Rapid Wetting Research Articles

A lysimeter experiment to investigate the effect of surface sealing on hydrology and pesticide loss from the reconstructed profile of a clay soil. 1. Hydrological characteristics

Abstract. A laboratory experiment was designed to assess the impact of surface seal development on the hydrological response of a clay soil. The influence of surface sealing on vertical macropore flow and lateral throughflow was of particular interest. The extent and development of the surface seal in repacked lysimeters was designed to match that recorded over two growing seasons at a clay field site in Essex, and was not extensive enough to reduce significantly the infiltration capacity of the soil. Consequently, the hydrological response of the lysimeters was similar under sealed and unsealed conditions, with a more rapid wetting response under sealed conditions being attributed to the higher soil moisture content required to create the surface seal. Macropore flow was initiated at the A/B soil boundary of the lysimeters, in response to the development of a saturated layer. The rate of macropore and throughflow in the soil was dictated by rainfall intensity at the soil surface as this controlled the depth of water in the perched water table. Simulation of the tensiometer response in the lysimeters demonstrated that it was possible to attribute the rapid movement of water through the A horizon to water displacement processes alone, without recourse to preferential flow processes.

Contamination by Slaked Fragments with Sorbed Compounds in a Structured Soil

An adequate understanding of the mechanisms involved in solute transport through porous media is needed to help design management strategies aimed at controlling ground water contamination. The objectives of this study were, first, to evaluate the effect of soil structure changes on the processes of water and solute transport, and second, to assess the contribution of the fine particles detached during rapid wetting (slaked fragments) to sorbed solute transport. A laboratory study was conducted on a silty loam soil wetted at three rates (slow, medium, and rapid) after adding a soluble compound (Br) and a highly adsorbed compound (radioactive 137 Cs) to sieved aggregates that were then deposited on the untreated soil surface. Soil particle migration (obtained from 137 Cs measurements), Br transport, mean weight diameter (MWD), and wetting rate were measured following the wetting events. The results showed that, from the soil surface down to a depth of 100 mm, sorbed soil particle transport occurred in significantly greater amounts under the rapid wetting treatment than under the slow and medium wetting treatments. For one single rainfall event, the sorbed fraction that left the surface represented about 0.3% of the surface applied amount. This occurred despite the fact that soil disintegration following the rapid wetting process increased the residual mass of Br in the soil from 77% (slow wetting) to 88% (fast wetting) and decreased the speed of water and Br transport through the soil profile. These results imply that contaminants sorbed onto external aggregate surfaces could be transported through the soil profile more quickly and in greater quantities than predicted by conventional contaminant transport models.

Spatial distribution of soil moisture over 6 and 30 cm depth, Mahurangi river catchment, New Zealand

Ground-based measurement of the spatial distribution of soil moisture can be difficult because sampling is essentially made at a point and the choice of both sample depth and sample spacing affects the interpretation of the measurements. Hydrological interest has generally been in soil moisture of the root zone. Microwave Remote Sensing methods are now available that allow the interpretation of spatial distributions of soil moisture, however, their signals respond to moisture in the upper few centimetres of soil. These instruments are still being developed, but one of the questions surrounding their application is how to interpret the surface moisture in a hydrological context. In this study we compare measurements of soil moisture in 0–30 cm of soil with those in 0–6 cm to examine how representative this surface measure is with regard to the root zone. Detailed spatial measurements of soil moisture were conducted at three pasture sites in the 50 km 2 Mahurangi River catchment of northern New Zealand as part of a comprehensive hydrology project; MARVEX (MAhurangi River Variability EXperiment). In three field sites, on each of three occasions, field measurements were made using both 30 and 6 cm dielectric-based instruments. Spatial grids of several hundred moisture measurements were collected over 0–30 cm and compared with those collected simultaneously over 0–6 cm. Results indicate that temporal and spatial issues interfere with correlation of the two sets of series. Rapid wetting of 0–6 cm compared with 0–30 cm is seen following storm activity. Some evidence of the decoupling of moisture content response is also evident when sites are measured on days following a storm. Rapid, but not unrealistic, response to intense rainfall was also observed. Implications are that detailed and accurate knowledge of local soil conditions and a sound model of soil water redistribution are required before surface soil moisture measurements can be used to infer root zone behaviour. Such knowledge was not available in this study, from either published data or field observation. In this study, without suitable a priori knowledge, soil property information was found via calibration.

Micronized powders of a poorly water soluble drug produced by a spray-freezing into liquid-emulsion process

The purpose of this paper is to investigate the influence of the emulsion composition of the feed liquid on physicochemical characteristics of drug-loaded powders produced by spray-freezing into liquid (SFL) micronization, and to compare the SFL emulsion process to the SFL solution process. Danazol was formulated with polyvinyl alcohol (MW 22,000), poloxamer 407, and polyvinylpyrrolidone K-15 in a 2:1:1:1 weight ratio (40% active pharmaceutical ingredient (API) potency based on dry weight). Emulsions were formulated in ratios up to 20:1:1:1 (87% API potency based on dry weight). Ethyl acetate/water or dichloromethane/water mixtures were used to produce o/w emulsions for SFL micronization, and a tetrahydrofuran/water mixture was used to formulate the feed solutions. Micronized SFL powders were characterized by X-ray diffraction, surface area, scanning and transmission electron microscopy, contact angle and dissolution. Emulsions containing danazol in the internal oil phase and processed by SFL produced micronized powders containing amorphous drug. The surface area increased as drug and excipient concentrations were increased. Surface areas ranged from 8.9 m 2/g (SFL powder from solution) to 83.1 m 2/g (SFL powder from emulsion). Danazol contained in micronized SFL powders from emulsion and solution was 100% dissolved in the dissolution media within 2 min, which was significantly faster than the dissolution of non-SFL processed controls investigated (<50% in 2 min). Micronized SFL powders produced from emulsion had similar dissolution enhancement compared to those produced from solution, but higher quantities could be SFL processed from emulsions. Potencies of up to 87% yielded powders with rapid wetting and dissolution when utilizing feed emulsions instead of solutions. Large-scale SFL product batches were manufactured using lower solvent quantities and higher drug concentrations via emulsion formulations, thus demonstrating the usefulness of the SFL micronization technology in pharmaceutical development.

Contamination by Slaked Fragments with Sorbed Compounds in a Structured Soil

An adequate understanding of the mechanisms involved in solute transport through porous media is needed to help design management strategies aimed at controlling ground water contamination. The objectives of this study were, first, to evaluate the effect of soil structure changes on the processes of water and solute transport, and second, to assess the contribution of the fine particles detached during rapid wetting (slaked fragments) to sorbed solute transport. A laboratory study was conducted on a silty loam soil wetted at three rates (slow, medium, and rapid) after adding a soluble compound (Br) and a highly adsorbed compound (radioactive 137Cs) to sieved aggregates that were then deposited on the untreated soil surface. Soil particle migration (obtained from 137Cs measurements), Br transport, mean weight diameter (MWD), and wetting rate were measured following the wetting events. The results showed that, from the soil surface down to a depth of 100 mm, sorbed soil particle transport occurred in significantly greater amounts under the rapid wetting treatment than under the slow and medium wetting treatments. For one single rainfall event, the sorbed fraction that left the surface represented about 0.3% of the surface applied amount. This occurred despite the fact that soil disintegration following the rapid wetting process increased the residual mass of Br in the soil from 77% (slow wetting) to 88% (fast wetting) and decreased the speed of water and Br transport through the soil profile. These results imply that contaminants sorbed onto external aggregate surfaces could be transported through the soil profile more quickly and in greater quantities than predicted by conventional contaminant transport models.

Enhanced aqueous dissolution of a poorly water soluble drug by novel particle engineering technology: spray-freezing into liquid with atmospheric freeze-drying.

The purpose of this work was to investigate spray-freezing into liquid (SFL) and atmospheric freeze-drying (ATMFD) as industrial processes for producing micronized SFL powders with enhanced aqueous dissolution. Micronized SFL powders dried by ATMFD were compared with vacuum freeze-dried SFL powders. Danazol was formulated with polyvinyl alcohol (MW 22,000), polyvinylpyrrolidone K-15, and poloxamer 407 to produce micronized SFL powders that were freeze-dried under vacuum or dried by ATMFD. The powders were characterized using Karl-Fischer titration, gas chromatography, differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, surface area, and dissolution testing (SLS 0.75%/Tris 1.21% buffer media). Micronized SFL powders containing amorphous drug were successfully dried using the ATMFD process. Micronized SFL powders contained less than 5% w/w and 50 ppm of residual water and organic solvent, respectively, which were similar to those contents detected in a co-ground physical mixture of similar composition. Micronized SFL powders dried by ATMFD had lower surface areas than powders produced by vacuum freeze-drying (5.7 vs. 8.9 m2/g) but significantly greater surface areas than the micronized bulk drug (0.5 m2/g) and co-ground physical mixture (1.9 m2/g). Rapid wetting and dissolution occurred when the SFL powders were introduced into the dissolution media. By 5 min, 100% dissolution of danazol from the ATMFD-micronized SFL powder had occurred, which was similar to the dissolution profile of the vacuum freeze-dried SFL powder. Vacuum freeze-drying is not a preferred technique in the pharmaceutical industry because of scalability and high-cost concerns. The ATMFD process enables commercialization of the SFL particle-engineering technology as a micronization method to enhance dissolution of hydrophobic drugs.

The effect of antecedent moisture conditions on sediment and phosphorus loss during overland flow: Mahantango Creek catchment, Pennsylvania, USA

AbstractThe loss of P in overland flow from most cultivated soils is controlled by erosion, and in‐turn soil moisture. We evaluated the effect of soil moisture on erosion and P transport in overland flow by applying rainfall (7 cm h−1) to packed soil boxes (1 m long and 0·15 m wide) and field plots (1 and 10 m long by 1 m wide) of silt loams in a central Pennsylvania (USA) catchment. Flow from packed soil boxes took longer to initiate as antecedent soil moisture decreased from field capacity (2 min) to air dried (8 to 9 min). Even in the more complex field plots (i.e. soil heterogeneity and topography), the wetter site (1 by 10 m plot; 70% field capacity) produced flow more quickly (3 min) and in greater volume (439 L) than the drier site (1 by 10 m plot; 40% field capacity, 15 min, and 214 L, respectively). However, less suspended sediment was transported from wetter soil boxes (1·6 to 2·5 g L−1) and field plots (0·9 g L−1) than drier boxes (2·9 to 4·2 g L−1) and plots (1·2 g L−1). Differences are attributed to their potential for soil aggregate breakdown, slaking and dispersion, which contribute to surface soil sealing and crusting, as dry soils are subject to rapid wetting (by rainfall). During flow, selective erosion and antecedent moisture conditions affected P transport. At field capacity, DRP and PP transport varied little during overland flow. Whereas P transport from previously dry soil decreased rapidly after the initiation of flow (6 to 1·5 mg TP L−1), owing to the greater slaking and dispersion of P‐rich particles into flow at the beginning than end of the flow event. These results indicate that soil moisture fluctuations greatly effect erosion and P transport potential and that management to decrease the potential for loss should consider practices such as conservation tillage and cover crops, particularly on areas where high soil P and erosion coincide. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Predicting hydraulic conductivity changes from aggregate mean weight diameter

Rapid wetting of structurally unstable soils results in aggregate disintegration, soil densification, reduced porosity, and changes in the pore‐size distribution and intake hydraulic properties. Knowledge of changes in these properties is critical for use in hydrological, solute transport, or erosion models. Multiple determinations of these parameters as they evolve over time are needed for short time step models. Routine measurement of all these properties is time‐consuming, and it would therefore be practical if some of them could be used to predict others. A model is proposed to predict hydraulic conductivity changes based on aggregate size distribution, stability, and wetting rate information in order to determine the stability of the pore network during wetting. Two independent experiments were conducted in the laboratory on two different soil types to evaluate the proposed model. The decrease in mean weight diameter of aggregates exposed to different rates of wetting was used to predict the mean weight diameter after wetting, which was then used in a model to predict changes in hydraulic conductivity. The results show that the changes in predicted and measured hydraulic conductivities between the potentials of −0.5 and −4 kPa were significantly correlated for both soils, despite an overestimation of the measured changes. The close correlation suggests that this bias could be empirically estimated from measurements of aggregate size and that information on aggregate size and stability would be useful for predicting changes in hydraulic conductivity.

Ortho ester‐based cleavable cationic surfactants

AbstractA new type of cleavable cationic surfactant, based on the ortho ester link, is described. Ortho ester amines with alkyl chain lengths from C8 to C16 were synthesized from a short‐chain ortho ester, a fatty alcohol, and an amino acohol. The ortho ester amine was subsequently quaternized with methyl chloride, yielding a cationic surfactant. The structures were confirmed by 1H and 13C nuclear magnetic resonance and the surface chemical properties were investigated by measuring both static and dynamic surface tensions. The results were compared to standard n‐alkyl trimethyl ammonium bromides. The critical micelle concentrations were found to vary rather widely, whereas the surface tensions in some cases were comparatively low. The short alkyl chain ortho ester quaternaries were found to have low surface tensions at short surface ages and to induce rapid wetting of a hydrophobic surface in dynamic tests. Additionally, the new surfactants were found to hydrolyze rapidly under mild acidic conditions, as measured by both titation and monitoring of the dynamic surface tension. Further, they showed excellent long‐term stability at pH 10 in dilute aqueous solutions. The effect of added electrolyte on rate of hydrolysis was also investigated. A marked enhancement in the hydrolysis rate was found when NaBr was added, whereas NaCl did not have the same effect. The new surfactants could potentially be used to provide temporary surfactant action in application areas such as disinfection, fabric care, personal care, chain lubrication, and mineral flotation as well as in textile processes such as dyeing or scouring.

PRE–WETTING EFFECT ON FURROW IRRIGATION EROSION: A FIELD STUDY&lt;/

Flowing water quickly saturates dry surface soil as water advances in irrigation furrows. Conversely, rain wets surface soil before runoff occurs. Rapid wetting destroys soil aggregates as water quickly displaces trapped air. Slowly increasing soil water content prior to saturation increases aggregate stability. We hypothesized that instantaneous wetting of dry surface soil during furrow irrigation results in greater soil erosion than if furrow soil was pre–wet immediately before irrigation. We conducted ten irrigation trials on 27–m long furrows in three different fields. Soil was pre–wet by surface drip irrigation (12 to 14 mm) or by lightly spraying with water (1.3 mm). Pre–wetting with drip irrigation significantly (P < 0.05) reduced soil loss for 5 of the first 7 irrigations compared to dry soil. The pre–wetting effect on soil loss was not always dramatic, but cumulative soil loss for the first seven irrigations was significantly different among the three treatments: 16, 30, and 56 Mg ha –1 for drip, spray, and dry treatments, respectively. The dry treatment never had less soil loss than either pre–wetting treatment. Pre–wetting furrow soil by spraying apparently did not add enough water to stabilize soil aggregates and decrease soil erosion for most irrigations. This study demonstrated that erosion was greater when water flowed over initially dry soil, which is typical with furrow irrigation, compared to water flowing over initially wet soil, which occurs during rain.

Stability of Structural Form during Infiltration Laboratory Measurements on the Effect of De‐inking Sludge

An adequate understanding of the mechanisms involved in the structural stabilization of soil by different sources of organic matter is needed to help design management strategies aimed at maintaining a stable soil structure. The objective of this study was to identify mechanisms involved in soil structure stabilization by paper sludge application, either by increasing the soil resistance to external stresses (aggregate stability) or by decreasing the magnitude of the external stresses (diminution of the wetting rate). A laboratory study was conducted on three different soil types with application of paper sludge at three rates (8, 16, and 24 dry t ha−1). The mean weight diameter, bulk density, hydraulic conductivity, and water retention properties were measured before and after a wetting event. The results indicate that most of the changes in physical properties resulting from rapid wetting took place at the soil surface (0–50 mm) and the magnitude of these changes gradually decreased down to a depth of 150 mm. Paper sludge application significantly improved the stability of 1‐ to 4‐mm aggregates to the destructive action of wetting in all three soil types. Paper sludge application increased porosity at potential &gt; −2 kPa, which resulted in higher hydraulic conductivity values (up to 88%) and a smaller increase in soil bulk density (down to 67%) relative to a control following rapid wetting. The wetting rates observed during the wetting event were similar regardless of the treatment, because the increase in the water potential at the wetting front was compensated for by an increase in hydraulic conductivity with increasing rates of sludge application.

The Influence of Wetting Rate and Electrolyte Concentration on the Matrix Failure Patterns of a Degraded Red Earth

The hydrologic response of a degraded red earth's topsoil to varying wetting rates (w) followed by falling electrolyte concentration (c) was investigated. Rapid wetting and the resulting structural failure increased total porosity and water retention across the 0 to 1500 kPa matric potential range. Substantial macroscopic swelling of rapidly wetted soil was attributed to air compressed into the largest available voids. Consequent soil matrix failure resulted in macroscopic swelling and increased sorption of water. Hydraulic conductivity appeared to be a non-linear function of wetting rate over the range w = 0-30 mm min-1. Similarly, for a given wetting rate, hydraulic conductivity increased with c, over the range 0.2-12 mmolc dm-3, according to the function log[1/Kr – 1] = a + blogc where Kr = K(c,w)/K(12,w). The slope b decreased as w increased indicating an increased sensitivity to low electrolyte concentrations at high wetting rates. Transactions of the Zimbabwe Scientific Association Vol. 74 2000: 25-32

Mechanisms and processes of crust formation on artificial aggregates

Crust formation process initiates from aggregate disinategration occurred through the combined effects of inherent soil properties and external actions. Rainfall plays a major role in determining the moisture conditions of the soil surface and in triggering aggregate disintegration mainly through slaking and the impact off raindrops. To analyze the relations between rainfall characteristics and crust formation, three artificially granulated aggregates were exposed to natural rainfall events. The rainfall characteristics were expressed in terms of the distribution pattern of rainfall intensity, I 10min; mean weighted drop diameters, D n; raindrop concentration, N Dn,; rainfall energy, Et;and kinetic energy, KE; as well as conventionally used parameters, such as total amount and mean intensity of rainfall. The degree of crusting was evaluated in terms of the water permeability and morphological observation just after the rainfall treatment. Based on a comparison of the rainfall characteristics and degree of crusting, we confirmed the following relations. Rainfall characteristics at the onset of the event considerably influence the degree of crusting, i.e. initial rainfall with high I 10min; and large drop size promotes aggregate disintegration by slaking due to rapid wetting, whereas initial gentle rainfall prevents slaking by slow wetting and subsequent crusting. When initial rainfall is not saaficient for slaking but Beads to a weakening sf the aggregates, subsequent disintegration and crusting are promoted by impact together with the residual influence of slaking. Impact is a dominant factor for aggregate disintegration under wet conditions after initial rainfall which is so gentle that slaking never occurs. Responses of the soils under different natural rainfall conditions were almost consistent with the results under simulated rainfall reported in the previous papers (Tanaka et $1.1997: Soil Sci. Plant Nutr., 43, 99-107, 109-115). Crusting was not observed in the Fujino 1–3 soil gender any rainfall events. In the Wokuei 1–3 soil, characterized by a susceptibility to slaking and stability to impact, crusting was only formed under the rainfall with high I 10 min at the onset. For the Shimazu 1 soil, impact played a dominant role in crusting under any rainfall events, and a larger degree of crusting was found when the effects of impact and slaking were combined. The characteristics of rainfall monitored in this study may be within a normal range as natural rainfall in terms of the amount and the mean intensity, suggesting that crusting likely to occur if the aggregate stability of soils is Bower than or similar to that sf the Shimaza 1 and the Hokuei 1–3 soils.

Effect of antecedent soil water content and rainfall regime on microrelief changes

Surface microrelief substantially affects surface sealing, runoff, and soil erosion processes on bare soils. Yet, the stability of microrelief for different antecedent soil water contents and rainstorms is not well understood. This study investigates the effect of surface microrelief and antecedent water content on the decay of microrelief under different rainstorm regimes. Two different rainstorm regimes were studied in laboratory experiments: continuous rainfall for a total amount of 60 mm applied at 30 mm/h intensity, and intermittent rainfall consisting of five successive rainstorms of 12 mm each, again with an intensity of 30 mm/h and separated by one week drying cycles. Rough, medium, and fine microrelief surface conditions representing different degrees of seedbed preparation were studied for three soils at antecedent soil water contents of 2–4% and 14–20%. Before and after rainfall, digital elevation models determining the surface microrelief were developed using a laser scanner with 2 mm grid spacing. The specific surface area calculated from microrelief data was used as an index to characterize microrelief. Microrelief stability increased with increasing initial roughness and was much higher for the antecedent wet soils than for the dry soils. Microrelief stability for the continuous rainstorm regime was higher than for intermittent rainfall. Differences in microrelief stability were mostly attributed to different aggregate stabilities. Additionally, the higher stability for the rougher microrelief surfaces was attributed to the lower drop impact density and splash density on the surfaces with larger specific surface area. Aggregate slaking due to air escape and rapid wetting was found to be responsible for the low microrelief stability at initially dry conditions.

Assessment of Basic Contaniination Withstand Voltage Characteristics of Polymer Insulators

An artificial contamination method for hydrophobic polymer insulators has been developed which can provide a uniform contaminant layer, similar to that found in nature, without either damaging the polymer surface or by adding any chemical agent to the conventional contamination slurry. Using this method, the basic contamination withstand voltage characteristics of polymer insulators were investigated compared with those of porcelain insulators. The results show that although hydrophobic polymer insulators generally have excellent contamination withstand voltage characteristics, critical reduction of withstand voltage sometimes occurs under rapid and heavy wetting and contamination conditions.

Wetting‐induced soil structural changes: The theory of liquid phase sintering

Rapid wetting of structurally unstable soils causes aggregate disintegration and settlement near the soil surface, thereby reducing soil porosity and intake properties. Beneath the slaked surface layer where unsaturated flow conditions develop, structural changes driven mostly by capillary forces take place at slower rates. The collapse of structural voids in the sublayer is due to coalescence of aggregates into larger units. We propose to adopt a model of liquid phase sintering of glass compacts to describe wetting induced densification of aggregated soil. The analogy between these processes is based on the observations that (1) the reduction in soil strength upon wetting is similar to the effect of high temperatures on glass powders and (2) in both systems capillary forces induce viscous flow. A sintering model which assumes a porous medium with tractable geometry was modified to accommodate soil conditions. The model considers the reduction in the water‐coated solid surface areas (i.e., water‐air interfaces) as the energy source for the soil viscous flow. Data from wetting experiments of aggregated silt loam were used to test the plausibility of the theory. The main unknown in the system was the viscosity of wet soil which was estimated to be in the range of 1.37×106 to 1.9×106 Pa s−1. The use of solid matrix with tractable geometry and energy considerations should prove useful for other studies of the dynamics of soil structure.

Soil Structural Stability during Rapid Wetting: Influence of Land Use on Some Aggregate Properties

A stable soil structure, resistant to the stresses induced by wetting, is important for maintaining agricultural productivity and reducing erosion. This study was undertaken to assess losses of soil structural stability resulting from long-term, continuous sugarcane (Saccharinum officinarum L.) cultivation (20 yr) in two Brazilian soils (an Oxisol and an Alfisol). When compared with virgin sites, sugarcane cropping decreased the organic C concentration and the wet aggregate stability (WAS) of the soil. The differences for organic C concentration and WAS between land use systems extended to the 1-m depth, in both soils, and were less prononced as depth increased. Cultivation also increased the extent of aggregate slaking relative to virgin sites. This increased sensitivity to structural damage could not be attributed to changes in aggregate cohesion; rather, it was observed that for the Alfisol, the greater slaking and mellowing (slaking without complete fragmentation) could be related to an increase in the rate of water uptake by the aggregate (1.9–3.4 times faster in the cultivated than in the virgin site) for the Alfisol. The pore tortuosity index was also 3.1 to 3.2 times higher in the virgin than in the cultivated site and indicated a pore network less efficient at transporting water within aggregates in the virgin site compared with the cultivated site. This suggests that the lower sensitivity to slaking in the virgin Alfisol was most likely caused by water entry within the aggregates slowed down by a clogged pore network.

Assessment of basic contamination withstand voltage characteristics of polymer insulators

An artificial contamination method for hydrophobic polymer insulators has been developed which can provide a uniform contaminant layer, similar to that found in nature, without either damaging the polymer surface or by adding any chemical agent to the conventional contamination slurry. Using this method, the basic contamination withstand voltage characteristics of polymer insulators were investigated compared with those of porcelain insulators. The results show that although hydrophobic polymer insulators generally have excellent contamination withstand voltage characteristics, critical reduction of withstand voltage sometimes occurs under rapid and heavy wetting and contamination conditions.

Effect of aggregate size and soil water content on emergence of soybean ( Glycine max, L. Merr.) and maize ( Zea mays, L.)

Rapid wetting of irrigated soils often leads to slaking and slumping, and on drying a surface crust and hard-set conditions may occur. This results in reduced crop emergence unless the surface is kept moist. The effect of aggregate size and water content on the emergence of soybean and maize from an Entic chromustert (heavy cracking clay) was determined using pots of sieved aggregates with size ranges less than 1, 1–2, 2–5 and 5–15 mm at soil water contents of 15, 20 and 25 g (100 g) −1. Unsieved soil was used as a control. Greatest emergence tended to occur from fine (1–2 mm) seedbeds compared with coarse (5–15 mm) seedbeds for both crops. A covered treatment, simulating a stubble mulch, resulted in greater emergence than an uncovered treatment for all water contents and aggregate sizes. Earlier emergence occurred from finer (less than 1 mm and 1–2 mm) seedbeds than from coarse (5–15 mm) seedbeds, and at the greatest water content used. Soil strength, measured with a shear vane, decreased with increasing water content and tended to be less on fine (1–2 mm) seedbeds compared with very fine (less than 1 mm) or coarse (5–15 mm) seedbeds. It is recommended that, for good emergence from this Entic chromustert, seedbeds be brought to a water content of 25 g (100 g) −1 by capillary wetting to prevent hardsetting and consist of 1–2 or 2–5 mm aggregates for soybean and maize, respectively, and have a stubble mulch on the surface. This corresponds to an equivalent depth of water of 15 mm and 9 mm for soybean and maize, respectively, in the top 50 mm of the profile.

Soil aggregation as influenced by cultural practices in Saskatchewan: I. Black Chernozemic soils

The impact of cultural practices on soil aggregate characteristics which determine the susceptibility of the soil to wind and water erosion was studied at two long-term (&gt; 30-yr) crop rotation sites on Black Chernozemic soils at Indian Head and Melfort, Saskatchewan. Surface soil (top 5 cm) taken in spring and fall, 1991, was air-dried and sieved by rotary sieve to measure aggregate size distribution. The water-stability of soil aggregates (1–2 mm) was determined after: (i) slow wetting, and (ii) fast wetting. Both rotation studies employed conventional tillage management until 1990 when the Indian Head experiment was converted to zero-tillage. Summerfallowing increased the wind-erodible (&lt; 0.84-mm) fraction of soil and decreased the geometric mean diameter (GMD) of aggregates. One year of cropping was sufficient to significantly reduce the proportion of wind-erodibile aggregates. Fertilization and legume green manure and hay crops reduced the wind-erodible fraction at Indian Head, but had no effect on the higher organic matter soil at Melfort. In monoculture wheat systems at Indian Head there was an inverse relationship between the wind-erodible fraction and cropping frequency; this was credited to the positive influence of cropping frequency on crop residue production. The wind-erodible fraction (Y) was related to GMD at Indian Head: Y = 11.8 + 117/GMD (r2 = 0.80***), and at Melfort, Y = 11.9 + 91/GMD (r2 = 0.82***). When subjected to rapid wetting, both the difference between cropped and native grassland soils, and the influence of cultural practices on water stable aggregates were pronounced. Aggregate stability was more closely related to the long-term management than to recent (&lt; 1 yr) cultural treatments. Frequent cropping, fertilization, and use of legumes increased water stable aggregates, particularly at the Indian Head site with its lower organic matter content. Key words: Wet sieving, dry sieving, legumes, fertilization, geometric mean diameter, wind erosion