bluuebunny/crossref_metadata_2025 huggingface dataset:
MODIFICATION OF A METHOD TO MEASURE THE ELECTROPHORETIC MOBILITY OF CLAYS
Authors: DIXIT S. P.
Published: July 2006

Modification of the electrode assembly of a counter current paper electrophoresis method kept the pH of the suspensions stable, and gave a method suitable for studying the electrophoretic behaviour of soil colloids.
OPTIMAL INTERPOLATION AND ISARITHMIC MAPPING OF SOIL PROPERTIES III CHANGING DRIFT AND UNIVERSAL KRIGING
Authors: WEBSTER R. , BURGESS T. M.
Published: July 2006

Universal kriging is a form of interpolation that takes account of local trends in data when minimizing the error associated with estimation. The presence of such trends, or drifts as they are known, is identified qualitatively, and their form found quantitatively by structural analysis, which simultaneously estimates semi‐variances of the differences between the drift and actual data. The resulting semi‐variograms are then used for the interpolation.

The method was applied to measurements of electrical resistivity made in the soil at 1 m intervals at Bekesbourne, Kent. Analysis showed that the data could be adequately represented as a series of linear drifts over distances of 4 m to 8 m and with negligible nugget variance. Semi‐variances of residuals from the drift were computed, and used to krige missing values and so complete an isarithmic map of the site.

The method is by no means universally applicable in soil survey, mainly because of the large nugget variances usually encountered. These effectively prevent any distinction between constant and changing drift. They arise in part because measurements are made on small widely separated volumes of soil. Universal kriging is likely to be profitable only where measurements are made on contiguous volumes of soil or after substantial bulking.
THE POTASSIUM STATUS OF MALAWI SOILS
Authors: MAIDA J. H. A.
Published: July 2006

Twenty surface soils from four main Soil Groups in Malawi and their sub‐soils were divided into three groups based on cation exchange capacity (group I, CEC < 50; group II, 50–100; and group III > 100μeq g–1). In each soil group the maximum amounts of K removed by successive extraction with 0.005 M CaCl2 solution were well related to the potassium potential pK–0.5p(Ca + Mg), exchangeable K, ‘step K’, and the quotient ‘step K’/CR.K, where CR.K is ‘constant‐rate’potassium. In Group III soils only, ‘step K’/CR.K values were significantly correlated with pH, clay, and CEC, and this suggested that the soils were relatively rich in K+ specific binding sites.

In 27 soils from an NPK factorial experiment on tea, the rate of depletion of extractable K reserve increased with ammonium sulphate treatment, whereas K fertilizers tended to off‐set significantly (P= 0.001) the depletion of K reserve. The values for the change in free energy ΔG =RT In aK/ (a(Ca+Mg)) ½, ranged from –12 to –16 kJ mol–1, and field observations showed that tea plants growing on soils having ΔG values less than –15 kJ mol–1 responded to K fertilizers. The investigation has indicated that heavily cropped soils are likely to show crop responses if the intensive cropping system does not include supplementation of K.
TOWARDS A DIRECT, NON‐DESTRUCTIVE METHOD OF CHARACTERISING SOIL HUMIC SUBSTANCES USING 13C N.M.R.
Authors: NEWMAN R. H. , TATE K. R. , BARRON P. F. , WILSON M. A.
Published: July 2006

The structure of a humic acid from a New Zealand pasture topsoil has been investigated by conventional solution 13C n.m.r. spectroscopy and by the cross polarisation technique. Relaxation results showed that a pulse spacing of about 0.2 s was optimum for n.m.r. of the humic acid dissolved in 0.5 M aqueous NaOH. Aromatic rings accounted for 26 per cent of the total carbon. Cross polarisation n.m.r. is a viable method for obtaining 13C spectra of solid humic acids, although aromaticity cannot be estimated as accurately as for coals.
THE USE OF ADSORPTION EQUATIONS TO DESCRIBE COPPER COMPLEXING BY HUMIFIED ORGANIC MATTER
Authors: SANDERS J. R.
Published: July 2006

Samples of humified organic matter were extracted from several sources by water or alkali, and divided into colloidal and dialysable fractions. The variation of the concentration of complexed copper with that of added copper was determined for these fractions using a cupric ion‐sensitive electrode. A computer program fitted five equations often used to describe adsorption of ions on soil surfaces to the results. The data did not fit Langmuir or Temkin equations very well, probably because of the heterogeneity of adsorption sites of the organic matter; they fitted double Langmuir, Gunary and Freundlich equations almost equally well, leaving very small residual sums of squares. The Freundlich isotherm is preferred as it has only two parameters and the values of these are fairly stable to changes in the range of copper concentrations considered, or to doubling of the complexant concentration.
SOLUBILITY AND SORPTION OF CADMIUM IN SOILS AMENDED WITH SEWAGE SLUDGE
Authors: SOON Y. K.
Published: July 2006

The mechanisms governing the retention and release of Cd in two soils, a loam and a loamy sand, pretreated with anaerobically digested sewage sludges or with chemical fertilizers, were studied using batch equilibration in 0.05 m Ca(NO3)2 solution containing up to 6 μg Cd/ml. Adsorption rather than precipitation as Cd3(PO4)2 limited solution Cd2+ concentration. With the addition of 50 μg Cd/g, however, precipitation as CdCO3 was likely at pH 7.6. Cadmium adsorption increased with increasing soil pH. The differences in Cd adsorption between different soil treatments were attributed mainly to the soil pH (6.9 to 7.9) induced by sludge application. About 82 to 92 per cent of adsorbed Cd was retained by cation exchange and complexing sites. Soils treated with sludge increased the amount of exchangeable Cd but reduced the amount of complexed Cd compared with the fertilized soil. Cadmium retention by cation exchange became more dominant as the amount of Cd in the soil was increased.
STUDIES OF SOIL ORGANIC MATTER USING FORMIC ACID SOLVENTS: FACTORS AFFECTING COMPOSITION AND DEGREE OF FORMYLATION OF ORGANIC ISOLATES
Authors: SINCLAIR A. H. , TINSLEY J.
Published: July 2006

Two soils were extracted under reflux with formic acid/H2O for 0.25 h, followed by formic acid/HF for 0.25 h. In one set of experiments 14C‐labelled formic acid was used in the first extraction and unlabelled formic acid in the second: in a second set this labelling sequence was reversed. After separation from the soil residues, the soil extracts were further refluxed for up to 3.75 h. Organic matter was then precipitated from the various extracts by addition of di‐isopropyl ether. Prolonged boiling of the extracts reduced the recovery of organic matter and nitrogen, and increased the inorganic contamination of the organic isolates.

The degree of formylation of the isolates from the first extraction with formic acid/H2O increased with reflux time to 0.50 h and thereafter decreased, but overall it was appreciably greater than that of the isolates from the second extraction with formic acid/HF. Formylation of the latter increased slowly with reflux time up to 4.00 h. In both solvents hydrolysis and formylation of organic solutes occur concurrently during refluxing.

Treatment of formic acid/H2O extracts with a TiCl4/HCl/propan‐2‐ol reagent to precipitate organic matter and washing the precipitate in a hydrous HF medium resulted in lower yields than precipitation with di‐isopropyl ether, but the final washed precipitate contained much less formyl groups and inorganic material.
SOIL SEGMENT OVERLAP IN CHARACTER SPACE AND ITS IMPLICATIONS FOR SOIL CLASSIFICATION
Authors: WEBSTER R. , McBRATNEY A. B.
Published: July 2006

A major problem in soil classification for soil survey is the lack, or uncertainty, of correspondence between mapping units in different localities. The problem is examined using multivariate soil data recorded at short regular intervals along transects in Oxfordshire and Aberdeenshire. The data for each transect were transformed to canonical variates, the first two of which were then used to locate soil boundaries and to show the relationships among the sampling points in that projection of the character space.

With few exceptions, the sampling points in adjacent segments on all three transects lay in different parts of the canonical variate (CV) plane, showing that the boundaries were well defined. On the transect near Witney, Oxfordshire, distant segments either occupied distinct parts of the CV plane, or were superimposed on it. For the other two transects the portions of the CV plane occupied by some segments overlapped only partially the space occupied by one or more other segments that were not contiguous on the transects. The results show that on two of the three examples, lack of correspondence between mapping classes in different localities is already present in small tracts of country.
THE BASIC FACTORS WHICH DETERMINE INHERENT SOIL FERTILITY IN UGANDA
Authors: FOSTER H. L.
Published: July 2006

Soil samples collected from approximately 1700 fertilizer trials carried out around 62 centres in Uganda were analysed. Mean levels of total N. organic P and cation exchange capacity at the 60 centres which had non‐volcanic soils were very closely related to mean organic matter levels and largely explained the strong correlation of indices of available N, P and base status with organic matter. The indices of available P and base status were additionally related to pH and the indices of available N and P were further negatively correlated with sunshine hours.

Soil organic matter itself was closely related to the content of clay, silt and free Fe. with which it is presumed it forms a stable complex, whilst soil pH decreased with rainfall in excess of evaporation. Thus soil texture, rainfall and hours of sunshine are the basic factors which largely determine inherent soil fertility in Uganda.
SOIL FRIABILITY
Authors: UTOMO W. H. , DEXTER A. R.
Published: July 2006

Soil friability is defined and a method for its measurement is developed from the theory of brittle fracture of soil aggregates. The variation in friability with soil water content is in good agreement with the results of tillage experiments done by earlier workers. It is found that Urrbrae and Strathalbyn sandy loams are most friable at water contents approximately equal to their Casagrande Plastic Limits.

The method is applied to study the effects of wetting and drying cycles, freezing and thawing cycles, and phosphoric acid treatment on the friability of the Urrbrae soil. It is found that all of these treatments increase soil friability as defined.
THE DETERMINATION OF IONIC DIFFUSION COEFFICIENTS IN FIELD SOILS. I. DIFFUSION COEFFICIENTS IN SIEVED SOILS IN RELATION TO WATER CONTENT AND BULK DENSITY
Authors: BARRACLOUGH P. B. , TINKER P. B.
Published: July 2006

A method for measuring diffusion coefficients in large cores of structured field soils has been developed and tested, in the first instance using sieved soil. Bromide‐chloride and bromide‐nitrate counter diffusion coefficients have been determined using ion exchange membranes as sinks for the diffusing ions. The effects of membrane selectivity, soil surface preparation, solution concentration, water content and bulk density have been investigated. The organic colloid content appears to have a considerable influence on the impedance factor‐water content relationship of a soil. Increasing the bulk density at constant volumetric water content linearly reduced the impedance factor by up to 30 per cent. The relationship between impedance factor and bulk density is discussed.
THE ADSORPTION OF COPPER BY SOIL MATERIALS AT LOW EQUILIBRIUM SOLUTION CONCENTRATIONS
Authors: MCLAREN R. G. , SWIFT R. s. , WILLIAMS J. G.
Published: July 2006

The adsorption of copper by individual soil components (organic matter fractions, oxides and clay minerals) was examined at equilibrium solution concentrations of copper within the range found in natural soils, the distribution of copper between solution and solid phases being measured by means of labelling with radioactive 64Cu. At these low solution concentrations it was found that the copper adsorption isotherms were essentially linear. The oxides and organic materials adsorbed the greatest amounts of copper. The concentration of copper in natural soil solutions will be controlled by these materials to a far greater extent than by the clay minerals, the influence of which may be negligible in some soils. Solution concentrations of copper are relatively unaffected by both the background concentration of major cations and by changes in pH within the ionic strength and pH range found in normal agricultural soils.

Copper adsorption studies with humic and fulvic acids showed that total solution copper concentrations could be greatly enhanced above the equilibrium levels for ionic copper by the presence of soluble organic complexes. The importance of taking into account the presence of such copper complexes in soil copper studies is emphasised.
SORPTION OF INORGANIC PHOSPHATE BY IRON‐ AND ALUMINIUM‐ CONTAINING COMPONENTS
Authors: McLAUGHLIN J. R. , RYDEN J. C. , SYERS J. K.
Published: July 2006

The amounts of inorganic P sorbed by a range of Fe‐ and Al‐ containing components varied appreciably and decreased in the order allophane > fresh Al gel > Fe gel pseudoboehmite > aged Al gel > dried Fe gel > Fe‐coated kaolinite > haematite > goethite > akaganeite > gibbsite = ground kaolinite > dispersed kaolinite. Al gel sorbed 30 to 70 times more P than gibbsite, and Fe gel sorbed approximately 10 times more P than its crystalline analogues (haematite, goethite. and akaganeite). Despite large differences in the extent of P sorption, the form of the isotherm was essentially the same for each sorbent. The ability of freshly‐prepared Al gel suspensions to sorb P decreased with ageing, a property not shown by Fe gel. Drying of Fe gel at 80°C, however, caused an approximately 4‐fold decrease in P sorption. Precipitation of Fe gel (2% Fe) on the surface of kaolinite increased P sorption by a factor of 10. The occurrence of Fe gel as a coating apparently presents more sorption sites to solution per unit weight of Fe gel than Fe gel alone. A linear relationship (r= 0.98) was obtained between the amount OH− sorbed per unit increase in pH value (‘hydroxyl buffering’) and the overall P sorption maximum for each sorbent. Hydroxyl buffering provided a better index of P sorption potential than specific surface area. Except for the crystalline Fe sorbents, isotherms obtained by plotting fractional sorption saturation against final solution P concentration for the sorbents were essentially coincident with those for several contrasting soils. For crystalline Fe components a lower relative amount of weaker sorption, as opposed to chemisorption, of the overall sorption maximum was obtained. Differences in the extent of P sorption. however, appear to be primarily related to the number of functional M‐OH groups presented at the solid‐solution interface.
QUANTIFICATION OF WEATHERING, SOIL GEOCHEMISTRY AND SOIL FERTILITY
Authors: KRONBERG B. I. , NESBITT H. W.
Published: July 2006

Continental chemical weathering is discussed with reference to a diagram, in which the ratio (CaO + Na2O + K2O)/(Al2O3+ CaO + Na2O + K2O) is plotted against the ratio (SiO2+ CaO + Na2O + K2O)/(Al2O3+ SiO2+ Na2O + K2O). The former ratio is a measure of the degree of feldspar breakdown, which is accompanied by the formation of secondary minerals (illite, smectite, etc.). The second ratio is a measure of the enrichment during weathering of Al, Si oxide phases such as kaolinite, quartz and gibbsite.

The application of the diagram to a series of global examples leads to the statement: 1) Chemical weathering is the principal process by which continental surfaces are modified. 2) The extent of chemical weathering is correlated with the age of continental surfaces. 3) Global agricultural productivity is correlated with geologically recent additions of fresh rock debris by processes of volcanism, glaciation or alluviation.
A METHOD FOR THE QUANTITATIVE MEASUREMENT OF ANISOTROPIC PLASMA IN SOIL THIN SECTIONS
Authors: HILL I. D.
Published: July 2006

A method is described to measure the total amount of anisotropic plasma visible in soil thin sections. The proportion occurring within 15μm of void and skeletal grain surfaces is also measured. The results can be presented as an orientation index.
SPECIFIC EFFECT OF MAGNESIUM ON THE HYDRAULIC CONDUCTIVITY OF SODIC SOILS
Authors: ALPEROVITCH N. , SHAINBERG I. , KEREN R.
Published: July 2006

Exchangeable Mg does not have a specific effect on the hydraulic conductivity (HC) and clay dispersion in calcareous soils. Conversely, in well‐weathered soils, which do not contain CaCO3, a specific effect of exchangeable Mg is evident, on both HC losses and clay dispersion, when the Na/Mg soils are leached with distilled water (simulating rain water). Soil clays, when Mg‐saturated, are chemically more stable and do not release electrolytes into the solutions: they thus disperse more easily when leached with distilled water. The presence of exchangeable Mg enhances the dissolution of CaCO3 in calcareous soils, and the electrolytes prevent the dispersion of the clay and HC losses in the Na/Mg calcareous soils.
TOTAL NITROGEN STATUS OF SOILS UNDER BUSH FALLOW IN THE FOREST ZONE OF SOUTH WESTERN NIGERIA
Authors: AWETO A. O.
Published: July 2006

A substantial increase of total nitrogen occurred in the top 10 cm of soil profiles during the first ten years under bush fallow but no significant increase occurred in the 10–30 cm layer. The increase of nitrogen in the topsoils is related to soil organic matter content, increase in tree size and in vegetation cover.
A NEW KIND OF GRINDING AND POLISHING PLATE FOR THE PREPARATION OF THIN SECTIONS
Authors: CENT J.
Published: July 2006

The construction, by disc casting of epoxy resin/carborundum grinding and polishing plates is described. Such plates are slow wearing and can be produced in the labouratory to a wide range of specifications.
A pulse method for studying effects of dead‐end pores, slow equilibration and soil structure on diffusion of solutes in soil
Authors: PINNER A. , NYE P. H.
Published: July 2006

A model is described for solute diffusion in an aggregated medium in which a proportion of the water‐filled pores are connected to a continuous pore network by narrow channels through which the diffusing ion can move only slowly. The experimental conditions for detecting departure from normal diffusion behaviour are discussed. A pulse technique for measuring the selfdiffision coefficient of 36Cl in both undisturbed and repacked soil samples at various moisture contents is described. No evidence of slow exchange between freely and slowly accessible pores was obtained. The self‐diffusion coefficients in large undisturbed peds were somewhat greater than in repacked sieved samples at the same volumetric moisture content.
Simulating diffusion within soil aggregates: a simple model for cubic and other regularly shaped aggregates
Authors: ADDISCOTT T. M.
Published: July 2006

Diffusion within a cube was simulated by a model which divides the cube into concentric volumes and computes by Fick's first law the diffusive flux between adjacent volumes and between the outermost volume and the external solution. The model can also be applied directly to a sphere and, with minor modifications, to other regular shapes. It was tested in two forms: (a) with the distances between the interfaces of adjacent concentric volumes made equal, and (b) with the volumes themselves equal, by using it to simulate the uptake of solute by a sphere from a stirred solution of limited volume. The output from the latter form of the model agreed the more closely with values obtained from a solution of the diffusion equations in radial co‐ordinates, and this form was used subsequently. The model was used successfully to simulate the diffusion of bromide from cubes of chalk and chloride‐36 from porous ceramic spheres, and its flexibility was illustrated by its ability to simulate experiments in which aliquots of the external solution were removed for assay.
Seasonal fluctuations in nitrogen fixation (acetylene reduction) by free‐living bacteria in soils contaminated with cadmium, lead and zinc
Authors: ROTHER J. A. , MILLBANK J. W. , THORNTON I.
Published: July 2006

Acetylene reduction by non‐symbiotic, heterotrophic micro‐organisms in a range of soils containing different concentrations of heavy metals was determined using intact soil cores. The suitability of this method for the soils used in this investigation was established. Samples were collected seasonally, and were incubated under standard conditions (darkness: 15°). Mean values of metal concentrations in the soil (μg g−1) were: Cd: 1–200; Pb: 60–8000; Zn: 70–26000, Cu: 20–40. Rates of acetylene reduction were generally low, from 2800 to 50000 nmol C2H4, m−2 day−1. Assuming a 3:1 ratio of C2H2 reduction to N2 fixation, this represents a rate of 0.3 to 5.0 g N fixed ha−1 day−1 in the surface 150 mm of soil. No consistent effect of heavy metal concentration was found. The most important factors determining activity were soil moisture content and possibly inorganic nitrogen concentration. It thus appears that the bacteria in polluted soils are capable of adapting to potentially toxic concentrations of heavy metals, or that these metals are present in the soils tested in unavailable or non‐toxic forms.
A reassessment of podzol formation processes
Authors: ANDERSON H. A. , BERROW M. L. , FARMER V. C. , HEPBURN A. , RUSSELL J. D. , WALKER A. D.
Published: July 2006

Translocated (oxalate‐soluble) Al and Fe are present predominantly in inorganic forms in the B2 horizons of the five pcdzol profiles examined: A1 as imogolite and proto‐imogolite allophanes, and Fe as a separate oxide phase. Below the top few cm of the B2 horizon, over 75 per cent of the extractable (acid‐plus alkali‐soluble) organic matter is present as Al‐fulvates, largely sorbed on allophanic material. The Bh horizons of the Iron Humus Podzol and Iron Podzol intergrades are distinguished by very high levels of organically bound Fe (soluble in EDTA solution), five to ten times more than in immediately adjacent A2 or B2 horizons, and also by larger humic acid contents than in comparable B2, levels in typical Iron Podzols. Inorganic forms of translocated Al and Fe are probably absent from two of the three Bh horizons examined, and also from the Bhg horizon overlying the thin iron pan in the Peaty Podzol. The organic matter in this Bhg horizon is saturated with Al rather than Fe. Chemical and physical processes which could lead to evolution of a profile along the genetic sequence, Iron Podzol, Iron Humus Podzol, Peaty Podzol, are postulated. During the formation of an Iron Podzol, positively charged inorganic sols carry aluminium, silicon and iron from the A2 and deposit them in the B2 horizon; subsequently, with the development of an H layer, colloidal humus migrates through the A2 and precipitates on the positive colloids at the top of the B2 horizon to form a Bh horizon, in which remobilized ferric species are trapped by the organic matter. In higher rainfall areas, occasional waterlogging above the oxide‐impregnated B2 leads to a thin iron pan, separating permanently oxidizing conditions below from seasonally waterlogged and reducing conditions above.
A technique for the measurement of gaseous diffusion in porous media
Authors: REIBLE D. D. , SHAIR F. H.
Published: July 2006

An apparatus composed of two well‐mixed vessels connected by a tube was used to measure the effective diffusivity of a gas (sulphur hexafluoride, SF6) in porous beds of sand, steel wool and glass beads. The general solution to Fick's second law for the apparatus was employed in the analysis. The ratio of the measured effective diffusivity of SF6 to SF6‐air diffusivity was found to equal approximately the void fraction of the porous bed raised to the 1.43 power. This relationship agreed well with previously collected data and with the theoretical models of Millington (1959) and Marshall (1959). The effective diffusivity of wet porous materials was also measured and, at higher moisture contents, found to deviate strongly from that measured in dry materials. The effective diffusivity of SF6 in sand with 20 per cent moisture (volume basis) was found to be about one‐fourth as large as the value expected from the dry‐bed experiments. No significant deviation was found for sand of 10 per cent moisture content or lower.
Predicting bare soil temperature. I. Theory and models for the multi‐day mean diurnal variation
Authors: BUCHAN G. D.
Published: July 2006

The energy balance equation for the soil‐air interface is expressed, by suitable approximations, in a form linear in the surface temperature T0. This forms the basis of two predictive models for the diurnal variation of T0, averaged over a many‐day interval, for potentially evaporating bare soil. Input data include total solar radiation, air temperature and vapour pressure, windspeed ua, cloud cover c, and soil radiative, aerodynamic and thermal properties (the latter assumed uniform). In the simpler model, based on harmonic analysis, ua and c are assumed constant. For Nt observations per day, an algebraic solution is found as a set of equations for the parameters of up to the first Nt/2 harmonics. In the second model, dynamic wind‐speed is introduced, and a more complicated linear algebraic method of solution is required. The models have agroclimatic value, since by averaging they predict the thermal ‘climate’ of soil, in contrast to instantaneous models which describe the time‐specific thermal ‘weather’.
Predicting bare soil temperature. II. Experimental testing of multi‐day models
Authors: BUCHAN G. D.
Published: July 2006

Two agroclimatic models described earlier are tested against measurements of temperature at the soil surface, T0, and in bulk soil, T(z,t), at an experimental site near Aberdeen during spring, summer and autumn. Soil temperature is predicted most accurately in spring and autumn, when the constant‐windspeed model ENBL1 predicts the mean within 0.3 K of measurement. Thedynamic‐windspeed model ENBL2 predicts within 0.1 K in all intervals, and with error<0.3 K to depth z=32 cm in spring and autumn. A sensitivity analysis of the output diurnal wave, T0(t), shows that maximum input economy requires only time‐dependent shortwave radiation, Rs(t), and air temperature, Ta(t), other inputs being constant. Two or three harmonics in ENBL1 should suffice, requiring four or six measurements per day, depending mainly on the time‐structure of the primary driving function Rs.
Effects of ionic strength and nature of the cation on the desorption of fluoride from soil
Authors: BARROW N. J. , SHAW T. C.
Published: July 2006

The reaction between soil and added fluoride was accelerated by incubating at a high temperature. Desorption of the fluoride was then studied using solutions of chloride salts of several cations at a range of solution : soil ratios and for periods which ranged from 1 h to 4 days. Fluoride desorbed was related to the experimental variables by a regression equation.

When the solution : soil ratio was small and hence only small amounts of fluoride were desorbed, decreasing the concentration of salts increased the concentration of fluoride in the solution. The concentration in the solution was lower for calcium chloride than for sodium or potassium chloride. Amongst the monovalent cations, the concentration of fluoride was highest for salts of the cations of lowest atomic number. Thus the greater the average distance between the charge conveyed to the surface by the adsorbed fluoride and the cation which balanced it, the higher the fluoride concentration in the solution.

As the solution : soil ratio was increased, the differences between the cations in their effects on fluoride desorption decreased and seemed to disappear as the solution : soil ratio became very large. This contrasts with previously observed effects on phosphate. It is suggested that the difference may have arisen because appreciable desorption of fluoride occurs by exchange with hydroxyl ions rather than by escape of the fluoride ion together with its counter ion.
The resistance patterns to metals of bacterial populations in contaminated land
Authors: OLSON B. H. , THORNTON I.
Published: July 2006

The resistance of populations of soil bacteria to heavy metals is determined using a dilution technique with media containing a range of metal concentrations. Metal resistance patterns are quantitatively related to ‘total’ concentrations of Cd, Zn and Pb in surface soils from historical mining areas in Somerset and Derbyshire. It is proposed that the tolerance of bacteria to metals may be used to determine their bioavailability and potential toxicity to other forms of life.
The use of gypsum for preventing soil sodification: effect of gypsum particle size and location in the profile
Authors: GOBRAN G. R. , DUFEY J. E. , LAUDELOUT H.
Published: July 2006

Laboratory studies were conducted on a mixture of surface soils from the Nile Delta (Egypt). Twenty‐two soil columns, initially saturated both with respect to their water‐holding capacity and to their base exchange capacity with calcium, contained 0.0, 0.25, 0.5, 1.0 and 2.0 per cent solid gypsum in the total weight of the solid material in the column. Three particle sizes of gypsum (>0.5, 0.5–1 and 1–2 mm) were mixed either with the top layer or with the whole soil column. The result of leaching these columns with saline water (36 meq 1−1 NaCl+4 meq 1−1 CaCl2) at 0.1 cm h−1 was compared with a mathematical model based on thermodynamic equilibria.

The three different particle sizes gave the same experimental results. Applying a given amount of gypsum to the surface soil was more effective in reducing the exchangeable sodium percentage (ESP) than mixing the same quantity through the soil. The mathematical model adequately predicted the changes in the soil column.
Analysing one‐step outflow experiments to calculate soil‐water diffusivities, using Gardner's Equation
Authors: TOWNER G. D.
Published: July 2006

Soil‐water diffusivity values, calculated using Gardner's analysis of measurements of water‐content changes with time of a horizontal soil column losing water from one end, are approximate. An iterative method is developed to improve their accuracy. Using the approximate values, computer simulations are made of the water‐content measurements. Gardner's analysis is then applied to these simulated values, thus giving the result of applying the analysis when the diffusivity function is known. This ‘calibration’ is used with the approximate values to infer the actual diffusivity function. The process is repeated until there is agreement between the simulated and the measured water contents.

The method is demonstrated, using computer modelling of the experiments, for four functions of diffusivity versus water contents: two linear and two exponential, increasing and decreasing with water content. The procedure is an improvement on recent alternative analyses in being applicable to decreasing as well as increasing diffusivity functions.
The influence of iron oxides on the surface area of soil
Authors: BORGGAARD O. K.
Published: July 2006

Soils from Denmark and Tanzania have been extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite‐EDTA (DE) to dissolve crystalline iron oxides. The surface areas of the extracted soils have been determined by applying the BET equation to nitrogen adsorption and by water adsorption at 19 percent relative humidity. High correlations (P < 0.001) were found (i) between EDTA‐extractable iron (amorphous iron oxides) and the decrease in the surface area following EDTA extraction, and (ii) between the difference between DE‐extractable iron and EDTA‐extractable iron (crystalline iron oxides) and the further decrease in the surface area following DE extraction. The calculated specific surfaces of both the amorphous and the crystalline iron oxides varied from soil to soil but without any definite trend. The means of all the soils investigated may therefore serve as reasonable estimates of the specific surfaces of amorphous and crystalline iron oxides in soil.
Modification of soil physical properties by manipulating the net surface charge on colloids through addition of Fe(III) polycations
Authors: SHANMUGANATHAN R. T. , OADES J. M.
Published: July 2006

The addition of 0.07 per cent Fe in the form of polycation of molecular weight 10000–50000 flocculated soil suspensions. Higher concentrations of Fe(III) caused redispersion of the clay. Electrophoretic and electron microscopic studies confirmed the flocculation‐dispersion phenomena. The soil suspensions with higher concentrations of Fe(III) gave points of zero charge (PZC) between pH values 5.0 and 6.0. The flocculation resulted in microaggregation and created pores 40–100 μm in diameter. This led to an increased water‐holding capacity and hydraulic conductivity and lower bulk densities and modulus of rupture. The soils treated with Fe(III) polycations were shown to be more friable than untreated soils.
The effect of lime on the response of soils to sodic conditions
Authors: SHAINBERG I. , GAL M.
Published: July 2006

The role of CaC03 in preventing clay dispersion and losses in hydraulic conductivity (HC) of sodic soils was determined directly by mixing two lime‐free soils with 0.5 and 2.0 per cent CaCO3. Whereas the HC of the lime‐free soils dropped sharply when 0.01 n solutions of SAR 20 were displaced with distilled water, mixing the soils with powdered lime prevented both HC losses and clay dispersion. The response of a sandy soil mixed with lime was similar to that of a calcareous sandy soil. The beneficial effect ofCaC03 was not so pronounced in soils equilibrated with solutions of SAR 30. The increase in electrolyte concentration, due to CaCO3 dissolution, was suggested as the mechanism responsible for the beneficial effect of lime.
Direct field measurement of N2 and N2O evolution from soil
Authors: LIMMER A. W. , STEELE K. W. , WILSON A. T.
Published: July 2006

A gas lysimeter has been designed and used to measure directly the evolution of N2and N2O in a soil profile under field conditions. Concentrations of N2 in the soil atmosphere within the lysimeter as low as 2000–5000 p.p.m. have been achieved by flushing with N‐free gas. A flow of gas into the base of the lysimeter forms a barrier against diffusion of soil air into the lysimeter during measurements.

After reducing the N2 concentration in the soil core, a low concentration of N2enriched in N‐15 is introduced. By monitoring changes in the 8 N value using a high‐precision isotope mass spectrometer, rates of N2 evolution down to 6 kg N2‐N ha−1 a−1 can be detected. N2O evolution was determined at the same time using the mass spectrometer in the single‐beam mode.
The variability introduced by various subsampling techniques
Authors: MULLINS C. E. , HUTCHISON B. J.
Published: July 2006

Subsamples of a granular material contain a finite number of grains. Practical methods of subsampling do not select these grains on an entirely random basis. Both of these features contribute to the observed variability between subsamples. As an example, the theoretical coefficients of variation attributable to statistical variation between 10‐g random subsamples taken from four different (<2 mm) air‐dry soils have been calculated. These are calculated to give CVs of between about 0.002 and 1 per cent between particle size analysis results. In an actual comparison between four different subsampling methods, CVs of between 4 and 18 per cent were observed in the proportion of coarse sand (2–0.2 mm) in 15‐g soil subsamples. Rotary sub‐sampling and chute splitting were found to be more reliable methods of subsampling than cone and quartering or random spoon sampling. Variability between subsamples is shown to depend not only on subsampler design, but also on the precise details of the procedure adopted, and the physical properties of the sample itself.
Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments
Authors: BALL B. C. , O'SULLIVAN M. F.
Published: July 2006

Cone resistance and vane shear strength were measured in the top 50–100 mm of seven soils. Bulk density and water content were also measured in the same layer by coring. At each site cone resistance and vane shear strength tended to decrease with increasing water content and decreasing bulk density down to 1300 kg m‐3, but at lower densities they were not related to bulk density. Cone resistance and vane shear strength tended to decrease with increasing coarseness of texture but also depended on soil structure and organic matter content.

Plant populations, mainly spring barley, were reduced in soils with cone resistances and vane shear strengths greater than 2500 kPa and 65 kPa respectively. Such high strengths in undisturbed soils were associated with wheeling during harvesting, were apparently independent of soil type and, at the only site of measurement, apparently decreased during weathering in the subsequent season.

In contrast to the cone penetrometer, the vane shear tester has minimal shaft friction in undisturbed soils and is more sensitive to soil differences and less sensitive to water content differences than the penetrometer. It is apparently more suitable for indexing the suitability of undisturbed soils for the penetration of direct drill coulters and for subsequent plant establishment.
Prediction of solute leaching under field conditions: an appraisal of three methods
Authors: CAMERON K. C. , WILD A.
Published: July 2006

The analytical equation of Rose was more successful than the models of Burns and Addiscott in predicting movement of chloride through a soil on Upper Chalk under irrigation and winter rainfall. The two models required water inputs to be divided into 5 mm increments. When this was done the model of Addiscott was more successful than that of Burns although both then gave predictions of acceptable accuracy for practical purposes. The analytical equation of Rose requires a value for solute dispersivity. This was obtained from the experimental measurements, but the value used (3 cm) was found not to be critical.
The effect of pH upon the copper and cupric ion concentrations in soil solutions
Authors: SANDERS J. R.
Published: July 2006

Soil samples whose pH had been adjusted to between 4.5 and 7.5 either for long periods in the field or short periods in the laboratory were incubated after wetting with water or 0.01 m CaCl2. Copper concentrations in the soil solutions decreased only slightly as the solution pH increased, but free cupric ion concentrations decreased considerably. The copper concentrations were smaller and the proportion of copper present in solution as cupric ion at a given pH was larger when CaCl2 rather than water was used. Complexed organic species made up most of the copper in all solutions. The duration of pH adjustment did not affect these results.

Copper adsorption isotherms were determined on the soils using low equilibrium solution concentrations. As a given copper concentration the quantity of copper adsorbed increased and the proportion of copper in solution present as cupric ion decreased with pH increase; again the duration of pH adjustment did not affect the results.
Sorption of isotopically exchangeable and non‐exchangeable phosphate by some soils of Colombia and Brazil, and comparisons with soils of southern Nigeria
Authors: LEMARE P. H.
Published: July 2006

Fourteen soils from Colombia and Brazil provided a wide range of sorption characteristics. Curves of sorbed phosphate that was exchangeable to 32P were described by Freundlich's equation, and of non‐exchangeable phosphate by Temkin's equation. Exchangeable phosphate was associated with aluminium in poorly‐crystalline oxides and in organic complexes. Non‐exchangeable phosphate was related to aluminium in organic complexes, and especially to the ratio of AI/C in them. In Nigerian soils similar mechanisms controlled sorption of phosphate but oxides and organic complexes of iron were important.

The concentration of phosphate in solution when affinities of soil for exchangeable and non‐exchangeable phosphate are equal, and the importance of organic matter, are discussed in relation to soil management and to responses of crops to fertilizer phosphate.

The results indicate that sorption curves should not be split into sections.
Effect of wetting and drying of soils on phosphate adsorption and resin extraction of soil phosphate
Authors: OLSEN R. G. , COURT M. N.
Published: July 2006

Sixteen topsoils from Denmark and the UK were subjected to two wetting and drying treatments: (i) moist incubation (wet), (ii) eleven wetting and drying cycles (W/D). The W/D treatment resulted in larger P adsorption and resin extraction of soil P than the wet treatment. The differences in P adsorption at the final concentration of 800 μM P were mainly above 20 per cent, whereas the differences in amount of resin‐extracted P were mainly less than 20 per cent. The effects were positively correlated with the cation exchange capacity of inorganic components. Furthermore, the increase in rapidly released P was positively correlated with pH. It is suggested that wetting and drying effects on P adsorption and desorption are associated with changes in soil structure caused by rewetting of dry samples.
The origin of oxygen in soil humic substances
Authors: DUNBAR J. , WILSON A. T.
Published: July 2006

The 18O/16O ratios of a number of soil humic and fulvic acids were measured and compared with those of lignin and cellulose samples originating from the same area. The average enrichments above ground water were: cellulose 32%0 humic and fulvic acid 29%0 and lignin 14%0, suggesting that the oxygen in humic and fulvic acid originates pricipally from cellulose or other plant carbohydrates and not lignin as has been suggested.
Nitrogen fixation by white clover (Trifolium repens) in grasslands on soils contaminated with cadmium, lead and zinc
Authors: ROTHER J. A. , MILLBANK J. W. , THORNTON I.
Published: July 2006

The nitrogenase activity (potential nitrogen fixation) of a mixed white clover‐grass sward growing on a range of soils containing up to 216 μg g−1 Cd, 30 000μg g−1 Pb and 20 000 μg g−1 Zn was determined using the acetylene‐reduction assay. The plants were incubated in situ using an intact soil‐core technique. Little change in the rate of C2H2 reduction was observed during the daylight hours although a marked seasonal fluctuation was found, the maximum activity during the spring and declining to 20% of this by the autumn. Some reductions of nodule and plant size, and in nitrogenase activity, was observed in the most heavily contaminated sites. These effects were small; the plants and nodules otherwise appeared healthy. The potential for nitrogen fixation was equally high in all the sites at up to 80 g N ha−1 h−1 in the spring.
N.M.R. spectroscopy of soils: structure of organic material in sodium deuteroxide extracts from Patua loam, New Zealand
Authors: WILSON M. A. , GOH K. M.
Published: July 2006

The structure of the organic matter from the ‘A’ and ‘B’ horizons of Patua loam (New Zealand) which is soluble in sodium deuteroxide has been investigated by conventional 13C‐nuclear magnetic resonance spectroscopy. Experiments using inverse gated 1H‐decoupling and a variety of pulse delays and weighting functions for Fourier transformation are reported. Sufficient number of transients have been collected for quantitative estimates of the aromaticity of the extract. The results complement an earlier qualitative study on this soil (Wilson & Goh, 1977) and illustrate conditions under which conventional 13C‐n.m.r. spectra of humic materials should be obtained.

The sodium deuteroxide soluble material from Patua loam ‘A’ and ‘B’ horizons has an estimated aromaticity of 33%. Different carbon types are not well resolved. However, there appear to be seven distinct resonances in the carboxyl region of the A horizon of which at least six are not derived from inorganic sources. Together with results already recorded in the literature, the results illustrate the variability of humic acids isolated from different soils. The nature of this variability is discussed.
Micromorphometric and micromorphological investigations of a clay loam soil in viticulture under zero and conventional tillage
Authors: PAGLIAI M. , LAMARCA M. , LUCAMANTE G.
Published: July 2006

Porosity, pore size distribution and the orientation pattern of pores were measured on thin sections prepared from undisturbed soil samples by means of electro‐optical image‐analysis. Total porosity was significantly higher at all sampling times in conventionally tilled plots, but the proportion of pores ranging from 30 to 500 μm, which are considered the most important both in soil‐water‐plant relationships and in maintaining a good soil structure, was higher in no‐tilled plots. Modifications of pore orientation pattern were also observed. Micromorphological observations revealed some differences between the two series of soil samples; the formation of surface soil crusts was strongly reduced in no‐tilled plots.
Management of the Cerrado soils of Brazil: a review
Authors: GOEDERT W.J.
Published: July 2006

A large proportion of the world tropical region is covered by savannas, which have been little used for agriculture. Locally known as Cerrado, this ecosystem occupies about 20% of Brazilian territory. Preliminary surveys indicate that at least 50 × 106 ha of the Cerrado are of potential use for intensive mechanized agriculture, but development has been limited by soil‐related constraints. Most soils are acid and with very low native fertility, especially in terms of phosphorus. Chemical barriers to root growth may cause severe yield losses during dry periods. Field crop responses to lime and fertilizers, obtained during the last 5–8 years, show that once soil‐related constraints are removed, high yields can be achieved. This review gives an account of the main lines of research which are leading to economic methods of developing the agricultural use of the Cerrado, and gives future research needs.
Effective stresses in unsaturated soils and their applicability in the theory of critical state soil mechanics
Authors: TOWNER G. D.
Published: July 2006

Cricital state soil mechanics, which expresses relationships between deformation and effective stresses, has not yet been established for unsaturated soils. Effective stresses in unsaturated soils are not only complicated functions of the applied stress and the pore‐water pressure, but are also hysteretic. Furthermore, they cannot yet be determined independently, because present theories are inadequate, and they must be obtained by the less satisfying method of comparing the deformation of unsaturated and saturated soils subjected to similar stress paths. The paper discusses the further measurements that need to be made to validate the extension of critical state soil mechanics to unsaturated soils, and concludes that unless the effect of hysteresis proves to be small, the theory may be too difficult, if not impossible, to apply quantitatively.