ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

Experiments designed to study the relationship between the hydraulic conductivity and the water content of a porous body were performed on two inert porous materials packed in long columns. The experimental results obtained are presented. They show that the relationship is hysteretic and that the repeated variation of the water content between any two limiting values results in a closed hysteretic loop. The relationship is subsequently examined in the light of the domain theory. It is found that the distribution functionHfor the conductivity elements may be considered as independent of the water contentsrVdandrVwat which the increment of water content reverses from negative to positive and from positive to negative, respectively, and therefore that the conductivity elements may be considered as independent. Distribution diagrams are obtained from an analysis of the primary depletion curves according to the independent domain concept. The hysteretic paths calculated by means of these diagrams show satisfactory agreement with the experimental paths.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

The successful fitting of the Wilson equation to solid phase activity coefficients for binary aqueous systems makes possible the construction of three-component graphs for the ternary systems: (Na-Rb-Cs) Wyoming montmorillonite, (Na-Rb-Cs) Chambers montmorillonite, and (Na-K-Cs) attapulgite. Given the composition of the liquid (solid) phase, one can predict the composition of the solid (liquid) phase. The results showed (i) that the ion exchanger in ternary systems still tended to prefer the counter-ion with smaller equivalent volume, (ii) that attapulgite ion-exchange properties were similar to montmorillonites and could be regarded as intermediate between Wyoming and Chambers, and (iii) that soil which contained montmorillonite with low CEC might be more susceptible to becoming sodic than would soil which contained montmorillonites with high CEC.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

The present paper evaluates the unsaturated flow mechanisms in low-swelling clays by studying the effects of dry bulk density, temperature, initial water content, and inlet suction on such unsaturated flow aspects as wetting front advance and soil moisture profile. The soil moisture profiles are plotted in a dimensionless form, and a statistical approach has been adopted to give a more quantitative comparison. In general, the effects of the internal forces of interaction and hence the swelling potential are not appreciable. This is well supported by the linear relationship between wetting front advance and square root of time under practically all testing conditions, thus indicating a stable pore geometry during the flow process. In most cases a simple capillary model based upon the initial pore structure seems to be able to explain most of the observed phenomena. However, the change in the shape of the soil moisture profiles with changes in initial water content and inlet suction still cannot be fully accounted for.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

Ion exchange resins were used for removal of carbonates from saline calcareous soils prior to determination of cation-exchange capacity based on Sr++retention.It was shown that CEC values measured as retained Sr++in the soils pretreated with a mixed-bed resin were approximately equal to those obtained using the NH4OAc-method. Untreated samples, however, showed a Sr++retention well over the measured CEC, mainly due to precipitation of Sr++as carbonate.The NaOAc method gave somewhat higher CEC values than the Sr++method, due probably to the higher pH (8.2). A comparison with the total amounts of exchangeable cations was also made.Use of an anion exchange resin alone for removal of carbonate ions, however, gave a Sr++retention well below the measured CEC. This may have resulted from blocking of exchange sites by resin particles.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

The distribution of organic and inorganic N forms in virgin soil profiles and their cultivated, fertilized analogues is compared. Eight agriculturally important soil series, representative of different U.S. cropping areas, are included. In six of eight cases, total N and organic C contents were lower in the cultivated soils. Comparing all soils on an average basis, total N and organic C losses for the cultivated surface horizons were 33 and 42 percent, respectively. Corresponding losses for the profiles were 13 and 17 percent. The chemical pattern of organic N forms in the virgin soil profiles tended to be preserved upon either a gain or loss of original N. Although nitrate levels were generally higher in the cultivated profiles, in most cases the potential to convert soil organic N forms to mineral N was considerably higher in the virgin profiles. Nonexchangeable ammonium levels as measured by HF extraction averaged 145 ppm N in the cultivated profiles, representing a 16 ppm increase. There was no evidence for inability of the regular Kjeldahl procedure to recover all the nitrogen in either the virgin soil profiles or their cultivated analogues. In general, findings observed on a profile basis were similar to those observed on a surface soil basis, but the magnitude of the differences tended to be less on a profile basis.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

To monitor changes in the capacities of rice soils to supply NPK over a period of intensive cropping, fertilizer trials were conducted for 10 consecutive cropping seasons from 1968 to 1972 at three experiment stations of the Philippine Bureau of Plant Industry. Each trial included three indicator varieties, with IR8 as the common variety, throughout the 10 crops and in all locations.At all locations, the efficiency of nitrogen use was highest in the first crop and declined with successive croppings; for phosphorus, the trend was opposite. Phosphorus responses were greater with K2O application, especially in the dry seasons. We observed a marked response to potassium, especially at Bicol. The efficiency of K use increased sharply over the years, especially in the dry seasons. Split application of K gave no clear-cut advantage over basal application. Some varieties responded better than others to K, but only in soils which responded well to K.Results indicate that in areas where one crop per year is grown, with adequate N supply, responses to P and/or to K may be marginal. Responses may become marked, however, under intensive rice cultivation.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

A simple model for a soil crust was formulated and experimentally tested. It is based upon the parallel orientation and close packing of clay particles perhaps like that in shale. The physical and chemical factors which disrupt the ordered arrangement of the plate-shaped particles were noted to be those which induce aggregate formation and which result in good soil structure. The factors which induce greater orientation and packing were noted to be those which induce crust formation and poor soil structure. The resultant deleterious effects upon seedling germination, moisture penetration and runoff, aeration, and penetrability to roots are well known.Experimental attempts were made to test the validity of the proposed model. In each instance, the data either provided support for the model or else could be reasonably reconciled with the model. The model appears to provide a reasonable basis for explaining cultural practices which are known to influence soil structure in either beneficial or deleterious ways.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

Twenty-three soils more developed than Andepts and derived from volcanic ash in Central America were studied. These soils had been previously classified by other authors as “Latosols.”Morphological, physical, and chemical properties in these soils are more uniform than in the Andepts. This reflects their warmer climate and older age. Base saturation, extractable Al, Ca, Mg, and K differ most; P, N, OM, and C/N ratio differ less; and pH, CEC, and clay content differ least. These soils have thicker sola, redder hues, heavier textures, clay illuviation, stronger structure, are more plastic and sticky, and have more Mg, CEC, extractable iron, and higher (Ca + Mg)/K ratios than the Andepts. They also have good physical properties, are moderately fertile and are suitable for cultivation. Sixteen of the 23 profiles classify as Ultisols (mostly Humults), three as Alfisols (mostly Udalfs), four as Inceptisols (mostly Tropepts), and none are Oxisols.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

A numerical model, based on the transport equations for water and noninteracting solutes and written in IBM S/360 CSMP language, was designed to compute the radial movement of water and salts to plant roots. The inputs are: the soil's suction and conductivity functions, the soil solution's content and concentration, root density and permeability, and the required uptake rate (whether constant or diurnally fluctuating). The output provides the time-dependent drawdown of matric and osmotic potentials in the immediate vicinity of the root, the gradients and flow rates of water and solutes in the soil, and the plant water potentials needed to maintain different uptake rates. The model is illustrated for various rooting densities and various initial water contents and salt concentrations. The effect of increasing root density and permeability is seen to be similar to the effect of increasing water content or reducing transpirational demand.

ISSN:0038-075X    

出版商:OVID    

年代:1975

数据来源: OVID