bluuebunny/crossref_metadata_2025 huggingface dataset:
Interlayer materials of partially interlayered vermiculites in Dystrochrepts derived from Tertiary sediments
Authors: MATSUE N. , WADA K.
Published: July 2006

Interlayer materials of partially interlayered vermiculites (PIV) in 15 Dystrochrepts derived from Tertiary sediments were analysed by dissolution with hot 1/3M sodium citrate or 0.15 M oxalate‐oxalic acid in combination with XRD and IR spectroscopy. Both the citrate and oxalate treatments dissolved Al from all soil clays. The dissolution of Al by the citrate treatment paralleled the dissolution of Si and the interlayer collapse of PIV, whereas that by the oxalate treatment did not, indicating that the interlayer materials contain not only Al but Si. The materials dissolved by the citrate treatment had a molar Si/Al ratio ranging from 0.72 to 0.24 that decreased with the increasing interlayering of PIV. Differential IR spectroscopy indicated the dissolution of aluminosilicates possibly having Si‐O, Al‐OH and Si‐O‐Al bonds, but not Si‐O‐Si bonds. The extent of interlayering of PIV showed a good correlation with the amount of Al dissolved by the citrate treatment and increased with increasing soil pH (H2O) from 4.5 to 5.2 and with decreasing exchangeable Al in soil.

Formation of a hydroxy‐Al sheet partially bonded with Si‐tetrahedra was suggested as a possible model of the interlayer structure of PIV. The differences between PIV in the studied Inceptisols and other soils are discussed.
Protonation reactions of fulvic acids
Authors: GREGOR J. E. , POWELL H. K. J.
Published: July 2006

The protonation reactions of fulvic acids are described in terms of a model involving a mixture of n monoprotic acids HA, HB, HC … pH titration data were analysed by nonlinear least squares methods, using a model having n independent ‘titration’ (protonation) constants, Ki i=1 to n, and n–1 independent acid concentrations. The best fit was obtained with n=4. The results for three fulvic acid samples were in the range log K1 6.5–6.7 (∼8% of total carboxyl acidity), log K2 5.3–5.6 (∼15%), log K3 3.9–4.8 (∼20–30%) and log K4 2.6–2.7 (∼50–70%).

The results indicate a comparatively small proportion (∼ 10–20%) of polyprotic carboxyl moieties in the diverse fulvic acid structures. They give some support to the Wershaw model of fulvic acids as strongly associated aggregates of acids, each of comparatively low molecular weight.
The soil solution in a soil treated with digested sewage sludge
Authors: CAMPBELL DUNCAN J. , BECKETT P. H. T.
Published: July 2006

Toxic trace metals may percolate to the ground water from sewage sludge disposed onto land. Analyses are presented of the soil solution from a slightly acid loamy soil treated 7 years earlier with single applications of digested sewage sludge in amounts equivalent to 0, 150 & 330 t dry matter ha−1

These very heavy dressings correspond to 2 & 4.5 times the recommended 30–year limit. Samples of soil and soil solution from four depths to 80 cm were analysed for Al, B, Ba, Ca, Cl, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, S, Sr, V, Zn, together with the OM of the soil, and the pH, alkalinity, dissolved organic carbon, and absorbance at 350 nm of the solutions.

These very heavy sludge applications were apparently still releasing substantial quantities of NO3, and some SO4 even after 7 years. Nitrate, SO4, Mg, Ca, Sr, B, and possibly Ba are still moving through the profile, possibly to the ground water. Solution concentrations of Cu and Zn are considerably higher at all depths than those in the untreated plot, but they fall off sharply with depth. It is unlikely that any Cu or Zn is now reaching the ground water.

The paper also presents a set of published solution analyses for soils, sludge–treated soils and digested sludge, as a basis for further studies.
The topology of pore structure in cracking clay soil. II. Connectivity density and its estimation
Authors: SCOTT G. J. T. , WEBSTER R. , NORTCLIFF S.
Published: July 2006

Pores in soil, especially in cracking clays, interconnect in more or less complex patterns. The complexity of the pattern can be represented by its connectivity. The connectivity of a network of cracks is the number of loops in it, and the number of loops per unit volume of soil is the connectivity density (Gv).

The patterns of cracks wider than 60 μm were exposed in parallel sections 50 μm apart, photographed and skeletonized. Loops were tracked from one section to another and counted. Other loops completed in three dimensions but not apparent in horizontal sections were recognized and added to the count. The counts were linearly related to the number of sections examined, and by regression analysis stable and precise estimates of Gv were obtained for the clay subsoils of Windsor and Swanwick soil series from as few as eight sections. The connectivity density of the crack pattern in the Windsor subsoil was approximately 300 cm−3 and that of the Swanwick series about 195 cm−3.
Calculation of gross nitrogen immobilization and mineralization in soil
Authors: BJARNASON S.
Published: July 2006

In a laboratory experiment, soil was treated with 15NH4‐ and 15NO3‐N compounds at various times during the incubation. Several approaches in the calculation of gross immobilization and gross mineralization between two sampling dates were compared and a new method was developed. It is based on a dynamic simulation model designed to interpret isotopic data and an optimization procedure used to determine the best fit between model output and experimental data. This flexible method was used to examine the validity of the assumptions usually made when calculating the gross transformations.

The results indicate that seriously erroneous estimates of the gross transformations can follow if it is assumed that remineralization of immobilized N does not occur. Less serious errors can result from the assumption that both opposing processes occur at a constant rate during the interval between sampling dates.

The combined use of the model and the optimization procedure has several advantages over traditional methods and some of the gross transformation estimates reported would not have been obtainable using older methods.
Effect of liming and phosphate additions on sulphate leaching in soils
Authors: BOLAN N. S. , SYERS J. K. , TILLMAN R. W. , SCOTTER D. R.
Published: July 2006

The effect of lime (CaCO3) and phosphate additions on surface charge characteristics and their effect on the leaching of sulphate were examined for two soils (Patua loam and Tokomaru silt loam) which differed in their adsorption capacities for sulphate.

Incubation of soils with either CaCO3 (0–600 mmol kg−1) or phosphate (0‐208 mmol kg−1) resulted in a two‐ to five‐fold increase in the net negative charge and a similar decrease in the adsorption of sulphate. The effect of either lime or phosphate addition on both the surface charge and sulphate adsorption was more pronounced for the allophanic Patua soil than for the Tokomaru soil containing mainly vermiculite.

In a column experiment, liming induced the leaching of sulphur either by the desorp‐tion of adsorbed sulphate or by the mineralization of organic sulphur. During a miscible displacement study, addition of either CaCO3 or phosphate resulted in an early breakthrough of sulphate in the leachate. In a pulse experiment, in which soils were incubated with sulphate (3.12 mmol kg−1) for 1 week and subsequently leached with water, more added sulphate was lost in the leachate of the soils previously incubated with either CaCO3 or phosphate.
Metal binding with root exudates of low molecular weight
Authors: MENCH M. , MOREL J. L. , GUCKERT A. , GUILLET B.
Published: July 2006

Root exudates were collected from maize plants (Zea mays L.), grown under sterile and hydroponic conditions. Exudates of low molecular weight were isolated by filtration and dialysis (MW 1000) of the nutrient solution and analysed. Metal binding properties of the exudates were investigated at 25°C in 0.1 m NaCIO4 ionic medium, using a potentiometric method. The total acidity of the exudates and the successive stability constants of complexes formed with Cu2+, Pb2+ and Zn2+ were determined.

Sugars and organic acids were the major components of the exudates. Measured total acidity was resolved into strong (5.85meq g−1C), weak (2.10meq g−1C) and very weak (2.20 meq g−1 C) acidities. Mono‐complexes were formed with Cu, Pb and Zn and bis‐complexes with Cu. At pH 5, stability constants followed the order Cu>Pb>Zn.
Metal‐humus complexes in A horizons of Thai and Korean red and yellow soils
Authors: VIRAKORNPHANICH P. , WADA S.‐I. , WADA K.
Published: July 2006

Carbon, Al and Fe (Cpyr, Alpyr and Fepyr) were extracted with 0.1 m Na4P2O7 from 26 A horizon samples of tropical Thai and temperate Korean soils (Ultisols, Alfisols, Oxisols and Inceptisols). The soils, except for one Thai Inceptisol, had similar total C (0.35–3.29%) and Cpyr/total C ratios (0.20–0.41). There were approximately linear relationships between total C or Cpyr and clay content; two groups of soils gave different linear relationships. A curvilinear relationship between Cpyr and (Al + Fe)pyr (milli‐atom kg−1) that can be approximated by an equation: Cpyr= 53 (Alpyr+ Fepyr)1/2– 24 was also found for most Thai and Korean soils. The above relationships indicated that total C and Cpyr would be close to zero at zero clay or zero (Al + Fe)pyr. It was inferred that clay‐humus interaction has a primary importance in the determination of humus content in red and yellow soils in tropical and temperate regions and that the main role of clay is to supply Al and Fe that complex and stabilize humus against microbial degradation.
The formation and characteristics of splash following raindrop impact on soil
Authors: GHADIR H. , PAYNE D.
Published: July 2006

Splash droplet characteristics were studied using cine‐photography and a simple droplet collection method. Two sizes of falling drop, two heights of fall and a number of target materials including soil, sand, water and rotating sandpaper were used. For a 6 mm drop falling at near terminal velocity about 5000 splash droplets were collected ranging in size from 10 um to more than 3 mm in diameter. The amount of soil material carried by splash droplets was much less in the presence of surface water and increased with the size of water stable aggregates. The energy and momentum balance showed a higher proportion of impact energy and momentum in the resulting splash droplets as the impact energy increased. This suggests that kinetic energy is not a reliable indicator of the erosivity of rain drops.
Aggregation in soils with small amounts of swelling clays. I. Aggregate stability
Authors: BARTOLI F. , PHILIPPY R. , BURTIN G.
Published: July 2006

The respective role of organic materials and poorly‐ordered Al and Fe hydrous oxides on soil aggregate stability was studied in silty soils with little swelling clay, using both multivariate analysis and physico‐chemical approaches. Soil disaggregation is a function of the hydrophobic character of organic matter (OM), which depends on the nature of the organic materials, cationic environment and the aggregating effect of Al and Fe hydrous oxides.

Two kinds of aggregates >50 μm can be distinguished in organic soils, one being about six times more stable than the other. In soils poor in organic matter, weak aggregates dominate; the binding agents are Ca or Al OM rich in polysaccharides and peptides.
Testing a mechanistic model. VII. The effects of pH and of electrolyte on the reaction of selenite and selenate with a soil
Authors: BARROW N. J. , WHELAN B. R.
Published: July 2006

The pH of samples of a soil was altered by mixing them either with acid or lime, and incubating the moistened samples at 60°C for a day. The sorption of selenate and of selenite was then measured using as background electrolytes, 0.01 M, 0.1 M and 1.0 M sodium chloride and also 0.01 M calcium chloride. The results were compared with previous studies with phosphate and fluoride.

Selenite was sorbed more strongly than selenate, but not as strongly as phosphate or fluoride. Sorption of both selenite and selenate decreased with increasing pH. This decrease was more marked for selenate than for selenite; more marked in a sodium system than in a calcium one; and more marked with a dilute background electrolyte than a concentrated one. Under certain conditions, the steeper curves for the dilute electrolyte crossed the curves for the concentrated electrolyte giving points of zero salt effect. For selenite, these points of zero salt effect occurred near pH 6 and the greater the sorption the lower the pH for zero salt effect. For phosphate, the analogous value was near pH 5. For selenate, if a point of zero salt effect occurred, it was at such a high pH and such a low amount of sorption that it could not be measured. Thus, the larger the amount of sorption the lower the pH for the point of zero salt effect. This generalization applied both within and between different kinds of sorbates.

The results were closely described by a model that had previously been applied to phosphate and fluoride. The model postulates that ions react with charged surfaces. The electric potentials of the reacting surfaces are affected by the identity and concentration of the background electrolyte and this produces the interactions between pH and electrolyte concentration. The model also postulates that there is a distribution of electric potentials. Anions react with surfaces which occur in the more positive tail of this distribution. The smaller the amount of the reaction the more positive the potential of the reacted surface and, therefore, the higher the pH required to decrease this potential to zero.
The effects of lime on adsorption and desorption of phosphate in five Colombian soils
Authors: LE MARE P. H. , LEON L. A.
Published: July 2006

The effects of lime (applied in the field) on the amounts of total and isotopically‐exchange‐able phosphate adsorbed from solutions were measured in five soils. The total amount of phosphate adsorbed without lime was in the range 200 to 1700 μg P per g of soil at 0.05 μg P cm−3 of solution. Lime diminished the amount of phosphate adsorbed at all concentrations of solution in an oxisol and a dystropept; in an ultisol and another dystropept, lime tended to increase sorption at small concentrations and diminish it at large concentrations; in a dystrandept that contained spheroidal allophane and a great deal of organic matter, lime increased adsorption at all concentrations up to 1 μg P cm−3. Lime increased the proportion of added phosphate that was isotopically exchangeable in the oxisol and one dystropept, had no effect in the other dystropept, and diminished the proportion in the ultisol and dystrandept.

Adsorbed phosphate was subsequently desorbed by suspending the soils in solutions without phosphate. After desorption the quantity of exchangeable phosphate in all soils was closely correlated with aluminium extracted by ammonium oxalate; buffer power was correlated in all except the dystrandept, in which it was larger per unit of aluminium than in the other soils; possibly the cause was aluminium associated with organic matter. In all soils lime diminished buffer power allowing a specific amount of exchangeable phosphate to maintain a larger concentration in solution. The beneficial effects of lime on exchangeable phosphate after desorption were consistent among soils, despite inconsistent results when the phosphate was adsorbed.
Acid rain, cation dissolution, and sulphate retention in three tropical soils
Authors: PATIL S. G. , SARMA V. A. K. , VanLOON G. W.
Published: July 2006

Surface and subsurface samples of three tropical soils were examined with respect to their interaction with dilute solutions of sulphuric acid of pH 3. In calcareous clayey samples with a large cation exchange capacity the H+ was replaced by an equivalent concentration of metal cations which remained in solution along with SO2‐4 as counterion. In a coarse‐textured neutral soil with small cation exchange capacity, there was less chemical interaction and a major proportion of the H2SO4 remained unchanged in the equilibrium solution. Another soil exhibited considerable ability to remove SO2‐4 from solution and, therefore, the total ionic concentration was greatly reduced. Other samples showed behaviour which was intermediate to these three types.

The ability to adsorb SO2‐4 is one of the most important factors which determines the nature of the interaction of soil with dilute sulphuric acid. This ability was shown to be affected by the content of hydrous sesquioxides and organic matter in these soils.
Carbohydrate composition of hay and maize soils and their possible importance in soil structure
Authors: BENZING‐PURDIE L. M. , NIKIFORUK J. H.
Published: July 2006

The soils of three fields under continuous maize culture for 2,4 and 15 yr, and the soil of an adjacent hayfield were analyzed for carbohydrates by gas chromatography. A sharp decrease (30%) in total carbohydrates, paralleled by a smaller decrease in organic matter (9%), was observed after 2 yr of maize cultivation. The 4‐yr maize soil showed an upward trend in both organic matter and carbohydrate content but this was subsequently reversed. The 15‐yr highly‐compacted maize soil had 40% less organic matter and 40% less carbohydrates than the hay soil. When compared to the well structured 2‐yr maize soil, this compacted soil had less arabinose, xylose, glucose, galactose and mannose; carbohydrates which are probably of plant root origin. These sugars are likely to be involved in the structural stability of soil aggregates. Conversely, neither the amino sugars nor the uronic acids seemed to contribute to good soil structure.
Steady‐state denitrification in aggregated soils: a mathematical model
Authors: ARAH J. R. M. , SMITH K. A.
Published: July 2006

A model is presented which calculates steady‐state denitrification rates as a function of more readily‐measured soil parameters: the soil moisture characteristic, the probability distributions of aggregate size and oxygen reduction potential, the nitrate concentration, and the moisture tension. The model does not depend on curve‐fitting. It indicates that aggregates of intermediate size may be more efficient denitrifiers than very large ones. It provides a theoretical explanation for the reported observation of proportionality between denitrification rate and calculated anaerobic fraction of incubated soil cores. This proportionality extends over some five orders of magnitude and appears to be independent of moisture tension. Calculated whole‐soil denitrification rates are affected principally by soil texture, structure and moisture tension, and less so by nitrate concentration. In addition to predicting denitrification rates, the model may be extended to predict the fraction of the gaseous products of denitrification emitted as nitrous oxide.
Genesis of a Xeralf on feldspathic sandstone, South Australia
Authors: CHITTLEBOROUGH D. J.
Published: July 2006

A Xeralf on feldspathic sandstone was sampled from a hillslope in the Mount Lofty Ranges in order to study its genesis and, in particular, the origin of the strong texture contrast. Micromorphological study demonstrated that the clay present in void argillans and papules in the B2 horizon accounted for only a small amount of the clay present. Elemental analysis of the whole soil (< 2 mm) and sand, silt and clay fractions showed that there had been considerable weathering of both quartz and microcline, which were the dominant minerals present. Kaolinite is the dominant clay mineral weathering product. Illite appears to be forming from vermiculite in the A horizon. Using zircon as an internal standard, it was shown that elemental losses of SiO2 and reductions in weight and volume were similar in A and B horizons. Losses of aluminium and potassium were greatest in the A horizon, least in the B3. There has been an absolute increase in the amount of iron. A possible source is iron from heavy mineral bands upslope. It was concluded from the similarity of the quartz particle‐size distributions of the A2, B2 and B3 horizons that the intensity of weathering of quartz was the same in A and B horizons. In the case of feldspar (mostly microcline), there is a greater proportion of feldspar in the fine sand and silt fractions of the A2 horizon than in the B horizon. Weathering of feldspar is greatest in the A horizon. The texture profile is principally a function of greater lateral loss of clay from the A horizons compared to the B horizons.
Effect of rewetting air‐dried soils on pH and accumulation of mineral nitrogen
Authors: HAYNES R. J. , SWIFT R. S.
Published: July 2006

The effect of rewetting a number of air‐dried soils on pH and on accumulation of mineral‐N was examined in a laboratory incubation study. When rewetted‐soils were incubated at 25°C three patterns of change in soil pH and in accumulation of mineral‐N were observed. Ammonification and nitrification proceeded together in soils with pH values greater than 6.0; soil pH decreased whilst concentrations of nitrate rose and those of ammonium remained low. By contrast, in soils with pH values less than 5.0, although ammonification proceeded there was no appreciable nitrification; soil pH increased whilst concentrations of ammonium rose and those of nitrate remained very low. In a third group of soils with pH values between 5.0 and 5.5, there was a delay in nitrification, but ammonification was not retarded; soil pH initially rose as concentrations of ammonium increased, but when nitrification subsequently commenced the pH decreased, concentrations of nitrate rose and those of ammonium declined. When microbial activity in rewetted soils was inhibited by incubation at 3°C, or in a chloroform atmosphere at 25°C, there was little change in concentrations of ammonium and nitrate, and soil pH remained relatively constant.

Such changes in soil pH, induced by ammonification and nitrification, are likely to have important consequences to soil chemical studies where pH‐dependent reactions are being studied using rewetted soils. Changes in pH can be minimized by using field moist rather than air‐dried soils.
Loss of availability of phosphate in New Zealand soils
Authors: PARFITT R. L. , HUME L. J. , SPARLING G. P.
Published: July 2006

Phosphate sorption was measured by the method of Barrow (1980) using a laboratory incubation procedure for up to 60 d on four soils which had different mineralogies but medium to high phosphate retention. All the soils had slow reactions where phosphate sorption continued, but at a decreasing rate, with time. The rate of decrease in the slow reactions was similar on all the soils. Phosphate became less available to plants during the slow reactions, and results of a pot trial with white clover showed that, on all the soils, phosphate incubated with the soils for 218 d was about 65% as effective as phosphate incubated for 10d.

When 700 mg P kg−1 was added to allophanic soils (Andisols), about 100 mg kg−1 was strongly adsorbed, about 200 mg kg−1 became unavailable in about 200 days and the remainder was weakly adsorbed. A similar result was obtained on Waiarikiki soil (Inceptisol), which contained ferrihydrite and Al‐humus as the predominant reactive species. On the Kerikeri soil (Oxisol) about 150 mg P kg−1 became unavailable with time as a result of reactions with geothite, hematite and Al‐humus.

The phosphate uptake by the microbial biomass was similar to the uptake by the clover, and immobilization of phosphate in the biomass can contribute to the loss of availability of phosphate in soils.
Effects of long‐term phosphatic fertilizer applications on amounts and forms of phosphorus in soils under irrigated pasture in New Zealand
Authors: CONDRON L. M. , GOH K. M.
Published: July 2006

In this study a detailed sequential fractionation scheme was used to examine the effects of long‐term annual applications of phosphatic fertilizer on the amounts and forms of phosphorus (P) in soils under intensively grazed irrigated pasture over a 25 year period (1952–1977). Results showed that increases in soil inorganic and organic P occurred in response to continued P fertilizer addition, although inorganic P decreased with time following the cessation of P fertilizer inputs in the residual fertilizer and control treatments. The rate of organic P accumulation in soils decreased with time, while a marked decline in soil organic P in all treatments between 1971 and 1974 was attributed to increased mineralization as a result of lime addition in 1972. Furthermore, changes in the forms of inorganic P which accumulated in the continually fertilized soils after 1971 were attributed to the combined effects of liming and changes in the chemical nature of applied fertilizer P.
Effect of microorganisms on extractability of Cd from soil with NaOH and DTPA
Authors: BOLLAG J.‐M. , CZABAN J.
Published: July 2006

Because Cd is extremely toxic, an understanding of the factors that allow or prevent Cd entrance into the food chain is important. The fate of Cd in soil is strongly influenced by microbial activity. Thus, an attempt was made to develop a method for studying the effect of microorganisms on Cd sorption to soil and availability to plants. It was found that the amount of Cd extracted from soil to soil suspensions with 0.1 m NaOH is related to microbial proliferation. In addition, an increase of Cd in NaOH extracts occurred concurrently with a decrease of Cd in DTPA extracts. Our observations suggest that the binding of Cd to microorganisms or their products can immobilize Cd and consequently affect its bioavailability.
Characterization and stability of transition metal complexes of chestnut (Castanea sativa L.) leaf litter
Authors: SENESI N. , SPOSITO G.
Published: July 2006

The water‐ and acid‐insoluble fractions of a chestnut (Castanea sativa L.) leaf litter sample and their complexes with Cu(II), Fe(III), and Mn(II) prepared in the laboratory were characterized by major elemental analysis, total Cu, Fe, and Mn content, infrared (IR), and electron spin resonance (ESR) spectroscopy. The IR spectra revealed a broad typology of functional groups (particularly carboxyls) in the solid litter, whereas the ESR spectra showed the existence of indigenous organic free radical species, inner‐sphere Fe3+ complexes, and outer‐sphere Mn2+ complexes. The litter exhibited a high residual binding capacity for Cu, Fe, and Mn in chemical forms of differing stability against water leaching and proton exchange. The ESR spectra of the metal complexes prepared in the laboratory indicated that Fe3+ and Cu2+ formed highly water‐stable, inner‐sphere complexes, whereas Mn2+ formed water‐labile, outer‐sphere complexes. Oxygen ligands of the litter were involved in metal complexation in all cases. The litter showed the highest affinity for Cu2+, followed by Fe3+ and Mn2+, when it was reacted with a single metal, whereas it complexed Fe3+ preferentially in the presence of both Cu2+ and Fe3+. Only a limited portion of the metal ions retained at the pH of distilled water remained bound in stable forms by the litter when the pH was lowered. Thus, variations of pH in forest soils will significantly affect micronutrient metal content and mobility in leaf litter.
Optimal interpolation and isarithmic mapping of soil properties. VI. Disjunctive kriging and mapping the conditional porbability
Authors: WEBSTER R. , OLIVER M. A.
Published: July 2006

Disjunctive kriging is a technique for estimating values of spatially distributed random variables by combining data non‐linearly and with minimum variance. Data are transformed using Hermite polynomials to a normal distribution, and the resultant transformation is assumed to produce a stationary bivariate normal distribution for all pairs of data. Conditional probabilities that the true values exceed or are less than a specified critical threshold can then be calculated. For soil survey both the estimated values of soil properties and their associated probabilities can be mapped isarithmically as aids to decisions on land management.

The technique is described and illustrated from two case studies. The pH, exchangeable potassium and available phosphorus are mapped over 77 ha of the Broom's Barn Farm, and in the eastern Border Region of Scotland the available copper and cobalt in the topsoil are mapped. The estimates are similar to those obtained by simple (linear) kriging. The conditional probabilities were estimated for values falling short of the minimum recommended thresholds, and maps of them showed where farmers would be advised to remedy or forestall deficiencies in lime and plant nutrients and to take prophylactic measures for their livestock.
A simple predictive approach to solute transport in layered soils
Authors: DYSON J. S. , WHITE R. E.
Published: July 2006

Solute transport through layered columns (repacked aggregates overlying sand) was studied under steady flow conditions. Predictions of transport were simplified by assuming that the distribution of solute travel times in one layer was not correlated with that in the other. The implications of this assumption were developed for the transfer function model (TFM) and the convection‐dispersion model (CDM) of solute transport. The parameter values in each model were obtained from experiments carried out on columns containing only aggregates or sand.

The solutes used were nitrate (surface‐applied) and chloride (previously distributed); predictions of the chloride movement were made using the parameter values for the nitrate. The predictions were tested against experimental values of drainage effluent concentration and solute concentration with depths in the columns (measured at the end of the experiments). The TFM (with an assumed lognormal distribution of travel times) and the CDM did not differ significantly, mainly because the spatial scale of the experiments was small.

Because the parameter values for the columns of aggregates or sand were determined from the drainage effluent data, they were average values for whole columns. These parameters were satisfactory for predicting drainage effluent concentration from the two‐layer columns. However, they were not satisfactory for predicting the depth distribution of solute, particularly in the sand, because the water content of the sand increased with depth, unlike that of the aggregates, which was approximately constant with depth. The overall results of this study on materials of differing transport characteristics suggest that the assumption of uncorrelated travel times between layers has a potentially wide application. The approach taken here needs to be tested on undisturbed layered soils.
A usable mechanistic model of nitrate leaching II. Application
Authors: BARRACLOUGH D.
Published: July 2006

Simulations produced by a mechanistic model are compared with field observations of nitrate leaching from 0.35 ha hydrologically isolated experimental plots. The parameters used in the model are obtained in two ways. First by fitting the model to field observations in one year. These parameters are then used to simulate leaching in other years. Second, model parameters are obtained by fitting eluant profiles from pulse inputs of solute to undisturbed cores in the laboratory. When used with the field‐derived parameters, the model simulates total leaching losses well in other years, although the pattern of loss is only approximately reproduced. The simulation suggests that water and solute flow in drained, structured soils is complex; preferential flow in the upper horizons resulting in 20% of the water‐filled pore space carrying most of the solute flow, and by‐pass flow in the subsoil causing rapid movement of water and solute to the drains. The result is that much of the nitrate in the upper horizons appears to be protected from leaching. When used with laboratory‐derived parameters, the model was a poor predictor of both the pattern and quantity of nitrate leached.
The hydrolysis of kaolinitic soils as affected by the type of the exchangeable cation
Authors: LEVY G. J. , ALPEROVITCH N. , van der MERWE A. J. , SHAINBERG I.
Published: July 2006

The hydrolysis of four kaolinitic soils from the Transvaal (RSA), and the effects of exchangeable Na, Ca and Mg were studied by monitoring the changes in the electrical conductivity of the soil suspension with time. The rate of the hydrolysis was found to be linearly related to the square root of time. The rates of the hydrolysis of the Half‐Way House and Krugersdorp soils were appreciably lower than those of the Potchefstroom and Amsterdam soils, and rates reported in the literature for smectite soils. This was probably because of the low content of hydrolysable bases in the Half‐Way House and Krugersdorp soils, resulting from the low cation exchange capacity of their clay fraction. The extensive hydrolysis observed in the Potchefstroom and Amsterdam soils could be ascribed to the presence of significant amounts of smectite in the former and high silt content in the latter; both factors are known to enhance the rate of weathering and hydrolysis. With the exception of the Potchefstroom soil, the rate of hydrolysis was not affected by the cationic composition of the adsorbed phase. Contrary to expectations, exchangeable Mg did not inhibit hydrolysis in the soils studied, probably because of the limited isomorphic substitutions in kaolinite which lead to very small amounts of octahedral Mg in the crystal. The hydrolysis of the Potchefstroom soil was enhanced in the presence of exchangeable Mg compared to exchangeable Ca; the reason for this is not fully understood.
Selenite fixation by soil particle‐size separates
Authors: CHRISTENSEN B. T. , BERTELSEN F. , GISSEL‐NIELSEN G.
Published: July 2006

The fixation of selenite by clay‐ (< 2 μm), silt‐ (2–20 μm) and sand‐size (20–2000 μm) separates from two arable soils was examined in solutions of 75Se‐labelled sodium selenite using a Se/sample ratio of 1/106. Size separates were isolated by ultrasonic dispersion and gravity sedimentation. Selenite fixation was determined after equilibration periods ranging from 5 min to 26 h. Hydrogen peroxide‐treated samples were included to examine the effect of organic matter on selenite fixation capacity.

The relative distribution of native Se, C, dithionite/citrate‐extractable Fe and Al between size separates was similar. Concentrations in clay were four to nine times higher than in whole soils, silt showing two to five times higher concentrations and sand being very low in Se, C, Fe and Al.

After 1 h, clay, silt and sand fixed 64–65%, 45–61% and <5% of the selenite added, respectively. The fixation on whole soils was 14–18%. After 1 day, fixation on clay, silt, sand and whole soil increased to 78–87%, 67–79%, 3–14% and 31–39%, respectively.

Hydrogen peroxide‐treatment reduced the selenite fixation capacity of whole soil, silt and sand to very low levels. Fixation on peroxide‐treated clay was in accord with values for pure clay minerals reported in the literature. Generally, the fixation capacity of peroxide‐treated natural clay and pure clay minerals was only half that observed for intact clay‐size separates, demonstrating the importance of organic matter in soil selenite fixation capacity.
A critical comparison of methods to monitor the decay rate of straw in field soils
Authors: HARPER S. H. T.
Published: July 2006

Studies of straw decay in soil have concentrated primarily on three separate issues: the loss of potential to produce phytotoxins, the disappearance of surface mulches and the contribution of the products of decay to the carbon cycle. Cost and availability prohibit the use of 14C‐labelled plant materials in extensive field trials and mesh bags can be criticized for providing conditions which are potentially unlike those within the bulk soil. In consequence, a range of chemical and physical tests for estimating the extent of straw decay have been compared. Strength measurements are of direct value where mechanical impedance of machinery is involved, but otherwise are satisfactory only when straw is both harvested and returned to the soil by hand. The increase in lignin content was found to provide the most satisfactory method for the estimation of weight losses of up to 60%.
Modelling volume changes in peat under conifer plantations
Authors: PYATT D. G. , JOHN A. L.
Published: July 2006

It is proposed that the shrinkage of well‐decomposed peat beneath conifer plantations owing to the irreversible loss of water takes place in two stages. The first stage of pure subsidence is followed by a second stage of the formation of natural aggregates with a network of large cracks and a slower rate of subsidence. A plot of specific volume against gravimetric water content for natural aggregates or for excised blocks dried artificially shows a linear relationship throughout the range of water content. The assumption of equi‐dimensional shrinkage in stage two allows a prediction of the relative amounts of subsidence and crack formation. The shrinkage of non‐cracking fibrous peat is also dealt with. A preliminary attempt at verification of the predictive model from measurements of crack volume or subsidence of ground level shows that the model gives values of the right order of magnitude.
Brief and vigorous N2O production by soil at spring thaw
Authors: CHRISTENSEN S. , TIEDJE J. M.
Published: July 2006

In an acid sandy loam soil (pH 3.8), field production of N2O was two orders of magnitude higher at thaw in the spring than at any time during the rest of the year. Soil thaw in midwinter did not result in any increase in N2O flux. Soil water content remained at, or above field capacity during measurements; nitrate was added in excess. This effect could be reproduced in the laboratory: thawing soil cores at controlled temperature, nitrate and moisture yielded a large flush of N2O compared to an unfrozen control. The results indicate the importance of microbial N2O production during thaw for total annual N2O‐emission.
Alysimeter study of the effects of a ryegrass catch crop, during a winter wheat/maize rotation, on nitrate leaching and on the following crop
Authors: MARTINEZ J. , GUIRAUD G.
Published: July 2006

The effects of an intercrop catch crop (Italian ryegrass) on (i) the amounts and concentrations of nitrate leached during the autumn and winter intercrop period, and (ii) the following crop, were examined in a lysimeter experiment and compared with that from a bare fallow treatment. The catch crop was grown in a winter wheat/maize rotation, after harvest of the wheat, and incorporated into the soil before sowing the maize. A calcium and potassium nitrate fertilizer labelled with 15N (200 kg N ha−1; 9.35 atom per cent excess) was applied to the winter wheat in spring. Total N uptake by the winter wheat was 154 kg ha−1 and the recovery of fertilizer‐derived N (labelled with 15N) was 60%. The catch crop (grown without further addition of N) yielded 3.8t ha−1 herbage dry matter, containing 43 kg N ha−1, of which 4.1 % was derived from the 15N‐labelled fertilizer. Two‐hundred kg unlabelled N ha−1 was applied to the maize crop. During the intercrop period the nitrate concentration in water draining from the bare fallow lysimeters reached 68 mg N1−1, with an average of 40 mg N1−1. With the catch crop, it declined rapidly, from 41 mg N I−1 to 0.25 mg N I−1, at the end of ryegrass growth. Over this period, 110 kg N ha−1 was leached under bare fallow, compared with 40 kg N ha−1 under the catch crop. 15N‐labelled nitrate was detected in the first drainage water collected in autumn, 5 months after the spring application. The quantity of fertilizer‐N that was leached during this winter period was greater under bare fallow (18.7% of applied N) than when a catch crop was grown (7.1 %). In both treatments, labelled fertilizer‐N contributed about 34% of the total N lost during this period. With the ryegrass catch crop incorporated at the time of seedbed preparation in spring, the subsequent maize grain‐yield was lowered by an average of 13%. Total N‐uptake by the maize sown following bare fallow was 224 kg N ha−1, compared with 180 kg ha−1 with prior incorporation of ryegrass; the corresponding values for uptake of residual labelled N were 3% (bare fallow) and 2% (ryegrass) of the initial application. Following the maize harvest, where ryegrass was incorporated, 22.7% of the previous year's labelled fertilizer addition was present in an organic form on the top 30 cm of lysimeter soil. This compares with 15.7% for the bare fallow intercropping treatment. Tracer analyses showed overall recoveries of labelled N of 91.7% for the winter wheat/ ryegrass/maize rotation and 97% for the winter wheat/bare fallow/maize rotation. The study clearly demonstrated the ecological importance of a catch crop in reducing N‐leaching as well as its efficient use of fertilizer in the plant‐soil system from this particular rotation. However, the fate of the organic N in the ploughed‐down catch crop is uncertain and problems were encountered in establishing the next crop of maize.
Chemical nature of organic phosphorus in cultivated and uncultivated soils under different environmental conditions
Authors: CONDRON L. M. , FROSSARD E. , TIESSEN H. , NEWMANS R. H. , STEWART J. W. B.
Published: July 2006

A sequential alkali extraction procedure followed by ultrafiltration and quantitative 31P nuclear magnetic resonance (NMR) spectroscopy was used to examine organic phosphorus in a Brown Chernozem, and an adjacent Gleysol developed under native prairie vegetation, and a Grey Luvisol formed under aspen forest in Saskatchewan, Canada. Differences in the nature of organic P in the native soils were related to moisture status and vegetation. In the grassland soils, a greater proportion of orthophosphate diester P was found in the bottom‐slope Gleysol. This difference was partly attributed to less favourable conditions for mineralization in the bottom slope soil compared with the mid‐slope Brown Chernozem. Teichoic acid P occurred only in the native Grey Luvisol (NMR δ p.p.m. 0.36‐0.95) under boreal forest and not under native grassland. At all three sites, soils under long‐term cultivation were also examined and while orthophosphate monoester P (83.4‐94.6% total Po), orthophosphate diester P (3.9–8.7% total Po) and teichoic acid P (12.7% total Po in forested Grey Luvisol) were detected in native soils, only orthophosphate monoester P was found in the corresponding soils that had been cultivated for 70–80 years. These findings suggest that orthophosphate diester P and teichoic acid P are more readily mineralized in the soil environment than orthophosphate monoester P forms.
Stability of soil aggregates in relation to organic constituents and soil water content
Authors: HAYNES R. J. , SWIFT R. S.
Published: July 2006

The effects of soil organic matter content, soil water content and duration of wet‐sieving on aggregate stability of soils with contrasting cropping histories were investigated. Long‐term pasture samples had a greater aggregate stability than long‐term arable samples. However, air‐drying aggregates before wet‐sieving increased the aggregate stability of long‐term pasture samples, but decreased that of long‐term arable samples. With increasing duration of wet‐sieving, the proportion of water‐stable aggregates declined until a near‐constant value was reached for each sample. Thus, within a sample there are aggregates possessing a wide range of stabilities; with increasing time under arable cropping there is an increase in the proportion of unstable aggregates present, and the measured aggregate stability, therefore, declines. Unstable aggregates (defined as those dispersed after wet‐sieving for 1 min) generally had lower organic matter content than stable ones (those still intact after sieving for 15 min).

The aggregate stability of a regrassed site (13 years of arable plus 2 years of pasture) was markedly higher than that of a corresponding site from 15 years of arable cropping. Nonetheless, levels of organic matter (organic C, total N and hydrolysable carbohydrate) were almost identical at the two sites. However, aggregates from the regrassed site did have a higher biomass C and water‐extractable carbohydrate content than those from the 15‐year arable site. For a group of soils with varying cropping histories, aggregate stability was significantly more closely correlated with hot water‐extractable carbohydrate content than with organic C or hydrolysable carbohydrate content. It is suggested that the hot water‐extractable carbohydrate fraction may represent a pool of carbohydrate involved in the formation of stable aggregates.
The control of aluminium solubility in some acidic Australian soils
Authors: CONYERS M.
Published: July 2006

The variation in the solubility of aluminium in some acid soils of south‐eastern Australia has been investigated. The hypothesis was that leaching of silicic acid influenced the extent of clay decomposition, and hence aluminium solubility, in accord with Le Chatelier's principle. In lower rainfall areas (< 800 mm) control of Al solubility was consistent with a poorly crystalline kaolinite as the source phase, while at higher rainfall (> 1200 mm) it was consistent with gibbsite control. However, soluble silica concentrations were too high for gibbsite and kaolinite to be thermodynamically stable phases. The hypothesis was modified to take account of a quasi‐equilibrium in the open soil system, in which clay dissolution and A1 solubility appear to depend on the relative rates of H+ input and of silica leaching. These may result in a supersaturated, or possibly even undersaturated system, at any point in time. The relative sizes of the organic sink and mineral Al source determine the influence of organic matter on Al solubility.
Seasonal variation in field‐saturated hydraulic conductivity in two swelling clay soils in Sweden
Authors: MESSING I. , JARVIS N. J.
Published: July 2006

A simple method (the inversed auger hole method) for measuring field‐saturated hydraulic conductivity (Kfs) was investigated. Measurements were carried out in the spring, summer and autumn at three depths in two Swedish clay soils (Ultuna and Limsta, with clay contents of 45–60%0 and 65–80%, under barley and grass/clover ley respectively). Seasonal fluctuations in Kfs at Limsta were more pronounced, and were observed deeper in the profile. This was attributed primarily to larger structural changes due to a higher capacity for swell/shrink (normal shrinkage over the available water range) and an earlier drying up of the soil under grass/clover ley.

It was shown that the measured Kfs values were strongly correlated with the total inter aggregate (macro‐) porosity (et), estimated from a simple model of soil shrinkage. Combining the data from both soils, a single power‐law relation was adequate (r= 0.73) to describe the variations in Kfs with et
Soil and soil solution chemical composition at three sites within the Loch Dee catchment, SW Scotland
Authors: GRIEVE I.C.
Published: July 2006

Soil solution chemical composition was monitored over a 2–year period at three plots near the edge of an igneous intrusion in SW Scotland. Soil solutions were all strongly acidic with mean pH of approximately 4.0 at 0.20 m depth and 4.5 at 0.60 m. Chemical composition of the deeper soil solutions varied with geology. Greatest acidity and aluminium concentrations were found over metamorphosed Ordovician sedimentary rocks, with mean pH of 4.3 and A1 concentration of 22 mmol m−3 at 0.60 m. At a site on similar rocks nearer the intrusion, mean pH at 0.60 m was 4.8, with very low A1 (3 mmol m−3) and correspondingly greater Ca. At a peaty site on igneous rocks, increased pH with depth was associated with no change in base cations and A1 but a reduction in mean SO, from 41 mmolm−3 at 0.20m to 10mmol m−3 at 0.60m. Ion‐exchange selectivity coefficients calculated from soil and solution chemistry differed slightly from currently used values for peaty upland soils, with larger coefficients for A1/K exchange and smaller coefficients for A1/Ca and A1/Mg exchange. Relations between A1 and H ion concentrations confirmed under‐saturation of solutions with respect to gibbsite at 0.20 m. The differences between plots were consistent in different seasons, despite significant seasonal variability in Na, C1, dissolved organic carbon and Fe.