摘要:

SUMMARYIt is proposed that the shrinkage of well‐decomposed peat beneath conifer plantations owing to the irreversible loss of water takes place in two stages. The first stage of pure subsidence is followed by a second stage of the formation of natural aggregates with a network of large cracks and a slower rate of subsidence. A plot of specific volume against gravimetric water content for natural aggregates or for excised blocks dried artificially shows a linear relationship throughout the range of water content. The assumption of equi‐dimensional shrinkage in stage two allows a prediction of the relative amounts of subsidence and crack formation. The shrinkage of non‐cracking fibrous peat is also dealt with. A preliminary attempt at verification of the predictive model from measurements of crack volume or subsidence of ground level shows that the model gives values of the right order of magn

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01310.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe effects on a number of soil physical and aeration parameters of compaction during spring pre‐sowing operations were measured on a clay soil (49% clay). A soil‐tyre contact stress of 200 kPa was applied by tractor tyres.Yield of an oat crop was reduced by 30% as a result of compaction. Total porosity of the soil was reduced by 6% v/v owing to loss of pores>60 μm, and water retention was increased. The resultant decrease in air‐filled porosity greatly reduced gas diffusion and air permeability coefficients of the soil, and, for a time, O2content of the soil atmosphere was significantly lowered in the compacted treatment. Penetrometer resistance after sowing was 3.5 MPa in the control and 4.5 MPa in the compacted treatment; in the latter, root growth was inhibited until the soil dried and cracked. By the end of June, canopy temperature measurements indicated water stress in the oat crop on compacted soil but not in that on the control.The results obtained indicated that air permeability, measured in the field, of 1 mm s−1provides a satisfactory single value below which crop growth is likely to be

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01311.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe effects on soil aeration of varying both the intensity and quantity of water applied per irrigation were investigated in a field experiment on a clay soil. Soil physical and soil aeration parameters were measured before, and for up to 7 days after irrigation. Irrigation increased the volumetric water content of the surface layer by approximately 8% v/v. However, periods of waterlogging were short on this soil. Significant decreases in soil oxygen content in the days after irrigation were caused by increased soil and root respiration. In terms of subsequent crop growth, the practice of applying 30 mm irrigation water at an intensity of 5 mm h−1was shown to be superior to irrigations providing half this quantity (15 mm) or double this intensity (10 mm h−1). The lower intensity irrigations decreased soil permeability by homogeneous wetting and swelling, and thus more of the water applied was recovered in the soil. Approximately one third of the water applied at the higher intensity was lost through cracks, and wetting of the soil was less unif

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01312.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYAlfisols exhibit a high degree of spatial variability in their physical properties. As a result, it is difficult to use information on physical parameters measured at one location to model larger‐scale hydrologic processes. In this study, the saturated hydraulic conductivity,KS, of an Alfisol was determined on 109 undisturbed monoliths using the falling‐head permeameter method. The model developed by Arya&Paris (1981) was used to calculate the pore volume from sand and clay fractions. Scaling factors were calculated from the measuredKs, sand pore‐volume, clay pore‐volume, clay content and effective porosity, using the similar media concept. Prediction ofKsof gravelly Alfisol using clay pore‐volume is confounded by high gravel content which, when discounted, improves the prediction remarkably. The scaled mean saturated hydraulic conductivityK*for all horizons of the Alfisol was approximately l.0x

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01313.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe relative importance of soil bulk density, water content and potential on the self‐diffusion and impedance factors of36C1 in a sandy loam and loamy clay were studied. The soil bulk densities used represented a range of conditions from freshly tilled seedbeds to compacted soils. The volumetric water contents and pF were the main factors controlling the soil impedance factors, with bulk density making a small but significant contribution.Soil type affects the impedance factors through differences in anion exclusion volumes, the water contents of poorly connected pores that contribute little to the diffusion process, and tortuosity of the diffusion pathway

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01314.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe kinetics of isotopic exchange of phosphate in a soil were studied using three techniques: (a) in shaken suspension, (b) in a system where solution circulated through a thin disc of soil, and (c) in moist soil from which solution was extracted by centrifugation with a dense immiscible liquid.The exchange kinetics of isotope diffusing in moist soil were inferred from the concentration profile of the isotope after diffusion periods of 10‐60 days. The proportion of the exchange that is effectively instantaneous in this diffusion system was much lower than in systems in which isotope is uniformly applied to the soil.It is postulated that the difference arises because exchange of diffusing isotope is limited by access to exchange sites and not, during the period considered, by slow exchange at the solid‐liquid interf

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01315.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYThree models of the self‐diffusion of phosphate in moist soil are presented, and their predictions compared to experimental data. The latter appeared to show that isotopic exchange was limited by access to exchange sites. The apparent rate of access was first described by a series of first‐order reactions, then as the diffusion of isotope into, and within, uniformly distributed, porous spherical aggregates, where the majority of exchange sites were situated, and finally as anNthorder (N1) reversible reaction. All models were solved using Crank‐Nicolson finite difference methods.The first two gave similar predictions; they were not capable of giving an adequate fit to experimental results. The third, although purely an empirical model, not based on a possible mechanism, was able to simulate data well.The mechanistic implications of each model, and of its relative success in simulating phosphate diffusion are disc

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01316.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe effects of pH value and of drying on phosphate adsorption by Al‐peat and Al‐humate associations were investigated. Raising the pH of the Al‐organic matter associations from 3 to 7, by addition of Ca(OH)2, greatly increased their phosphate adsorption capacity. There was a pronounced increase in phosphate adsorption as a result of drying the limed, Al‐organic matter associations, but when equilibration time was increased phosphate adsorption by moist Al‐peat samples approached that of the dried samples, indicating that the drying effect was at least partially caused by incomplete equilibration of P in the moist samples. In contrast, increasing the equilibration time did not alter the magnitude of the pH effect.A detailed study was made of the adsorption of phosphate both by Al‐peat limed to pH 7.0, and by its constituent components Ca‐peat (pH 7.0) and hydroxy‐Al (pH 7.0), synthesized separately then mixed together. For limed Al‐peat, drying increased phosphate adsorption, whereas for the individually synthesized components in combination the reverse was true. Thus, the limed Al‐peat behaved differently to its components.The increasing adsorption of phosphate by Al‐organic matter complexes with rising pH was attributed to increasing hydrolysis and polymerization of hydroxy‐Al associated with the organic matter. However, the interaction of organic matter with hydroxy‐Al would block many phosphate adsorption sites on the hydroxy‐Al. Upon drying, it is suggested that the organic matter condenses and the accessibility of phosphate to adsorption sites on the hydroxy‐Al is increased. Thus, drying resulted in i

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01317.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYRelease of potassium from 15 surface samples of benchmark Alluvial, Red and Black soils of India to 0.01 M solutions of BaCl2, CaCl2, NH4Cl and NaCl was studied in soils either untreated or pretreated with 5 × 10−3M KCl. In the untreated soils, the efficacy of the extractants declined in the sequence: BaCl2>NH4Cl>CaCl2>NaCl. Cumulative K‐release was greatest from Black soils, followed by Red and Alluvial soils. From soils pretreated with 5 * 10−3M KCl, more K was released than retained, and more ‘native’ K was released than that from untreated soils. Increase in the release of ‘native’ K decreased in the sequence: Red>Alluvial>Black soils. The amounts of surface and internal K, desorption rate constants and parabolic diffusion constants were calculated from K release to the variou

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01318.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SUMMARYA mathematical model, previously developed, is tested here for various clay minerals and their mixtures with iron oxides. The model yields good predictions of the permanent charge for non‐swelling clay minerals, but clearly underestimates the permanent charge of montmorillonite. This result is interpreted as being caused by partial accessibility of the internal surface of the mineral.In all cases, the values estimated for the permanent charge of the clay minerals fit the data for their mixtures with iron oxides. Nevertheless, important differences are found between the values of maximum densities of adsorption sites of potential‐determining ions of the various minerals estimated when they are alone and when they are in mixtures, particularly in the case of montmorillonite mixtures. The differences are explained in terms of an intimate association between the particles in the mixtures, and some SEM and EDX results point to the same conclus

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1989.tb01319.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY