bluuebunny/crossref_metadata_2025 huggingface dataset:
Fate of fertilizer nitrogen applied to grassland. I. Field leaching results
Authors: BARRACLOUGH D. , HYDEN M. J. , DAVIES G. P.
Published: July 2006

Results are presented from a 3 year investigation into nitrate leaching from isolated 0.4 ha grassland plots fertilized with 250, 500 and 900 kg N ha−1 a−1. Cumulative nitrate leaching over the 3 years was equivalent to 1.5%, 5.4% and 16.7% of the fertilizer applied at 250, 500 and 900 kg N ha−1 rates respectively. Over a whole drainage season, mean nitrate leachate concentrations at 250 kg N ha−1 did not exceed 4 mgl−1, although maximum values of 13.3 mgl−1 were observed. In contrast, at 900 kg N ha−1, the mean nitrate leachate concentration in two of the years exceeded 90 mgl−1. Mineral nitrogen balances constructed for the 1979 growing season indicated that leaching at 250 kg N ha−1 was low because net mineralization of soil organic nitrogen was small, and crop nitrogen uptake almost balanced fertilizer application. Although the pattern of nitrate leaching suggested that by‐passing occurred in the movement of water down the soil profile, it was not possible to confirm this using simulation models of leaching. Possible reasons for this, including the occurrence of rapid water flow down gravitationally drained macropores, are discussed.
Nitrate loss in soil drainage waters in relation to by‐passing flow and discharge on an arable site
Authors: SMETTEM K. R. J. , TRUDGILL S. T. , PICKLES A. M.
Published: July 2006

In a well‐structured soil at or close to pedal saturation, rainfall intensity in excess of pedal Ksat is predicted to result in by‐passing flow through well connected structural voids.

The depth of solute penetration (tracer dye and nitrate) under by‐passing flow has been predicted by Df=(P‐Cp)/θf, where Df is the depth of penetration of the leading edge of solute in by‐passing flow, P is gross precipitation, Cp is the input volume transmitted through the soil peds and θf is the operational volumetric moisture content in by‐passing. The timing of surface applied nitrate arrival at a channel bank is demonstrated to be related to by‐passing flow rather than uniform displacement.

The relationship between nitrate load in soil drainage waters (Y, mg s‐1) and water discharge (X, dm3 s−1) was Y∼6X after nitrate fertilizer application but subsequently declined to Y 0.5X It is deduced that the decline resulted from inefficient leaching under by‐passing conditions once surface applied nitrate had become relocated within the soil peds.
Some effects of mound‐building termites on the soils of a semi‐arid area of Kenya
Authors: POMEROY D. E.
Published: July 2006

The nutrient status of soils from Macrotermes termite mounds, in and around Kajiado District, reflects that of the subsoils more closely than in some other studies, notably Watson (1977). Consequently the growth of vegetation on and around mounds was not noticeably enhanced, except in grasslands and in higher rainfall areas. There was no evidence that mounds acted as wicks.

Mound soils contained greater concentrations of calcium, magnesium and potassium than subsoils in almost all cases where subsoil values were low, suggesting that the termites’ activities enhanced the nutrient status of mound soils. The nutrients were probably derived from their food, and the effects increased in some cases by their selection of clay on clay‐poor soils. Nevertheless, the overall effects of Macrotermes on rangeland soils are probably small.

The distribution of Macrotermes is remarkably unaffected by soil types; they construct mounds on almost all soils except those consisting largely of montmorillonite.
Extraction of inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon
Authors: FARMER V. C. , RUSSELL J. D. , SMITH B. F. L.
Published: July 2006

A range of extractants commonly used in soil studies, both to clean up the crystalline clay minerals by dissolving poorly ordered phases, and to assist in the classification of soils, has been used to assess their efficiency in extracting inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon. A 4 h extraction with ammonium oxalate pH 3 is the most efficient single procedure for these phases. It is concluded that satisfactory characterization of a podzol B horizon requires the use of two extractants: acid oxalate to assess the total amounts of Al, Fe and Si in non‐crystalline weathering products, and pyrophosphate to assess the amounts in organic forms. Dithionite‐citrate‐bicarbonate gives a measure of total free iron oxides, but extracts only an ill‐defined fraction of the allophane‐imogolite complex.
Nitrogen uptake by wheat in relation to water regime and depth of N placement in a loamy sand soil: measurement and model simulation*
Authors: SHARMA B. R. , CHAUDHARY T. N.
Published: July 2006

Nitrate‐N distribution in soil, root‐length, rate of water uptake and flux of nitrate‐N to wheat roots as influenced by moisture regime and depth of nitrogen placement were studied in 60 cm diameter by 180 cm deep metal columns filled with loamy sand soil. The contribution of the different soil depths in the root zone towards total N uptake was calculated employing a convective‐diffusion model and compared with the experimentally measured N uptake by wheat plants. Under non‐irrigated conditions with surface‐mixing of N(Ns) concentrations of nitrate, root‐lengths and water uptake rates were high only in the upper soil layers. With deep placement of N(Nd), the lower layers also contributed significantly towards total N uptake and total N uptake was greater with Nd than with Ns. In spite of many simplifying assumptions and certain estimates the predicted values of N uptake were quite close to those measured experimentally.
The strength of unsaturated mixtures of sand and kaolin and the concept of effective stress
Authors: MULLINS C. E. , PANAYIOTOPOULOS K. P.
Published: July 2006

Pastes containing sand and 0, 5, 20 and 80 g kaolin kg−1 mixture were equilibrated at matric potentials of – 2, – 10, – 100 and – 1000 kN m−2 or allowed to air‐dry. The strength of cylindrical samples was determined in unconfined compression and by the indirect tensile strength test. Measured strengths are explained using the Coulomb‐Mohr theory and the concept of effective stress. At pore water tensions greater than 10 kN m−2 effective stress was the dominant factor in determining compressive and tensile strength. The contrast between this behaviour and that of friable topsoils is discussed.
A combined penetrometer and surface relief meter for studying drill coulter performance
Authors: GOODMAN D. , ROWSE H. R.
Published: July 2006

A miniature penetrometer which measures soil strength in a vertical section across a drill line is described. It has a cone diameter of 3.6 mm and a range and resolution of 0–5000 and 5.4 kPa respectively. Data are recorded directly on to a portable digital tape logger which interfaces directly with a microcomputer for data analysis. The use of a horizontal reference height, rather than the soil, enables measurement of the soil surface relief. Results are presented showing the mean soil strength in a vertical section (200 mm wide by 130 mm deep) in drill lines in autumn and spring cultivated soil, with a horizontal and vertical resolution of 10 and 3 mm respectively.
Definition and use of the term ‘pedoderm’
Authors: WALKER P. H. , BECKMANN G. G. , BREWER R.
Published: July 2006

A new system for soil studies has been proposed by Butler based on his earlier Kcycle concept. In doing so, he has re‐defined ‘pedoderm’, the soil stratigraphic term originally defined in the Australian Soil‐Stratigraphic Proposal (ASSP). Some aspects of soil stratigraphy are discussed and the ASSP outlined. From these considerations we conclude that the re‐definition of pedoderm is inappropriate.

We strongly recommend the original definition and usage of the term pedoderm. It was widely discussed among earth scientists and was developed by the careful testing of a number of alternative proposals. The term with its original definition has recently been adopted by the INQUA/Paleopedology Working Group on Soil Stratigraphy.
The effect of soil pH and high urea concentrations on urease activity in soil
Authors: SINGH RACHHPAL , NYE P. H.
Published: July 2006

The rate of hydrolysis of urea in soil over the wide range of concentrations, up to 10 moles N per dm3 soil solution, found in fertilizer practice, was examined in Begbroke sandy loam adjusted to different pH values.

On rewetting air‐dry soil, urease activity increased rapidly, reached a maximum within the first 24 h and then decreased slowly to level off after about 4 days. Pretreatment of the soil with urea or ammonium had no effect on the urease activity.

Urease activity increased with substrate concentration, reached an optimum value and then decreased with rising urea concentration. The results could be explained by substrate inhibition at higher urea concentrations, and the data are well described by a modified Michaelis‐Menten equation involving three parameters, Vmax, Km and Ki where Ki is an inhibition constant. Km decreased linearily with rise in pH whereas Ki increased slightly between pH 4.9 and 7.0 and steeply between 7.0 and 8.4. Vmax increased with rise in pH, reached a maximum value at pH 6.0 and then declined at higher pHs.

There was a further reaction, reaching a maximum rate at a urea concentration of about 0.2 molar N in the soil solution, that followed Michaelis‐Menten kinetics. Km for this high affinity reaction increased up to pH 7.2 and then decreased at higher pH values; Vmax increased up to pH 6.8 and then decreased. The contribution of the high affinity reaction was small except at low concentrations of urea.
Soil properties relating to the sediment volume of Ando soils
Authors: EGASHIRA K. , WATANUKI T.
Published: July 2006

Soil properties relating to the sediment volume of Ando soils varied depending on their physical state. The water content is a useful index property for the sediment volume of field‐moist soils. The sediment volume of air‐dried soils increased with decreasing clay content and with increasing organic carbon content. The water retention at 52% relative humidity was related to the sediment volume of soils subjected to a sonic wave treatment. An explanation of this behaviour is given in terms of the tendency of Ando soils to form water‐stable aggregates.
The automatic recognition and measurement of soil pore types by image analysis and computer programs
Authors: RINGROSE‐VOASE A. J. , BULLOCK P.
Published: July 2006

A system, ANOPOR, has been developed which uses the Quantimet 720 image analyser to recognize and measure the different types of pores in impregnated soil blocks. The system is concerned with three types of pores: channels, planar voids (cracks and fissures) and vughs but is not suitable for highly interconnected pore patterns. Each of the three types has a different origin and function and presents a different two‐dimensional (2‐D) shape when sectioned.

A learning set consisting of these three pore classes was used to teach the system how to recognize statistically soil pores in images using measurements made by Quantimet. To describe the pore outlines in 2‐D, shape factors giving the best class separation of the learning set in the pattern space were derived. Bayes equation was used to give the probability of a pore belonging to a particular class by comparing its position in the pattern space with the learning set. Class boundaries were determined which ANOPOR uses to allocate each pore in any image to the most likely class. The system measures the proportion of the pore space attributable to each class and the perimeter and intercept density for each. The system is used to measure the pore patterns in three soil horizons.
Traces of 2:1 layer‐silicate clays in Oxisols from Brazil, and their significance for potassium nutrition
Authors: ARKCOLL D. B. , GOULDING K. W. T. , HUGHES J. C.
Published: July 2006

The recuperation of the soil during bush fallows after a period of cultivation is an important part of the success of shifting cultivation in the lowland humid tropics. However, the sources of the nutrients which enable the observed rapid growth on apparently exhausted soils has never been satisfactorily explained. Some soils from the Manaus region of Brazil have been examined in an attempt to identify reserves of potassium. X‐ray diffraction, physical and chemical analyses, and microcalorimetric measurements of K‐Ca exchange show that small reserves of K occur in 2:1 clay minerals which exist in trace amounts in the soils. The results suggest that the clay fraction of the soil may play a more important role in nutrient supply than previously realized. The reserves are considered in terms of the land use of the area.
Mineralogical changes with depth in a layered Andosol near Bandung, Java (Indonesia)
Authors: CHARTRES C. J. , VAN REULER H.
Published: July 2006

The mineralogy and some physical and chemical properties of an Andosol developed in four distinct layers of volcanic ash have been investigated. Analytical methods used include X‐ray diffraction, transmission electron microscopy, particle‐size analyses, infrared spectroscopy, pyrophosphate extracts of C, Al and Fe, and acid‐oxalate extracts of Al, Fe and Si. Three buried A horizons in the ash were also dated by the 14C method. The dates obtained indicate burial of A horizons c. 17700, 14500 and 8600 years B.P., respectively. Halloysite dominates the weathered fraction in the ash older than 17700 B.P., whereas allophane and imogolite with molar A1/Si ratios of 1.7–2.0 are predominant in the younger ash layers. Some kaolinite also occurs in the upper two layers of ash. It is considered possible that the halloysite in the deepest ash layer formed under different climatic conditions from those prevailing at present, rather than due to resilication of allophane under the overburden of ash. The results also demonstrate the mineralogical complexity of soil developed in more than one layer of volcanic ash.
Nature and distribution of soil phosphorus as revealed by a sequential extraction method followed by 31P nuclear magnetic resonance analysis
Authors: CONDRON L. M. , GOH K. M. , NEWMAN R. H.
Published: July 2006

Soil samples from long‐term plots annually fertilized with superphosphate since 1952 and also unfertilized (control) plots of an irrigated and intensively grazed pasture in Canterbury, New Zealand, were subjected to a sequential extraction procedure followed by 31P nuclear magnetic resonance (NMR) analysis of the soil phosphorus (P) forms present.

Overall, 80% of the total organic P (as determined by ignition) was removed from the soil by the sequential extraction procedure. Most of the organic P (90%) detected by NMR was in the monoester fraction. Small quantities of diester and pyrophosphate were also found. Choline phosphate constituted a significant proportion (18–25%) of the monoester P in the acetylacetone and 0.5 M NaOH extracts. Long‐term superphosphate additions resulted in almost all of the P accumulated in the monoester P fraction (up to 99%).
The effect of mole drainage on soil temperatures under pasture
Authors: SCOTTER D. R. , HORNE D. J.
Published: July 2006

Drainage is often claimed to increase soil temperatures in early spring by decreasing the soil's heat capacity. Measurements of water table depth, soil water content and soil temperature were made during winter and spring on mole‐pipe drained and undrained plots of a silt loam soil under pasture. Despite differences in water table depth and soil water content, drainage had no observable effect on soil temperature.

Laboratory measurements of the thermal properties of soil cores at a matric potential of −4kPa and then at saturation showed the volumetric heat capacity increased from 3.1 to 3.3 MJ m−3 K−1, with a proportional increase in the thermal conductivity from 1.1 to 1.2 W m−1 K−1. The thermal diffusivity remained unchanged. These values were used in a numerical simulation of the effects of drainage on the seasonal and diurnal oscillations in soil temperature. It is argued that the soil heat flux under pasture in spring is unaffected by drainage. The predicted temperature differences due to drainage are of the order of 0.2°C. As differences of this magnitude were observed between replicate thermometers in the field, it follows from the calculations above that any differences in soil temperature due to drainage would be too small to detect.
The cause of redness in some buried and non‐buried soils in eastern England
Authors: KEMP R. A.
Published: July 2006

Relationships were examined between colours and the amounts of extractable‐iron or of individual iron oxides present in <2μm fractions of samples from buried and non‐buried soils in eastern England. Colour was expressed in terms of a redness rating (RR). Oxalate‐extractable iron (Feo), dithionite‐extractable iron (Fed) and goethite contents were not significantly correlated with RR. There was, however, a strong correlation between RR and hematite content.

These preliminary results provide some justification for the assumptions concerning relationships between reddish colours and hematite content, which are implicit in the paleo‐argillic concept of the Soil Survey of England and Wales. However, reddish colours (particularly within the 7.5 YR and possibly the 5 YR hue range) should not be used in isolation as an indiscriminate indicator of pre‐Devensian pedogenesis in Britain.
Crystallochemistry and surface properties of biogenic opal
Authors: BARTOLI F.
Published: July 2006

The very low solubility of phytoliths or biogenic silicas (which are minor constituents of most soils) and their ability to stabilize nitrogenous fulvic acids in podzols are attributed to chemisorbed AlVI on the surface making the silica less soluble and the surface OH groups very reactive towards organic anions in an acid medium. The surface Si/Al ratio of phytoliths is very low (1.5–8) in comparison with the Si/Al ratio (8–20) for the whole sample. The pK values of the reactions in which surface groups are involved are pK1= 1.8 for deciduous phytoliths to 3.8 for more aluminous coniferous phytoliths; the pK2 range is 8.2–9.0. The zero point of charge which is correlated with pK values varies from 5 to 6.
Interactions between fertilizer nitrogen and soil nitrogen—the so‐called ‘priming’ effect
Authors: JENKINSON D. S. , FOX R. H. , RAYNER J. H.
Published: July 2006

Experiments with 15N labelled fertilizers often show that plants given fertilizer N take up more N from the soil than plants not given N—the priming effect or ‘added nitrogen interaction’(ANI). This paper is a theoretical study of ANIs and how they can affect the interpretation of experiments with 15N labelled fertilizers. ANIs can be ‘Real’, if for example, fertilizer N increases the volume of soil explored by roots, or ‘apparent’, caused by pool substitution or by isotope displacement reactions. Pool substitution is the process by which added labelled N stands proxy for native unlabelled N that would otherwise have been removed from that pool. Microbial immobilization of N, whether driven by the decomposition of soil organic matter or by the decomposition of plant roots, can lead to pool substitution and is the dominant cause of apparent ANIs. Denitrification and plant uptake of N can also, under special circumstances, lead to pool substitution and thus give rise to apparent ANIs. Isotope displacement reactions, in which the added labelled N displaces native unlabelled N from a ‘bound’ pool, can lead to apparent ANIs but are only likely to be of significance in exceptional circumstances.

The relationship between ANIs, ‘A’ values and N fertilizer uptake efficiencies are examined by means of a simple model for uptake of 15N‐labelled fertilizer by a crop. A positive ‘apparent’ ANI is accompanied by an ‘A’ value that changes as fertilizer applications increase. Likewise, a positive ‘apparent’ ANI also causes fertilizer uptake efficiency to appear lower when measured by the uptake of 15N than when measured by the non‐isotopic ‘difference’ method.
Compaction by wheels: a numerical model for agricultural soils
Authors: SMITH D. L. O.
Published: July 2006

A simple, numerical model was developed to predict changes in soil dry bulk density resulting from the passage of wheels. The model uses Söhne's solutions to estimate stress increases, and empirical isotropic stress‐strain data to predict soil volume changes. Predicted and measured increases in bulk density, resulting from the passage of various agricultural vehicles, compared favourably at most depths but the model underestimated the compaction of loose soil in close proximity to underlying dense layers. The model may also be used to compare the compaction caused by various types and arrangements of wheels and to assess the contribution made by a particular input variable. Examples of such uses are given.
Simultaneous nitrification and diffusion in soil. II. The effects at levels of ammonium chloride which inhibit nitrification
Authors: DARRAH P. R. , WHITE R. E. , NYE P. H.
Published: July 2006

The physicochemical and microbiological changes occurring in a fine sandy loam soil following the application of ammonium chloride were followed experimentally and with a simulation model. Two levels of ammonium addition were used corresponding to application rates of 37 and 143 kg ha−1. The measured concentration profiles of ammonium nitrate and pH, which developed in soil columns as a result of the diffusion and simultaneous nitrification of the added NH4+, were measured at different incubation times. The measured profiles suggested that nitrification was inhibited at the site of application of the ammonium salt. This inhibition was attributed to an effect of increased osmotic pressure or chloride ion in the soil. A simulation model was developed to account for the inhibition by examining and testing two hypotheses about the response of nitrifiers to a fluctuating osmotic pressure. These were the irreversible inhibition model, which assumed that exposure to high osmotic pressures irreversibly inactivated a portion of the nitrifier population, and the reversible inhibition model, which assumed that the nitrifiers would recover after exposure to high osmotic pressures. The model included terms for the adsorption equilibria of NH4+ and soil acidity with the soil solid phase, and the influence of other ions on the rate of diffusion of each diffusing ion. The inputs to the model were based on parameters obtained independently of the diffusion experiments. Good agreement was found between experimental and predicted concentration profiles for both models although the reversible inhibition model gave the better simulation of the data.
Nitrous oxide emission from a grassland soil fertilized with slurry and calcium nitrate
Authors: EGGINTON GWEN M. , SMITH K. A.
Published: July 2006

Concentrations of nitrous oxide (N2O) and oxygen were monitored over a 2‐yr period in an imperfectly drained grassland soil receiving applications of N as cattle slurry or Ca(NO3)2. In both years N2O concentrations in the different treatments were in the order nitrate > slurry > control. Gaseous diffusion coefficients were determined in soil cores by a krypton‐85 tracer method and used to calculate approximate N2O fluxes from the soil. Only 1–5 kg N ha−1 was lost as N2O after a single application of > 1200 kg N ha −1 as slurry compared with 3–11 kg N ha −1 lost after 100 kg was added as NO3−. Total gaseous losses (N2O+N2) could be expected to be higher in both cases.
Straw incorporation and soil organic matter in macro‐aggregates and particle size separates
Authors: CHRISTENSEN BENT T.
Published: July 2006

Two field experiments in which straw has been removed or incorporated for 17 yr (loamy sand) and 10 yr (sandy clay loam) were sampled to examine the effect of straw on the C and N contents in whole soil samples, macro‐aggregate fractions and primary particle‐size separates. The particle size composition of the aggregate fractions was determined. Aggregates were isolated by dry sieving.

Straw incorporation increased the number of 1–20 mm aggregates in the loamy sand but no effect was noted in the sandy clay loam. Straw had no effect on the particle size composition of the various aggregate fractions. After correction for loose sand that accumulated in the aggregate fractions during dry sieving, macro‐aggregates appeared to be enriched in clay and silt compared with whole soil samples. Because of the possible detachment of sand particles from the exterior surface of aggregates during sieving operations, it was inferred that the particle size composition of macro‐aggregates is similar to that of the bulk soil.

The organic matter contents of the aggregate fractions were closely correlated with their clay + silt contents. Differences in the organic matter content of clay isolated from whole soil samples and aggregate fractions were generally small. This was also true for the silt‐size separates. In both soils, straw incorporation increased the organic matter content of nearly all clay and silt separates; for silt this was generally twice that observed for clay.

The amounts of soil C, derived from straw, left in the loamy sand and sandy clay loam at the time of sampling were 4.4 and 4.5 t ha−1, corresponding to 12 and 21% of the straw C added. The C/N ratios of the straw‐derived soil organic matter were 11 and 12 for the loamy sand and sandy clay loam, respectively.
A recording penetrometer to measure the strength of soil in relation to the stresses exerted by a walking cow
Authors: SCHOLEFIELD D. , HALL D. M.
Published: July 2006

A penetrometer has been designed for field use to simulate the stresses exerted on the ground by the hoof of a walking cow, and to measure the resulting deformation of the soil. The extent of deformation produced by the penetrometer in two intact topsoils under pasture was found to be small and independent of water content, whilst significant correlations between soil strength and water content were obtained using a cone penetrometer and a hand‐held shear vane. The results imply that the deep hoofprints associated with poached pasture are not produced immediately upon treading wet soil, but only after a progressive loss of soil strength due to repeated treading. Thus the rate of loss, which is deduced to be a measure of the susceptibility of pasture to poaching, cannot be estimated directly from a single measure of soil strength.
The preparation of unsmeared soil surfaces and an improved apparatus for infiltration measurements
Authors: KOPPI A. J. , GEERING H. R.
Published: July 2006

summary

A method is described for using a quick setting epoxy resin to prepare a soil surface for infiltration measurements. It gives an unsmeared soil surface where the biological pores and structural surfaces are revealed. Experiments with an alluvial Prairie Soil and a Red Podzolic Soil, comparing steady state infiltration rates through cut smeared surfaces and resin treated ones, showed that the resin‐prepared surface permits 2.5–6 times more rapid entry of water. It is postulated that the greater rate more closely reflects the field structural condition. Apparatus, with a water reservoir remote from the hole, is also described for measuring constant head subsoil infiltration through a specially prepared auger hole.
Testing a mechanistic model. I. The effects of time and temperature on the reaction of fluoride and molybdate with a soil
Authors: BARROW N. J.
Published: July 2006

A mechanistic model that had been developed to describe the reaction of phosphate with soil was applied to the reactions of fluoride and molybdate with soil. The model assumes that: there is an initial adsorption reaction between ions and charged surfaces; that the surfaces are not uniform in their properties; and that the initial adsorption is followed by diffusive penetration.

The model was developed beyond that previously published to permit the rate of the initial adsorption reaction to be included. The model reproduced the effects of solution concentration, and of increasing period and temperature of incubation, on the retention of both fluoride and molybdate. Increasing the temperature of incubation increased the retention, or decreased the solution concentration, and this was reproduced by the effect of temperature on the rate of diffusive penetration. Desorption of fluoride was also reproduced. This supports the argument that slow desorption is mostly due to the need to reverse the diffusive penetration. After incubation at constant temperature, increasing the temperature at which solution concentration of molybdenum was measured, increased the concentration. The direction of this effect of temperature was also reproduced. It arises because of the effect of temperature on the position of the equilibrium between adsorbed molybdate (in the strict sense of the term) and molybdate in solution.
Testing a mechanistic model. III. The effects of pH on fluoride retention by a soil
Authors: BARROW N. J. , ELLIS A. S.
Published: July 2006

The pH of a soil was altered by incubating it with either lime or acid at 60°C for 1 day. Subsamples were then mixed with fluoride solutions in order to measure the effects of pH on fluoride retention. The results were compared with those previously obtained with phosphate on the same soil.

At equal concentration of total fluorine in solution, fluoride retention was greatest at about pH 5.5 and decreased at both lower and higher pH. The decrease at low pH appeared to be caused by the formation of complexes between fluoride and aluminium in solution. As a result, only a small proportion of the total fluorine in solution was present as fluoride ions. At equal concentrations of fluoride ions, fluoride retention decreased with increasing pH. It was shown that this decrease could be explained by decreases in the electrostatic potential of the variable charge materials. The decrease in potential was steeper than that required to describe phosphate retention. This is consistent with the plane of adsorption of fluoride ions being closer to the plane of adsorption of H+ and OH− ions.
Testing a mechanistic model. VI. Molecular modelling of the effects of pH on phosphate and on zinc retention by soils
Authors: BARROW N. J.
Published: July 2006

Both phosphate and metal cations such as zinc can be retained by oxides of iron and aluminium. Yet the phosphate‐retaining and zinc‐retaining materials in soil appear to differ in their behaviour from that of pure oxides of iron and aluminium. In most soils, the electrostatic potentials of retaining materials appear to be negative at normal soil pH values. The changes in potential with pH, and consequent change in retention, also appear to differ from those of pure oxides. A model of a pure oxide was modified in an attempt to simulate the observed behaviour of soil. The most effective modification was to include a negative charge in the oxide. Such a charge could have arisen from solid–state diffusion of anions into the oxide, or by close association with permanently charged clays. The inclusion of this charge changes the direction of the effect of pH on phosphate retention. If the amount of negative charge is large enough, the paradoxical situation can arise in which the potential of a ‘variable‐charge’ surface is not affected by pH. This is consistent with observed effects of pH on zinc retention by soils.
Determination of erodibility of a subtropical clay soil: a laboratory rainfall simulator experiment
Authors: ELWELL H. A.
Published: July 2006

Erodibility of tropical soils changes according to land use and management. It is important to monitor these changes so that farming practices can be designed to conserve the soil.

On a fersiallitic clay soil in Zimbabwe, the mean weight diameter of water‐stable aggregates was highly significant in explaining variations in both soil loss and runoff. Percent water‐stable aggregates > 2 mm diameter was also significant and had the practical advantage of being easier to measure. Organic carbon in the soil largely explains the proportion of water‐stable aggregates. These indices will aid management decisions on arable and grazing lands on this soil type, within the limits of the ranges tested.
Desorption of potassium from five soils of North India using electro‐ultrafiltration
Authors: BRAR M. S. , SEKHON G. S.
Published: July 2006

Samples were collected from five soil series, developed on different parent materials and occurring widely in intensively cultivated areas of Northern India. Potassium in these soils was desorbed by electro‐ultrafiltration, and collected in different fractions by applying successively higher voltages and temperature. K was also determined by extraction with M neutral ammonium acetate and M boiling nitric acid. Potassium desorption using EUF differentiated between soil series and between low K and high K soils. The K‐buffering capacity of the soil was associated with soil texture.
Mechanisms controlling zinc solubility in soils
Authors: PULFORD I.D.
Published: July 2006

The removal of zinc from solution has been studied for seven soils of varying properties. The presence of a background electrolyte as an ionic strength buffer and the nature of the anions and cations in solution have also been examined. This process has been considered as both a surface adsorption and a precipitation mechanism. Evidence presented suggests a role for a precipitation mechanism involving zinc and iron, which is independent of pH. Iron concentration in solution decreased with increasing zinc concentration; the ion product pZn+2pFe was constant (15.00, SD 1.00) over all seven soils. A further indicator of a precipitation mechanism was the shape of the Langmuir plot at low equilibrium concentrations of zinc in solution.
Acid pyrophosphate extraction of soil fulvic acids
Authors: GREGOR J. E. , POWELL H. K. J.
Published: July 2006

A simple three step method is described for isolation of soil fulvic acids in high yield. The complexing agent H2P2O72− (at pH 2) is used to release soil‐bound fulvic acids. Extraction of humic acids is minimal. Selective separation of the protonated fulvic acids from the ionic extractant is achieved on a non‐ionic polyacrylate resin (Amberlite XAD‐7); after washing the resin, fulvic acids were retrieved in >98% yield by adjusting the pH to 6.5. Two problems associated with the classical alkali extraction method are avoided: possible alkaline oxidation of phenolic components, and their oxidation by Fe3+ under the acidic conditions employed to precipitate humic acids. The product typically has an ash weight of <0.6% after one XAD treatment. The method has been applied to three soils and one IHSS peat sample.
Changes in the mineralogy of a cultivated marsh soil caused by simulated weathering
Authors: DE ARAMBARRI P. , TALIBUDEEN O.
Published: July 2006

Differential heats and isotherms for K‐Ca exchange and CEC values show that simulated weathering by grinding and NaTPB treatment of the coarse clay fraction of a rice‐growing marsh soil exposed vermiculitic layers to reaction with ions in solution, and also created more montmorillonitic layers and uncharged clay. Calcium selectivity was also increased, especially at K saturations > 50%. Changes in the CEC values of the fine clay, fine silt and coarse silt by such treatment, compared with that in the coarse clay fraction, were used to predict changes in the layer charges of their component phyllosilicates.
A retentivity function for use in soil–water simulation models
Authors: HUTSON J. L. , CASS A.
Published: July 2006

A sigmoidal, non‐hysteretic two‐part retentivity function having only two constants in addition to porosity was developed for use in soil‐water flow simulation models. Values of the constants, shapes of the retentivity curves, and soil textural properties were related by fitting the retentivity function to retentivity data generated using regression equations. Two sets of regression equations, which relate water content at specific matric potentials to soil texture and bulk density, were used, one for British soils and one for South African soils. Hydrologically inhomogenous soils may be modelled by varying the values of the retentivity constants through the profile to reflect changing soil properties.
A comparison of the convection‐dispersion equation and transfer function model for predicting chloride leaching through an undisturbed, structured clay soil
Authors: DYSON J.S. , WHITE R.E.
Published: July 2006

The transfer function mode) (TFM) and convection‐dispersion equation (CDE) were compared for predicting Cl − transport through a calcareous pelosol during steady, nearsaturated water flow. Large, undisturbed soil cores were used at constant irrigation intensities (q0) between 0.3 and 3 cm h−1, with a step‐change in Cl− concentration. The assumption of a lognormal distribution of travel times–characterized by the mean (μ) and variance (σ2)–permitted the flux‐averaged breakthrough curves (BTCs) to be modelled very accurately by the TFM. The BTCs could be modelled equally well by the CDE when both the mean pore water velocity (v) and dispersion coefficient (D) were optimized simultaneously by the method of least squares, but not when v was put equal to q0/v, where V was the mean volumetric water content.

The best estimate of v was consistently > q0/v, which suggested that not all the pore water was effective in chloride transport. An operationally defined transport volume (θst) was calculated from the mean () or median (τm) travel times derived from the TFM. Chloride exclusion was not solely responsible for θst() being <V: immobile water also contributed. The positive skewness of the travel time distributions meant that θst(τm) < θst(), indicating the effectiveness of macropore flow in solute transport. Dαv1.42 (from the CDE), and σ2αv (from the TFM), confirmed that Cl− dispersion increased as flow velocity increased.

Flux‐averaged concentrations were used to calculate the volume‐averaged resident concentrations. They matched the measured Cl− concentrations most closely when there was a gradual decrease in measured Cl − concentration with depth, but not when Cl − decreased sharply below c. 10 cm. Calculations assuming that all the water was effective in chloride transport gave less accurate results. Comparison of the measured and predicted concentrations of solute demonstrated that this must be a critical part of the evaluation of any model of solute transport.
Comparison of several spatial prediction methods for soil pH
Authors: LASLETT G. M. , McBRATNEY A. B. , PAHL P. J. , HUTCHINSON M. F.
Published: July 2006

A survey of topsoil pH has been designed specifically to compare the performance of several two‐dimensional spatial prediction methods. These methods have been classified as global or local, and interpolating or non‐interpolating, and smooth or non‐smooth predictors. The techniques tested were global means and medians, moving averages, inverse squared distance interpolation, Akima's interpolation, natural neighbour interpolation, quadratic trend surface, Laplacian smoothing splines and ordinary kriging. All methods showed some deficiencies; for example, prediction sums‐of‐squares were higher than expected for all methods. Interpolating methods either were very poor predictors, or suffered from theoretical drawbacks, or both. Of the non‐interpolating methods, Laplacian smoothing splines and kriging generally performed best. Estimates of variance derived from models which assume independent errors were greater than estimates of variance derived from neighbouring pairs of data sites, suggesting that short range correlations definitely exist, and should be taken into account in any predictive modelling.
A one‐parameter model for the soil water characteristic
Authors: GREGSON K. , HECTOR D. J. , McGOWAN M.
Published: July 2006

The two‐parameter model In φ= a+b ln θis widely accepted in the literature as providing a good fit to the soil moisture characteristic for a wide variety of soil types. This model is examined and it is shown that the parameters a and b are not independent, and hence that the SMC may be represented by a one‐parameter model with little loss of accuracy. Thus it is possible to define the SMC from one paired measurement of soil water content against potential. Because this single paired measurement is all that is required it also avoids the necessity of classification of the soil type–indeed it may provide an additional means of soil identification.
Retarded leaching of nitrate measured in monolith lysimeters in south‐east Nigeria
Authors: WONG M. T. F. , WILD A. , JUO A. S. R.
Published: July 2006

At Onne in South‐east Nigeria, drainage water was collected from four monolith lysimeters and analysed for nitrate. The lysimeters contained an acid sandy loam. At the start of the first rainy season two lysimeters received urea labelled with 15NO3– and two received no nitrogen fertilizer; all four were uncropped in the first year.

The peak concentrations of 15NO3– and of unlabelled (soil) NO3– were found after 2.5 pore volumes of water had passed through the lysimeters. Using the same soil in the laboratory after fine sieving, the peak concentration of tritiated water was found at 1 pore volume whereas nitrate leaching was retarded. The pattern of nitrate leaching was well described by miscible and immiscible models which included an adsorption coefficient for nitrate. Over the 2 years 81.4% of the 15N added at the start of the first rainy season was recovered in the drainage water.
Hydraulic and soil mechanical aspects of rill generation on agricultural soils
Authors: RAUWS G. , COVERS G.
Published: July 2006

An attempt is made, using literature data, to clarify the relationship between the detachment capacity of overland flow and its hydraulic characteristics. Attention is paid to the influence of sediment properties and rainfall.

Using these relationships and the concept of effective shear velocity, it was possible to establish a relationship for rill prediction which covers a variety of soils and a wide range of field conditions. Raindrop impacts do not seem to have an important influence on rill generation on cohesive materials.
Dependence of tensile strength of soil aggregates on soil constituents, density and load history
Authors: HADAS A. , LENNARD G.
Published: July 2006

The compressive‐tensile stress ratio at failure under unidirectional loading of air‐dry samples of 10 soils was compared with their clay and CaCO3 contents, and their surface areas. Deviations from the commonly assumed Griffith theory were found for two soils. No special relationships were found between soil constituents and associated compressive tensile stress ratio, even though this ratio tends to decrease slightly with increasing clay percentage.
The effect of sodium and calcium on physical properties and micromorphology of two red‐brown earth soils
Authors: GREENE R. S. B. , RENGASAMY P. , FORD G. W. , CHARTRES C. J. , MILLAR J. J.
Published: July 2006

The effect of treatment with either gypsum or sodium chloride on the saturated hydraulic conductivity (Ks) of repacked soil columns and modulus of rupture (MOR) was studied on surface samples of two red‐brown earth soils from SE wheat belt in Australia.

When the exchangeable sodium percentage (ESP) of the two soils was increased to >80, KS was substantially reduced and MOR increased relative to the untreated soil; the values of the parameters were nearly equal for these pairs of high ESP soils. However, after treatment with gypsum the Raywood soil had a Ks twice, and a MOR less than half, the corresponding values for the Glenloth soil.

Micromorphological and scanning electron microscope (SEM) observations suggest that the increase in Ks following gypsum treatment is associated with an increase in visible macropores and reduced clay dispersion; Na treatment increased dispersion at the soil surface, with the clay particles forming an impermeable surface seal and illuviation argillans.
The influence of earthworms and cranefly larvae on the decomposition of uniformly 14C labelled plant material in soil
Authors: CHESHIRE M. V. , GRIFFITHS B. S.
Published: July 2006

Amounts of the individual carbohydrate components derived from uniformly 14Clabelled grass added to soil were monitored in incubations lasting for up to 2 years. Decomposition was enhanced by the presence of the earthworms. Allolobophora caliginosa and Lumbricus rubellus and the cranefly larvae Tipula paludosa. After 28 d larvae had increased the loss of 14C‐arabinose by 8.8% (compared to soil without larvae), ‐xylose by 15%, ‐non cellulose glucose by 5.3% and ‐cellulose glucose by 12.5%. Earthworms had increased the loss of 14C‐xylose by 5%, ‐galactose by 21% and ‐total glucose by 11% after 6 months, and by another 29%, 14% and 8% respectively after 12 months. The losses of 14C, initially about 60% from soil with tipulid larvae after 1 month, and 50% from soil with earthworms after 3 months, became progressively less. Total 14C losses in the presence of earthworms were 66% after 12 months and 60% after 2 years, respectively. Increased decomposition is considered to be the result of the mixing of soil and substrate by the invertebrates, rather than an effect of their digestive capabilities.
On the role of expert systems and numerical taxonomy in soil classification
Authors: DALE M. B. , McBRATNEY A. B. , RUSSELL J. S.
Published: July 2006

Expert systems and numerical classification in general are reviewed and their relative ability to improve soil classification systems are discussed. The inductive and deductive elements of expert systems are seen as corresponding to the class establishment and class assignment phases of classification; computerized expert systems have so far been set up only for very simple assignments. To date, numerical classification has been useful in the analysis and organization of local low‐level soil data but has been largely untried at the higher global levels of soil classification because of a lack of suitable data and scientific commitment. It is concluded that numerical classification has a potentially useful part to play in establishing soil classes and generating rules for assignment in expert systems. The inference procedures and user interface of expert systems should allow more integration and realistic assignment of available data and increase the usefulness and predictability of soil information, especially if presented in a user‐friendly mode.
Seasonal changes in the structure of clay soils in relation to soil management and crop type. II. Effects of cultivation and cropping at Compton Beauchamp
Authors: MACKIE‐DAWSON L. A. , MULLINS C. E. , GOSS M. J. , COURT M. N. , FITZPATRICK E. A.
Published: July 2006

The effects of direct drilling and ploughing on the structure and macroporosity of a clay soil at Compton Beauchamp, Oxfordshire, growing winter wheat, were studied using large thin sections (10 * 5 * 3.5 cm) and image analysis of photographs of fluorescent resin impregnated soil blocks.

Composite coarse angular to subangular blocky and medium to fine granular aggregates were present in the top 3 cm of the direct drilled soil and in the top 10 cm of the ploughed soil. Settling of the soil and coalescing of these aggregates took place throughout the year under both treatments, but was most marked in the ploughed soil. In the subsoil (20 to 30 cm) most of the structural changes could be attributed to changes in the soil water content. Swelling in the winter led to the closure of the most macropores and shrinking in the spring and summer led initially to the development of vertical planar macropores and then to large cracks. Freezing and thawing in the winter created many very small subcuboidial aggregates at the soil surface which later coalesced as a result of raindrop impact.