摘要:

A calibration equation describing the relationship between electrical resistance and gravimetric soil moisture was generated for resistance-type soil-moisture-sensing units to be used in a high-clay-content soil. Soil moisture percentages measured gravimetrically and percentages predicted by entering resistance values from a validation set of moisture-sensing units into the equation did not differ. Subsequent analyses indicated that with a calibration set of five soil moisture units, a reliable equation could be generated with data from as few as three wetting-drying cycles in a high-clay-content soil.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

Kinetics of proton desorption at the hematite (α -Fe2O3)/electrolyte solution interface were studied to investigate the surface charge characteristics of the system. The experiments were performed over a period of 1 min, and a two-step process was observed. A rapid desorption of protons from the hydrated surface of the oxide was always followed by a relatively slower desorption step. A first-order kinetic equation was applied to describe the second step of the process, whereas kinetic parameters at time zero (t= 0) were calculated to obtain indications about the first rapid step. Results support the hypothesis that a diffuse space-charge may be created in the solid phase of the oxide, and that the overall process is largely dependent on diffusion phenomena. Changes in the specific rate values as a function of either the nature or concentration of the supporting electrolyte were also investigated.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

The relationships between maleic hydrazide (MH) adsorption and selected properties for 22 soils were studied by determining adsorption isotherms in aqueous media and calculating simple correlation coefficients amongKfvalues, from Freundlich equation isotherms, and the soil properties. The results of these correlations showed that MH adsorption by soils was essentially dependent upon specific surface area, clay content, and pH, but not organic matter content. When the soils were grouped according to their mineralogical components, MH adsorption-soil property relationships were shown to be dependent upon the soil clay mineralogy. For one group of soils with phyllosilicates as essential clay mineral components and free iron content (Fet) below 5%, MH adsorption was directly related to clay content and surface area, and the free iron oxides content (Fef) was inversely related to MH adsorption capacity. For a second group of soils with iron oxides and oxyhydroxides as essential clay mineral components and Fef> 5%, soil pH was the only variable related to MH adsorption. Desorption of MH from soils showed a hysteresis effect that was attributed partially to MH precipitation, or biodegradation, or both.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

The objective of this study was to characterize the reaction products of fertilizer phosphorus (P) and the phosphate rocks (PRs) that remained unreacted in a Colombian Oxisol (Tropeptic Halplustox) under field conditions cropped withBrachiaria decumbensduring the 5 yr after application of triple superphosphate (TSP) and six sources of PRs at a rate of 176 kg P ha-1. A strong agronomic response to P fertilizers was still observed at the 21st cutting of the crop, which was harvested after soil sampling.Soil chemical analyses revealed that soil pH, exchangeable Ca, total Ca, total P, Al-P, Fe-P, Ca-P, and Bray-1 P were all increased in the soil samples treated with PRs. The fertilizer P treatments appeared to have no significant effect on soil-exchangeable Al, oxalate-extractable Al and Fe, and P sorption isotherm. Data show that the amounts of the undecomposed PRs in the soil ranged from 2.5% (Fosbayovar rock) to 20.9% (Pesca rock) of the initially applied PRs. Almost 88% of the excess total P content in the soil as a result of PR treatments was found in the clay fraction.The results obtained in the magnetic separation study suggest that the reaction products formed from PRs may be less crystalline than those from TSP, even though the reaction products are probably in the same forms, namely, Al-P and Fe-P. This may explain why the residual available P of PRs was more effective than that of TSP for the plant.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

We investigated in the laboratory the distribution of applied Zn at two levels (5 and 10 ppm) in two soils (Alfisol and Ultisol) under two different moisture regimes, viz., continuously submerged and alternately submerged, with two levels of organic matter (starch, 0 and 0.5% of soil weight). The soils were sequentially extracted at different periods after incubation with (1) 1.0MNH4OAc (pH 7.0), (2) 0.05MCu(OAc)2, (3) 0.2M(NH4)2C2O4(pH 3.0), and (4) CBD solution to extract water-soluble plus exchangeable, organic complexed, amorphous sesquioxide-bound, and crystalline sesquioxide-bound zinc, respectively. Zinc in the extracts was estimated using atomic absorption spectrophotometry and expressed as the percentage of the applied amount after deducting the corresponding control values (0 level).The results showed a greater amount of applied Zn distributed to NH4OAc-and Cu(OAc)2-extractable fractions in the Alfisol than in the Ultisol soil, while the reverse was true for the (NH4)2C2O4-and CBD-extractable fractions; this has been attributed to the lower CEC higher sesquioxide, and lower organic matter content in the latter soil. A larger amount of distribution of applied Zn to (NH4)2C2O4-and Cu(OAc)2-extractable fractions occurred under the continuously than under the alternately submerged moisture regime, and the reverse occurred in respect of the other two Zn fractions. Organic matter application encouraged distribution of the applied Zn to the Cu(OAc)2-and NH4OAc-extractable fractions, though it discouraged such distribution to the (NH4)2C2O4-and CBD-extractable fractions, particularly during the later period of incubation. This has been attributed to the dissolution of sesquioxides under the reduced conditions developed in soil. These transformation processes of applied Zn are discussed in relation to Zn nutrition of wetland rice.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

Five soils were sampled for a study with the following objectives: (1) to quantify the K distribution in various fractions of a group of sandy feldspathic soils not currently under cultivation; (2) to estimate, using laboratory analyses, the capability of these soils to supply adequate K from mineral sources for plant growth, and (3) to determine whether soils series adequately group soils with respect to their K-release behavior. All soils have enough available K to supply any immediate crop growth requirements, and most are rated high or very high, based on current soil test calibrations. Even though textural and mineralogical differences exist among the soils, all have over 400 mg K/kg soil in the slowly available form, which can be released over time. The sources of slowly available K in the coarse-textured soils include mica from the clay separate and feldspars from the very fine and fine sands. The contribution of the coarse size fractions to K availability is confirmed because the clay alone accounts for only about half of the slowly available (HNO3-extractable) K. The A horizons of all soils have consistently more immediately and slowly available K than the C horizons, a characteristic possibly related to corresponding differences in organic matter content. Calculation of discriminant functions allows the soils to be grouped by series based on their K-release properties. The most important factors in the grouping appear to be those describing the quantity of slowly available K and the soil clay content.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

In 1856, J. B. Barth, a Norwegian forester, explained the formation of a podzol profile. Although his theory was correct in principle, he unfortunately rejected it and supported an older but wrong one.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

The Guelph permeameter method was assessed using laboratory and numerical procedures.Sand tank experiments using air-dry Caledon sand confirmed the theoretical basis of the method, that flow out of a small, uncased well into unsaturated soil rapidly decreases to a steady rate (Qs) and involves a finite wetting region. Saturated flow and field-saturated flow experiments found that the truly saturated hydraulic conductivityKs, (no entrapped air present) for repacked Caledon sand was approximately a factor of 3.5 greater than the corresponding field-saturated hydraulic conductivity,Kfs(entrapped air present). Horizontal infiltration experiments, which were conducted primarily to measure the metric flux potential (&phis;m) of air-dry Caledon sand, suggested that the discrepancy betweenKsandKfswas the result of an approximate 17% undersaturation due to the entrapped air. Application of the Laplace (K1fs) and Gardner (&phis;1m) analyses to the sand tank results yields approximately a factor of 12 overestimate ofKfsbyK1fs, but only about a 10% overestimate of &phis;mby &phis;lm. This indicates that capillarity (parameterized by &phis;m) dominated field-saturated flow (parameterized byKfs,) in the sand tank experiments.A numerical model based on Richards' equation was able to accurately simulate the sand tank results usingKfsfrom the field-saturated flow experiments and &phis;mfrom the horizontal infiltration experiments.Qsand C values were predicted within ≤2% of the experimental values. The numerical results suggest thatKfs, (rather thanKs) and &phis;mare appropriate parameters for describing flow out of a well into unsaturated, structureless soil; and that the integrally correct representation of theK(ψ) relationship provided by the exponential form of Gardner (1958) and by the method of calculating α, can be adequate for describing both transient and steady flow.A three-member family of numerically derived, unsaturated flow C value curves representing “sand,” “loam and structured clay,” and “unstructured clay” appears to compensate adequately for the neglect of the gravity-capillarity interaction in the Guelph permeameter equations.Sensitivity analyses indicate that for the Richards analysis the correct choice of C value curve from the three-member family of curves is least critical for the hydraulic parameter of primary importance in describing the flow. When field-saturated flow is the dominant component of flow out of the well, the error induced inKfs, by incorrect choice of C is on the order of ≤30% for the flow conditions analyzed. When capillarity is the dominant component of flow out of the well, the error induced in &phis;mby incorrect choice of C is on the order of ≤10% for the flow conditions analyzed. The sensitivity analyses also confirm that the Laplace analysis is reasonably accurate when the field-saturated component of flow dominates, and that the Gardner analysis is reasonably accurate when the capillarity component of flow dominates.

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1987

数据来源: OVID