摘要:

Root growth is studied, and a growth absorption model for high concentrations is proposed, i.e., the free boundary Michaelis-Menten model. Nutrient availability and transport between the root surface and the rhizosphere are studied through the Michaelis-Menten type absorption for high concentrations. The resultant equations of the present free boundary problem are used to compute the growth of root radius. Graphics of interface concentrations versus interface position and interface root-soil position versus time are shown as a function of parameters such as the flux velocity, constant efflux, rhizosphere soil solution concentration, diffusion coefficient, and buffer power.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Cadmium and copper adsorption by δ-MnO2was investigated by potentiometric titration over a range of pH. Adsorption increased with increasing pH because of hydrolysis of metal cations and/or variable charge sites on δ-MnO2δ-MnO2. The surface acidity constant for δ-MnO2was determined using the Triple Layer Model to be pKint,a2= 5.34. The data from titrations with pH as master variable were analyzed to determine the Triple Layer Model intrinsic stability constants, p*Kint,cd2+= 0.81, p*Kint.cdoH+= 6.89, p*Kint,cd2+= 1.66, and p*K+int,CuOH+= 3.79 for cadmium and copper adsorption onto δ-MnO2. Titrations with metal ion as the master variable were performed at constant pH to estimate the adsorption capacities. The data conformed to a Langmuir isotherm and could be modeled with the Triple Layer Model constants. For cadmium, at pH 5.5, 7.0, and 8.0, the adsorption capacity is 0.434, 1.08, and 1.92 mmol/g, respectively. The affinity of δ-MnO2for Cu, 1.54 mmol/g at pH 5.5, is greater than that for cadmium. The results show δ-MnO2has high adsorption capacities and high adsorption affinities for cadmium and copper even in acidic conditions.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Mean weighted pore diameter and coefficient of uniformity concepts are introduced to describe the soil pore size distribution using only soil moisture characteristic data. Mean weighted pore diameter gives the mean pore size and coefficient of uniformity details the heterogeneity of the pore size distribution. Representing soil water characteristic information with single variables allows this information to be easily included in statistical analysis and readily interpreted with respect to other agronomic factors. The cumulative percentage of pore volume versus pore diameter (calculated from pore water tension) is plotted, an equation fitted to the curve, and the area above it integrated, resulting in an areal determination from which the mean weighted pore diameter is calculated. From the same plot, the pore diameter of the 75th and 25th percentiles are determined, and the coefficient of uniformity equals their ratio; it is ranked on an indexed scale to assess the degree of pore size heterogeneity. Different boundaries may be defined for the pore size range under consideration to allow one to investigate specific changes in macro-, meso-, or micropores. Examples are shown for a typic Paleudult and a typic Hapludult.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Weighing lysimeters are often used as a standard for evaporation measurements and are typically constructed with steel walls, a material with a thermal conductivity approximately 40 times that of soil. This highly conductive material can increase the rate of vertical energy transfer in the lysimeter and alter lysimeter soil temperatures. The objective of this study was to quantify effects of the vertical energy transfer by lysimeter walls on soil temperatures in lysimeters with surface areas of 0.175, 0.75, and 3.0 m2. Hourly average soil temperatures were measured at 0.1 and 0.4 m from 7 through 29 May 1985 in a field and at various positions in the lysimeters at the Blackland Research Center, Temple, Texas. The diurnal amplitude of hourly temperatures was smaller in the field than in lysimeters, was inversely related to lysimeter surface area, and decreased with distance away from the lysimeter wall. Mean daily temperatures in the lysimeters were up to 3°C greater than those in the field. Temperature differences were greater for smaller lysimeters. Diurnal temperature differences between the field and lysimeters were primarily caused by energy incident upon the exposed steel at the surface which was conducted into the soil by the wall. Temperature differences could impact early-season root and shoot growth and thus affect representativeness of lysimeter evaporation measurements. Lysimeters constructed wtih steel walls should be designed to minimize the exposed area of steel at the surface.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

The relation between changes in water content and swelling and shrinkage processes was studied by exposing an undisturbed heavy clay soil in a lysimeter to evaporation at controlled conditions in the laboratory during a period of 82 days. Changes in water content were measured with tensiometers and by weighing the lysimeter. Swelling and shrinkage were determined by measuring the surface subsidence. The loss of water from the clay soil amounted to 45 mm, 40% less than the loss of water from a comparable silty soil lysimeter. Drying of the clay soil was restricted to the top 15 cm of the soil. As much as 67% of the water loss originated from the top 7.5 cm of the soil. Simultaneous shrinkage in the clay soil resulted in a three-dimensional decrease in volume of 34 mm, consisting of a crack volume of 22 mm and a surface subsidence of 12 mm. The clay soil exhibited the successive occurrence of structural shrinkage, isotropic normal shrinkage, isotropic residual shrinkage, and isotropic normal shrinkage again. The occurrence of normal and residual shrinkage could be predicted by the water content changes in the soil and the shrinkage characteristics of soil aggregates. Water loss in the structural shrinkage phase occurred from interaggregate pores and could therefore only be qualified from the lysimeter experiment.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

The hydric behavior of various samples of peats and composts which are commonly used in horticulture were studied. The wettability was assessed from contact angle measurements performed on air-dried pellets. Void index and water index curves were given in the process of first dehydration and then after air drying and rewetting, in a second dehydration process. Concerning the solid-liquid contact angle on air-dried material, peats turned out to be hydrophobic; this character was more obvious for woody (122.1°) and herbaceous (116.8°) peats than sphagnum peat (110.9°). These results were correlated with the humification degree (von Post index and pyrophosphate index). In comparison, fresh sphagnum is quite hydrophilic. The contact angles measured on composts are lower than 90°, whereas the woody materials used as litter are hydrophobic. Contact angle decreased during the composting process. The hydrophobic character of herbaceous and woody peat, together with their lack of rigidity, explains the irreversibility of the shrinkage. The opposite sphagnum peat, due to its hydrophilic character and elasticity, presented a reversible behavior after air drying. After air drying, the hydric behavior of composts, which are made of rigid and hydrophilic materials, was not greatly modified.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Field experiments were conducted for 5 years, starting from the dry season of 1984–85, to study the effects of rice straw management practices such as straw incorporation, straw mulch, straw burning, farmyard manure incorporation, farmyard manure plus straw incorporation, and no straw as control on soil physical properties and yield of winter wheat (Triticum aestivumL.). The rate of rice straw and farmyard manure was 5 t·ha−1. The soils of the site are classified as clayey thermic typic Hapludalf with pH 5.8; organic C 0.66 percent; cation exchange capacity 11.8 cmol P+100−1g; hydraulic conductivity of the saturated soil 0.80 x 10−6ms−1in the surface (0–0.3 m) depth. The treatments of farmyard manure and farmyard manure plus straw incorporation had higher percentage of water stable aggregates >0.25 mm diameter (80.9 percent, larger mean weight diameter (0.82 mm), higher porosity (54.2 percent, lower bulk density (1.19 Mg·m−3), higher available water capacity (0.125 m3·m−3, and higher hydraulic conductivity of saturated soil (3.21 x 10−6ms−1). The soil temperature at 0.1-m depth was not substantially affected by any of the treatments under study except the straw mulch where minimum temperature was raised by about 2°C over the control. The improvement in physical properties coupled with supply of nutrients from farmyard manure and rice straw resulted in consistently higher grain yields under farmyard manure plus straw incorporation followed by farmyard manure and straw mulch for the 5 years. The grain yields under straw burning, straw incorporation, and control were comparatively low.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Mineralogical and micromorphic techniques were used to investigate secondary minerals that occur in duripans (Durargids and Durixeralfs) on the Owyhee Plateau region of southwestern Idaho. Opaline silica and sepiolite were the principal secondary minerals found. Opaline silica exhibited three morphogenetic forms. Clearopal, consisting of opal-A with included calcite and unknown crystallites, occurred throughout the duripan, principally as pseudomorphic replacements after calcite pendants. Dirty-opal, a gray, semitran-sparent and nearly x-ray amorphous material, was found in highly altered cores of complex pedogenic concretions. Some opaline silica occurred in a web-like arrangement intimately mixed with calcite and was located in the matrix among loess agglomerates, in the laminar cap and in loess pedotubules. The opal in this microgranular calcite-silica was opal-A with included quartz and appeared to form, in part, by silica supplied in situ from alteration of primary aluminosilicate minerals. Transmission electron microscopic examination of colloidal silica revealed the presence of two types of coatings. Gray-gel was visually homogeneous and semielectron opaque whereas mottled-gel was -less electron opaque and contained many, minute, more electron opaque spheroids. The reorganization of these gels seems to control the formation of opaline silica spheres. In addition, most opaline silica spheres exhibited bubble-like substructures heretofore unrecognized. The presence of poorly crystalline sepiolite was indicated by x-ray diffraction. Imaged with the transmission electron microscope, the sepiolite exhibited a hierarchical structure. At the highest scale of magnification, laths with a crosssectional area of 13 by 23 nm were evident. The laths were organized in bundles. The most visually distinctive bundles were 950 by 1200 nm in length and lacked gel-like coatings. Other bundles were less distinct, more curvilinear, and exhibited less parallel orientation or mesh-like arrangements. These laths and fibers were from 250 to 375 nm in length, coated wtih structurally amorphous material resembling the grey-gel, and surficially sprinkled with opaline silica spheres and spherical units of mottled-gel. In the duripans studied, sepiolite was largely limited to pedogenic concretions in which it occurred as acicular crystals that radiated outward from concretionary laminae of opaline silica. This finding and the association of opal spheres with sepiolite laths suggest that some sepiolite forms via the addition of Mg to opal.

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1991

数据来源: OVID