摘要:

This paper describes an improved numerical method to estimate the gas diffusion coefficient (D) of a finite soil sample, giving as a by-product an estimation of the effective porosity (e). This method is applied to an earlier apparatus used to measure the rate of transfer of tracer krypton-85 through a finite soil sample: the soil sample is enclosed by two gas cells, and the concentration in each gas cell is regularly measured after gas injection using β radiation count rates from the test gas. The proposed numerical approach combines the finite-element method to solve Fick's second law of diffusion, and a nonlinear, iterative procedure to find the estimated parameters&OV0429;andê, where the residual differences between the measured and simulated count rates at opposite ends of the sample are minimized.An error analysis that takes into account the random process of β emission of tracer is numerically simulated: the gas diffusion coefficient is shown to have a low sensitivity to this source of error, whereas the affective porosity has a larger sensitivity.The proposed method is applied to a large range of soils, including (1) wet and dry undisturbed field soil cores, (2) compacted, water-saturated aggregates, (3) dry and wet textural soil cores. The parameters&OV0429;andêwere fitted satisfactorily to the measured data on each soil sample: the diffusion coefficient estimates are compared with existing estimation methods, whereas the estimated effective porosity is approximately the same when using the iterative proposed approach or a mass balance method applied to the experimental apparatus. Furthermore, it is shown that there is no simple and unique relationship for all the porous media between the calculated gas diffusion coefficient and soil sample air-filled porosity: such a relationship is likely to be highly dependent on the physical characteristics of the pore space, such as pore continuity, tortuosity, and morphology.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

The adsorption of the plant growth regulator chlorocholine chloride from aqueous solutions by homoionic montmorillonitesof K+, Ca2+, Mn2+, Co2+, and Ni2+takes place through a reversible cationic exchange and by a coadsorption onto the external surface of the silicate. The isotherms fitted Langmuir's equation, and the total retention capacity was close to the CEC (106 meq/100 g), although some 20% of the exchange cations were not displaced by the chlorocholine ion. Data from the x-ray powder diffraction and infrared studies showed that the chlorocholine molecule was adsorbed in the interlamellar spaces; it had neutralized the layer charge and was oriented so that the quaternary ammonium ion C3axis was perpendicular to the silicate layers.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

For reclaiming alkali soils, pyrite is often used as an amendment. We studied the removal of Fe and Mn through leaching in columns and their subsequent presence in various forms in the soil after applying pyrite and farmyard manure. A highly alkali soil was treated with three levels each of pyrite (equivalent to 0, 50, and 100% of gypsum requirement), alone and in combination with farmyard manure (0, 12.5, and 25 Mg/ha) and was then leached with distilled water. After being leached, the soil in the columns was used to grow rice plants.The results revealed that a major portion of Fe and Mn applied through pyrite was retained in the soil itself, and only a small fraction was removed through drainage water. The contents of available, reducible, and active forms of iron and exchangeable manganese in the soil increased considerably due to pyrite application. Farmyard manure increased the reducible-Fe content, but had little effect on manganese content of the soil. Pyrite also increased the dry matter accumulation and Mn content of rice plant.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

Volcanic ash-flow tuffs are common in the Pacific Rim countries of Latin America. In addition to being consolidated by geologic processes, they are cemented pedogenically with illuviated silica and sometimes also with carbonates. These deposits, calledtepetatein Mexico, have very low permeability and the overlying soil erodes rapidly when cultivated. This study characterized the density and strength of typical tepetate. Density varied from 1.00 to 2.11 Mg/m3, and the mean of all 18 samples was 1.38 Mg/m3. Unconfined compressive strength varied from 3 to 160 kg/cm2, and the mean was 24.7 kg/cm2. The strongest, most indurated tepetate appears to occur when disseminated carbonates combine with silica as cements.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

In a laboratory study we evaluated the chemical significance of PCO2buildup, visà-vis reduction in soil pH and sodicity on the associated increase in hydraulic conductivity (HC) for rice culture conditions. Soil HC was determined at constant hydraulic head in soil columns packed to a bulk density of 1.47 g cm−3and flushed with CO2to obviate air entrapment in the presence and in the absence of farmyard manure (FYM) and rice.Hydraulic conductivity of alkali soils generally was least improved by continuous submergence, but was more improved in FYM-treated soils. Growing rice further increased the HC of soils. Exchangeable Na removal and reductions in pH appear to be directly related to the amount of CO2production in calcareous strongly alkali soils. It seems that the physical presence of an active root system yields a certain HC, but chemical reclamation is primarily through more CO2production. Organic manures improve rice growth because they raise PCO2to reduce pH and sodicity through greater mobilization of soil lime.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

A method was devised to determine the diffusion coefficient of ions in soil using miniature ion-selective electrodes. Ions were allowed to diffuse from one side of a soil column for a certain time, and then the potential (E) of the electrode, positioned at different distances (x) from the diffusing source, was measured. The diffusion coefficient (D) was obtained from a plot ofEagainstx2. The results were fairly reproducible under specified experimental conditions. The diffusion coefficients of Cl−, NO3−, K+, and Ca2+ions in several electro-dialyzed soils were determined, which showed the effect of the nature of ion species onD.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

Tillage-induced pans in Atlantic Coastal Plain Paleudults exhibit high cone index (CI) values and restrict crop rooting. This study evaluated the residual effects of deep tillage and landscape position on the CI of E horizons from a complex of Typic and Arenic Paleudults. Soil cores were sampled from three tillage treatments [disk only (D), chisel-plowed (C), and subsoiled and bedded (SB)] across footslope and linear backslope landscape positions. Cone index, saturated hydraulic conductivity (K), and the soil water characteristic were measured on the soil cores in the laboratory. A tillage x landscape position interaction for CI was significant at the 0.01 level. The linear slope position had higher CI values than the footslope position for both the C and D treatments. Subsoiling resulted in lower CI values at the linear slope position than either the C or D treatments. Using a multiple stepwise regression procedure to develop description models, we found log10of the saturated hydraulic conductivity (Klog) to be the best single variable for predicting CI when data for all tillage treatments were pooled. The CI of E horizons, which had not previously been subjected to either chisel-plowing or subsoiling, was best predicted by depth to the EB or B horizon. A model containingKlog, bulk density, and depth of sample within the E horizon (EDP) explained 64% of the CI variability when all treatments were pooled.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

We used two sets of experiments to evaluate the main and interactive effects of humic acids (HA) and anionic (AS) and nonionic (NS) surfactants on aggregate stability of two soils: a fragile, sandy loam, Typic Psammaqnent and a strong, clay, Vertic Xerochrept. For the main effects, the soils were treated at six rates, 0, 10, 50, 100, 1000, and 10000 mg/kg, with either HA, AS, or NS. A 3 x 4 factorial design, comprising three HA rates (0, 50, and 1000 mg/kg) and four AS or NS rates (0, 10, 100, and 1000 mg/kg), was used to assess the interactive effects. Changes in aggregate stability were measured at both the macro- and micro- levels by wet-sieving and clay dispersion techniques, respectively.On both soils, the percentage of water-stable aggregates increased with increasing rates of HA and NS, whereas aggregate stability was reduced progressively with increasing concentration of AS. The HA was, however, relatively more effective on the more fragile soil, whereas the converse was true for the NS. At higher NS application rates (100 mg/kg), clay dispersion decreased on the clay but increased slightly on the sandy loam soil, indicating, respectively, increased and decreased microaggregate stability. The HA and AS, however, increased clay dispersion on both soils at higher rates.The interaction of HA with NS was positively synergistic in increasing the stability of aggregates at macro- and micro- levels in both soils. The HA-AS interaction, however, reduced the stability of the sandy loam, but increased that of the clay soil at both levels of aggregation. The possible modes of action of these additives and the management implications of their effects on soil structure are emphasized.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

In 1964 amelioration treatments were begun in an 87-yr-old stand of Scots pine (Pinus sylvestrisL.) in northeast Bavaria to restore soil fertility lost through four centuries of litter raking and other human influences. This paper reports research to measure nitrogen mineralization differences in substrates from the degraded pine forest that had received contrasting amelioration treatments over a 19-yr period. Field plots with a “fertilizer” treatment received Ca, P, K, Mg, and repeated N fertilization; those with a “lupine” treatment were fertilized with Ca, P, K, and Mg, then tilled and underplanted withLu-pinus polyphyllusL. Laboratory N mineralization was measured on organic (F + H horizons) and mineral soil (0− to 10-cm) substrates from control, fertilizer, and lupine field plots. The effect of lime and gypsum laboratory amendments on N mineralization was also studied. Substrates were incubated at 35°C and moisture content equivalent to 0.08 mPa for 10 wk. Samples were leached at 2, 4, 7, and 10 wk with 0.01MKCI and analyzed for inorganic N. Total N mineralized (10 wk) was about 20-fold higher in organic layer substrates than in mineral soil substrates, and about 2-fold higher on both substrates from the lupine plots than from control or fertilizer plots. The substrates responded differentially to lime and gypsum treatments; these treatments had no effect on mineralization of the organic layer, but liming resulted in a 3-fold increase in N mineralization of the mineral soil. Gypsum had no effect on either substrate.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

We incubated two highly weathered soils from the humid coastal region of Brazil, using lime at 1.5 times the exchangeable acidity and with 400 μMg.g−1of phosphate. The effects of these treatments on the soils' variable charges and on the leaching of added potassium chloride and soil cations from soil columns of reconstituted A and B horizons were evaluated. Both liming and ammonium phosphate treatments resulted in neutralization of exchangeable acidity and creation of new negative charges. Treatment effects were additive in only one soil. Increases in negative charge were attributed to pH increases and the unblocking of Al-hydroxide-occupied exchange sites. Significant amounts of native and applied K were leached, and leaching was not reduced by new charges from liming or P addition. Both applied Ca and P were leached in significant amounts to below the B horizon.

ISSN:0038-075X    

出版商:OVID    

年代:1989

数据来源: OVID