ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

摘要:

Soil-moisture characteristics can be established directly from the physical properties of soils and a single measurement of water potential, &phis;, in bars, at some moisture content, &phis;, on a volume basis. The method is applicable for those soils for which the &phis; - &thetas; relations can be expressed as &phis; = &phis;e(&thetas;/&thetas;o)-β, where &phis;eis the air-entry water potential in bars, &thetas;ois the saturated water content in volume per volume, and β is an empirically determined constant. The procedure is useful when a reliable soil moisture characteristic curve is not available. We noted considerable agreement between estimated values using this procedure and those experimentally determined and published for eight soils.

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

摘要:

It is proposed that the standard state for exchangeable cations be selected as mole fraction one-half, rather than the conventional choice of unity. This leads to a more rational interpretation of the thermodynamic equilibrium constant as ion affinities for a surface and greatly simplifies the calculation of thermodynamic functions. The necessity of a complete ion exchange isotherm is eliminated. An example of Li+-Mg2+exchange on Yolo loam soil is given.

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

摘要:

Although methyl bromide (CH3Br) is used extensively as a soil fumigant, quantitative descriptions of its behavior after fumigation are limited. Methyl bromide undergoes a variety of processes, such as sorption and dissolution, and irreversible sink processes, such as hydrolysis. To evaluate these simultaneously occurring processes, a miscible displacement technique was used in which a pulse of pure CH3Br gas was passed through 10-centimeter-long soil columns at flow velocities ranging from 6 to 60 centimeters per hour (cm h-1). The concentration of the effluent gas was measured using gas chromatography. Experiments were conducted at one flow rate on dry sand, moist sand, a loam, and a peaty muck soil to determine the effect of water, clay minerals, and organic matter on the reversible sink processes. Additional experiments were conducted on Yolo loam at four different flow velocities to determine the effect of flow rate on the reversible sink terms. Mathematical models of transport and transformation of a reactive gas were evaluated by fitting calculated to measured effluent curves. A first-order kinetic model for the reversible sink term more adequately described effluent curves than did a linear, equilibrium model, although both models were apparently inadequate in completely describing the adsorption-desorption process. Irreversible sink processes had negligible effect on the quantitative description of the flow of gas in the columns. Although the rate of transformation of CH3Br gas to Br−had negligible effect on the shape and position of effluent curves, rates of Br−production were significant and influenced by soil type, as determined from incubation experiments. Rates of Br−production were greatest with muck, intermediate with loam, and least with sand.

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

摘要:

We performed chemical analysis of 240 soil samples collected from two surface layers of plots planted with a forest tree crop (Gmelina arborea), alone and interplanted with single and multiple food crops, in addition toGmelina arborea. The purpose was to investigate the effect of agrisilviculture, or farm forestry, on soil fertility.In addition to the controlGmelina arboreaplot, four otherGmelinaplots were interplanted separately with three food crops, and the three food crops combined in three ecological zones of southern Nigeria. Intercropping of the young forest tree crop with food crops increased (though insignificantly) soil nitrogen and phosphorus in two of the three ecological zones. Total exchangeable bases, exchangeable acidity, and percentage of carbon did not vary significantly as a result of agrisilviculture. The increased soil N and P were related to observed insignificant reduction in organic C in the respective plots. The findings gave scientific support to the practice of agrisilviculture as a means of increasing food crop production in Nigeria.

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

摘要:

Loss of four disparate pesticides (prometryne, carbaryl, trifluralin, and metribuzin) from Congaree sandy loam with time is best represented by two-stage, first-order curves. The relatively steep slopes of stage 1 are characterized by a relatively large, unique rate constant, k1, reflecting a relatively short-lived episode; the relatively shallow slope of stage 2 is characterized by a relatively small, less distinguishable rate constant, k2, reflecting a relatively long-lived episode. The times to 50 percent loss (t0.5), to onset of stage 2 (t2), and to 90 percent loss (t0.1) and selected ratios (k2/k0.5(1)/t0.1) complete the individual characterization. Assuming that residues do not become marginally safe until 90 percent of the applied pesticide has disappeared, t0.1is perhaps the most useful danger index. For the subject pesticides, t0.1ranges from 23 days (metribuzin) to 1040 days (prometryne).It is suggested that the two-stage disappearance curves observed for the disparate pesticides can serve as a model for other pesticides and that the slopes of the two legs of individual pesticide curves can be predicted, approximately, from limited laboratory sorption, movement, and loss data.

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

摘要:

Two splitting schedules of a 100 kg/ha nitrogen dressing applied to winter wheat as Na15NO3and (15NH4)2SO4have been compared in a microplot study through a four-course rotation (winter wheat, oats, fodder maize, fodder maize) for their influence on soil and fertilizer N. The soil was a well-drained silt-loam containing 0.25 percent CaCO3by weight. The treatments investigated are referred to as 3-split (NO3), 2-split (NO3), 3-split (NH4), and 2-split (NH4).The percentage uptake of the fertilizer N by the winter wheat is markedly related to the splitting schedule and, to a lesser extent, to the kind of carrier.The 3-split (NH4) treatment leads to a significant 25 percent increase in soil N uptake as compared with the O-N reference microplots; the priming effect calculated for this dressing amounts roughly to 12 kg N/ha.The evolution of the residual fertilizer N in the 0–70 cm soil profile shows higher half-life values for the 2-split than for the 3-split applications, namely 10.0 versus 5.1 years and 13.4 versus 10.9 years for the NO4and the NH4carriers, respectively.A tentative evaluation of the denitrification and of the NH3-N volatilization losses related to each split, and of the residual fertilizer N left beyond the 70-cm depth has been made. This approach is based on the evolution of the soil conditions (actual water deficit, drainage, temperature) during the first cropping season and on the regression curves of the residual first year fertilizer N during the last three years of the crop rotation.The NH3-N volatilization loss represents 5 percent of the surface-applied ammonium sulfate.Residual fertilizer N beyond 70 cm does not exceed 2 percent of the applied N dressing.The percentage of N in the crop derived from fertilizer (NDFF) is related to the splitting schedule. Significantly higher NDFF values have been found for the 3-split dressings. Higher values have been calculated for the grain than for the straw. This is true for the fertilizer N taken up by the wheat crop and for the residual N taken up by the oat the second year of the crop rotation.

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

摘要:

The fate of fertilizer N applied to winter wheat as Na15NO3and (15NH4)2SO4according to two splitting schedules has been followed in a silt-loam for fixed ammonium turnover and nitrogen reversion. The experiment has been conducted in microplots through a four-course rotation: winter wheat, oats, fodder maize, and fodder maize. The treatments involved in this experiment are referred to as 3-split (NH4), 2-split (NH4), and 2-split (NO3).The residual fixed NH4+-N in the 2-split (NH4) treatment amounts to 7 percent of the residual fertilizer N left in the 0− to 20-cm layer after the wheat harvest. This fraction decreases by 55 percent over the following three years of the crop rotation, which represents a faster release when compared with the decrease of the residual fertilizer N as a whole.The amounts of residual fixed NH4+-N found in the 0− to 10-cm layer correspond to 0.82 and 0.17 percent of the applied fertilizer N for the 2-split (NH4) and the 3-split (NH4) dressings, respectively. The influence of the splitting pattern on the fertilizer NH4+-N fixation seems to be related to the method and timing of the NH4+and K+fertilizer applications.A redistribution of fertilizer NH4+-N into the lower soil layers has been observed after the harvest of the oat crop. Intensive defixation (24–39 percent) of native NH4+-N due to the oat crop has been found in the lower layers of the cultural profile with practically no release in the 0− to 10− and 10− to 20-cm layers. Release of native fixed NH4+-N seems to be inhibited to a greater extent by the addition of annual K+dressings than that of recently fixed NH4+-N.The availability ratio established after the first three crop harvests for the 2-split (NH4) treatment decreases from 7.83 to 2.93 and from 12.12 to 3.62 in the 0− to 10− and 10− to 20-cm layers, respectively.

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1980

数据来源: OVID