摘要:

Two relatively simple models are proposed for describing the soil water retention curve. The expressions define sigmoidal or bimodal type retention functions with four or five parameters, respectively. The sigmoidal retention model may be combined with predictive pore-size distribution theories to yield closed-form equations for the unsaturated hydraulic conductivity. Parameters in the proposed hydraulic functions were estimated from observed retention data using a nonlinear least-squares optimization process. The models were tested on hydraulic data for more than 20 soils. Good agreement between predicted values and measured retention and conductivity data was found for most of the soils. The soil hydraulic models can be effectively utilized as inputs for numerical models of water flow and solute transport

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Salt-affected cracking clay soils are common to many irrigated, relatively and regions of the world. The structural changes of these soils having changing water contents lead to a pattern of macro-porosity of concern to water management and groundwater contamination. Herein, we briefly describe the different morphologic observations and attempts at quantifying aspects of the soil-cracking phenomena. After a brief review of fracture mechanics theory, we outline the equations appropriate for its application to soils. Results of the theoretical analysis are compared with field observations of crack morphology in Tulare basin soils irrigated with Na2SO4waters having Total Dissolved Solids (TDS) concentrations of approximately 400, 4500, and 9000 mg/LThe theoretical analysis indicates that widely opened shallow cracks require high tensile stresses for propagation and that sodium-affected soils require yet greater energy to extend crack depth. Combining the theory with field observations indicated that changes in crack width and depth in the high salinity treatment soils required roughly twice the energy as that in the low salinity soils. In addition, we found that the change in water content necessary to induce crack growth increases as the soil dries and that at any given water content, the stress associated with crack growth was less in the high salinity soils. Overall, fracture mechanics, coupled with measurements of soil physical properties can be used to determine quantitatively the propensity of soils to crack and the extent to which cracking will occur. However, additional laboratory and field studies are necessary to verify estimates of strain energy release rates from the fracture mechanics theory

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Water uptake by plant roots was successfully simulated by use of a volumetric sink term S(z) added to the continuity equation for soil moisture flow, which generally requires detailed information about the root system as functions of root density, root distribution, and root length. Unfortunately, these factors are difficult to evaluate. This paper describes a simple method for the estimation of soil water extraction by roots based on root discrimination of selected solute species such as chloride. A silt loam soil planted with carrots and affected by groundwater at different depths was used for the investigation. The soil was characterized by soil matric potentials close to hydrostatic equilibrium conditions. Aeration in the root zone was impeded by high moisture content. Because chloride was strongly discriminated by the roots, root water uptake was found to be related to the increase in soil36chloride solution concentration. Consequently, the chloride in the soil water was found to be an ideal indicator of water uptake in plants. Based on this proposed approach, patterns of water extraction by carrot roots could be described using a quasi steady-state model. We also found that in the groundwater-affected silt loam soil with impeded aeration, about 80% of the water transpired was extracted from the top 5 cm of the root zone

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The emission and consumption of N2O by 18 soils having a wide variety of soil characteristics were determined in the laboratory during a 20-day incubation at three different moisture regimes: field capacity, saturation, and waterlogged conditions. The highest N2O production and consumption occurred under saturated conditions, confirming that conditions developing marginal anaerobic conditions, favor N2O accumulation. Furthermore, it indicates that, in the 18 soils tested, the N2O emission was not primarily determined by the process rate, but by the relative N2O production, which is the percentage of reduced (denitrification: N2O*100/[N2O + N2]) or oxidized (nitrification: N2O*100/[N2O−+ N2O]) substrate being transformed into N2OMultiple regression analysis with step-wise selection of variables showed that soil pH was the soil characteristic with the highest predictive value of the emission and maximum concentration of N2O, probably through its direct effect on nitrification and denitrification. Other important characteristics were CaCO3and sand content, having an effect on the diffusion characteristics of the soil, and the NO2−concentration before the experiment, whose effect remains to be explained. The lack of predictive value of organic matter, water soluble organic matter, NO3−and NH4+concentration may indicate that those factors were not limiting N2O emission or consumption in most soils

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Land disposal of bauxite refining residues produced by the aluminum industry has resulted in areas devoid of plants because of the high salinity and alkalinity of the residues. In addition, the fine fraction (red mud) of these residues is prone to wind and water erosion, which can pollute the surrounding soils and surface waters. A glasshouse pot-leaching study was performed to evaluate the effectiveness of waste gypsum (CaSO4) and copperas (FeSO4) as ameliorants for the red mud fraction. Red mud was amended with the wastes at rates of 0, 2, 5, and 8% (w/w), and leached with 1200 ml (126 min rainfall) of deionized water before conducting a seedling emergence test usingAgropyron elongatum.Leachate and soil analyses indicated that gypsum and copperas amendments were able to reduce the pH, electrical conductivity (EC) and Na content of red mud significantly. Copperas was more effective in reducing soil Na contents and maintaining a lower soil EC. On the other hand, gypsum-amended red mud maintained a lower pH and a higher Ca content because of the low solubility of gypsum. Both amendments also resulted in significant reductions in soluble Al content and exchangeable sodium percentage (ESP). The reduced pH, ESP and Al content were correlated to the improved seedling emergence ofAgropyronat application rates of >2%. The results demonstrate that both gypsum and copperas are effective ameliorants for red mud. However, gypsum appears to provide a more persistent pH buffering capacity and lower ESP for red mud

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Iron oxide-coated strips can serve as a P sink to continuously remove P from solution. In this way, P extraction is analogous to P absorption by plant roots. Because the relationship between soil P sorption capacity and P extraction by the Fe oxide-coated strips can affect the interpretation of the P test values, it needs to be examined. The effectiveness of the Fe oxide-coated strips in extracting sorbed P, and its relationship with soil P sorption capacity and with the commonly used NaHCO3P test, were evaluated in acidic soils amended with varying amounts of P. Both the Fe oxide strip and NaHCO3P tests were correlated with corn (Zea maysL.) dry matter yields to determine their relative effectiveness in describing plant growth response to increased P availability in soils. Iron oxide strip-extractable P, like NaHCO3-extractable P, increased with increasing amounts of P added to the soils. The recoveries of sorbed P by the two tests were similarly affected by the P sorption capacities of the soils at a solution P concentration of 9.7 X 10−6Mor at a constant P addition of 20 mmol kg−1, which explains the close correlation between the two tests (R2= 0.85,P< 0.001). The Fe oxide strip-extractable P reflected more appropriately an intensity index, rather than a quantity index, in the characterization of soil P availability. Decreased P extraction by the Fe oxide strips at high ionic strengths indicated that desorption of sorbed P from soil limited P extraction by the Fe oxide strips and that maintaining similar ionic strength is important in assessing P availability among diverse soils. Both the Fe oxide strip- and NaHCO3-P tests correlated well with corn yields, tissue P concentration, and P uptake. The Fe oxide strip P test did not seem to have much advantage over the NaHCO3-P test in describing corn response to increased P availability in the soils; however, the Fe oxide strips can be stored easily for later analysis or sent to centralized locations for P determination, if necessary

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Little information exists about the extent to which fixed ammonium in soils is released under conditions imposed during analyses that are bases of nitrogen availability indexes and under conditions of very low soil solution concentrations of ammonium and potassium. This information is needed because fixed ammonium constitutes a significant portion of the total N in many soils. In this study, 5 to 45% of the total N in 10 surface soils and 10 to 85% in their associated subsoils was fixed, representing from 45 to 1763 mg fixed ammonium-N kg−1. These agriculturally important U.S. soils, including Alfisols, Aridisols, Inceptisols, and Mollisols, were subjected to analytical procedures designed to provide chemical (i.e., autoclave-distillable N) and biological (i.e., aerobic and anaerobic N mineralization) indexes of available N. Results indicated only small fractions (generally

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Tillage practices impart physical changes to the solum that may alter patterns of nutrient availability in soil profiles. The soil solution composition may be indicative of altered patterns of nutrient availability. Soil solution compositional analysis was used to compare the effects of 7 years continuous no-tillage (NT) vs. conventional tillage (CT) management of corn (Zea maysL.), and differing times of N fertilization, on solute distribution in a Loring (fine-silty, mixed, thermic Typic Fragiudalf) soil subjected to continuous CT for >20 years. Tillage plots received 168 kg N ha−1as NH4NO3, broadcast either at planting (Early) or 5 weeks after planting (Late). Soil solutions obtained by vacuum displacement for three sampling dates (3 April, 8 May, and 24 June) were analyzed for pH, electrical conductance (EC), dissolved organic C (DOC), and total concentrations of K, Na, Ca, Mg, NO3, Cl, SO4, and Al (24 June only). Tillage and time of N application frequently interacted to influence solute distribution in the soil profile. Comparisons of CT Early and CT Late with NT Early and NT Late, respectively, consistently indicated higher EC, NO3, Ca, and Mg for subsoils (>30 cm) sampled under CT. The imposition of NT on a soil that had been in long-term continuous CT resulted in altered patterns of nutrient distribution as reflected in the composition of soil solution sampled throughout the growing season

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

A mineralogical examination of the reddish brown (mainly 5YR 4/4) shales from the Triassic Culpeper Basin of Maryland was performed to determine the mineralogic composition of the dominant rock type of this basin. Results by x-ray diffraction (XRD) indicated that the silicate mineralogy consisted predominantly of quartz and mica, with lesser amounts of chlorite and feldspars. Results by XRD following the 5MNaOH treatment showed hematite was the only iron oxide identified in the shales. The low ratio of oxalate-extractable to dithionite-extractable iron is consistent with the occurrence of hematite as the only iron oxide in the shales. Aluminum substitution of the hematite in the shales ranged from 0.2 to 6.5 mol % and averaged 2.8 mol %. Mean crystallite dimensions (MCD) using the (110) peak of the hematite averaged 77 nm, which is consistent with MCD values of hematite from other Triassic red beds and terra rossa samples. Because thermodynamics favor the formation of goethite over hematite, and because hematite forms only rarely relative to goethite in Maryland soils, this monomineralic iron oxide parent material provides an opportunity to study Fe (hydr) oxide transformations in the soils derived from these hematitic parent materials

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID