摘要:

Using three computation models, I simulated transport of the insecticides methomyl and diazinon in the root zone of a greenhouse soil. Two models simulated nonuniform flow of water in a slab geometry and in a cylindrical geometry. Soil regions with vertical and horizontal flow were distinguished. In those models, nonuniform flow produced more leaching of methomyl from the root zone than uniform flow did. The differences in leaching computed with the slab and cylindrical geometries were small. Narrowing of the region with downward flow increased the simulated leaching to a certain level. Reduction in scale of the irregularities increased the interchange of pesticide between the regions with vertical and horizontal flow, which resulted in less leaching. The effects of rate of water supply and of different transformation rates in greenhouse soils were demonstrated. When the properties of diazinon were introduced into the computations with nonuniform flow, there was hardly any leaching according to the model. However, when the compound was simulated to be applied in the same place as the irrigation, the extent of leaching strongly increased.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

A theory for the effects of expansion and contraction of encapsulated air in soils, as water pressure changes, is developed and applied to constant head permeameters.On commencement of flow from a static ponded column, water pressure at distances from the exit surface reflects the dampening effect of the air. Pressure and flow are described by a diffusion coefficient equal toK/C, whereKis the hydraulic conductivity, andCis the compressibility of the entrapped air.Experimental results confirm the model and show that steady-state flow is reached only after a time interval that is a function ofK/Cand the length of the column. The model is relevant to results from constant head permeameters, particularly with clay soils. For example, for samples 30 cm long, with entrapped air equal to 6% of total volume and a nominalKof 1 mm h-1, the first 11 min of outflow is enhanced, and it would take 49 min for constant potential to be reached throughout the column after outflow is stopped.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

In situ unsaturated hydraulic conductivities of calcareous soils have not been intensively investigated. The objective of this investigation was to compare the hydraulic properties of adjoining calcareous and noncalcareous soils in the field. Soils used for this investigation were the calcareous Estacado series (fine-loamy, mixed thermic, Calciorthidic Paleustoll) and the noncalcareous Friona series (fine-loamy, mixed, thermic, Petrocalcic Paleustoll). In situ, unsaturated hydraulic conductivities were determined in the Ap1, Ap2, Bt1, and Bt2 horizons of each soil. Water was ponded daily and allowed to infiltrate under falling head conditions. Water additions were terminated when the tensiometers in the Bt2 horizons indicated no change of soil water potential from the previous day. Plots were covered with Styrofoam and plastic to prevent evaporation. During infiltration and drainage, water potential was monitored with two replicated tensiometers within each soil horizon. Two replicated gravimetric soil samples were taken concurrently from each soil horizon and converted to volumetric water content. Soil water release curves were constructed from in situ soil water potentials and volumetric water content. Hydraulic conductivities were calculated using in situ change of soil water contents and soil water potentials with time. The noncalcareous soil had higher hydraulic conductivities than the calcareous soil at the same water content. The calcareous Ap1 and Ap2 horizons had higher hydraulic conductivities than the noncalcareous soil at the same soil water potential. In situ soil water release curves indicated that calcareous soil retained more water than the noncalcareous soil at the same soil water potential in the paired horizons. Unsaturated hydraulic conductivities increased with depth for both soils.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

This article reports simple relationships among soil water content, &thetas;, soil water potential, &phis;, and hydraulic conductivity,K, tested on five uniform soil materials, for which the necessary parameters were obtained in the laboratory, and on nine field soils, where the parameters were obtained in situ. Agreement between experimentalK(&thetas;) relationships and those calculated from saturated hydraulic conductivity and the water retention curve &phis;(&thetas;) was good for four laboratory soils, but was only adequate in the high water content range for a fine sand. For the field soils, agreement was good for four soils, adequate in the higher water content range for two, and inadequate for three soils in the sandy loam to light clay range.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

The sorption and desorption of phenol vapors by sediment material were measured at 35, 45, and 55°C. Considerable amounts (0.10 to 8.57 mmol/g) of monosubstituted chloro- and methylphenols and 2-nitrophenol were sorbed. The majority of sorption occurred quickly (∼1 h), except for methylphenols at the higher temperatures where multilayer formation occurred. Hydrogen-bonding was the main mechanism of sorption and was sensitive to some steric effects. Sorption was much greater at 55°C than at 35 or 45°C, indicating that pretreating the sediment at the highest temperature had a significant effect on the sorption surface, probably by dehydration of hydrogen-bonding sites. Vapor pressures of the compounds at the three temperatures examined were not related consistently with amounts sorbed. Sorption of all compounds was reversible at 35 and 45°C, but the rate of desorption was slower than the rate of uptake. A small amount of 2− and 3-methylphenol was irreversibly held at 55°C because of transformation at the sorbent surface.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

In lowland rice (Oryza sativaL.) soils, NH4+-N is considered the most important inorganic nitrogen form. Therefore, we examined the influence of NH4+fertilizer application and N uptake of the crop on the level of exchangeable and nonexchangeable soil NH4+Field experiments at three sites in major rice-growing areas in the Philippines showed that NH4+fertilizer application was clearly reflected by an increase of exchangeable and nonexchangeable soil NH4+, while N uptake resulted in a decrease of both NH4+fractions. We found major NH4+contributions from nonexchangeable soil NH4+only in Maligaya silty clay loam, which is rich in vermiculite. On all soils in the wet and dry seasons the exchangeable soil NH4+was depleted during the first 8 wk of crop growth, indicating that this fraction is easily available to the crop. The net release of exchangeable and nonexchangeable NH4+throughout the growth period ranged from 0 to 100% of the N uptake of rice and depended greatly on soil and weather conditions. In the wet season most of the N taken up by the crop could be accounted for by the net release of NH4+, whereas in the dry season most of the N taken up by the crop originated from soil organic N.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

Results from short-term, absorption experiments with roots in Ca-free culture solutions have led to the concept that phosphorus is absorbed largely or exclusively as the monovalent ion, H2PO4-. In an effort to see if this concept has a useful application in soils, we treated an acid Ultisol surface soil with three rates of Ca(OH)2and six rates of Ca(H2PO4)2to give a wide range in pH and P contents. Rye (Secale cerealeL.) was grown for 4 wk, and top growth was analyzed for P content. Soil solution was collected for each treatment and analyzed for inorganic ion composition. Soil-solution pH ranged from 4.6 to 7.5, and soil-solution P ranged from 1.9 to 191 μMin unlimed soil and from 1.4 to 32 μMin the highest limed soil. The critical P concentration for maximum growth was about 10 μM.Lower P uptake (at equal P rates) at higher soil pH was primarily the result of lower solution P concentration. Across soil pH levels, P uptake was better correlated with total solution P concentration than with H2PO4-concentration.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

We investigated the mineralogical and chemical properties of Tertiary limestones of different ages and characteristics in northern Jordan and also studied a soil profile formed on Tertiary limestone. According to our results, palygorskite or smectite dominated the silicate mineralogy of the limestone, but no clear relationship could be established between palygorskite and dolomite or total carbonate contents. Hard limestones contained more palygorskite and less smectite than marly limestone. Thus the mode of crystallization of the limestone may be a factor in palygorskite formation, as is the chemical composition of formation environment. Cristobalite and zeolite were present in the clay fraction of some limestones, especially the marly limestone with high smectite content. Soil formed on one of the limestones contained more smectite and less palygorskite in the topsoil than the subsoil, indicating that smectite may have formed from palygorskite as a result of weathering.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

We measured selected properties of a naturally saline and sodic soil in pedons that had been reclaimed a minimum of 0, 5, 8, 15, or 25 yr. More than 7.2 x 104kg of soluble salt were leached from the upper 100 cm of the soil after 5 yr, with little or no additional removal with time. A steady-state soil solution composition was reached after approximately 15 yr based on electrical conductivity values of the saturation extracts. The sodium adsorption ratios (SAR) of the steady-state solutions were either greater or less than the SAR of the irrigation water and appeared to be related to mineral weathering. Smectite minerals were unstable in the reclaimed soils and were transformed to kaolinite. This may have been the cause for the reduction in water-dispersible clay in soils that had been reclaimed for 15 yr or more.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID

摘要:

In this paper, we describe a method for measuring cation and anion exchange capacities (CEC, AEC) and both exchangeable cations and anions in low-activity clay soils, using NH4NO3solutions; CEC and AEC are determined by equilibrating soil with solution strengths that reflect field conditions, as indicated by osmotic potential measurements. Comparative analyses illustrate that the initial 0.5MNH4NO3extraction provides an effective alternative to routine exchangeable cation, anion, and Al analyses. The methodology is simple, sensitive, and suitable for studying the effect of Cl-and SO42-on the surface charge characteristics of Oxisols and Ultisols.

ISSN:0038-075X    

出版商:OVID    

年代:1985

数据来源: OVID