摘要:

Water quality in agricultural areas is often influenced by chemicals used in crop production. The application methods of agricultural chemicals can affect how these solutes move through soil. Precision management of zones within each crop row is often achieved via tillage (e.g., mechanical weed control in a standing crop), but few methods for specific management to reduce transport of chemicals from treated soils have been suggested. Management of separate soil zones during chemical application was investigated using solute breakthrough studies with undisturbed soil cores (fine-loamy, mixed Aquic Hapludoll). Treatments were the application of 13.8 g/m2chloride (Cl−) using three strategies: (i) ponded application of the solute; (ii) unsaturated solute infiltration; and (iii) a zonal management technique, in which a small zone (volume) of soil was disturbed and mixed with solute before leaching. Column effluent was analyzed for Cl−. Columns were leached with Cl−-free solution before another treatment was applied. Each column received each treatment in succession. The solute breakthrough patterns during the first 2.5-cm drainage were influenced by both solute application method and soil column macroporosity. These results suggest that compared with broadcast applications, solutes restricted to soil management zones had slower transport through soil that, overall, had an abundance of macropores.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Freezing and thawing affect water and nutrient movement within soil profiles. Understanding the freezing and thawing processes and their effects on water movement and soil structure is necessary to develop improved management strategies. Experiments were conducted to measure the effects of freeze/thaw on soil water and solute movement in a Webster silty clay loam (fine-loamy, mixed mesic Typic Haplaquolls). Polyvinyl chloride (PVC) cylinders (0.13 m inside diameter, 1.2 m long) were packed with topsoil, and potassium bromide tracer was placed in the top 0.05- to 0.15-m soil layers in some of the columns. The soil columns were buried vertically in the field and exposed to the winter freeze/thaw conditions at Ankeny, Iowa. Soil columns were removed from the field throughout the winter and sectioned into 0.05-m layers. Each layer was analyzed for water content, bulk density, and electrical conductivity. Water moved upward to the freezing zone, carrying some solutes along. Electrical conductivity values verified the movement of solutes during the freeze/thaw periods. Bulk density changed abruptly as a result of expansion and compression of the soil matrix during freeze/thaw periods. Thawed soil retained some of the physical property changes caused by freezing and remained more variable than unfrozen soil. Freezing action increases the heterogeneity of soil properties in the frozen zone. This additional variability increases the complexity of predicting overwinter changes in the soil profile and the difficulty of developing management practices for maximizing nutrient efficiency while minimizing pollution.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The objective of this study was to develop and evaluate a simple field method to determine unsaturated hydraulic conductivities using measurements of water flux into a tensiometer. The tensiometer consists of a ceramic cup glued to one end of a piece of plastic tubing. A suction is first applied to the inside of the tensiometer, which is closed to the atmosphere. The reduced pressure in the tensiometer causes water to flow into the tensiometer from the soil. As the water flows into the tensiometer, the volume of air in the tensiometer decreases, and pressure increases. The rate of water flow into the tensiometer, water flux, is calculated from the measured pressures using a form of the ideal gas equation, PV = Constant, and its full differential, PdV/dt + VdP/dt = 0, where P is the measured pressure, and V is volume. The water flux is obtained from the change in volume with time, −dV/dt. The parameters for the unsaturated conductivity equation are determined by using a two-dimensional finite element soil model (2DSOIL) coupled with a Marquardt-Levenberg algorithm to fit calculated fluxes to measured ones. For comparison purposes, unsaturated hydraulic conductivities were also determined for the same soil within 25-cm-diameter rings from measured water contents and matric potentials during drainage for two locations. Fitted and measured fluxes agreed well. Unsaturated hydraulic conductivities obtained from the tensiometer inflow data, however, were much less than unsaturated hydraulic conductivities measured during drainage. We attributed the differences to anisotropy and scale effects although clogging of the tensiometer pores by fine soil material could also be a contributing factor. The method is relatively quick, uses inexpensive materials, provides consistent results and is not limited greatly by the conductivity of the cup.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The depth and duration of soil freezing have important implications for the hydrology, biology, and chemistry of ecosystems. Four existing soil models capable of simulating subfreezing soil temperatures and frost depth were evaluated for their ability to predict the depth and timing of soil frost at sites in North America. The evaluation was carried out by comparing model simulations with field data collected in Alaska, characterized by a Cryaquept with grass cover, and in Minnesota, characterized by a Haploboroll with corn stubble. The SHAW and SOIL models employ a finite difference solution to assess heat flow in the soil profile. Both models predicted frost depth with reasonable accuracy, at least when the simulated snow depth agreed with the recorded snow depth. The Benoit and Gusev models assess frost depth by balancing heat fluxes within the soil profile. These models generally overpredicted frost depth. The chief advantages of the simpler Benoit and Gusev models are the fewer data requirements and faster execution times compared with the SHAW and SOIL models. The latter two models, however, include provisions to reduce the data requirements by utilizing default data values in the simulation. The greater accuracy attained using the more sophisticated modern computer models may warrant their use for site-specific environmental applications. This study illustrates the difficulty of simulating snow cover, and, therefore, soil frost penetration, accurately.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

When dual-energy gamma radiation systems are employed for measurements in porous media, count rates from both sources are often used to compute parameter values. However, for several applications, the count rates of just one source are sufficient. These applications include the determination of volumetric liquid content values in two-liquid systems and salt concentration values in water-saturated porous media. Single-energy gamma radiation procedures for three applications are described in this paper. Through an error analysis, single-source procedures are shown to reduce the probable error in the determinations considerably. Example calculations and simple column experiments were conducted for each application to compare the performance of the new single-source and standard dual-source methods. In all cases, the single-source methods provided more reliable data than the traditional dual-source methods. In addition, a single-source calibration procedure is proposed to determine incident count rates indirectly. This procedure, which requires packing under saturated conditions, can be used in all single- and dual-source applications and yields accurate porosity and dry bulk density values.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Soil acidity and aluminum toxicity are serious environmental problems often found in humid temperate and tropical regions or in areas with acid rain. Iron oxidation in soils can also cause high concentrations of H+, which, in turn, causes an increase of Al3+in the soil solution. To examine this problem, a study was undertaken to discover the cause of crop damage in crops planted over buried anode bars. Anode bars are part of an impressed current cathodic protection system for pipelines near Decatur, Illinois. Soil samples were collected from the problem site and from a non-problem site for comparison. Results showed that Fe oxidation around anode bars at the problem site is stimulated by electric current, a situation that results in high concentrations of H+and reduces soil pH to less than 3.0. Under the low pH condition, the content of available Al is very high, and, therefore, the soil solution becomes toxic for soybean roots. Exchangeable Al was 360 to 700 ppm in soil immediately adjacent to anode bars but only 3 ppm in the soil midway between anode bars. The damage to the plants, such as reduced vegetative growth and lowered seed yield, developed in a circular pattern over the anode bars. Factors contributing to the problem were soil Fe content, rectifier voltage, and soil drainage.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Direct-land application of liquid hog manure is the most common practice for recycling this type of waste. However, liquid hog manure not only contains valuable agronomic substances, it also includes substances that may pose serious environmental concerns. Pyrolysis-Field Ionization Mass Spectrometry(Py-FIMS),13C Nuclear Magnetic Resonance (13C NMR), and Fourier-Transform Infra-Red (FTIR) were spectroscopy methods used to characterize colloidal fractions separated from three liquid hog manures. The spectroscopic data revealed that the colloids in the liquid manures contain relatively high concentrations of sterols(between 10.1 and 12.7% of Total Ion Intensity (TII)), which may have originated from animals and plants as well as from fortified feeds. Other significant colloid components, as percentages of TII, are lipids (5.3 to 11.9%), free fatty acids (3.4 to 9.5%), carbohydrates (2.5 to 6.0%), peptides (3.5 to 4.9%), n-alkylaromatics (2.9 to 4.0% C), N-compounds (3.1 to 4.7%), and phenols (2.5 to 4.0%). The presence of relatively high concentrations of sterols and phenols indicate that liquid hog manure has a high propensity for contributing to the contamination of soils and surface and subsurface waters.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID