摘要:

We tested a simple empirical method (called J & T here) for estimatingK(ψ) on sandy soils using only particle size information against in situK(ψ) values for seven sandy soil profiles in eastern North Dakota. We also compared the method with the Brooks and Corey method. On a composite of all data, the J & T method accounted for 60% of variability when compared with field values. Within a single soil series, and within individual soil profiles, better than 80% agreement between predicted and actualK(ψ) values was obtained in five of seven profiles with log-log calibration of the series (or profile) for the J & T method. In all cases, results using the proposed method were comparable to or better than those determined using the method of Brooks and Corey with a field-determined matching value. Both simplified methods adequately characterized the variability ofK, compared with in situ determinations.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

Evidence is presented for the presence of Ca-oxalate crystals at the soil-hyphae interface of mycorrhizalPascopyrum smithii. This prompted the development of a thermodynamic model that uses the ability of oxalate to scavenge Ca2+ion from the soil solution. The model predicts the increased solubility of Ca-apatite when oxalate is present in both the calcite-apatite (calcareous) system and in the exchangeable Ca-apatite (noncalcareous) system. The result is a marked increase of soluble (available) P in solution relative to the situation where oxalate is absent. The model is formulated on conditions prevalent in semiarid soils and presents a possible mechanism by which phosphorus uptake of plants is enhanced by mycorrhizal infection.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

By using x-ray diffractometry and13C-NMR spectroscopy, in combination with chemical and heat pretreatments, we have found good evidence for the occurrence of interlayer clay-organic complexes in two New Zealand soils. Here, the clay mineral is a regularly interstratified mica-smectite, and the organic species is a humic substance with a polymethylene chain structure. The organic material in the complex is resistant to peroxidation and pyrophosphate extraction. It is also thermally stable, being extensively decomposed only after the complex is heated at 400°C. A combination of soil factors seems necessary for interlayer complex formation. This includes a smectitic clay mineralogy, an accumulation of organic matter associated with a low microbial activity, and a highly acid soil reaction.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

This paper reports the results of a study of the effects of organic matter additions to anoxic soils on P transformations and sorption. We examined four soils alternately cropped to rice, where upland crops develop severe P-deficiency problems. The soils were amended with 0, 0.5, and 1.0% cellulose and kept flooded for 85 d. The P sorption, as affected by anaerobic decomposition of organic matter, oxalate extraction, and some anion additions, was investigated in the drained soils. The effect of pH, changed by ±2.0 pH units on soils flooded for the same time and then drained was studied for P sorption.Soil flooding and draining, with or without added organic matter, increased the P sorption significantly over unflooded soils. The added organic matter, however, significantly enhanced conditions for P sorption in drained soils. The increased P sorption in drained soil due to prior flooding with or without added organic matter was removed by extracting the soil with 0.2Mammonium oxalate acidified to pH 3.0. Extracting amorphous Fe by this treatment significantly reduced P sorption by the drained soil. Both pH adjustment and the introduction of citrate or silicate ions had some effect on P sorption, but their effects were much smaller than those caused by the anaerobic decomposition of organic matter. The anaerobic decomposition of organic matter, which is associated with accelerated soil reduction, increased amorphous soil Fe and P sorption.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

Previously published data from 625 paired soil samples were used to predict carbon in cultivated soil as a function of initial carbon content. A 30-cm sampling depth provided a less variable estimate (r2= 0.9) of changes in carbon than a 15-cm sampling depth (r2= 0.6). Regression analyses of changes in carbon storage in relation to years of cultivation confirmed that the greatest rates of change occurred in the first 20 yr. An initial carbon effect was present in all analyses: soils very low in carbon tended to gain slight amounts of carbon after cultivation, but soils high in carbon lost at least 20% during cultivation. Carbon losses from most agricultural soils are estimated to average less than 20% of initial values or less than 1.5 kg/cm2within the top 30 cm. These estimates should not be applied to depths greater than 30 cm and would be improved with more bulk density information and equivalent sample volumes.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

The objective of this study was to evaluate the effects of land-clearing methods and subsequent soil management on aggregate stability and soil organic carbon (OC) content in the 0− to 15-cm depth. The 2-yr-long study, beginning in July 1980, was conducted on Yurimaguas soil (fine-loamy, siliceous, isohyperthermic Typic Paleudult) in the Amazon jungle of Peru. The site was initially covered with a 20-yr-old secondary evergreen forest. Land-clearing methods were: (1) slash-and-burn, with logs removed by hand, but stumps remaining in place; (2) mechanical clearing with a bulldozer with a straight blade; and (3) mechanical clearing with a bulldozer with a shear blade. Soil samples were taken before clearing and at 3, 8, and 23 mo after clearing. After the second soil sampling, various land-preparation techniques were superimposed upon the three clearing methods to give a total of six land clearing-soil management treatments. Each treatment was split in three subtreatments: (1) flat-planted with no fertilizer or lime, (2) flat-planted with recommended rates of fertilizer and lime, and (3) bedding with fertilizers and lime. The third and fourth sets of soil samples (8 and 23 mo after clearing) were taken prior to harvesting the first and fifth consecutive crops, respectively. Straight-blade clearing reduced mean weight diameter (MWD) and OC by 39 and 21%, respectively. The least change in these two soil properties occurred for slash-and-burn clearing, with an 11 % reduction in MWD and no change in OC. No significant changes in the percentages of five water-stable aggregate size classes occurred between 8 and 23 mo after clearing. Soil-management practices, such as bedding and lime application, tended to maintain aggregate stability and OC levels of newly cleared land during the first 2 yr of continuous cropping.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

This paper presents an approximate analytical method for describing irreversible kinetics in soil columns. The method, which produces exact results for the limiting cases of zero- and first-order kinetics, uses an interpolation technique between those two orders and can be applied to an arbitrary kinetic rate. Using Michaelis Menten kinetics as an example, we apply the method to near zero-order kinetics, near first-order kinetics, and an intermediate case for a wide range of possible experimental conditions. The results are checked by comparison with a numerical solution. Conditions under which accurate results can be expected are given. Also, results are compared with a previous approximation, which is simpler to implement but less accurate when the effect of dispersion is considerable. Practical use of the method, to determine kinetic rates, is illustrated using an experimental break-through curve.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

The percolation test is commonly used to determine site suitability and filter field design for on-site wastewater treatment facilities. As now practiced, however, the test is neither standardized nor scientifically sound. The test hole radius, the depth of water in the hole, and the measurement procedure vary widely within and between jurisdictions; and more importantly, the present interpretation of the percolation test does not take into account the capillarity component of flow in unsaturated soils.This paper presents a new analysis of the percolation test, based on three-dimensional, saturated-unsaturated flow theory. It accounts for capillarity, hole radius, and water depth, and it explains much of the anomalous behavior previously observed in percolation rates.The new analysis identifies the field-saturated hydraulic conductivity and the matric flux potential, rather than the percolation rate, as the main soil hydraulic properties relevant to determining site suitability and filter field design.Procedures are suggested for determining the field-saturated hydraulic conductivity and matric flux potential, and for dealing with spatial variability.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID