摘要:

Description of one-dimensional water flow in saturated swelling soils is facilitated by using a space-like coordinate based on the distribution of solids; a water-based space-like coordinate facilitates analysis of solute movement during non-steady water flow in unsaturated soils. These space-like material coordinates are simply defined and measured for many practical situations. Furthermore, their mathematical formulations are similar and result in similar non-linear diffusion equations that may be solved for important initial and boundary conditions. The approach may be extended to solute transport during non-steady flow of water in a swelling soil. This paper illustrates the similarity of methods of formulating flow equations, it discusses the benefits of the use of such coordinates, and it comments on the need for dimensional consistency between coordinates and intrinsic soil properties.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

The sorption mechanism of glyphosate, one of the most frequently used herbicides in the world, resembles that of phosphate. This study quantifies the variation in glyphosate sorption and desorption to a coarse sandy soil and to a sandy loam soil with varying phosphorus content and pH. Using batch experiments, glyphosate adsorption and desorption isotherms were determined on soil samples taken from long-term field experiments that received different additions of phosphorus and lime during 60-year (coarse sand) and 100-year (sandy loam) periods. Sorption isotherms were non-linear and manifested adsorption desorption non-singularity. The isotherms were best fitted with an extended Freundlich model, which had earlier been shown to describe phosphate sorption data well. The phosphate content in the soils had a significant influence on the sorption of glyphosate. With 0.5Mbicarbonate extractable P (pH 8.5) increasing from 6.2 to 58.7 in the loamy sand and 9.1 to 87.4 in the coarse sand, the extended Freundlich adsorption coefficient (Kf,MF,ads) decreased from 214.7 to 106 and from 154.0 to 83.5, respectively. Liming of the coarse sandy soil resulted in stronger glyphosate sorption because of an increase of reactive amorphous aluminum and iron hydrous oxides with increasing pH values. Glyphosate competes with phosphate for sorption sites, a quality that might result in glyphosate being sorbed more weakly in soils with high phosphorus levels.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Copper(II) retention by Leached Chernozem (LCh), Gray Forest (GFS), and Dernovo-Podzolic (DPS) soils at pH 4, 5, and 6 is accompanied by the simultaneous displacement of both Ca2+and H+ions. The amount of Cu(II) sorbed can exceed the effective cation exchange capacity (ECEC) values because copper(II) replaces not only exchangeable metal cations but also H+ions that are not displaced while estimating ECEC. The amount of (Ca2++ H+) displaced for LCh and DPS was close to the amount of Cu(II) retained (varied from 0.77 to 1.05 molcmolc−1, depending on pH). For GFS the sum of the amounts of displaced ions was less than the amount of Cu(II) retained (from 0.53 to 0.74 molcmolc−1). Copper(II) sorption by LCh and DPS can be considered a ternary cation exchange, whereas in GFS we suggest that formation of a positively charged surface complex may be involved. The Cu(II) retention isotherms at all pH values studied for the three soils can be fit equally well to the Freundlich equation, based on the concentration and activity of Cu(II).

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Potential distribution and charge density on the clay colloid surface with interacting double layers was solved numerically by computer using the anion negative adsorption equation. The results showed that the interacting degree of double layers affected not only the potential, Ψd, at the midplane but also the potential Ψδat the outer Helmholtz plane (OHP). The magnitude of Ψdincreased remarkably as flat colloid particles approached each other at first and then remained almost constant at the distance of less than 70Å between two colloid particles. Therefore, it is more appropriate to use Ψd/Ψδto describe the interacting degree of double layers than Ψdas in previous studies. When Ψd/Ψδis smaller than 0.6-0.8, the surface charge density, σδ, is almost unchanged with the increase of the interacting degree of electric double layers. When Ψd/Ψδexceeds 0.6-0.8, Na+in the diffuse layer may enter into OHP and neutralize the partial surface charge so as to lead to a reduction of the charge density at OHP. The results obtained in this paper are in good agreement with theoretical expectation.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Very little data are available regarding the genesis of Inceptisols derived from pyroclastic materials in Taiwan because most of the earlier research focused on Andisols. This study was conducted to explain the formation of Inceptisols derived from the pyroclastic material in northern Taiwan. Three pedons representing the summit, backslope, and footslope positions of a volcanic landscape were examined and sampled for physical, chemical, and mineralogical analyses. Results revealed that the soils were moderately developed and characterized by an A-BA-Bw-BC horizon sequence, loam to clay loam texture, firm and subangular blocky structure, low bulk density, high water, and P retention. The soils were very acid, with low exchangeable bases but very high exchangeable Al as well as considerable amounts of Al associated with humus. Gibbsite and quartz were found to be dominant in the sand fraction, whereas gibbsite, kaolinite, and 2:1 minerals were the major minerals in the clay fraction. Formation of the soils was relatively fast because of the easy weather-ability of the pyroclastic materials, perhumid climate, and good drainage of the volcanic landscape. Since these Inceptisols possessed several Andic soil features, we hypothesize that they probably developed from earlier formed Andisols. Therefore, it is likely that the sequence of soil formation was Entisol→Andisol→Inceptisol. The most important soil-forming processes in the formation of the Inceptisols were likely structure formation, loss of bases and acidification, braunification, bioturbation, organic matter accumulation, weathering, and clay mineral formation.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Updated crop yield estimates for different soil types are required to meet the needs of farmers, land appraisers, and governmental agencies of most states within the United States. The objective of this study was to update wheat [Triticum aestivum L.] yield estimates for all soil types in the state of Illinois under the average management level used by all Illinois farmers in the 1990s. A crop yield soil properties (CYSP) model was developed from established 1970s wheat estimates and soil properties. Correlation and multiple regression analysis was used to establish the relationships between 16 physical and chemical soil properties and wheat yields. Using step-wise multiple regression, a final CYSP model with six variables (% silt in A and E horizons, % organic matter in A and E horizons, cation exchange capacity, rooting depth, bulk density of the B horizon, and Na content) explained 78% of the variation in 1970s wheat yields of (R2= 0.78 [34 soils]). A 22-year yield trend was applied to the 1970s predicted and established wheat yields to estimate the 1990s yields under an average level of management used by all Illinois farmers. The 1990s wheat yields (model predicted plus trend) were compared with both established (Circular 1156) plus trend yields and with farmer-reported yields by Illinois Agricultural Statistics (IAS) Staff for 161 soils in nine test counties. Predicted 1990s test county wheat yields showed no statistical differences between the three data sources. The final 1990s wheat yield estimates for all Illinois soil types were the mean of the estimates from: (i) the established 1970s yields plus 22-year yield trend, (ii) the model predicted plus 22-year yield trend, and (iii) the 1990s farmer-reported yields. The proposed approach to updating wheat yields worked well in Illinois and should be useful in surrounding states and/or countries.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

The adoption of alternative management practices has been shown to increase soil organic matter. However, the effect of adopting these practices on soil phosphorus (P), especially organic P, is not clear. We evaluated the effect of such practices-mainly no-tillage, zero and low-chemical input, organic-based, row crop agricultural systems-on soil P and compared them with conventional agriculture and perennial farming systems. We also compared soil P under conventional agriculture to an adjacent forest site and a never-tilled native successional community site in southwest Michigan. Sequential fractionation analysis of soil inorganic and organic P fractions showed that long-term conventional row crop agriculture resulted in a 79% reduction of NaOH-extractable organic P compared with adjacent forested sites. The calcium phosphate pool and the residual P fraction, however, increased under conventional agriculture compared with the forest site, probably because of fertilizer inputs. Adoption of no-tillage and low-chemical input systems with a winter leguminous cover crop in the rotation for 7 years did not increase organic P significantly in any of the fractions extracted from the annual cropping systems. However, organic P extracted by NaOH increased to 22.1% after adoption of continuous alfalfa for the same period compared with 11.4% extracted under conventionally tilled annual cropping systems. We conclude that continuous alfalfa can help restore soils to their native P fertility levels by taking up P from the stable residual fraction and transforming it into moderately labile organic P through root death. We found no evidence that low chemical input organic based systems are sustainable with regard to P as there is no evidence that organic P is accumulating as a result of the use of cover crops. Further investigation is warranted after these soils become P limiting and more years have passed under the same treatments.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID