摘要:

SummaryPublished literature is re‐examined in an attempt to understand the influence of the spatial distribution of soil nitrate on N uptake in order to devise a simple method for estimating the depth to which nitrate must be leached before it becomes unavailable to the crop. The evidence suggests that most crops can continue normal growth with less than 15% of their roots exposed to nitrate.A simple model of nitrate uptake is constructed in which nitrate is assumed to be totally available above a set depth and totally unavailable below it. This effective rooting depth is assumed to coincide with the depth at which uptake per unit length of root declines to half of the maximum rate. Estimates of effective rooting depth have been made from root distribution data for various vegetable crops grown at Wellesbourne and cereal crops at Rothamsted. The results were found to fit a simple regression equation which can be used to calculate effective rooting depths at any stage of growth from the dry weight and population density of the crop and the mean cross‐sectional area of its roots. This equation is used in the succeeding papers to estimate the effects of leaching on the N fertilizer needs of cr

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02073.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryThe model is based on the concept that there is an effective rooting depth above which all the inorganic N in the soil is equally available and below which all N is totally unavailable. The extent to which nitrate is leached below this depth is calculated from the excess rainfall over evapotranspiration and the water holding capacity of the soil.The model was tested against the results of N fertilizer experiments with lettuce, French beans and overwintered onions which had been carried out in at least 2 years (and often more) on adjacent sites of the same field. The yields from all of the experiments with each crop were plotted against the level of fertilizer applied and against the difference between that applied and that lost by leaching. Comparison of these graphs showed that correcting for leaching in this way greatly reduced the variability between the response curves. The model was also tested against the results of similar experiments with Brussels sprouts grown on different sites with different combinations of base and top dressing. The results showed that the poor response to top dressing could be explained by the relatively small amounts of N in the base dressing which were leached from the rooting zone of this crop.It appears that the model will enable worthwhile adjustments in N fertilizer dressings to be made for differences in rainfall, provided that leaching occurs during the early stages of crop growth when uptake is small.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02074.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryA model is described for predicting the amounts of nitrate residues remaining in the soil in the autumn and the extent to which these are lost by leaching during the winter. It is used to predict the effects of winter rainfall on the growth of spring crops. The model is based on the assumption that any nitrate which remains in the effective rooting zone of the spring crop after the winter will be equally available and any nitrate leached below it will be totally unavailable. The losses of nitrate were estimated from the excess rainfall and the water holding capacity of the soil.The model was tested against the results of published N response experiments in the Netherlands and the UK. It predicted the differential effects of winter rainfall with reasonable accuracy, but tended to overestimate the spring nitrate contents observed in the Dutch experiments and underestimate those for the English data. These deviations appeared to be associated with errors in the estimation of the amounts of nitrate remaining in the soil in the autumn.With certain reservations, the model would appear to provide helpful advice for adjusting spring fertilizer dressings for the effects of differences in winter rainfall.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02075.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryA method, described previously, is used to quantify the macrostructure within the surface layers of swelling soil. Dissimilarity coefficients are defined and used to quantify soil structural differences resulting from different treatments. Structural differences over small increments in depth are resolved. Effects resulting from different tillage and stubble management practices are distinguished. Differences resulting from burning or retention of stubble had at least as great an effect as tillage practice on the void and aggregate size distributions. However, tillage increased the macro‐porosity.The structures of two mechanically‐prepared seed beds are compared with the structure of soil on tillage and stubble management trials where seed beds had not been prepared.It is shown in Appendix I that the structures measured on the two sections through each impregnated soil sample are essentially independent.A method is presented in Appendix II for converting the structural transition probabilities at unit spacing (e.g.0.5 mm) to transition probabilities at double spacing (e.g.1.0 mm). This enables results from different sources, which have been measured on the two scales, to be compa

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02076.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryErrors of measurement of soil water content with the neutron probe can be made small enough for most water balance studies; and apart from the possible damage resulting from access tube implacement, they can be readily specified. A more important problem is estimating how much water is lost from soil by drainage.A simple, graphical method is described to distinguish between drainage and evaporation from soil, based upon identification of the maximum depth at which measurable quantities of water are extracted by roots. As a result of errors in soil water measurements and uncertainties in the distinction between drainage and evaporation, estimates of soil water deficit and hence evaporation for a single profile are likely to have a precision of about ±5 mm. Larger variability is commonly found in soil water storage changes measured in replicate profiles partly because of uneven wetting of the soil during rain and partly because of non‐uniform losses by drainage and root extraction. This variability requires careful attention to replication and location of access tubes when assessing the evaporation for a particular si

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02077.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryRates of evaporation, soil water deficits and extraction depths for spring cereals and grass were estimated from soil water measurements for a four year period. Estimates of evaporation were also compared with evaporation formulae and evaporimeter values.Evaporation and soil water deficit were highly dependent upon weather, particularly rainfall. Evaporation from grass was noticeably restricted whenever deficits exceed 50 millimeters. Cereals extracted water from a slightly greater depth of soil but developed deficits, up to harvest, very similar to those under grass. Deficits developed in sand and clay soils were also very similar, because water held in large dense clay aggregates is not readily accessible to roots.Although soil type had only minor influence on rates of evaporation, deficits and depths of drying, because of movement of water through fissures, slow drainage and delayed swelling effects soil type has a strong influence on stream behaviour.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02078.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryThe catchment of the Kingston Brook has an area of 57 km2, mainly under pasture (56%) and arable crops (36%). Changes of soil water content, measured with a neutron probe from April 1969 to March 1973, were analysed to determine evaporation (summer only) and drainage. From measurements of rainfall and runoff, supplemented by Penman estimates of evaporation (in winter), water storage is estimated month by month.Annual mean values (mm) were: rainfall (559), evaporation (398), runoff (157). During the summer, the measured decrease in soil water storage contributedca100 mm to evaporation and drainage and there is evidence of delayed recharge (about 30 mm) during the winter. A linear relation between annual rainfall and annual runoff is interpreted in terms of (i) a fixed catchment storage (125 mm); (ii) a small and nearly constant winter evaporation (ca49mm); (iii) summer evaporation of 125 mm from storage plus a constant fraction (0.57) of contemporary precipitation.Summer evaporation was restricted by the supply of rain in every year from 1969 to 1976. By estimation, 500 mm of summer rain is needed to maintain potential evaporation, and the deficit at which actual evaporation falls below the potential rate was about 40 mm. Replacing the pasture by cereals would increase runoff by about 10% because of the shorter growing season.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02079.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryA model has been developed of the variation in the extent of anaerobiosis in aggregated soils with variations in gaseous diffusion coefficients, respiration rate and aggregate size. It extends previously published models of radial diffusion into individual aggregates to a log‐normally distributed population of aggregate sizes such as is known to occur in the field. It considers in particular the situation in which relatively better drainage and aeration occurs in the inter‐aggregate pores than within the aggregates. The model indicates that in most circumstances the effects of variations in the gaseous diffusion coefficients outweigh comparable variations in aggregate size. It is demonstrated how the model may be applied to calculation of rates of denitrification, and two alternative approaches are proposed: one based on the quantities of nitrogen and nitrous oxide diffusing out of partially anaerobic aggregates, the other on the disappearance of nitrate following diffusion into anaerobic zo

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02080.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryThis paper describes and evaluates a new solid‐state recording hand‐held cone penetrometer developed forin‐situsoil studies. Force is measured by a strain‐gauged transducer, and depth by an accurate optical system. Data for up to 20 penetrations per plot are stored and processed within the instrument in the field.The new penetrometer has been evaluated in field and soil tank experiments in comparison with three widely contrasting existing penetrometers ranging from a simple hand‐held penetrometer to a trailer‐mounted power‐driven instrument.The results indicated that the simple hand‐held penetrometer gave appreciably different cone resistance profiles from the other instruments. In particular, spurious ‘treatment effects’ obtained from the tillage experiment were shown to be due to inadequate instrument performance. The new solid‐state recording hand‐held penetrometer gave results comparable with those obtained from the more complex trailer‐mounted instrument and another hand‐

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02081.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY

摘要:

SummaryAfter 100h reaction with Cl‐resin and 300h reaction with HCO3‐resin (approaching equilibrium), the concentration of anions complementary to phosphate was the critical variable affecting the transfer of P from soil to resin. Solution concentrations of H30+, Ca2+and phosphate indicated that desorption of P by OH‐, and dissolution of Ca phosphates, controlled P release from soils. P extracted by HCO3‐resin was much greater than by Cl‐resin from an acid soil, due to lower total anion and higher desorbing anion concentrations, but there was little difference between the two resins with a calcareous soil. HCO3‐resin extracted a constant proportion of isotopically‐exchangeable P from different soils whereas Cl‐resin did not. Anion exchange resins provide a convenient means for producing P desorption c

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1980.tb02082.x

出版商:Blackwell Publishing Ltd    

年代:1980

数据来源: WILEY