摘要:

Solution displacement processes are important to irrigated agriculture in salt-affected soils. The experiments described in this paper explore solution displacement processes during transient unsteady flow in unsaturated soil. Changing solution contents and salt concentrations were determined from nondestructive experimental techniques that employed a dual-source gamma-attenuation system. In general, complete displacement of antecedent water by infiltrating solution was observed. Complete displacement, however, depended on the boundary flux and initial solution content of the soil. Dependence of the dispersion coefficient on solution content is also determined from one of the experiments.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

We conducted a laboratory experiment to study the K-Ca exchange equilibria in some soils from semiarid regions of India. The soils were depleted of their available and fixed K prior to exchange studies in order to understand the exchangeability and availability of applied K in such soils. Various exchange selectivity equations were also tested for consistency of the relevant exchange selectivity coefficients. In light-textured and relatively low K soils, about 50% of the exchange sites preferred K; this figure was reduced drastically, to about 5%, upon fixed K depletion. The values of standard free energy change of the exchange reactions, ΔG°, were negative in all the soils and decreased with depletion of fixed K, indicating the reduced specificities for K. The Gapon selectivity coefficient,KG, decreased with increased K saturation. NH4OAc treatment enhancedKGvalues at all exchangeable K percentages. None of the selectivity coefficients tested remained constant throughout the exchange isotherm.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

We studied the influence of the biological immobilization rate on the mobility and fate of fertilizer N under the humid climate of Mauritius in soil columns using (NH4)2SO4labeled with 10% atom excess15N and sorghum as the test plant. Our results showed that biological immobilization of fertilizer N occurred rapidly and was almost complete within 2 mo after fertilization. Most (>70%) of the immobilized fertilizer N remained hydrolyzable in 6MHCl 10 mo after fertilization. Rapid biological immobilization, coupled with the fast uptake of fertilizer N by sorghum, led to less than 5 and 12% of fertilizer N being leached from the rock-free (3035 mm water added) and rocky soils (1325 mm water applied), respectively. This study draws attention to the fact that in wet tropical regions, such as Mauritius, where soil temperature is always conducive to biological immobilization and where the soil colloids fix negligible quantities of NH4+, biological immobilization plays a leading role in hampering the mobility of N in soils, thereby altering the fate of fertilizer N.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

In four greenhouse pot experiments, we studied the relationships of cationic ratios, K/(Ca + Mg), in top growth of corn (Zea maysL.) and grain sorghum (Sorghum bicolorL. Moench) to ratios of available cations in soil when growth was stimulated by application of multiple rates of P, K, Fe, or lime. Increases in dry matter yields were accompanied by marked reductions in concentrations of nontest nutrients in each individual experiment. These reductions were attributed primarily to the replacement of one cation by another and not to simple dilution. Potassium was the dominant cation in most situations with these largely Na-excluding crops. Plant concentrations of (Ca + Mg) were suppressed until available K in the soil was depleted; then (Ca + Mg) became dominant, and Mg partially replaced plant K under conditions of severe K depletion. The extent of replacement of plant K by Ca, Mg, or both increased with the degree of K depletion in soil.The cationic ratio, K/(Ca + Mg), in plant tops increased linearly with that in the soil, when the soil ratio was changed by adding K or lime. However, these linear relationships varied (i.e., slopes changed) with initial level of soil K. Plant ratios decreased when soil ratios were held constant and growth was stimulated by applied P. In soils low in available K, plant K concentrations were closely related to soil K levels, even though K/(Ca + Mg) ratios in soil ranged widely. Therefore, the K/(Ca + Mg) ratio in plants often is linearly related to the corresponding ratio of these cations in soil; however, this relationship varies with levels of these nutrients in the soil and also with crop response when other growth-limiting factors are alleviated.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

The goodness-of-fit of first-order and empirically determined equations to net nitrogen mineralization data obtained by various investigators in laboratory incubations and greenhouse and field experiments is compared. In the majority of instances the empirically determined equation has a lower root-mean-square than does the first-order model. Parabolic functions of the formY = AXBprovided a good fit to most of the data. All the greenhouse and field data were fitted closely by the parabolic model or by a linear model, which can be considered as a special case of the parabolic model whereB= 1. The first-order model, based upon the concept of a single pool of mineralizable N of discrete size, does not conform to reality and is inferior to empirically determined models as a means of describing nitrogen mineralization.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

We conducted 21 nitrogen experiments in two different areas of Sweden between 1980 and 1982, one area in western Sweden and one in the east. Nine experiments were conducted on soils containing less than 15% clay, and 12 on soils containing more clay. Nitrogen rates were 0, 30, 60, 90, 120, 150, 180, and 240 kg/ha N as calcium ammonium nitrate (Nitrochalk, 28% N).Soil mineral N in the spring (NH4-N + NO3-N) was determined before fertilizer application. Sampling depth was 90 cm.Nitrogen fertilizer raised grain yields significantly. Grain yields were the highest in the western region. The difference equaled 600 kg/ha of grain. They were also the highest in the >15% clay group. The latter difference may be a consequence of differences in soil mineral N between the soil groups, and the former mainly depends on precipitation differences. During the experimental period the precipitation totals from April until July in the western region were 10 to 99% above those in the eastern region.Mineral N in soil in the spring varied between 27.8 and 142.5 kg/ha in a 90-cm deep profile. The very high figure of 142.5 kg was due to application of farmyard manure the previous year. Mineral N was the lowest in the <15% clay group, and it was lower in 1981 than in the other two years.The grain yield response to 90 kg/ha, N (y90) showed a negative relationship with soil mineral N. This was most clearly demonstrated in the western region, probably because of more favorable precipitation conditions there. They90decreased 11.8 to 81.9 kg/ha by a 1-kg/ha increase of soil mineral N. The greatest impacts of soil mineral N on they90were in the <15% clay group.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

We studied the effect of mucuna (Mucuna utilis) cover on the physical properties of an Oxic paleustalf in western Nigeria, previously cleared by four different methods. The methods used were manual clearing and clearing with a shearblade, a tree-pusher, and a treepusher/root rake. The soil physical properties measured were total porosity, penetrometer resistance, infiltration, and soil water retention.In contrast to cropping, mucuna cover improved soil porosity, penetrometer resistance, and hydraulic properties in all treatments. Total porosity of the 0− to 100-mm depth increased with mucuna cover from 0.55 to 0.59, 0.54 to 0.57, and 0.57 to 0.59 m3m−3in the manual, shearblade, and treepusher clearing treatments, respectively. Porosity of the treepusher/root rake clearing method was unaffected by mucuna cover. Penetrometer resistance of the 50− to 70-mm depth decreased with mucuna from 426 to 409, 425 to 418, 436 to 398, and 412 to 403 kPa in the manual, shearblade, treepusher and treepusher/root rake methods, respectively. Cumulative infiltration amounts over 3 h of the plots sown to mucuna were 134.1, 55.2, 14.6, and 186.8% greater than the corresponding cropped plots for the manual, shearblade, treepusher, and treepusher/root rake treatments, respectively. Similar decreases were noted in soil water retention at a potential of 0 kPa.The beneficial effects of mucuna cover were greater in manually cleared and treepusher-cleared plots. It was indicated that where land clearing causes high soil compaction, mucuna cover for about 1 yr would be a useful practice to restore soil physical properties to favorable levels for arable farming.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

Experiments were designed to determine the mechanism by which drying soil causes an appreciable increment of exchangeable K+(not exchangeable from the wet soil withMNH4OAc) to become exchangeable withMNH4OAc after the soil is dried. Earlier, this release was identified with the presence of fine micas. The hypothesis that the release mechanism involved proton dehydration from hydronium (OH3+) ion could be tested because the dissociation of HOH leaves one OH−ion in solution for each proton consumed in the mica-K+release process. The moles H+consumed g−1soil were measured by the rise in pH+(OH−in solution = H+consumed = K+released). Soil materials containing clay-sized micas were washed with pH+5MNaOAc and CaCl2; subsamples were dried for 16 h at 24, 60, or 110°C. The moles exchangeable K+g−1soil extracted withMNH4OAc from different subsamples were determined by flame emission spectrometry. The respective moles OH−g−1soil produced were measured by the pH+of an unbufferedMKCI supernatant solution from a duplicate sample. Undried control samples were treated in parallel. The pH+increase (&phis;+) in the unbuffered supernatantMKCI solution served as a direct measure of the total proton uptake by the sample from HOH, some of which is still present even at 110°C, as shown by DTA. For ease of comparison with &phis;+, the increase on drying of moles K+L−1g−1soil was calculated similarly, as ΔpK+. A high correlation (24 to 110°C;r= 0.99;p< 0.001; slope of regression equation, 0.985) was obtained between &phis;+and ΔpK+. These expressions are the increased activity ratio of K+and of OH−as moles g−1soil, for the dried samples in excess of wet samples. The mechanism proposed is that hydrated protons are progressively dehydrated by the drying, so that they can escape the steric hindrance characteristic of hydrated or larger cations to penetration into the structural site of mica K+, and form -SiOH. The protons (about 10−8times the size of a water molecule) are able to penetrate into the interlayer of mica and exchange for structural K+of -SiOK, forming -SiOH as an isolated crystal disorder. A similar mechanism could possibly fit K+release from other potassium-bearing minerals.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

The soil water sampler is a widely used device for collecting water from the vadose zone. Previous users attempting to mathematically model the fluid flow around a sampler have treated the sampler as a prescribed potential (Dirichlet) boundary condition. Physically, the sampler is a finite volume of space in which the air pressure changes (as dictated by the gas law) as the water level rises within. This change in air pressure can be a dominant factor that controls the transient flow of water into the sampler. An important consequence of the air-pressure effect is that the volume of sample that can be collected and the radius of influence of the sampler are both functions of the void volume of the sampler. A numerical technique in the context of the integral finite different method (IFDM) is proposed to simulate the behavior of a pneumatically sealed soil water sampler that extracts water from a partially saturated soil. A detailed parametric analysis of the problem will be the subject of a future paper.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

Total salts leached from column-packed soil-manure mixtures were significantly less than the total salt inputs. The loss of cations, calculated from the total soluble plus exchangeable cations in the control column and in the added manure minus these quantities after leaching, was greater for calcium (Ca) and potassium (K) than for magnesium (Mg) and sodium (Na), which was attributed to reversible K fixation and precipitation of Ca salts from solution. The measured decrease in cation exchange capacity (CEC) ranged from 54 to 84% of the total cation losses. Assuming that fixation of K releases an equivalent amount of other cations, then the total loss of cations minus the decrease in CEC was used to estimate the precipitation losses of the cations, presumably as sparingly soluble compounds, such as carbonate, sulfate, and phosphates.Measured K fixation was greater in pulse-irrigated than in continuously irrigated columns. SAR values obtained in the effluent collected from soil manure mixtures were generally equal to or smaller than those measured in the manure-free columns.

ISSN:0038-075X    

出版商:OVID    

年代:1986

数据来源: OVID