摘要:

SummaryThe paper examines the ability of a critical‐state model to predict stresses and deformations of agricultural soil in a variety of laboratory shear and compression tests. The critical‐state model used is a simple extension to the well‐known Modified Cam Clay model. The extension provides a smoother transition from elastic to plastic behaviour and, amongst other things, introduces a capacity to model cyclic loading. The model is incorporated into a finite‐element program.The model predictions are compared with: experimental observations of simple and direct shear tests with both constant normal stress and constant volume conditions; cyclic uniaxial compression tests; compaction tests in U‐shaped and V‐shaped boxes; and observations of some gross structural features caused by shear in direct‐shear boxes. Predictions are made for both the compressing, strain‐hardening and the expanding, strain‐softening regimes of behaviour. In all cases the material properties for the model were obtained from tests other than those being used for the comparisons.The model predictions generally compare well with the various experimental results, although some numerical problems were encountered in strain‐softening conditions. This demonstrates the versatility of the critical‐state model for predicting fairly general stress and, deformation conditions in unsaturated soils using only five material‐property constants. It also demonstrates that common laboratory strength and compression tests are adequate to measure

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00506.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryCritical state parameters were determined in constant cell volume triaxial tests on three remoulded agricultural topsoils, a sandy loam, a clay loam and a clay. Tests were made at a range of water contents. The normal consolidation lines tended to be linear on a semi‐logarithmic plot up to a degree of saturation ofc. 0.85, above which the soil was incompressible but highly deformable. The slopes of the projected critical state lines were slightly greater than the slopes of the normal consolidation lines for all three soils. For each soil, both lines pivoted about a point as water content increased and, for the two lighter‐textured soils, the increases in compactibility tended to be greatest near the plastic limit. For all three soils, strength remained fairly constant with increasing water content until the soil was at around 70‐85% of the cone penetrometer plastic limit. Strength then decreased with increasing water content, with the smallest decrease in the sandy

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00507.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryNinety years after the start of the Askov long‐term fertilization experiment on sandy loam, bulk soil was taken from the 0–20 cm and 30–35 cm depths of unfertilized (UNF), animal‐manure (FYM) treated and mineral‐fertilized (NPK) plots and analysed for textural composition, carbon content, pH, CEC, particle density and plasticity limits. Undisturbed soil cores (100 cm3) from the 8–12 cm layer were brought to six different matric potentials and subjected to confined uniaxial compression, drop‐cone penetration and annulus shear tests. Water‐retention curves based on seven matric potentials were produced for undisturbed cores from the 8–12 and 30–35 cm layers. In the field, in situ shear strength of plough‐layer soil was determined by a vane shear tester and a torsional shear box.FYM and NPK treatments increased the soil organic carbon content by 23 and 11% of the amount in UNF, respectively. Corresponding increases observed in CEC were 17 and 11%. The water content at the lower and upper plastic limits both decreased from FYM to NPK to UNF. Soil bulk density in the 0‐20 cm layer was reduced in FYM and NPK treatments relative to UNF, whereas the volume of soil pores larger than 30 μm was unaffected by past fertilization.Soil receiving animal manure showed the greatest soil strength when exposed to annulus shear, drop‐cone penetration and confined uniaxial compression tests, Shear strength measurements indicated that the UNF sandy loam soil reacted like a sand, the increase in soil strength upon drying primarily being due to increased internal friction. In contrast, soil from FYM and NPK treatments showed reactions typical of a loamy soil, the increase in soil strength during drying being caused by increased cohesion in the soil matrix. The field tests employed were unable to detect the management‐induced differences in soil physical parameters found in the laboratory tests.This study shows that physical soil properties related to conditions for tillage and traffic, to crop development and erosion are significantly influenced by differences in soil organic matter levels resulting from contrasting methods of fertilizer management. Exhausting a loamy soil by long‐term lack of fertilizer application severely affects the p

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00508.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryA device that infiltrated water over a small circular surface area having a radius ranging between 1.45 mm and 2.5 mm was used to measure the sorptivity of initially air‐dry aggregates of size greater than 20 mm. The small infiltration area caused the water uptake to be dominated by capillarity that allowed the use of a simple formula to obtain the sorptivity from the steady‐state flow rate that occurred very early after the start of imbibition. The results of measurements of sorptivity made on a fine sand agreed with those obtained from measurements on one‐dimensional water infiltration into columns of the sand. Sorptivity measurements on stabilized aggregates of a clay soil and on air‐dried field aggregates of a clay soil were easily made. The simplicity and rapidity of the method allowed measurements to be conveniently rep

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00509.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryAggregates (9.5–12.7 mm) from ten soils were equilibrated at a range of matric suctions (Ψa) between 1 kPa and 100 MPa before immersion in water or wetting on a porous plate at zero suction. The soils were from cultivated and grassland sites and included hardsetting and non‐hardsetting Australian and British soils as well as a Vertisol. The initial rate of wetting of each aggregate, and the composition and size distribution of the slaked fragments were measured. There was a significant inverse linear relation between the amount of slaking produced by plate wetting air‐dry soil (Ψa=100 MPa) and its organic carbon content (r= 0.82***). The three cultivated hardsetting soils shared several common features. Their slaking was the most pronounced after plate wetting and occurred at the smallest Ψa(10 kPa). Their slaking also increased linearly with rate of wetting and the particle‐size distribution of their slaked fragments varied significantly and considerably with Ψa. This last observation demonstrates that it is not always helpful to call the fragments produced by slaking, microaggregates.Possible explanations for our results and their agricultural implications ar

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00510.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryPhysical disruption of a clay‐loam soil by slaking, grinding and compaction was used to determine the specific effects of soil structure on soil protozoa. Individual air‐dry aggregates (1–2 cm diameter) were wetted slowly, or had their structure disrupted by slaking or grinding. They were then moistened with nutrient solution and incubated, at a matric potential of −10 kPa. The nutrient solution had to supply at least 400 μg C g−1dry soil before protozoan populations increased measurably. Numbers of ciliate protozoa were enhanced by grinding, regardless of the amount of nutrient added. The moisture content and, therefore, the final nutrient concentration of the disrupted aggregates, changed markedly and could account for the observed increase in protozoan biomass from slowly wet to slaked to ground aggregates. There were no differences in protozoan biomass when the applied nutrient concentration was adjusted so that all treatments contained the same amount of nutrient. Soil cores were prepared from sieved (2–4 mm diameter), ground and ground/compacted soil. Thin sections revealed that the pore space accessible to protozoa decreased in these three treatments from 24% to 13% to 9%, respectively. Protozoan biomass was unaffected by grinding but showed a greater than 30‐fold decrease following compaction that could not be accounted for solely by the reduced pore space. Grinding and compacting could have favoured anaerobic conditions in the core which would have reduced protozoan activity. Soil structure had no measurable direct effect on protozoan populations, but it had a much greater indirect effect through its influence on moisture content and a

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00511.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryConventional soil survey stratifies a region into mapping classes and characterizes each by a representative soil profile within it. The efficacy of the procedure for predicting particle‐size fractions, bulk density, water retention, and available water capacity (AWC) of the soil at previously unvisited sites on the Plain of Languedoc in southern France is evaluated for three scales of survey (1/10 000, 1/25 000 and 1/100 000) and is compared to that of prediction from stratified random and simple random samples. Data from 85 soil profiles on a random transect were used for evaluation.Classification partitioned the variation of the measured properties, except for AWC, well at the 1/10 000 and 1/25 000 scales, whereas classification at the 1/100 000 scale was less effective. At the 1/10 000 and 1/25 000 scales both classification and stratified random sampling were better for prediction than simple random sampling for the same total sample. On average the representative profiles proved substantially better predictors than the stratified random samples, but in most situations where soil stratification performed well efficiencies of the two predictors were similar. In essence, the more successful the classification was the more difficult it was to improve prediction by selecting representatives instead of sampling randomly within classes. These results confirmed statistically that the soil surveyor can exercise intuition and judgement to classify and select representative

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00512.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryA major problem in using soil‐water models for land evaluation is to define which combination of model complexity and sampling density provides the most reliable predictions for a given investment. The overall error which affects the predictions arises from different sources. For instance, one is the error due to spatial estimation of soil data from soil classifications. The paper describes, for two models of different complexity and two soil maps of different resolution, how this error is propagated to the predictions of crop evapotranspiration.Errors arose and were accumulated during the course of the simulations, but they were not amplified. The variance of the errors depended on the climatic conditions of the simulations. Nevertheless, whatever the conditions, uncertainties in soil properties were propagated to the predictions to a lesser extent by the simple model than by the complex one. For example, combining the simple model and the 1/10 000 map required the same experimental investment as combining the complex model and the 1/100 000 map, but the variance of propagated errors was 53% greater for the complex model than for the simple one. Thus, if we consider only the simulation error derived from estimation errors in soil properties and the sampling costs, it is justifiable to use simple models for predicting the soil water balance in space. However, decisions should be based on the overall precision of the simulations which is also affected by other sources of error, such as the error arising from the model itsel

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00513.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryOne of the characteristics of soils located within the cores of stockpile storage mounds at opencast mine sites is their accumulation of ammonium‐nitrogen. Two areas of restored land were constructed from soil stockpiled for 3 years; one consisted of mound‐surface (‘aerobic zone’) soil, and the other of deeply buried (‘anaerobic zone’) soil. In that constructed from mound‐surface soil, concentrations of both ammonium‐ and nitrate‐nitrogen remained fairly stable throughout the first 6 months of restoration at about 12–20 μg g−1, but in the site constructed from deeply buried soil, concentrations of ammonium‐N decreased from an initial high of 160 to 14 μg N g−1soil after 14 weeks, and increased again to 42 μg N g−1soil by week 29. In contrast, concentrations of nitrate‐nitrogen at the latter site increased from an initial 9 μg to a maximum recorded level of 77 μg N g−1soil by week 14, before subsiding to 9 μg N g−1soil by week 29. Nitrate was considered to have been lost from the restored soils by a combination of leaching and denitrification, as no vegetation was established at these sites. After a short‐term stimulation following restoration, soil microbial biomass levels remained fairly constant, though soils (up to 3 years after restoration) were characterized by a

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00514.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummarySoils stored in stockpiles during opencast mining operations accumulate significant quantities of ammonium (of the order of 200 μg NH4+‐N g−1soil) within the predominantly anaerobic cores of mounds. Upon stockpile dismantling and land restoration, this NH+4‐N is rapidly oxidized to NO−3‐N, which is readily lost from newly restored soil ecosystems by leaching and denitrification. Experiments were set up to examine how these significant reserves of mineral N might be conserved in such situations. Application of the nitrification inhibitor dicyandiamide was successful in minimizing NO3−‐N lost by leaching, though large concentrations of NH4+‐N were detected in drainage waters. Straw incorporation decreased nitrate leaching by up to 40%; biomass C was some 40% greater in straw‐amended than in unamended soils after 14 weeks, though biomass N was similar in both. Addition of nitrogen‐free organic materials (glucose, starch and cellulose) produced different results, with glucose amendment showing the greatest reduction in nitrate leaching in the short term (due to an apparent stimulation of denitrification) whereas addition of cellulose resulted in the most effective conservation of nitrogen over 14 weeks; this was due, at least in part, to uptake of mineral N by the so

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00515.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY