ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

This paper considers some aspects of the hydrodynamic dispersion of tracer which occurs under the combined influence of molecular diffusion and convection when a fluid flows through a finite column of porous material. In particular, the paper discusses (i) the use of the principle of superposition to obtain breakthrough curves for pulsed and periodic inputs of trace, (ii) the form of dispersion coefficients in terms of dimensionless groups of parameters, and (iii) the mechanisms underlying dispersion.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

Recent models of gaseous diffusion in soils have taken account of the effects of soil structure, and explain the development of anaerobic zones within aggregates. Aggregate sizes commonly fit log-normal distribution. In clayey soils anaerobic conditions persist for long periods in the larger aggregates, but quantitative assessment of anaerobic volumes is complicated by uncertainty about the value of the diffusion coefficient for oxygen in wet soils.Methods of measuring soil aeration, and the results of field studies, are reviewed. It is concluded that climatic factors have more effect than cultivation or compaction, and drainage produces little improvement in the heaviest soils. Under these conditions considerable losses of nitrogen occur by denitrification, and significant concentrations of physiologically active compounds such as ethylene are released.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

The rate of uptake of a nutrient may be limited by its rate of diffusion through the soil or by the ability of the root to absorb it from low concentration in the soil solution. In the growth-response range of concentration, uptake of N, P, and K appears to be limited by diffusion. Transpiration rate, in these conditions, should have little influence. The root may be able to increase the soil solution concentration of sparingly soluble nutrients.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

Data are assembled from the Rothamsted classical field experiments on the effects of long-continued cropping and manuring on the amount of organic matter in soil, on the age of this soil organic matter, on the amount of microbial biomass in the soil, and on the rate at which plant residues decompose in these soils. These data were then fitted to a model in which soil organic matter was separated into five compartments: decomposable plant material (DPM, half-life 0.165 years); resistant plant material (RPM, 2.31 years); soil biomass (BIO, 1.69 years); physically stabilized organic matter (POM, 49.5 years) and chemically stabilized organic matter (COM, 1980 years) For unitary input of plant material (1 t fresh plant C ha-1year-1) under steady-state conditions, after 10,000 years, the model predicts that the soil will contain 0.01 t C in DPM, 0.47 t in RPM, 0.28 t in BIO, 11.3 t in POM, and 12.2 t in COM. The predicted radiocarbon age is 1240 years (equivalent age). The fit between predicted and measured data is sufficiently good to suggest that the model is a useful representation of the turnover of organic matter in cropped soils.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

Criteria for identifying Spodosols are compared with those used to identify podzolic soils in England and Wales, with particular reference to upland soils classed as brown podzolic soils and stagnopodzols. Applied to British soils, the chemical criteria for the spodic horizon seem unduly restrictive, whereas those for the podzolic B entail the identification as brown podzolic of soils with nonilluvial B horizons which may resemble those of Andepts. Some stagnopodzols also fail to qualify as Spodosols, but grouping them in different classes at the highest categorical level is considered inadvisable on both genetic and practical grounds.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

A routine test programme for studying the side-effects of pesticides on soil microorganisms is described. It includes methods for determining functional processes of the microbial population (respiratory activities, nitrogen mineralization, and nitrification) and is supported by tests which describe the microflora per se (enumeration of microorganisms, measurements of microbial ATP, potential for specific enzymic activities, and determination of symbiotic nitrogen fixation). The sampling and processing of soil for such investigations is discussed.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

Hill land occupies one-third of the agricultural area of Britain but its output is currently restricted by temporary and permanent limitations. The distribution and properties of hill soils, and their limitations for production, are described. Priorities for future research and development are considered.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

Soil mechanics has been developed to solve civil engineering problems in which compact soil is caused to fail at relatively low stress levels, and it is therefore applicable to primary tillage operations like ploughing. However, the much more important agricultural problem of the compaction of loose soils by the relatively high stresses induced by rolling and slipping tyres needs a new theoretical basis. The ideas of critical state soil mechanics provide an integrated mathematical system comprising three models describing elastic, brittle, and plastic compressible behaviour. The latter component is new and provides a means for applying plasticity theory to the problem of soil compaction. The theory also provides a means for determining which is the appropriate mathematical model to use. The paper attempts to describe these new ideas in soil mechanics in terms that are intelligible to soil scientists in general.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID

摘要:

Slurry is a suspension of faeces in urine, normally with some additional water. It can be a valuable source of plant nutrients, but when large quantities are spread on limited areas of land, there may be problems of nutrient accumulation and imbalance, pollution of drainage water, and the production of anaerobic soil conditions. So nutrient input and output on any area should be balanced and slurry dressings should be applied only when soil and climatic conditions are suitable.

ISSN:0038-075X    

出版商:OVID    

年代:1977

数据来源: OVID