest fraction of any metal extracted by EDTA, Cu of any metal extracted by NaOH and Cd of any metal extracted by CaCl2. The same extractions were made of the sewage sludges that were applied to the field, and the distributions of the metals differed from those found in the treated soils. It was particularly apparent that more Pb and Cu was present as the ‘residual’ (aqua regia) fraction in sludges than in the soils.
b‐Fabric of some benchmark Vertisols of India in relation to their mineralogy
Authors: KALBANDE A. R. , PAL D. K. , DESHPANDE S. B.
Published: July 2006

At present precise information on the b‐fabric of Vertisols is inadequate for gaining a comprehensive knowledge about such soils formed in alluvium of basic igneous and metamorphic rocks and spread in varying agro‐climatic zones of peninsular India. The aim of the present study, on five benchmark Vertisol series (Sarol, Aroli, Nimone, Bellary and Kovilpatti), was to assess the possible reasons for differences in the b‐fabric of the subsoils despite their common shrink‐swell properties, as evidenced by the presence of sphenoids and/or slickensides, and similar clay contents and amounts of fine clay smectite. In Sarol and Aroli soils from the sub‐humid region, the b‐fabric is porostriated, whereas in Nimone and Kovilpatti soils of the semi‐arid and Bellary soils of the arid regions the fabric is either mosaic‐speckled or granostriated, indicating weak plasma separation, a result of restricted swelling of clays. The related distribution pattern of the s‐matrix in all these soils was open porphyric.

Recent literature has suggested that weak plasma separation is an effect of dissolution and crystallization of calcite. However, generally low concentrations of soluble calcium (c. 1 mmol dm−3) in soil solutions of Vertisols in arid and semi‐arid agroclimates suggest that the presence of calcite does not guarantee the presence of sufficient soluble calcium to impair swelling of clays by contracting the diffuse double layer. Weak plasma separation in Bellary and Kovilpatti soils compared with Sarol and Aroli soils is explained in terms of a decrease in the internal surface area of fine smectite, as evident from a very high degree of chloritization in smectite interlayers, and smaller proportions of coarse smectite. In Nimone soils the weak plasma separation is primarily due only to a decreased internal surface area of fine smectite.
The influence of vegetational origin and degree of humification of organic soils on their chemical composition, determined by solid‐state13C NMR
Authors: KROSSHAVN M. , SOUTHON T. E. , STEINNES E.
Published: July 2006

Organic soil samples with different vegetational background and others with variation in the degree of humification, were investigated with solid‐state13C NMR. This work indicates that the vegetational origin and degree of humification of the organic matter appear to influence the distribution of functional groups in organic soils considerably, but one year of decomposition under controlled laboratory conditions gave only small changes in the chemical composition.
Oriented and random sedimentation of plate‐like clay particles in high centrifugal fields
Authors: WHALLEY W. R. , MULLINS C. E.
Published: July 2006

Plate‐like particles can sediment in an oriented manner or completely randomly. The hydrodynamic conditions that lead to an oriented sedimentation are reviewed. The tendency for plate‐like clay particles to adopt oriented sedimentation in high centrifugal fields is demonstrated. Maximum centrifugal fields that still permit random sedimentation are suggested on the basis of experimental observation.