bluuebunny/crossref_metadata_2025 huggingface dataset:
Clay and iron minerals in soils of the clay plains of Central Sudan
Authors: YOUSIF A. A. , MOHAMED H. H. A. , ERICSSON T.
Published: July 2006

Surface soil samples from seven profiles from the central plains of Sudan were investigated using XRD, chemical analysis and Mössbauer spectroscopy measured at 15K. Dioctahedral montmorillonite was the main constituent with different proportions of kaolinite and chlorite. Goethite was found as fine iron oxyhydroxide in all the samples, with haematite detected in two pedons. Crystallinity of the clays was observed to correlate negatively with the amount of free iron oxide in the soil. The influence of the climatic factors on the soil and its formation is discussed.
The reactions of copper and zinc with calcium carbonate surfaces
Authors: PAPADOPOULOS P. , ROWELL D. L.
Published: July 2006

The reaction of copper and zinc were studied by adding the metal nitrate to a 4g:50ml suspension of calcite and water which had been equilibrated for two days, and then equilibrating for a further three days. Zinc behaved in a similar manner to cadmium in forming a surface‐solid solution of ZnχCa1‐χCO3 as a result of adsorption, and the pIAP of the equilibrium solutions were close to those expected from the Thorstenson & Plummer equation. However, the continuity from adsorption to precipitation was broken by the formation of Zn5(OH)6(CO3)2 which has a higher stability than ZnCO3. The CaCO3 surface appears to constrain the adsorbed Zn to conform to a carbonate structure despite its lower stability than the hydroxy carbonate.

The adsorption data for Cu can also be explained in terms of a surface‐solid solution of CuχCa1‐XCO3, but CuCO3 is not found naturally because of much greater stability of Cu(OH)2, and so there is no independent value for the solubility of a CuCO3 end‐member if a surface‐solid solution is formed. Secondary reactions are more likely to occur for Cu, especially close to the adsorption‐precipitation boundary. Continuity from adsorption to precipitation again was not found. Theory predicts that Cu is less likely to form a surface‐solid solution than Zn.

Upper Chalk (calcite with 4% other minerals, mostly quartz) behaved in a similar way but with increased adsorption due to its higher surface area.
Seasonal changes in Cu, Mn, Zn and Co concentrations in soil in the root‐zone of barley (Hordeum vulgare L.)
Authors: LINEMAN D. J. , SINCLAIR A. H. , MITCHELL M. C.
Published: July 2006

Soil solutions were isolated by centrifugation of moist soil taken from the root‐zone of field‐grown barley plants and bulk soil from adjacent fallow areas. Low concentrations of Cu, Mn, Zn and Co were found in soil solutions in late winter with increases in spring and early summer. Discrete maxima are generally achieved at some time between May and early July. The precise timing of such maxima appears to depend on the extent of the development of the growing crop and on seasonal influences. The extent to which micronutrient concentrations increase in fallow soils is less than in the root‐zone soil solutions although similar trends occur. It is suggested that the increased micronutrient concentrations might result from their mobilization from insoluble forms by biologically produced chelating ligands.
The variation of soil water regime, oxygen status and rooting pattern with soil type under Sitka spruce
Authors: NISBET T. R. , MULLINS C. E. , MACLEOD D. A.
Published: July 2006

Soil water regime, oxygen status and rooting pattern under Sitka spruce were characterized in five soil types: a waterlogged peaty gley, a peaty gley, a flushed peaty gley, a surface water gley and a brown forest soil during 1982. The waterlogged peaty gley had a high water‐table which only dropped below 30 cm depth for 46 d during the summer. Below the water‐table the profile was anoxic and rooting was restricted to a depth of 12cm. The peaty gley experienced seasonal waterlogging with a winter water‐table around 15cm depth. Roots that grew further down the profile during the summer as the topsoil dried to –100 kPa matric potential were later killed when the water‐table rose. The top 30 cm of the flushed peaty gley remained oxygenated even below the water‐table. Roots survived below the winter water‐table. The top 30 cm of the surface water gley and brown forest soil were freely draining and remained aerated throughout the year. The platinum electrode polarographic method demonstrated that there was a sharp transition to anoxic conditions just above the water‐table at the first two sites.
The effect of wet cultivation on the structure and fabric of a Vertisol
Authors: McGARRY D.
Published: July 2006

Changes in soil structure and fabric of a Vertisol in northern New South Wales, Australia following pre‐sowing cultivation at two different soil water profiles (dry and wet) were investigated. Both the dry and wet cultivated plots had similar, intensive pre‐sowing cultivation typical of commercial practice for irrigated cotton. For thin section analysis two profiles were sampled prior to this cultivation, and three from each of the dry and wet plots were sampled after cultivation. Samples were also available from soils in the area which had never been cultivated. All soils had porphyric fabric with vo‐ma‐skel‐insepic plasmic fabrics and a wide range of pedological features. The cultivated soils had other pedological features not present in the ‘never’‐cultivated soils, i.e. zones within which the plasma exhibited a moderately‐ to strongly‐striated orientation pattern. The more strongly‐striated units included occasional, very fine to fine joint planes. These zones were domi‐nantly equant to planar with sharp boundaries. Internally, they consisted of layer silicate plasma and mineral skeleton grains most commonly similar to the enclosing material. The proportion of these features differed between treatments especially in the 0.05‐0.25 m layer. Soil cultivated dry, and land prior to cultivation had < 1 % of their area composed of zones of striated clay, compared to up to 17% in the wet cultivated soil. This may be related to a greater incidence of shearing of soil in a plastic state by tillage implements and tractor wheels. In the dry cultivated soil shear appears to be of minimal significance.
Soil scientists as members of the scientific community*
Authors: WILD A.
Published: July 2006

I have tried to show in this paper that there is a need for our expertise in areas outside agriculture, though in the immediate future employment is likely to remain largely through applications to agriculture. At the same time, in broadening our interests we must not neglect to develop our understanding of the properties of soils, their behaviour and the processes that occur in them, because it is fundamental knowledge which finds applications. A related requirement is for us to be aware of innovation in other sciences and technologies. We also have the difficult task of selling our subject to the public while maintaining our scientific integrity. I believe that our image will improve if we can do all this.

As interests broaden and the range of applications increases, as I expect they will, there is a danger of a Society like ours becoming fragmented. If one part isolates itself it can become sterile because soil biology, chemistry, mineralogy, physics and pedology interact with each other. In this narrow sense we should all be holists at heart even though developments in science require us to specialize.
Testing a mechanistic model. X. The effect of pH and electrolyte concentration on borate sorption by a soil
Authors: BARROW N. J.
Published: July 2006

Borate sorption by a soil was measured with 0.01, 0.1, or 1.0 M sodium chloride as background electrolyte and samples of soil with a range of pH values achieved by incubating with either calcium carbonate or hydrochloric acid for 24 h at 60°C.

Borate sorption generally increased with increasing pH. The more concentrated the electrolyte, the steeper the increase. At low pH, increasing the salt concentration decreased borate sorption; at high pH, it increased sorption. There was an intermediate pH at which salt had no effect on borate sorption. The point of zero salt effect on borate sorption was at a higher pH than the point of zero salt effect on pH. This result was explained by a mechanism in which borate ions react with variable charge surfaces which are heterogeneous and for which part of the heterogeneity is in the electric potential of the surfaces. It cannot be explained by mechanisms which do not take into account the effects of the electric potential of the reacting surfaces on the reaction with borate ions. Although the behaviour of borate was broadly consistent with that of other anions, it differed in that about half of the heterogeneity had to be allocated to the binding constant for borate ions. It was suggested that this was because reaction with organic matter was more important for borate than for other anions.
Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. II Modelling the extent and rate of reaction
Authors: BARROW N. J. , GERTH J. , BRÜMMER G. W.
Published: July 2006

A model of the reaction of metal ions with a variable charge surface was modified and applied to goethite that had been formed in the presence of silicate. The data included the effects of initial concentrations of Ni, Zn and Cd ranging from 1 to 100 μm, of periods from 2 h to 42 d, of temperatures from 5 to 35°C, of pH from about 4 to about 8, and of background electrolyte from 0.01 to 1.0 M.

The observed effects of concentration and pH were explained by assuming that the reacting surface sites were heterogeneous and that they reacted with the MOH+ ions in solution. The heterogeneity was described by assuming that the affinity of the sites for MOH+ ions decreased logarithmically as the log of the amount adsorbed increased. This gave a Freundlich relation between sorption and surface activity. The effects of increasing the concentration of the background electrolyte were explained as being caused by decreased activity of MOH+ ions in solution counteracted by decreases in the electric potential of the surface. To explain the effects of time and temperature, it was assumed that an initial rapid adsorption reaction was followed by slow diffusion of metal ions into the goethite. Using these assumptions, a comprehensive description of the data was obtained.
Depletion and movement of water beneath cereal crops grown on a shallow soil overlying chalk
Authors: GREGORY P.J.
Published: July 2006

Changes in water storage and gradients of water potential were measured beneath cereal crops grown on Andover soil series for three seasons. The measurements showed that depletion of water from the soil (0‐0.3 m) was 20–29% of the total profile depletion and that from the chalk/soil and chalk layers was 71–80%. Rain showers rewet the soil so that depletion from the non‐soil layers contributed about 35% of the total water use despite containing only 12% of the roots.

Water in the chalk/soil and chalk layers was held at matric potentials between – 200 and – 800 kPa and gradients of hydraulic potential indicated upward movement from depths of almost 3 m. Comparisons of measured water depletion beneath the root zone with estimates of upward movement to the rooting zone indicate that substantial upward movement is possible, although the estimates were generally larger than the measured depletion.

The estimated contribution of the water moved to the root zone for shoot dry matter production was 8% and 22% for winter and spring cereals respectively.
A usable mechanistic model of nitrate leaching I. The model
Authors: BARRACLOUGH D.
Published: July 2006

A mechanistic model to simulate the leaching of nitrate in soil is described. Three flow patterns are accommodated: conventional convective‐dispersion; preferential flow, in which water and solute moving in the larger water‐filled pores fails to equilibrate with that in the smaller pores; and by‐pass flow, in which water (and solute) moves rapidly down cracks and fissures without displacing the soil water. The pattern of flow depends on the rainfall intensity and the hydraulic conductivity of the water‐filled pore space. The performance of the model is examined in terms of its ability to reproduce solute profiles calculated from analytical solutions to transport equations, and in the sensitivity of the output to changes in the main parameters. A second paper (Barraclough, 1989) illustrates the use of the model to reproduce nitrate leaching patterns observed in the field.
Exchange‐induced dissolution of gypsum and the reclamation of sodic soils
Authors: FRENKEL H. , GERSTL Z. , ALPEROVITCH N.
Published: July 2006

A study was undertaken to define the dissolution kinetics of gypsum in the presence of ion exchange resins and to study sodic soil reclamation in laboratory soil columns.

Gypsum pellets were prepared at 1.5 MPa pressure and the time course of their dissolution followed by measuring the electrical conductivity of the solution. Dissolution experiments were carried out in distilled water with and without Na‐ and Cl‐saturated exchange resins or a combination of the two. The results indicate that in the presence of resin the reaction is first order as calcium and/or sulphate are removed from solution by the resin driving the reaction to completion. In water alone the reaction follows second‐order kinetics and depends on the rate of mixing as the reaction is transport controlled.

The effect of gypsum placement (uniformly mixed with the entire soil, mixed with the top third of the soil, applied to the soil surface or applied as a saturated gypsum solution) on the efficiency of exchangeable sodium removal, leaching of soluble salts and soil hydraulic conductivity was studied. In addition, the effect of various flow rates on reclamation efficiency was investigated.

The amount of leachate required for reclamation was found to be dependent on gypsum placement, tending to decrease in the order mixed < top third mixed < saturated gypsum solution < gypsum surface application < water. Soil hydraulic conductivity was much higher in the mixed gypsum column than in the gypsum applied on the surface; a result of the higher effective gypsum solubility. Sodic soil electrical conductivity in the presence of solid‐phase gypsum is linearly related to the total amount of exchangeable sodium expressed in mol dm−3.
Carbon mineralization in soil size fractions after various amounts of aggregate disruption
Authors: GREGORICH E. G. , KACHANOSKI R. G. , VORONEY R. P.
Published: July 2006

Aggregates (1–2mm) were subjected to shaking, increasing intensities of ultrasonification, or a peroxide treatment and then physically fractionated into sand‐, silt‐ and clay‐size fractions. CO2 evolution was measured during a 20‐day incubation of the sand‐, silt‐ and clay‐size fractions and was used to assess the decomposability of the organic matter within aggregates and associated with these size fractions.

All of the size fractions showed a large increase in the amount of readily decomposable C when the ultrasonic energy input increased from 300 to 500 J ml−1 and disruption of microaggregates occurred. The data suggest that some readily decomposable organic matter is sequestered within microaggregates and protected from microbial attack.

Following complete dispersion, the C mineralized (mg C g−’C) upon incubation was greatest in the sand particles and least in the clay. The levels of potentially mineralizable C (C0) in the sand‐size fraction increased with increased dispersion energy whereas the mineralization rate (k) remained about the same. The levels of C0 in the clay‐size fraction decreased and the estimates of k increased abruptly upon the disruption of micro‐aggregates.
The effect of non‐instantaneous exchange on the self‐diffusion of phosphate in soil
Authors: STAUNTON S. , NYE P. H.
Published: July 2006

The kinetics of isotopic exchange of phosphate in a soil were studied using three techniques: (a) in shaken suspension, (b) in a system where solution circulated through a thin disc of soil, and (c) in moist soil from which solution was extracted by centrifugation with a dense immiscible liquid.

The exchange kinetics of isotope diffusing in moist soil were inferred from the concentration profile of the isotope after diffusion periods of 10‐60 days. The proportion of the exchange that is effectively instantaneous in this diffusion system was much lower than in systems in which isotope is uniformly applied to the soil.

It is postulated that the difference arises because exchange of diffusing isotope is limited by access to exchange sites and not, during the period considered, by slow exchange at the solid‐liquid interface.
The effects of pH and drying on adsorption of phosphate by aluminium‐organic matter associations
Authors: HAYNES R. J. , SWIFT R. S.
Published: July 2006

The effects of pH value and of drying on phosphate adsorption by Al‐peat and Al‐humate associations were investigated. Raising the pH of the Al‐organic matter associations from 3 to 7, by addition of Ca(OH)2, greatly increased their phosphate adsorption capacity. There was a pronounced increase in phosphate adsorption as a result of drying the limed, Al‐organic matter associations, but when equilibration time was increased phosphate adsorption by moist Al‐peat samples approached that of the dried samples, indicating that the drying effect was at least partially caused by incomplete equilibration of P in the moist samples. In contrast, increasing the equilibration time did not alter the magnitude of the pH effect.

A detailed study was made of the adsorption of phosphate both by Al‐peat limed to pH 7.0, and by its constituent components Ca‐peat (pH 7.0) and hydroxy‐Al (pH 7.0), synthesized separately then mixed together. For limed Al‐peat, drying increased phosphate adsorption, whereas for the individually synthesized components in combination the reverse was true. Thus, the limed Al‐peat behaved differently to its components.

The increasing adsorption of phosphate by Al‐organic matter complexes with rising pH was attributed to increasing hydrolysis and polymerization of hydroxy‐Al associated with the organic matter. However, the interaction of organic matter with hydroxy‐Al would block many phosphate adsorption sites on the hydroxy‐Al. Upon drying, it is suggested that the organic matter condenses and the accessibility of phosphate to adsorption sites on the hydroxy‐Al is increased. Thus, drying resulted in increased phosphate adsorption.
Release of potassium from some benchmark soils of India
Authors: DHILLON S. K. , SIDHU P. S. , BANSAL R. C.
Published: July 2006

Release of potassium from 15 surface samples of benchmark Alluvial, Red and Black soils of India to 0.01 M solutions of BaCl2, CaCl2, NH4Cl and NaCl was studied in soils either untreated or pretreated with 5 × 10−3 M KCl. In the untreated soils, the efficacy of the extractants declined in the sequence: BaCl2 > NH4Cl > CaCl2 > NaCl. Cumulative K‐release was greatest from Black soils, followed by Red and Alluvial soils. From soils pretreated with 5 * 10−3 M KCl, more K was released than retained, and more ‘native’ K was released than that from untreated soils. Increase in the release of ‘native’ K decreased in the sequence: Red > Alluvial > Black soils. The amounts of surface and internal K, desorption rate constants and parabolic diffusion constants were calculated from K release to the various electrolytes.
Description of titration curves of mixed materials with variable and permanent charge by a mathematical model. 3. Influence of the nature of the permanent charge mineral
Authors: MADRID L. , DIAZ‐BARRIENTOS E. , SÁNCHEZ‐SOTO P. J.
Published: July 2006

A mathematical model, previously developed, is tested here for various clay minerals and their mixtures with iron oxides. The model yields good predictions of the permanent charge for non‐swelling clay minerals, but clearly underestimates the permanent charge of montmorillonite. This result is interpreted as being caused by partial accessibility of the internal surface of the mineral.

In all cases, the values estimated for the permanent charge of the clay minerals fit the data for their mixtures with iron oxides. Nevertheless, important differences are found between the values of maximum densities of adsorption sites of potential‐determining ions of the various minerals estimated when they are alone and when they are in mixtures, particularly in the case of montmorillonite mixtures. The differences are explained in terms of an intimate association between the particles in the mixtures, and some SEM and EDX results point to the same conclusion.
Chemical change in organic matter decomposing in and on a forest Rendzina under beech (Fagus sylvatica L.)
Authors: JOERGENSEN R. G. , MEYER B.
Published: July 2006

The chemical characteristics of the organic matter of five horizons from a Terra fusca Rendzina under beech were compared by fractionation. Two litter layers, sampled in autumn, were compared: fresh leaf litter and the remains of the previous years’ litter. Three mineral horizons, A, B and C were also sampled.

While decomposition in the litter layer led only to slight redistribution of the fractions, much greater changes occurred when the organic matter became incorporated into the mineral soil. The proportion of C and N in amino sugars rose markedly down the profile. The percentage of more readily soluble substances, such as the hot water‐soluble fraction or the unidentified, hydrolysable component, in the mineral soil was twice the amount found in the leaf litter. The percentage of non‐hydrolysable compounds in the mineral horizons was nearly half that found in the leaf litter. The C/N ratio fell from 44 in fresh leaf litter to 10 in the C horizon. The amino acid C/amino sugar C ratio fell from 45 in fresh leaf litter to 3.0 in the B horizon. The glucosamine/galactosamine ratio fell from infinite in galactosamine‐free fresh leaf litter to 1.2 in residual litter and increased again to 2.5 in the C horizon. The hexose/pentose ratios were virtually the same in all horizons. The glucose/ mannose ratio showed a steady fall down the profile, from 7.7 to 1.6. The glucuronic/ galacturonic acid ratio increased from 0.1 and 0.2 in the two litter layers to 4.7 in the C horizon.
An evaluation of some oxidative degradation methods of humic substances applied to carbohydrate‐derived humic‐like polymers
Authors: ALMENDROS G. , LEAL J. A.
Published: July 2006

Artificial polymers prepared by acid‐catalysed dehydration of glucose were examined by 13C nuclear magnetic resonance (NMR), and subjected to degradation procedures frequently used in the study of humic substances: acid hydrolysis, persulphate degradation, alkaline permanganate oxidation, perborate depolymerization and cupric oxide oxidation. The ethyl acetate–soluble degradation products were identified by combined gas chromatography‐mass spectrometry. The compounds released in the perborate and in the cupric oxide treatments are as would be expected from a polymer derived from glucose dehydration products. Such degradation patterns contrasted with those obtained by alkaline permanganate oxidation, which resembled those of humic‐ or lignin‐like substances.
Aluminosilicate phases during initial clay formation: H+‐Al3+–oxalic acid–silicic acid–Na+ system
Authors: BILINSKI H. , HORVÁTH L. , INGRI N. , SJÖBERG S.
Published: July 2006

The solid phases and the precipitation boundary characterizing the system H+‐Al3+‐oxalic acid‐silicic acid‐Na+ are discussed. Model experiments have been used to throw more light on two environmental problems: the formation of sparingly soluble aluminium silicates in oceans and alkaline lakes, which could be determining aluminium and silicate concentrations in pore waters of sediments, and the validity of inorganic and organic mechanisms of podzolization and their significance for soil science.

pH and Tyndallometric measurements were performed at constant ionic strength of 0.6 M NaCl at 25°C. Three phases Al(OH)4, H4SiO4 (phase Via), Al2, (OH)6.H4SiO4 (phase VIb) and NaAl(OH)4.(H4SiO4), (phase VIII) determine the precipitation boundary. Phase NaAl(OH)4.H4SiO4 (phase VII precipitates at 0.4pH units above NaAl(OH)4.(H4SiO4)2. Using a set of previously determined binary and ternary complexes, and phases of the subsystems, the following formation constants were deduced:

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Phases VIa and VIb are described as end‐members of the allophane series with Si: Al ratios of 1:1 and 1.2. Phase VIb was identified with protoimogolite allophane. These two phases are good model clays for podzolic soils and are extremely soluble at pH < 4. Sodium phases could be hydrous feldspathoids. These phases are possible in sediments of seawater or saline lakes.

It is suggested that organic and inorganic mechanisms of podzolization operate sequentially and that neither of them alone can completely describe the process.
Effect of liming on phosphate sorption by acid soils
Authors: NAIDU R. , SYERSY J. K. , TILLMAN R. W. , KIRKMAN J. H.
Published: July 2006

The sorption of phosphate (P) by four strongly acid Fijian soils from 0.01 M CaCl2 decreased with increasing pH up to pH 5.5–6.0 and then increased again. The initial decrease in P sorption with increasing pH appears to result from an interaction between added P, negative charge, and the electrostatic potential in the plane of sorption. The results of a sorption study, involving KCl or CaCl2 of varying concentrations as the background electrolyte and using Nadroloulou soil incubated with KOH or Ca(OH)2, suggested that the increase in P sorption at pH values > 6.0 was caused by the formation of insoluble Ca‐P compounds. For some soils this is consistent with the results of an isotopic‐exchange study in which incubation with lime caused marked reductions in the amounts of exchangeable P at high pH.
The development, calibration and field testing of a soil loss and a runoff model derived from a small‐scale physical simulation of the erosion environment on arable land in Zimbabwe
Authors: ELWELL H. A.
Published: July 2006

Soil loss and runoff prediction models were developed for a clay soil from a five‐factor small‐scale physical simulation of the field environment by following a central composite rota table experimental design.

The uncalibrated models efficiently ranked observed annual soil losses and runoff over a 4–year period between nine field treatments consisting of two bare fallows, two weed fallows and five plots cropped to soyabeans.

When calibrated against the field data, the soil loss model predicted the 4–year mean losses from cropped and bare fallow treatments to within 6% for two of the plots, to within 12% for five of the plots and to within 14% for all seven treatments.

Over the same period, the runoff model predicted mean annual runoff for the cropped and bare fallow treatments to within 4% for four of the plots and to within 16% for all seven treatments.

Percentage vegetal cover proved to be an adequate parameter for describing the role of the soyabean crop in runoff and soil loss processes for a wide range of planting densities of the crop. However, it did not prove to be an efficient index for weed fallows and it was apparent that factors other than simple above‐ground cover became important soon after germination.
Paragonite in Spanish ‘raña’ soils
Authors: GARCÍA‐GONZÁLEZ M. T. , ARAGONESES F. J.
Published: July 2006

This study reports the presence of paragonite in the fine fraction of soils developed from Pliocene‐Quaternary aged formations (raña) in central Spain and investigates the factors that determine its presence. Paragonite was found in two of the four areas studied. These two areas had a lesser degree of alteration, although of the two, the more altered area had a higher proportion of paragonite. No significant correlation was found between paragonite and illite content, although the areas which contain paragonite generally had a higher illite content. The presence of paragonite was determined by the origin of the materials in each of the zones.
Effect of tillage on soil organic carbon mineralization estimated from 13C abundance in maize fields
Authors: BALESDENT J. , MARIOTTI A. , BOISGONTIER D.
Published: July 2006

Three methods of cultivation, conventional tillage (CT), superficial tillage (ST) and no‐tillage (NT), were applied for 17 years to continuous maize. Their effect on soil organic carbon content was investigated through measurements of carbon and 13C/12C ratios, using the natural difference in 13C content between C3 plants and maize, which is a C4 plant. Because the soil had carried C3 plants before the experiment started, the organic carbon remaining from that time (C3,‐carbon), was distinguished from the carbon derived from maize. Comparison between continuous wheat and maize plots showed that organic matter from both maize and wheat decomposed without significant 13C enrichment, whereas older C3‐carbon was enriched by 1.5% compared to that of fresh wheat material. From the initial 3.6 kg C m−2 in the topsoil (0–30 cm), 0.95 were mineralized in the CT treatment, but only 0.45 in NT. The mineralization was the same in the tilled layer of ST as in CT. The CT treatment accumulated 1.1 kg C m−2 of maize‐derived carbon and the NT treatment 0.8. The mineralization of initial C3‐carbon was the same at all depths between 0 and 30 cm in the NT treatment; 75% of the carbon derived from maize was found in the 0–5 cm layer.
Soil controls on recharge to aquifers
Authors: COOPER J. D. , GARDNER C. M. K. , MACKENZIE N.
Published: July 2006

Case studies are presented of the use of soil water measurements to determine the pathways, timing and quantities of natural recharge to groundwater at sites on the Chalk and Triassic Sandstone aquifers in England. Water content and potential have been monitored for several years at seven sites and unsaturated hydraulic conductivity measured in situ at three of the chalk sites. Recharge was estimated using a combination of the zero flux plane (ZFP) method and a simple water balance. Examples of the water balance are presented for three sites.

It is shown that a shallow (∼1 m) depth of permeable drift and soil increased the annual drainage from the soil to the underlying formation by about 34% at a pair of sites on the Chalk aquifer. Effects upon drainage timing are equally dramatic. On thin soil directly on chalk, drainage responds quickly to rainfall and, once evaporation starts to exceed rainfall, declines rapidly. On drift‐covered sites, the drainage continues for a much longer period, as it does from the loamy soil overlying the sandstone site.
The effect of volatilization on the mass flow of a non‐aqueous pollutant liquid mixture in an inert porous medium: experiments with kerosene
Authors: GALIN T. , McDOWELL C. , YARON B.
Published: July 2006

Kerosene, an industrial petroleum product characterized by the presence of a large number of petroleum hydrocarbons (C9‐C15), was selected as an example of a non‐aqueous pollutant liquid (NAPL) mixture for our studies. Three inert materials (fine, medium and coarse sand) were used in the experiments and the initial amount of kerosene applied ranged from residual to saturated retention capacity.

Volatilization in the air phase and saturated mass flow of kerosene in the three sands were studied in the laboratory under controlled conditions. The volatilization was the major physicochemical process affecting the fate of kerosene in the inert porous medium. During volatilization the liquid kerosene changed its composition by gradually losing its light components (C9‐C13), and the viscosity of the remaining liquid kerosene increased. The increase in viscosity led to a decrease in the infiltration rate, for example, by about 20% when the viscosity increased from 1.3 × 10−3 to 2.0 × 10−3 Pa s. The relationship between the composition of the residual kerosene following volatilization and its mass flow in a sand illustrates the behaviour of non‐aqueous pollutants in inert porous media.
Sulphate sorption by variable charge soils
Authors: GUADALIX M. E. , PARDO M. T.
Published: July 2006

The sorption of sulphate (SO2−4) by three variable charge soils from the Canary Islands (Spain) was studied. Sulphate sorption decreased with increasing pH. Only negligible amounts of SO2−4 were sorbed above pH 6.5. When the soils were washed with an indifferent electrolyte (0.01 M KCl), more SO2−4 was recovered than had been sorbed. This indicated a release of native SO2−4 Sulphate replaced hydroxyl ions (OH) and co‐ordinated H2O molecules, as well as very small amounts of silicate (Si). No measurable amount of phosphate (P) was released. On average hydroxyl release accounted for 50% of SO2−4 sorbed, the rest being accounted for by the increase in negative charge as measured by K+ adsorption. The results presented here are consistent with the sorption of SO2−4 through a ligand exchange mechanism, but in a different plane of sorption to that of phosphate.
Testing a mechanistic model. XI. The effects of time and of level of application on isotopically exchangeable phosphate
Authors: BARROW N. J.
Published: July 2006

A model for the reaction of ions with soil was improved to permit time trends to be followed at a given level of phosphate addition. Difference equations were also developed to describe the rate of reaction of ions with both vacant sites and occupied sites, while diffusive penetration of the surface was occurring. The model was applied to data for the effects of time and of level of application on exchangeable phosphate.

Many of the observed values for isotopically exchangeable phosphate could be well‐described if it was assumed that equilibration of 32P with surface sites was very rapid and this was followed by a diffusive penetration into the adsorbing particles. However, for short periods of contact between soil and 32P, it was necessary to also take into account the rate of the reaction between 32P and surface sites. This reaction was largely with vacant sites. Reaction with occupied sites–that is, true exchange–was unimportant.

It is suggested that the electric potential of the surface may determine whether reaction is with occupied or vacant sites. In contrast to reaction of 32P with occupied sites, reaction with vacant sites involves a net transfer of charge. Reaction with vacant sites would be slow if the potential was large and negative. It is shown that when reaction with vacant sites is slow, the proportion of previously added 31P recorded as exchangeable increases with level of addition of 31P. This may explain published observations of slow and non‐linear exchange in some soils.
The distribution of humic substances between the solid and aqueous phases of acid organic soils; a description based on humic heterogeneity and charge‐dependent sorption equilibria
Authors: TIPPING E. , WOOF C.
Published: July 2006

A description of the partitioning of humic substances (HS) between the solid and solution phases of acid organic soils is proposed, based on the opposition of hydrophobic and hydrophilic properties. Soil HS are considered to be of two types, mobile, i.e. potentially transferable to the soil solution (HSM), and immobile (HSI). The HSM consist of a series of fractions, each with the same ion‐binding properties, but differing in hydrophobicity. Partitioning of HSM between the solid and solution phases depends upon the net humic charge (Z), which is determined by the binding of protons and other cationic species. For each fraction, the extent of transfer to the solution phase increases as Z becomes more negative. The most hydrophilic HSM require only a small negative charge for transfer to the solution phase to be favoured, while the most hydrophobic require the development of a large negative charge. A mathematical version of the model, combined with an existing model of ion‐binding by HS, can explain reasonably well the release of HS from agitated aqueous suspensions of organic soil samples. The model also explains the release of HS when the solution phases of organic soil suspensions were repeatedly replaced with NaCl solution.
Factors affecting the formation of dark, thick epipedons beneath forest vegetation, Michigan, USA
Authors: SCHAETZL R. J.
Published: July 2006

Extremely gravelly, coarse‐textured soils (frigid Udorthents and Rendolls) with different thicknesses of Oa+A horizon sequences were studied to identify factors that have influenced their genesis. These well‐drained, forested soils occur on geomorphic surfaces that range in age from 3200 to 6000 years BP. The soils all have more than 500 g kg−1 coarse fragments by mass; most contain less than 300 g kg−1 fine earth. In the lower solum of most pedons, content of cobbles increases and amount of fine earth decreases. Most coarse fragments are dolomite and chert.

Thick, gravelly Oa and A horizons are weakly correlated with parent material characteristics such as high pH and carbonate contents. Organic matter concentrations in, and thicknesses of, upper horizons are enhanced by an abundance of coarse clasts, as soils with the most gravel exhibited the thickest and darkest epipedons. Relatively high amounts of crystalline clasts in the fine gravel fraction, as well as feldspathic minerals in the fine sands, also appear to promote the development of mollic epipedons.
The use of region partitioning to improve the representation of geo statistically mapped soil attributes
Authors: McBRATNEY A. B. , HART G. A. , McGARRY D.
Published: July 2006

An attempt to improve the representation of a geo statistically mapped soil attribute, clay content of the surface soil, through partitioning of the study area into two new regions was made. A topographic boundary divided the study area into hill and plain regions.

Possible global non‐stationarity or non‐stationarity within the two newly defined regions was dealt with through the use of intrinsic random functions (IRF) of order k. Cross‐validation of generalized covariance functions suggested that ordinary kriging might also have been appropriate. Exponential variogram models were subsequently fitted to the experimental variograms for each region.

IRF‐k block kriging and ordinary block kriging were then used as the primary methods of estimation. Both IRF‐k and ordinary kriging performed badly in the vicinity of the topographic boundary when global models were used. This discontinuity was removed, at the expense of the introduction of some additional edge effects, when the hill and plain regions were kriged using models appropriate to each region.

Independent zero‐order generalized covariance functions with nugget and linear terms and exponential variogram models produced similar representations of clay content within each region, when used with their respective estimators. Splitting the region resulted in a 6% reduction in mean absolute deviation and a 14% reduction in mean squared deviation of predicted clay contents compared with a global model.
An image analysis evaluation of soil compaction in grassland
Authors: KOPPI A. J. , DOUGLAS J. T. , MORAN C. J.
Published: July 2006

In the fifth year of a traffic management experiment on grassland comparing conventional (C), reduced ground pressure (R) and zero (Z) systems, differences in soil structure were characterized in the upper 100 mm. Soil bulk density and vane shear strength increased between systems in the order Z, R, C. A more detailed description of the soil structural differences between traffic systems, and between depths within systems, was obtained by an image‐interpretation technique. Rapid‐hardening epoxy resin, containing fluorescent dye, was added to the soil in the field condition, and the macroporosity was quantified by image analysis. All measured attributes (bulk density, vane shear strength and those from image analysis) showed that soil structure was superior in the zero (Z) and reduced ground pressure (R) traffic systems compared to that of the conventional (C) system. Porosity and pore or solid surface area of the Z system were greater, and aggregate size was smaller, than in the C system. In general, R was intermediate between Z and C. At about 10–30 mm in the reduced ground pressure system, there was a zone of low porosity that resembled much of the soil in the conventional system. Macroporosity and roots were concentrated close to the soil surface under the C and R systems. In the absence of traffic, or under the R system, the surface of the ground was rougher than that under the C system.
Carbendazim adsorption on montmorillonite, peat and soils
Authors: CANCELA G. DIOS , TABOADA E. ROMERO , SÁNCHEZ‐RASERO F.
Published: July 2006

The adsorption of carbendazim by peat and montmorillonite was studied as a function of the exchangeable cations and temperature. The adsorption on soils was also studied.

The kinetics of carbendazim adsorption on peat showed that adsorption equilibrium was reached within 1 h. The order of adsorption of carbendazim on peat was as follows: H+‐peat > Cu2+‐peat > Co2+‐peat > Mg2+‐peat > K+‐peat, and the thermodynamic parameters appeared to suggest an adsorption mechanism involving hydrogen bonds, although in the H+, Cu2+ and Co2+ samples a protonation process and adsorption of the protonated species were also likely.

The kinetics of carbendazim adsorption on montmorillonite (mont.) showed that equilibrium was reached within 1 h. The order of adsorption was: H+‐mont. > Cu2+‐mont. > Co2+‐mont. > Ca2+‐mont., the adsorption on the H+ and Cu2+ samples being much greater than that on the other samples. For the H+ and Cu2+ samples, the thermodynamic parameters appeared to suggest a double mechanism: physical adsorption, and protonation and adsorption by ion exchange. The most probable mechanism for the adsorption of carbendazim on the Co2+ and Ca2+ samples was physical bonding.

The capacity for adsorption of this fungicide on soil was dependent on the organic matter, nitrogen and clay content, as well as on the cation exchange capacity. No significant correlation was found with pH, C/N ratio or free iron content.
Sample adequately to estimate variograms of soil properties
Authors: WEBSTER R. , OLIVER M. A.
Published: July 2006

The variogram is central in the spatial analysis of soil, yet it is often estimated from few data, and its precision is unknown because confidence limits cannot be determined analytically from a single set of data. Approximate confidence intervals for the variogram of a soil property can be found numerically by simulating a large field of values using a plausible model and then taking many samples from it and computing the observed variogram of each sample. A sampling distribution of the variogram and its percentiles can then be obtained. When this is done for situations typical in soil and environmental surveys it seems that variograms computed on fewer than 50 data are of little value and that at least 100 data are needed. Our experiments suggest that for a normally distributed isotropic variable a variogram computed from a sample of 150 data might often be satisfactory, while one derived from 225 data will usually be reliable.
Changes in soil solution composition and pH in urine‐affected areas of pasture
Authors: HAYNES R. J. , WILLIAMS P. H.
Published: July 2006

Changes in soil solution composition and concentrations of exchangeable cations and mineral N in undisturbed cores of pasture soil were investigated in two experiments following applications of sheep urine to the cores. The major cations applied in the urine were K+ and Na+, and the major anions were HCO3− and Cl−. Addition of urine increased concentrations of exchangeable K+, Na+ and NH4+ and measured ionic strength of the soil solution throughout the surface 15 cm of soil, demonstrating that the urine moved through the core by macropore flow immediately following addition. Immediately following urine application the ionic strength in soil solution in the surface 2.5 cm of soil increased from 4–6 MM to 24–41 mM.

Hydrolysis of urine‐urea was extremely rapid, and in less than 1 d high concentrations of NH4+‐N (i.e. 270–370 mg N kg−1) had accumulated in the surface 0–2.5 cm of the urine patch, and soil pH had risen by over one unit. Nitrification then proceeded and, after approximately 15 d, NO3− became the dominant form of mineral N present. During nitrification, soil pH declined and the ionic strength of the soil solution increased substantially with NO3− becoming the dominant anion present in solution. There were concomitant increases in the concentrations of Ca2+ and, to a lesser extent, Mg2+ in the soil solution as NO3− concentrations increased. After approximately 30 d, concentrations of exchangeable NO3− had risen to 250–330 mg N kg−1, soil solution NO3− concentrations had increased to about 80 mmol, dm−3, and ionic strength in the soil solution had increased to 130–140 mM. These results demonstrate the dominating effect of N transformations in causing large fluctuations in the pH, ionic composition and ionic strength of the soil solution in the urine patch. It was concluded that nutrient availability in the patch was affected directly by nutrient addition in urine, and also probably indirectly through the fluctuations in soil solution pH and ionic strength that occur.
Composition of carbon fractions and potential denitrification in drained peat soils
Authors: JÖRGENSEN R. G. , RICHTER G. M.
Published: July 2006

The organic matter of five low‐moor peat soils and one eutrophic raised‐bog peat soil was chemically characterized by C fractionation and ion‐exchange chromatography of amino acids and carbohydrates. C fractions were related to potential denitrification, Dpot, as a measure of microbial activity and C availability, determined by the acetylene inhibition technique. Chemical and physical properties vary distinctively between different kinds of peat, and show increasing C/N ratio and decreasing bulk density and ash content within the profile. Generally, the carbon composition reflects the geobotanical origin of the peat. In most samples more than 65% of organic C consists of non‐hydrolysable C. Readily hydrolysable neutral sugar C represents up to about 12% of organic C, usually decreasing with depth. The recalcitrant fraction of neutral sugar C is much smaller (1 to 4.2% of organic C) and does not vary with depth. The content of readily hydrolysable glucose exhibits a strong profile differentiation that decreases with depth, whereas the higher contents of recalcitrant glucose carbon (12/0.5 M H2SO4) in the lower peat horizons reflect their cellulose character. Regression analysis between Dpot and single C components explains up to 51.5% of the variability. Combining fractions which point to C availability (readily hydrolysable glucose) and microbial metabolism (amino acids), it is possible to estimate Dpot with a certainty of more than 80%.
Changes in microstructure of soils following extraction of organically bonded metals and organic matter
Authors: WIERZCHOS J. , CIRIA C. ASCASO , GARCIA‐GONZALEZ M. T.
Published: July 2006

The influence of selective removal of organically bonded metals and organic matter on soil microstructure was investigated. Two samples of soils with different mineralogical, chemical and mechanical composition were treated with acetylacetone in both polar and non‐polar solvents to dissolve amorphous organic iron and aluminium, and with hydrogen peroxide to destroy organic matter.

Transmission electron micrographs of ultrathin sections and scanning electron micrographs of <5−μm fractions of the extracted soils showed distinct changes of microstructure of clays after successive removal of cementing agents. Although untreated soils showed flocculent or honeycomb structure, soils with organic matter and organically bonded metals removed showed turbostratic domain structure with stepped clusters. The changes in microstructure of soils following extraction were confirmed by determination of pore‐size distribution and total cumulative volume of pores using the mercury porosimetry method. In addition, the surface area of the extracted soils was determined by water adsorption. The results showed that organically bonded iron and aluminium and organic matter distinctly influence the fabric of microstructure as flocculating agents.