摘要:

SUMMARYThe development of soil science has, in the past, been largely driven by the need to improve agricultural production. It is an applied science.Soils and soil processes have had a profound impact on the development of agriculture, and civilization, throughout the ages. Mistreatment of soil has, over the millennia to the present day, been responsible for horrific human tragedy. The most pressing challenge to soil science is to devise ways of improving food production, at minimum cost, in those areas of the world where food shortages are most acute.The new era of reducing price support for major agricultural commodities will increase competition in the EC and elsewhere. It will become imperative to improve efficiency of crop production, especially by the introduction of novel techniques, if arable and horticultural cropping in the UK is to flourish.Soil science is becoming increasingly important in assisting governments to develop policies and in assisting commercial organizations to screen and introduce new chemicals.There is a wide range of environmental and pollution problems that demand inputs from soil science. They include industrial degradation of soil, entry of toxic materials into the food chain, waste disposal, industrial contamination of the atmosphere and the sub‐sequent effects on soil and vegetation, forest decline, and the improvement of industrially damaged areas to attract new industries and reduce unemployment.Far greater use should be made of soils information in urban and rural planning and in building design as, at least in some countries, this practice has resulted in considerable financial savings.Advances in theory, measurement and computing are revolutionizing soil science and will greatly facilitate the application of the subject to practical problem

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00445.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYQuantification of soil ped shape is discussed from three viewpoints:1The theoretical reasons why quantification of ped shape should be useful (objective classification of shape, improved understanding of ped/pore relationships, soil genesis and ped development and as a measure of soil heterogeneity);2The theoretical division of irregular shape into three components (circularity, angularity and surface roughness) and how circularity and surface roughness can be quantified using Cox's R‐statistic and the Fractal dimension (D). Angularity remains unquantified;3The testing of a computer program written to quantify ped shape against six criteria that were presented (Clark, 1981) to allow an assessment of shape quantification methods and the calibration of the program using Pettijohn's (1 957) standard shape chart.It is concluded that R and D are able quantifiers of irregular particle shap

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00446.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYA procedure is developed which allows centrifugal photosedimentation to be used for analysing samples, such as clays, containing a broad range of particle sizes. The implicit assumptions that have to be made are discussed and the calculation of the particle size distribution from photosedimentation data is described. In particular, it is important to be certain that any light scattering theory, used to correct for the variation in light scattering with particle size, accurately represents the light scattering behaviour of clays. Appropriate theories of light scattering that can be used to correct photo sedimentation data for the variation in light scattering with particle size are identified. For kaolinite, over the size range 100 nm−1 μm, the anomalous diffraction theory for oblate ellipsoids was found to be a suitable theory for correcting photosedimentation da

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00447.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYTemperatures were measured in a disturbed and a structured loess soil to study the influence of aggregation on thermal properties. The disturbance was done by mechanically destroying soil aggregates, and the structured soil was obtained by subjecting the disturbed soil to several irrigation and drying cycles. In all soils, five harmonics of a Fourier‐series representation accounted for ≥99% of the variance of daily soil surface temperatures, and two to three harmonics did so below the soil surface. For soil water contents between 0.04 and 0.23 m3m‐3, the apparent thermal diffusivity, computed by the harmonic method, was higher in the structured soil than in the disturbed soil. The same was true for the apparent thermal conductivity, since the volumetric heat capacities of both the disturbed and structured soil were similar. The differences in the apparent thermal diffusivity and conductivity were attributed to increased heat conduction and water vapour transport in the structured

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00448.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYThe energy applied to a soil‐water suspension by an ultrasonic probe was determined for seven vertisol soils using calorimetric techniques. The rate of energy consumed by aggregate dispersion during sonification was calculated as the difference between the energy components measured before and after complete dispersion. Dispersive energy consumption was found to vary significantly during sonification and significant differences (P<0.05) were found between soils for the total dispersive energy required for complete dispersion. The soil dispersion characteristic curves, which relate the dispersive energy consumption during sonification to the quantity of<2 μm and<20 μm material dispersed, were also significantly different between soils. Error analysis of the calorimetric technique revealed that the energy consumed by complete dispersion is calculated with a precision of ±0.5 J g−1. For the soils studied, this was smaller than the observed variance in total dispersive energy consumed suggesting sample variability was the major source of variation.The ultrasonic technique was also used to determine the equivalent total energy applied and the amount of energy consumed by aggregate breakdown and dispersion during 30 min of end‐over‐end shaking. No significant difference (P<0.05) was found between soils for the total energy applied. However, a significant difference (P<0.05) was observed between soils for the quantity of dispersive energy required to produce the equivalent end‐over‐

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00449.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYThe effects of cropping history (pasture or arable) and sample pretreatment (field‐moist, air‐dried or air‐dried and then tension or vacuum rewetted) on aggregate stability as measured by wet sieving or turbidimetry were compared. When field‐moist samples were used there was a tendency for aggregate stability, as measured by wet sieving, to decline with increasing time under arable cropping (i.e. decreasing soil organic matter content). Air‐drying samples caused a pronounced decline in stability of soils from under arable management and as a consequence there was a marked decline in stability with increasing time under arable. Use of tension or vacuum rewetted samples resulted in high values of stability which were unaffected by cropping history.For turbidimetry, there was a marked decline in measured stability with increasing time under arable cropping when field‐moist samples were used. Air‐drying caused an increase in measured stability that was relatively greater for the less stable samples. In comparison with air‐dried samples, tension and vacuum rewetting caused a decrease in stability values for relatively unstable soils.It is suggested that, upon air‐drying (and contraction of aggregates), additional intermolecular associations were formed between soil constituents thus conferring greater stability on aggregates. This resulted in reduced dispersion (and the release of particles<0.04 mm in diameter) from the surfaces of aggregates and slaked aggregate fragments following rapid rewetting. As a consequence stability as measured by turbidimetry was increased by drying. For aggregates from a predominantly arable history, this stabilization was not great enough to prevent slaking occurring following rapid rewetting, with the formation of a large proportion of stabilized fragments<0.5 mm in diameter. The stability of these aggregates as measured by wet sieving was therefore de

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00450.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYEquations are developed for the simultaneous movement and reaction in flooded soils of urea, ammoniacal species, carbonate species, and acids and bases, the simultaneous uptake of N by a developing rice root system, and losses of NH3and CO2by volatilization from the floodwater. The equations allow for the initial distribution of urea and other reactants in the soil and floodwater, for movement both by diffusion and convection, and for the rates of urea hydrolysis, floodwater algal activity, organic‐C and ‐N mineralization, and H+ release from roots to balance excess uptake of cations over anions. The equations are combined in a computer model which can predict the fate of N fertilizer in lowland ricefields for any combination of crop and environmental conditi

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00451.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYA sensitivity analysis of the model is given for conditions in which there is no plant uptake of N and over the range of conditions likely in tropical lowland ricefields, and the mechanisms governing inorganic carbon, nitrogen and acidity dynamics in the floodwater and soil are discussed. Changes in the rate of CO2production in the soil, floodwater algal activity and floodwater carbonic anhydrase activity altered diurnal changes in floodwater pH but little influenced the daily mean pH, and consequently little influenced cumulative N loss by NH, volatilization. Soil pH buffer power and rate of urea application also little influenced cumulative percentage N loss. Increases in soil ammonium buffer power, floodwater depth and downward water flux, and decreases in floodwater‐ and air‐turbulence, moderately decreased N loss. N loss greatly decreased with decrease in urea hydrolysis rate and with urea incorporation in the soil. Incorporation decreased N loss more in the absence of standing floodwater. With low urease activity, deep floodwater gave consistently lower N loss; with high urease activity, shallow floodwater gave lower N loss when urea was incorpora

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00452.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYStarting from the basic equations of chemical equilibrium, an analytical mathematical expression is derived that relates pH to base saturation, concentration of acid anions (SO42−, NO3−, etc.) and other properties of the soil and soil solution. The equation is particularly valid in acid soils (low base saturation and relatively large,>100 μmolcdm−3, concentration of acid anions) in the range in which cation exchange is the buffering mechanism. Values of pH, alkalinity and degassed pH calculated with the aid of this equation compared well to values measured in three forest‐floor horizons; calculated pH values also compared well to values measured on a number of acid soils. The equation is also used to derive analytical mathematical expressions for alkalinity, soil leaching sensitivity (a measure of the sensitivity of a soil solution to become dominated by A13+), and other variables of interest in the context of acidi

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00453.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYFor chemical speciation calculations concerning soil solutions it is essential to convert ionic concentrations to activities. For dilute solutions with ionic strengths up to 0.3 mol dm−3, relations are given for calculating the single‐ion activity coefficients of 26 ions. Using the mean‐salt method, measured mean activity coefficients have been converted into single‐ion activity coefficients. For each ion, the variation in ion activity coefficient with ionic strength has been curve‐fitted with the first degree extension of the Debye‐Hückel equation. The ion‐dependent parameters in the extended Debye‐Hückel equation have been quantified.The derived set of relations can be incorporated in geochemical models, which makes calculating ion activities in soil solutions with variable ionic strengths up to 0.3 mol dm−3simple and accurate. With an ion speciation model it is shown for various solutions with ionic strengths up to 0.3 mol dm−3that the type of equation chosen for calculating single‐ion activity coefficients can lead to differences in

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00454.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY