bluuebunny/crossref_metadata_2025 huggingface dataset:
The structural improvement of damaged clay subsoils
Authors: McGOWAN M. , WELLINGS S. R. , FRY G. J.
Published: July 2006

A manually damaged, smeared layer (pan) was created at plough depth in two clay soils. Soil physical properties, soil water regimes and plant responses were measured in panned and non‐panned soil from 1972 to 1975. Continuity of the pan was readily lost on drying by formation of shrinkage cracks which did not fully close unless the soil had been wet for a long time; thus the occurrence of perched water tables above the pan was rare. The pans had no effect on crop yields. Leaf water potential, effective rooting depth and subsoil water deficits all showed that the pans affected water uptake by roots, but these effects were transient. The results indicate that natural amelioration of pans in these clay soils occurs even after slight drying and that the degree but not duration of drying is important.
Multiscale sources of spatial variation in soil. I. The application of fractal concepts to nested levels of soil variation
Authors: BURROUGH P. A.
Published: July 2006

Soil variation has often been considered to be composed of‘functional’ or ‘systematic’ variation that can be explained, and random variation (‘noise’) that is unresolved. The distinction between systematic variation and noise is entirely scale dependent because increasing the scale of observation almost always reveals structure in the noise. The white noise concept of a normally distributed random function must be replaced to take into account the nested, autocorrelated and scale‐dependent nature of unresolved variations. Fractals are a means of studying these phenomena. The Hausdorff‐Besicovitch dimension D is introduced as a measure of the relative balance between long‐ and short‐range sources of variation; D can be estimated from the slope of a double logarithmic plot of the semivariogram. The family of Brownian linear fractals is introduced as the model of ideal stochastic fractals.

Data from published and unpublished soil studies are examined and compared with other environmental data and simulated fractional Brownian series. The soil data are fractals because increasing the scale of observation continues to reveal more and more detail. But soil does not vary exactly as a Brownian fractal because its variation is controlled by many independent processes that can cause abrupt transitions or local second order stationarity. Estimates of D values show that soil data usually have a much higher proportion of short‐range variation than landform or ground water surfaces. The practical implication is that interpolation of soil property values based on observations from single 10 cm auger observations will be unsatisfactory and that some method of bulking or block kriging should be used whenever longrange variations need to be mapped.
A 31P nuclear magnetic resonance study of the phosphorus species in alkali extracts of soils from long‐term field experiments
Authors: HAWKES G. E. , POWLSON D. S. , RANDALL E. W. , TATE K. R.
Published: July 2006

The different forms of phosphorus (P) in 0.5 m sodium hydroxide extracts of soils from long‐term field experiments at Rothamsted were characterized by 31P‐nuclear magnetic resonance spectroscopy (NMR).

The extract from an old grassland soil (pH 4.6) from a plot of the Park Grass Continuous Hay Experiment that had received no fertilizer or lime for at least 125 years contained the following forms of P: inorganic orthophosphate (22% of the extracted P), orthophosphate monoesters (49%), orthophosphate diesters (14%), phosphonates (3%), pyrophosphate (4%) and two unidentified forms of P (7%). The soil extract from a Park Grass plot given inorganic phosphate fertilizer (35 kg P ha−1) annually for 121 years contained the same forms of P and, in addition, a small amount of polyphosphate. There was also evidence of an increase in the orthophosphate monoester fraction.

Another old grassland soil, of pH 6.1, contained more total and organic P than Park Grass but the extract contained fewer forms of P: inorganic orthophosphate (14% of the extracted P), orthophosphate monoesters (39%), orthophosphate diesters (34%) and an unidentified form (13%). An area of this grassland that had been ploughed up 20 years previously, and kept bare since, contained less organic P. The extract contained less of the phosphate diesters but the more stable monoesters remained relatively unchanged.
Soil organic matter maps by nuclear magnetic resonance
Authors: WILSON M. A.
Published: July 2006

Preliminary studies are reported on a soil and a litter fraction which have the aim of exploring the potential of two‐dimensional 13C nuclear magnetic resonance spectroscopy for the analysis of soil organic matter structure. It has been shown that contour plots can be obtained in which one co‐ordinate gives chemical shift information and the other the magnitude of the dipolar interaction. For aromatic and oxygenated aliphatic carbon, the latter parameter is a measure of the degree of protonation. Hence the method is useful for distinguishing between soils containing highly substituted and condensed aromatic structures and those with protonated aromatic carbon. Likewise, it is possible to distinguish between soils containing dioxygenated carbon which is. protonated (e.g. ketal).
The influence of organic matter on aggregate stability in some British soils
Authors: CHANEY K. , SWIFT R.S.
Published: July 2006

The stability of aggregates from 26 soils selected from agricultural areas was measured by wet‐sieving and the results correlated with sand, silt, clay, nitrogen, organic matter and iron contents and with cation exchange capacity. Highly significant correlations were obtained for the relationships between aggregate stability and organic matter and some properties associated with it. No other soil constituent investigated had a significant relationship with aggregate stability, indicating that organic matter is mainly responsible for the stabilization of aggregates in these soils.

The relationships between aggregate stability, and organic matter content plus some of its component fractions were examined in more detail using 120 soils. Total organic matter, total carbohydrate and humic material extracted by various reagents each gave highly or very highly significant correlations with aggregate stability. However, whilst it was not possible to distinguish whether any one organic component was more important than another, the results indicate that soil organic matter levels can be used diagnostically to identify soils which may show problems of structural instability.
The influence of clay mineralogy and diagenesis of Upper Carboniferous shales on soil formation in parts of Devon
Authors: GRAINGER P.
Published: July 2006

There is a correlation between variations in the clay mineral assemblages of the Crackington Formation (Upper Carboniferous) shales in parts of Devon, and the soil series mapped in the same areas. The differences in soil‐forming characteristics of the shales are not due to mineralogy alone, but to various states of diagenetic induration which they have achieved. Brown earths (Dunsford series) are formed on steeper slopes of more durable, illite‐chlorite dominated shales, whereas clayey soils (Tedburn and Halstow series) are typical of flatter, more poorly drained slopes of weaker illite and illite‐kaolinite clay mineral assemblages. This result adds the geological component to earlier views that the different soil series are simply a consequence of physiographic and hydrologic position.
Simultaneous nitrification and diffusion in soil. I. The effects of the addition of a low level of ammonium chloride
Authors: DARRAH P. R. , WHITE R. E. , NYE P. H.
Published: July 2006

The nitrification of NH4+ and the simultaneous diffusion of NH4+, NO3− and H+ following the addition of ammonium chloride to a fine sandy loam soil was analysed experimentally and theoretically. Experimentally, the concentration profiles of mineral N and pH were analysed 140h and 284 h after the homogeneous addition of 11 μmoles NH4Clcm−3 of soil to one part of a composite soil column. The mathematical model presented includes a kinetic model of nitrifier growth and activity, the adsorption equilibria of NH4+ and soil acidity with the soil solid phase and the influence of other ions on the diffusion characteristics of each diffusing ion. The predictions of the model were generated using parameters derived from independent experiments so that the predictions did not depend on data derived from the experimental concentration profiles. Good agreement was found between experimental and predicted profiles. The use of the model for predicting the penetration of NH4+ and NO3− into the soil is demonstrated.
Influence of organic matter content, moisture status and time after reworking on soil shear strength
Authors: DAVIES PATRICIA
Published: July 2006

Recovery of soil shear strength was measured in disturbed samples of a calcareous clay loam soils, with four different organic matter contents. Repacked soil cores were put on tension tables at 25 or 50 cm water tension. Shear strength and water content were measured at one of eight time intervals, from 48 h to 7 weeks after the tension was applied. Soil strength increased with increasing organic matter content or increasing the applied tension. The major factor affecting strength, however, was the length of time that tension was applied. Because water contents showed little or no change with time on the tension table these results demonstrate thixotropic recovery of shear strength. This property is discussed with relevance to the soil's potential for mole drainage.
Ammonia volatilization from urea‐treated pasture and tillage soils: effects of soil properties
Authors: O'TOOLE P. , McGARRY S. J. , MORGAN M. A.
Published: July 2006

Mean NH3 losses after nine days incubation at 18°C and 60% FC were 3.1±2.9% and 7.6±6.0% of applied urea‐N from the pasture and tillage counterparts of 10 soil series. These losses were highly correlated with buffered CEC and maximal pH values (pHm) generated three days after urea application. NH3 volatilization was apparently controlled by buffered CEC and initial pH (R2= 72–87%) and was related to variations in soil organic matter and texture (R2= 77–81%). Losses in the acid pasture soils were attributed largely to initial pH differences, and in the tillage soils to buffered CEC only.

Evolution was greater from the tillage than from the pasture equivalent in eight series. This was attributed to differences in CEC, including buffered CEC and pH‐dependent charge, caused by differences in OM content primarily but also in texture between the two soil groups. Differences in NH3 evolution from urea in pasture and tillage soils, in general, are not related to pH differences.
Studies on aggregate stability. I. Re‐formation of soil aggregates
Authors: CHANEY K. , SWIFT R. S.
Published: July 2006

When natural soil aggregates were destroyed by crushing, techniques traditionally used for re‐forming aggregates, such as wetting/drying and freezing/thawing cycles, did not produce any stable re‐formed aggregates. Incubation without amendment, was similarly unsuccessful, whereas incubation with glucose amendment did produce stable aggregates, and their stability was related both to the natural soil organic matter levels and to the original stability of the natural aggregates. However, the stability induced by incubation with glucose was of a transient nature and declined over a period of 12 weeks. This behaviour was attributed to the production of microbial, extracellular polysaccharides and their subsequent decomposition. Addition of microbial polysaccharides of known structure confirmed that such polymers were capable of producing stable re‐formed aggregates without the assistance of further microbial activity. Longer term incubation showed that the stability of the re‐formed aggregates also declined as soil micro‐organisms broke down the polysaccharide material.

Neither the glucose incubation nor addition of extracellular polysaccharide was very successful in producing stable aggregates when used with soil which had been washed with salt solutions, and dialysed, to form mono‐ionic soils.
Sorption and desorption of cobalt by soils and soil components
Authors: McLAREN R. G. , LAWSON D. M. , SWIFT R. S.
Published: July 2006

The sorption of Co by individual soil components was studied at solution Co concentrations that were within the range found in natural soil solutions. Soil‐derived oxide materials sorbed by far the greatest amounts of Co although substantial amounts were also sorbed by organic materials (humic and fulvic acids). Clay minerals and non‐pedogenic iron and manganese oxides sorbed relatively little Co. It is considered that clay minerals are unlikely to have a significant influence on the sorption of Co by whole soils.

Cobalt sorbed by soil oxide material was not readily desorbed back into solution and, in addition, rapidly became non‐isotopically exchangeable with solution Co. In contrast, Co was relatively easily desorbed from humic acid and a large proportion of the Co sorbed by humic acid remained isotopically exchangeable. Cobalt sorbed by montmorillonite was more easily desorbed than that sorbed by soil oxide but less easily than that sorbed by humic acid.

Cobalt sorption isotherms for whole soils at low site coverage were essentially linear and the gradients of isotherms increased with pH. A comparison of isotherm gradients for whole soils and individual soil components supported the suggestion that Co sorption in whole soils is largely controlled by soil oxide materials.
The classification and mapping of potassium reserves in soils of England and Wales
Authors: GOULDING K. W. T. , LOVELAND P. J.
Published: July 2006

Potassium was extracted from samples of 21 surface soils and nine subsoils using a calcium‐saturated cation exchange resin and concentrated HCI under reflux. Most of the important arable soils of England and Wales were represented in the samples. The two methods permitted determination of the amounts and rates of release of exchangeable K, K fixed in the weathered periphery of vermiculitic and illitic minerals, and K in the unweathered core of mica minerals. Potassium reserves in the soils were classified as High, Medium or Low according to the rate of release of fixed K and the amount and rate of release of mica K. The K reserves of soils in England and Wales could be mapped by soil series using this method of classification. Such mapping would provide data on the extent and availability of reserves in both the short term (i.e. to a crop over a growing season) and long term.
Choosing functions for semi‐variograms of soil properties and fitting them to sampling estimates
Authors: McBRATNEY A. B. , WEBSTER R.
Published: July 2006

The semi‐variogram is central to geostatistics and the single most important tool in geo‐statistical applications to soil. Mathematical functions for semi‐variograms must be conditional negative semi‐definite, and there are only a few families of simple function that meet this demand. These include the transitive models with finite a priori variance deriving from moving average processes. The spherical and exponential schemes are the most often encountered members. The other major group is that of unbounded models in which the variance appears to increase without limit. The linear model is the most common in this group. If more complex models are needed they can be formed by combining two or more simple models.

The usual estimator of the semi‐variance is often considered inefficient and to be sensitive to departures from normality in the data. It is compared with a robust estimator and shown to be generally preferable in being unbiased and having confidence intervals that are no wider. For routine analysis, fitting models to sample semi‐variograms by weighted least squares approximation, with weights proportional to the expected semi‐variance, is preferred to the more elaborate and computationally demanding statistical procedures of generalized least squares and maximum likelihood. The Akaike information criterion is recommended for selecting the best model from several plausible ones to describe the observed variation in soil, though for kriging it may be desirable to validate the chosen model.

Examples of models fitted to soil semi‐variograms are shown and compared.
A comparison of tests of soil structural stability
Authors: MATKIN E. A. , SMART P.
Published: July 2006

Samples from 70 horizons from UK soils were tested for structural stability, and a comparison was made between the results from the following six tests: (i) Emerson, (ii) Janse & Koenigs, (iii) Thorburn, (iv) Childs (soil moisture characteristic), (v) Williams & Cooke slump test, and (vi) wet sieving. Both correlations and systematic differences were observed between the tests, suggesting that different aspects of stability were emphasized by different test procedures. The tests leading to classes (i), (ii) and (iii) were found to be more subjective than those leading to indices. It is suggested that (v) may be applicable to less stable soils, and (vi) to more stable soils. Childs’ test was time consuming and subject to some erratic results; two new indices of stability are proposed to increase the reliability of this test.
Some problems associated with the use of the neutron probe in swelling/shrinkling clay soils
Authors: JARVIS N.J. , LEEDS‐HARRISON P.B.
Published: July 2006

Comparison of neutron probe count rate ratios measured at 10 cm depth, with bulk soil water contents measured 3–5m away from the access tubes, confirmed that in swelling/ shrinking clay soils, recharge may be over estimated. This is due to the drying soil shrinking form the access tube, creating an air gap and radial cracks which allow preferential re‐writing of the soil near the access tube. Installation of access tubes into dry soil was only a short‐term answer.

Comparison of results obtained for bulk soil and for small colds shows that calibrating the neutron probe using small diameter cores may be lead to serious under estimates of changes in water content in shrinking clay soils.
Inorganic anion sorption and interactions with phosphate sorption by hydrous ferric oxide gel
Authors: RYDEN J. C. , SYERS J. K. , TILLMAN R. W.
Published: July 2006

The sorption of inorganic anions by hydrous ferric oxide gel (Fe gel) from 10 −1 M NaClO4 at pH 6.5 decreased in the order: orthophosphate (H2PO4)>arsenate (H2AsO4) = selenite (HseO3) > silicate (H4SiO4) > molybdate (MoO24−) > sulphate (SO24−) > selenate(SeO24−)>chloride (Cl−) = nitrate (NO−3). When each anion was added to Fe gel with an equimolar quantity of H2PO−4, there was no detectable effect of SO24−, SeO24−, Cl−, and NO−3 on the amount of H2PO−4 sorbed. Other anions depressed H2PO4 sorption in the order H2AsO4 >HseO3 > H4SiO4 > MoO24. At the lowest level of anion addition (500 mmol kg −1), H2PO4 sorption was depressed by no more than 6% of the sorption level in the absence of a competing anion. In contrast, at the highest level of anion addition (1700 mmol kg‐1 of each), H2AsO4 decreased H2PO4 sorption by 44%. The sorption of SO4− was completely eliminated when this anion was added with equimolar amounts of H2PO4. The ability of anions to compete with H2PO4 for sorption sites could not be explained solely by the results obtained for the sorption of each anion alone. Thus, H2AsO4 was more competitive than H2PO4 when added together, even though more H2PO4 than H2AsO4 was sorbed when each anion was added alone. Although H2PO4 was sorbed in larger amounts, there is no evidence to suggest that H2PO4. H2AsO4, and HseO3 were sorbed on different sites.
The elucidation of soil pattern in the Wyre Forest of the West Midlands, England. II. Spatial distribution
Authors: OLIVER M. A. , WEBSTER R.
Published: July 2006

The spatial distribution of soil in the Wyre Forest of England was analysed in two phases. In the first the soil was examined at sites chosen using a five‐stage nested design with spacings increasing geometrically from 6 m to 600 m. Some 80% of the variance was contributed by components for the spacings between 6 m and 60 m. Measurements were then made on transects at 5 m intervals and semi‐variograms estimated to 70 m. Except for those of pH the semi‐variograms of the soil properties had the same general transitive form and a common effective range of about 40 m. This short range meant that very intensive sampling, approximately one point per 400 m2, is needed to map the soil variation. A survey was made of a small portion of the forest on a 20m × 25 m grid to test the inference. Maps of clay and sand content were made successfully by kriging from the data. The mutual correlations between soil properties and the common range of their semi‐variograms enabled a general purpose and spatially coherent soil classification to be created from the data. Its mapping confirmed the intricacy of the soil pattern in the Forest.
Andosols developed from non‐volcanic materials in Galicia, NW Spain
Authors: GARCIA‐RODEJA E. , SILVA B. M. , MACÍAS F.
Published: July 2006

A set of well‐drained soils in Galicia, formed from gabbros, amphibolites and schists, with udic moisture and mesic temperature regimes, are studied. All but one of the surface horizons show an‘exchange complex dominated by amorphous material’(ECDAM); consequently, most soils are classified as Andosols (FAO) or as different subgroups of Dystrandepts (Soil Taxonomy). The soils on gabbros and amphibolites frequently have‘andic properties’so, following the ICOMAND proposals, five of the 12 soils studied may be characterized as Andisols. Many of the five soils have properties comparable to those of soils from volcanic areas, especially to those called‘non‐allophanic Andosols’, which have an abundance of active‐Al, mainly in the form of Al‐humus complexes.
Extraction and characterization of water–repellent materials from Australian soils
Authors: MA'SHUM M. , TATE M. E. , JONES G. P. , OADES J. M.
Published: July 2006

Organic materials responsible for water–repellency in some Australian soils were extracted with an amphiphilic mixture of iso‐propanol/15.7 m ammonia (7:3, v:v) in a Soxhlet apparatus, after which the water–repellent soils were rendered wettable. The successful extraction by an organic solvent system indicates that the bulk of hydrophobicity in these soils is not covalently linked to the surface of the sand. The extracted materials restored hydrophobicity on acid washed sands or ignited sands at levels comparable to the original soils.

Spectroscopic and chromatographic examination of the extracted materials indicated that both free and esterified long–chain, 16–32 carbon atom, fatty acids were present with a bimodal distribution showing maxima at C16 and C22. The 13C–NMR and infrared spectra of the most hydrophobic extract suggest that hydrophobicity is caused by molecules with extensive polymethylene chains. Calculations with model compounds indicate that at least a close packed monolayer is required before measurable hydrophobicity can be detected with the molarity of ethanol droplet penetration test.
The significance of gley features in soils derived from grey parent materials
Authors: MOFFAT A. J. , JARVIS M. G.
Published: July 2006

Detailed observations of gley features in soils derived from grey Upper Greensand rocks have been compared with their moisture regimes as determined by water‐levels in dipwells. There was good correlation between the incidence of grey ped face and matrix colours of low chroma, prominent ochreous mottling within the matrix and duration of waterlogging. Careful examination of soil macromorphology can be used, despite inherent greyness, to assess the drainage status and Wetness Class of these soils.

Physical and micromorphological analyses suggest that the wetness of some Upper Greensand soils is due to the combination of low vertical permeability in the underlying rock, and moderate horizontal permeability in the overlying horizons. Such circumstances allow perched water‐tables to form in wet weather, sustained by lateral flow when rainfall has ceased.
The role of diffusion in the kinetics of phosphate desorption: the relevance of the Elovich equation
Authors: PAVLATOU ATHENA , POLYZOPOULOS N. A.
Published: July 2006

Various kinetic expressions were tested with phosphate desorption data from four contrasting soils and found inadequate, but the data supported a model recently proposed by Aharoni and Suzin. This model assumes that chemisorption reactions are kinetically controlled by diffusion and integration of Pick's equation, under appropriate boundary conditions, results in an expression for the time dependence of adsorption or desorption, that can be approximated by a sequence of equations‐parabolic, Elovich and exponential–at small, intermediate and large times, respectively. Thus, full or even partial conformation of data to such a sequence would suggest diffusion control of the rate determining step, a suggestion usually relying on a time relationship alone.

Since true rate constants could not be derived from the other expressions tested, calculation of physically meaningful activation energies was not possible. An approach to the calculation of activation energies at different stages of the process, based on the model of Aharoni and Suzin, and the difficulties encountered are also discussed.
The use of ignition loss to estimate bulk density of forest soils
Authors: HONEYSETT J. L. , RATKOWSKY D. A.
Published: July 2006

136 samples were collected from soils under wet schlerophyll forest in southern Tasmania. The samples were from different depths and covered a wide range of bulk density and organic matter content. A simple linear relationship between specific volume (the reciprocal of bulk density) and low‐temperature ignition loss was established. Since the variance about the regression tended to be homogeneous with bulk density rather than specific volume, the relationship was fitted as a generalized linear model using bulk density as the dependent variable with an ‘inverse’ link and normal error assumption.

For soils with loss‐on‐ignition at 375 ± 5°C (I) ranging from 1.8% to 87.9%, the bulk density, pb was given by an equation of the form l/pb=α+βI, where α and β are constants related to the bulk densities of the mineral and organic matter of the soils studied.

The existence of the simple linear relationship in the deterministic part of the model implies that these soils are made up of a mineral framework and organic matter, with neither of the two components penetrating the interstices of the other.
Testing a mechanistic model. IX. Competition between anions for sorption by soil
Authors: BARROW N. J.
Published: July 2006

A model which describes the reaction of anions and cations with soil when they are added separately was applied to the results of Roy et al. (1986a,b) in which pair‐wise combinations of phosphate, arsenate, and molybdate competed for sorption on three soils. The model postulates that the reaction is between ions in solution and charged surfaces of the soil particles. The surfaces are assumed to be non‐uniform and to differ in their electric potential. Competition comprises both ordinary competition between ions for adsorption sites and the electrostatic interactions which occur because reaction with one anion makes the surface more negative and, therefore, less favourable for reaction with a second anion. The equations of the model were expressed in such a way that the outcome was related to the initial anion concentrations.

With appropriate choice of parameters, the model described sorption both when the anions were added singly and when they were added in competing pairs. This shows that the model is comprehensive in that it can be extended to competing pairs of ions. It also shows that simpler descriptions of competition which neglect the electrostatic effects are unlikely to be comprehensive.
A method for the dehydration and impregnation of clay soil
Authors: MORAN C. J. , McBRATNEY A. B. , RINGROSE‐VOASE A. J. , CHARTRES C. J.
Published: July 2006

Difficulty is sometimes experienced in achieving dehydration and acceptable impregnation of undisturbed specimens of clay soil. It is necessary to avoid shrinkage when removing water and disruption of brittle dehydrated specimens when impregnating. Methods for dehydrating clayey soil for structural studies are reviewed. A method using continuous flow and dehydration of acetone or 1,4 dioxane followed by slow impregnation is outlined. Inexpensive techniques for measuring the water content of these two solvents are described.
The effect of bulk density, water content and soil type on the diffusion of chloride in soil
Authors: SO H. B. , NYE P. H.
Published: July 2006

The relative importance of soil bulk density, water content and potential on the self‐diffusion and impedance factors of 36C1 in a sandy loam and loamy clay were studied. The soil bulk densities used represented a range of conditions from freshly tilled seedbeds to compacted soils. The volumetric water contents and pF were the main factors controlling the soil impedance factors, with bulk density making a small but significant contribution.

Soil type affects the impedance factors through differences in anion exclusion volumes, the water contents of poorly connected pores that contribute little to the diffusion process, and tortuosity of the diffusion pathways.
Three approaches to the simulation of the self‐diffusion and non‐instantaneous isotopic exchange of phosphate in soil
Authors: STAUNTON S. , NYE P. H.
Published: July 2006

Three models of the self‐diffusion of phosphate in moist soil are presented, and their predictions compared to experimental data. The latter appeared to show that isotopic exchange was limited by access to exchange sites. The apparent rate of access was first described by a series of first‐order reactions, then as the diffusion of isotope into, and within, uniformly distributed, porous spherical aggregates, where the majority of exchange sites were situated, and finally as an Nth order (N 1) reversible reaction. All models were solved using Crank‐Nicolson finite difference methods.

The first two gave similar predictions; they were not capable of giving an adequate fit to experimental results. The third, although purely an empirical model, not based on a possible mechanism, was able to simulate data well.

The mechanistic implications of each model, and of its relative success in simulating phosphate diffusion are discussed.
A rapid method for predicting the lime requirement of acidic temperate soils with widely varying organic matter contents. II. Testing the lime requirement model
Authors: BAILEY J. S. , STEVENS R. J. , KILPATRICK D. J.
Published: July 2006

A model for predicting lime requirement (Bailey et al., 1989) was tested using data from independent liming experiments on soils from Northern Ireland and New Zealand. Lime requirements predicted by the model were also compared with those obtained by the methods currently used by the Department of Agriculture for Northern Ireland and by the Ministry of Agriculture, Fisheries and Food in England and Wales.

The model predicted lime requirement precisely for the independent set of soils from Northern Ireland, and was as reliable at predicting lime requirement for New Zealand soils as the best of seven other quick‐test methods. The model was more reliable at predicting lime requirement for soils from Northern Ireland than the method currently used by the Department of Agriculture for Northern Ireland, and appeared to be as reliable as the method used by the Ministry of Agriculture, Fisheries and Food in England and Wales.
Molecular weight distribution of soil organic phosphorus under irrigated pasture in New Zealand
Authors: CONDRON L. M. , GOH K. M.
Published: July 2006

Gel filtration chormatography (Sephadex G‐100) was used to examine the combined effects of time and single superphosphate fertilizer application on the molecular weight distribution of organic phosphorus (P) extracted from soils under irrigated pasture. Results showed that while most (61‐83%) of the extracted organic P was present in the low molecular weight fraction (i.e. < 100 000 MW), the proportion of high molecular weight organic P (i.e. > 100 000 MW) increased with time from 17‐19% in 1958 to 38‐39% in 1983. Furthermore, organic P, which accumulated in the soil as a result of annual P fertilizer additions appeared to be evenly distributed between the low and high molecular weight fractions.
A simplified analysis of soil water flow to a mole drain
Authors: SCOTTER D. R. , HENG L. K. , HORNE D. J. , WHITE R. E.
Published: July 2006

An analysis is described which allows transient mole‐drain flow, surface runoff and water table depth to be computed, given measured or assumed values for rainfall, evaporation, deep percolation, mole‐drain spacing and depth, surface detention, the drainage coefficient, and the saturated hydraulic conductivity, total porosity and macroporosity of the topsoil and subsoil. Dupuit‐Forchheimer flow below the water table and vertical hydraulic potential equilibrium above the water table are assumed. The analysis was successfully tested by simulating the hydrographs from a mole‐pipe drained silt loam, and comparing the results with the measured hydrographs for three contrasting rainfall events.
Frost action in clay soils. I. A temperature‐step and equilibrate differential scanning calorimeter technique for unfrozen water content determinations below 0°C
Authors: BROWN S. C. , PAYNE D.
Published: July 2006

A modified differential scanning calorimeter procedure was used to determine the relationship between temperature and unfrozen water content (UWC) in seven clays between 0°C and – 15°C to – 24°C. The procedure, which involved nucleation by a cooling followed by a rapid warming to – 0.l°C, avoided the difficulty of supercooling. Problems caused by temperature lag between the sample and its holder and by slowness of water movement within the sample, important even when cooling clay samples of about 17 mm3 at a rate of 0.31 K min−1, were overcome by cooling in a stepwise fashion which allowed time for equilibration between each temperature step. The clays studied included sodium and calcium Wyoming bentonites, < 1 μm and 1–2 pm fractions of St Austell kaolinite and three subsoil clays from England. After nucleation and as temperature fell below OT, water froze in two stages in most of the clays. UWCs at the minimum temperatures reached were up to 38% for the bentonites, between 9% and 20% for the subsoil clays and <4% for the kaolinites.
A critical evaluation of the factors influencing the dissolution of Gafsa phosphate rock
Authors: ROBINSON J. S. , SYERS J. K.
Published: July 2006

A carbonate‐substituted phosphate rock (Gafsa, GPR) was allowed to dissolve in the presence of one or more of the following: an acid pH buffer (a source of protons), hydrous ferric oxide gel (Fe gel, a sink for P), and a cation exchange membrane (CEM, a sink for calcium). This allowed a quantitative evaluation of the individual and combined influences of three of the most important soil‐related factors (pH, P sink, and Ca sink) which control phosphate rock (PR) dissolution in soils.

At 44 d, pH alone (pH 6.1 compared to 4.5) had little effect on the extent of dissolution (1.5 and 6.0% dissolution of P, respectively), probably because of the Ca common‐ion effect resulting from the preferential dissolution of the more‐soluble CaCO, accessory phase. The inclusion of a P sink at pH 6.2 resulted in only a 3.2% dissolution of P after 44 d, whereas at pH 4.5 it increased dissolution from 6.0% to 9.8%. This suggests that pH and P sorption have an interactive effect in influencing GPR dissolution.

Inclusion of a Ca sink had the largest influence on GPR dissolution. This was due to the ability of the Ca sink to remove accessory CaCO2. At pH 6.2 the dissolution of P was 10 times larger in the presence of a Ca sink than it was in the presence of a P sink. Despite the smaller ratio of Ca sink size to GPR‐Ca content relative to that of the P sink size to GPR‐P content, the Ca sink was more effective in promoting GPR dissolution. The presence of the Ca sink resulted in twice as much dissolution of P at pH 4.5 than at pH 6.5. At pH 4.5, and in the presence of both the Ca sink and the P sink, 95% dissolution of GPR occurred over the 44‐d experimental period.
Influence of natural and synthetic humic substances on the activity of urease
Authors: VAUGHAN D. , ORD B. G.
Published: July 2006

The activity of a purified urease, obtained from Bacillus pasteurii, was inhibited by humic and fulvic acids obtained from an agricultural soil. Enzyme kinetic studies showed that the humic substances affected the affinity of the enzyme for its substrate (Km) and the maximum velocity of the reaction (Vmax). The Vmax was inhibited to the same extent by both humic (HA) and fulvic (FA) acids, the precise effect depending on the pH and concentration of humic substance. At pH 4.0, HA concentrations of 25 pg cm−3 and 10 μg cm−3 inhibited the Vmax by 38.5% and 20% respectively. HA and FA had similar effects on the Km but in this case the lowering of the affinity of the enzyme for its substrate was not concentration dependent in the range 0–25 μg cm−3 of humic substance. Typically, the affinity was decreased from a KM of 50 mM in the control to 67 mM in the presence of HA and FA.

The effects were not due primarily to the ash or N contents of the humic substances because de‐ashed humic acid and synthetic model humic (made from catechol, guaiacol, pyrogallol, resorcinol and protocatechuic acid) and fulvic acid (made from polymaleic acid), containing virtually no ash or N, were equally as effective. The effect was not related to the phenolic monomers which, before polymerization, had no effect on urease activity.
Characterization of the Fe(III)—fulvic acid reaction by Mössbauer spectroscopy
Authors: GOODMAN B. A. , CHESHIRE M. V. , CHADWICK J.
Published: July 2006

Fulvic acids have been isolated from a sandy loam (Countesswells series) and a clay soil (Tipperty series) and the products of their reaction with different amounts of iron over a range of pH from 0.5 to 11 analyzed by Mössbauer spectroscopy. Three distinct types of spectral component were detected at 77 K, a sextet from magnetically dilute Fe(III) and doublets from Fe(II) and Fe(III), the last arising from both organic complexes and poorly crystalline oxide species.

In iron‐fulvic acid mixtures the proportion of iron as Fe(II) increased as the pH was lowered from 5 to 1 by the addition of hydrochloric or nitric acid at all Fe to fulvic acid ratios (1:5 to 1:500). When the pH was lowered below 1 the amounts of Fe(II) decreased with the lower Fe to fulvic acid ratios, but increased with the higher ratios. The amounts of the Fe(III) component contributing to a doublet signal decreased with decreasing Fe:fulvic acid ratios.

At low iron concentrations the iron appears to be strongly bound to the fulvic acid, but when the iron content is of the order of 1–2% uncomplexed Fe(III) species can be present. At pH > 2 these are hydrolysed ions which form poorly‐crystalline oxides at higher pH. This was confirmed by analysis of spectra at 4.2 K. At pH < 2 free ions are present in solution.

In solutions with high fulvic acid contents (greater than 100‐fold excess) the reactions with iron are completely reversible, but in solutions with a lower proportion of fulvic acid to iron, where free ions are present, there is a lack of reversibility.
Measuring the diffusion coefficients or rhizosphere exudates in soil. II. The diffusion of sorbing compounds
Authors: DARRAH P. R.
Published: July 2006

A mathematical model was developed which was capable of simulating the diffusion of compounds with a concentration‐dependent diffusion coefficient, by virtue of their sorption on the solid phase of the soil. Methods were developed which allowed the sorption characteristics and the effective diffusion coefficient of several organic compounds to be inferred from their measured diffusion profiles.
Comparison of various methods for the determination of specific surfaces of sub soils
Authors: CHURCHMAN G. J. , BURKE C. M. , PARFITT R. L.
Published: July 2006

Retention of ethylene glycol monoethylether (EGME) and ethylene glycol (EG), and sorption of nitrogen and water vapour, were studied on a wide range of soils from New Zealand and Fiji. EGME retention was mainly affected by the amount of EGME liquid present in the desiccator, the interaction with the desiccant (CaCl2), the amount of residual water in the samples, and the time taken to establish end‐points. The extent to which each of these factors influences the weight of EGME retained was different for each sample. Surface areas from nitrogen sorption (BET) were not related to those derived from EGME retention in the same way for all soils; EGME retentions did not vary systematically with clay mineralogy. A constant conversion factor cannot be used to convert retentions of polar liquids (EGME, water and EG) to specific surface values, and retention data by itself cannot be used to measure the absolute surface areas of soils.
Evaluation of porous cup soil‐water samplers under controlled field conditions: comparison of ceramic and PTFE cups
Authors: BEIER C. , HANSEN K.
Published: July 2006

The value of soil water samples used in ecological studies is highly dependent on the quality of the samplers. Tension soil‐water samplers are widely used to extract soil solutions, and the samplers are often tested in the laboratory under conditions that differ significantly from field conditions. This study describes a field procedure useful for comparison of two different tension soil‐water samplers. Ceramic and PTFE cups are compared. There were no differences in the concentrations sampled by the two different types of sampler for Na+, K+, Ca2+, Al3+, NH4+, H+ and NPOC (non‐purgeable organic carbon). Change in the applied vacuum in the range 0 to –0.4 × 105 Pa did not change the concentrations of chemical species in the collected soil water. The ceramic cups collected significantly larger amounts of water due to differences in the hydrostatic characteristics of the two samplers. It was found that the ceramic samplers collected the highest concentrations of Mg2+ in some situations. The results are evaluated and discussed in relation to the possible sources of errors and the temporal and spatial variabilities.
Forms of soluble aluminium in acidic topsoils estimated by ion chromatography and 8‐hydroxyquinoline and their correlation with growth of subterranean clover
Authors: WHITTEN M G. , RITCHIE G. S. P. , WILLETT I. R.
Published: July 2006

Labile Al in the soil solution measured by 8‐hydroxyquinoline (AlHQ) was a better predictor of plant growth than trivalent Al (AlIC3+) measured by ion chromatography (IC). HQ reacted with some organic Al complexes which did not separate during chromatography. In the presence of oxalic acid, AlHQ was greater than AlIC3+, which was the same as the greater than Al3+ (Al3+Calc), whereas in the presence of citric acid, AlHQ was greater than AlIC3+, and both were greater than Al+3Calc, In extracts of soils that had been acidified, Al3+IC was less than AlHQ, which was similar to Al3+Calc, when it was assumed that the only complexing ligands were OH− and F−. The proportion of Al3+IC in the soil solutions decreased more than AlHQ as the pH increased. Organic ligands appeared to form complexes with Al at the expense of AIF complexes. Forms of Al detected by IC differed in CaCl2 extracts and soil solutions. AlHQ in the CaCl2 extracts and soil solution were closely correlated, although the proportion of AlHQ was higher in the CaCl2 extracts. And soil solutions. AlHQ in the CaCl2 extracts and soil solution were closely correlated, although the proportion of AlHQ was higher in the CaCl2 extracts.
The effects of pH and chloride concentration on mercury sorption. II. By a soil
Authors: BARROW N. J. , COX V. C.
Published: July 2006

The pH of a soil was altered by incubation with either acid or lime, and the sorption of mercury was measured in the absence of chloride and at three chloride concentrations. In the absence of chloride there were only small effects of pH on sorption between pH 4 and 6; sorption decreased at higher pH. Addition of chloride decreased sorption at low pH but had little effect on sorption at high pH. Consequently, in the presence of chloride, sorption increased with increasing pH between pH 4 and 6 and then decreased. Many of the mercury sorption curves were sigmoid. This was explained by assuming that a small amount of complexing material was present in the solution after mixing with the soil. Calculation of the mercury species present in solution was made difficult by uncertainties about the postulated complexing material. Nevertheless, between pH 4 and pH 5.8, it was possible to explain the effects of pH and of chloride concentration on sorption as entirely due to changes in the HgOH+ concentration.
Nature of soil carbohydrate and its association with soil humic substances
Authors: CHESHIRE M. V. , RUSSELL J. D. , FRASER A. R. , BRACEWELL J. M. , ROBERTSONS G. W. , BENZING‐PURDIE L. M. , RATCLIFFE C. I. , RIPMEESTER J. A. , GOODMAN B. A.
Published: July 2006

The fulvic acid and alkali‐soluble polysaccharide fractions of a sandy loam arable soil of the Countesswells series have been subjected to acid hydrolysis or methylation and the products examined by infra‐red and NMR spectroscopy, pyrolysis mass spectrometry and chemical analysis. Infra‐red and NMR spectroscopy of the polysaccharide fraction indicated that the substance was predominantly carbohydrate, although sugars accounted for less than one‐third of the weight by chemical analysis. Pyrolysis mass spectrometry con‐firmed the presence of sugars by sugar anhydride formation, but also showed the presence of ‘secondary’ or ‘pseudo’ polysaccharide. The fulvic acid contained only 2–3% sugars by chemical analysis, whereas a much larger carbohydrate component was suggested by physicochemical analyses. Infra‐red and pyrolysis mass spectrometry difference spectra for the residues after acid hydrolysis indicated the release of material with some of the characteristics of glycoprotein. Most of the carbohydrate present in the fulvic acid was of the secondary or pseudo polysaccharide type. The hypothesis that the secondary polysaccharide could be a degraded polysaccharide structure in which some sugar residues have been partly transformed to melanoidins by Maillard reaction is explored.
The physical and chemical composition of mounds of Macrotermes carbonarius (Hagen) (Termitidae, Macrotermitinae), in Penang, Malaysia
Authors: McCOMIE L. D. , DHANARAJAN G.
Published: July 2006

The mound of the fungus‐growing termite Macrotermes carbonarius (Hagen) contained significantly larger proportions of clay (P<0.05) than the adjacent soil. The largest clay content was found in the brood chamber. Concentrations of organic carbon, nitrogen, potassium and calcium in various parts of the mound were enriched compared to the adjacent soil at 10–40 cm depth from which they might have been derived. The activities of M. carbonarius did not significantly alter the pH or the bulk density of the mound soil compared to the adjacent topsoil.