Showing papers in "Soil Science in 1998"

Database-related accuracy and uncertainty of pedotransfer functions

Abstract: Pedotransfer functions (PTFs) are becoming a more common way to predict soil hydraulic properties from soil texture, bulk density, and organic matter content. Thus far, the calibration and validation of PTFs has been hampered by a lack of suitable databases. In this paper we employed three databases

Estimating soil water retention characteristics from limited data in Brazilian Amazonia

Abstract: The application and validation of complex atmosphere-soil water transport models demands knowledge of the parameters that describe hydraulic properties over extensive areas. Such information is rarely available, but Pedo Transfer Functions (PTFs) provide a means of predicting these parameters from soil survey data. However, most PTFs have been derived and validated using information from soils of temperate regions and have not been tested for the soils of tropical areas, for which chemical, physical, and pedogenetic processes are different. The equations of Rawls and colleagues, for example, overestimate water content when applied to the soils of Brazilian Amazonia. In this paper, we have developed a PTF to predict Brooks-Corey parameters from texture using data from soils of Amazonia. Multiple linear regressions were fitted to estimate, from soil texture (% sand, silt, and clay), the bulk density and porosity and the water content at a range of matric potentials. Brooks-Corey parameters were then derived and correlated independently with soil texture, providing a straightforward method for deriving soil retention parameters from the percentage of clay and silt. The method was validated using an independent data set for which textural and water release data were available. The agreement between the observed and measured values was very significant, but the results showed that the differences between predictions and measurements also depended on bulk density. However, the Amazonian soil survey data, which may be used to extrapolate these results spatially, do not generally include bulk density, and for this reason they were not included in the regressions.

Spatial variability of soil hydrophobicity in fire-prone eucalyptus and pine forests, Portugal

Abstract: Because of its implications for slope hydrology and soil erosion in the region and the lack of previous work on (i) spatial variability of hydrophobicity and (ii) hydrophobicity in a wet Mediterranean environment, this paper assesses the in situ severity and spatial variability of hydrophobicity of surface soils in dry summer conditions in burnt and unburnt Pinus pinaster and Eucalyptus globulus forests in north-central Portugal. Results of experiments to explore the origin of hydrophobicity are also reported. The molarity of ethanol droplet (MED) technique was employed. The average severity of hydrophobicity (MED > 24%) in both long-unburnt and recently burnt forests is among the highest recorded. In contrast to other studies, spatial variability of hydrophobicity is generally low for all land types. This is thought to be caused by a comparatively high release rate and thorough distribution of hydrophobic substances aided by the relatively wet climate combined with the fairly uniform character of the commercial forest stands investigated. Although forest fires are usually thought either to increase (for low ground temperatures) or to destroy (for high ground temperatures) surface soil hydrophobicity, burning in the study area had little impact on surface hydrophobicity. This is attributed to (i) preburn hydrophobicity already so severe that the organic compounds released from the litter during burning contribute no detectable additional hydrophobic effects and (ii) fire temperatures insufficient to destroy surface hydrophobicity. The results suggest that the relative spatial uniformity of hydrophobicity in the study area is induced by the planting of E. globulus or P. pinaster. The litter layers of both species, and the root zone in the case of E. globulus, are identified as sources of hydrophobic substances. Extreme hydrophobicity in E. globulus stands is found to develop within 2 years of planting on previously hydrophilic plowed terrain.

Adsorption and oxidation of arsenite on goethite

Abstract: Arsenic toxicity, mobility, and bioavailability in soil-water systems are highly dependent on its oxidation states and chemical species. In this study, both indirect (FTIR) and direct (X-ray Absorption Near Edge Structure (XANES) spectroscopic techniques were applied to investigate the adsorption

Chemical Fractionation of Organic Phosphorus in Selected HISTOSOLS1

Abstract: It is often reported that mineralization of less stable forms of organic phosphorus (Po) in Histosols contributes to the P load in drainage waters entering the Water Conservation Areas of the biologically sensitive Everglades wetlands. Consequently, a detailed evaluation of the degree of Po stabilit

Effect of drying temperature on the severity of soil water repellency

Abstract: Soil water repellency is often recognized in surface layers of soils that dry out frequently. The degree of water repellency of a soil can be measured by using the water drop penetration time (WDPT) test on field-moist or dried samples, referred to as actual and potential water repellency, respectively. A soil layer is actually water repellent below and actually wettable above its critical soil water content. Findings of the present study indicated that the degree of potential water repellency might change with different drying temperatures. For four of the seven sandy soil sites studied in the Netherlands, potential water repellency was greater after drying at 65° C relative to drying at 25°C, whereas it decreased at two sites and remained unchanged at one. The most reliable estimate of water repellency was obtained from undried samples collected during dry periods. Wetting rate measurements illustrated that water repellency increasing as a result of high drying temperatures led to decreasing water absorption by samples. Micromorphological investigations indicated that high drying temperatures resulted in an increase in the

Rates of Dissolved Organic Matter Release and Sorption in Forest Soils

Abstract: Rapid percolation enhances dissolved organic matter (DOM) transport in soils. One reason for this is nonequilibrium conditions, which make the time-dependence of the sorption/desorption an important parameter. We investigated the time-dependent DOM release and sorption of four soils (one topsoil, three subsoils) with different properties (organic carbon (OC): 2-56 g kg -1 ; dithionite-citrate-bicarbonate-extractable Fe: 2.4-21.9 g kg -1 ; clay: 30-450 g kg -1 ) in batch experiments during a 24-h period. Release of dissolved organic carbon (DOC) when a DOC-free solution was added occurred mainly within the first 15 min and then decreased drastically. After 2 to 4 h, the release almost stopped, but steady-state conditions were not reached within the experimental period. DOM fractionation with XAD-8 adsorber resin showed that the major portion of the released DOC exhibited hydrophilic properties. At an addition of 14 mmol DOC kg -1 , the topsoil still released DOC. One subsoil, which was rich in Al and Fe oxides and low in OC, sorbed both hydrophilic and hydrophobic DOC strongly, with a preference for the hydrophobic fraction. The other two subsoil horizons showed a sorption of hydrophobic DOC that was accompanied by a steadily increasing release of hydrophilic DOC. This indicates competition between the two DOC fractions, with hydrophobic DOC having the stronger affinity to sorbent and displacing soil-bound hydrophilic substances. The reactions approached steady-state conditions after 2 to 4 h. The time-dependency of the release and sorption can be described adequately by the Elovich equation and the fractional-power model. In a few cases, the DOC release from soil was represented better by the parabolic-diffusion equation. The slope of the Elovich equation, which may be considered the apparent rate constant, was higher for the hydrophilic than for the hydrophobic DOC fraction. It is related to the ratio between OC and Al and Fe oxide content.

Factors affecting molybdenum adsorption by soils and minerals

Abstract: Molybdenum adsorption behavior was investigated on a variety of crystalline and X-ray amorphous aluminum and iron oxide minerals, clay minerals, and arid-zone soils as a function of solution pH, molybdenum concentration, ionic strength, particle concentration, competing anion concentrations, and

Cd, Cu AND Zn SOLUBILITY IN ARABLE AND FOREST SOILS: CONSEQUENCES OF LAND USE CHANGES FOR METAL MOBILITY AND RISK ASSESSMENT

Abstract: The effect of land use, total metal content and soil parameters on the distribution of Cd, Zn, and Cu between the solid phase and soil solution is examined in a large field survey. In situ soil solution samples were obtained by centrifugation of field moist soil samples from six depth layers (0 to 80 cm) in 30 Dutch forest and arable soils. The soils vary in texture, pH, organic matter content, and land use and represent major soil types in the Netherlands. Total metal contents in arable soils exceeded those of forest soils because of manure and fertilizer application and decreased with depth in both land use types. Cd and Zn solution concentrations were higher in forest soils and increased strongly below pH 5.5 despite the low total metal content. Cu solution concentrations were higher in agricultural soils and increased with dissolved organic carbon. Multiple linear regression showed that CEC and pH explained 49% (for Cu), 79% (for Cd), and 83% (for Zn) of the measured variation in distribution coefficients (K d ). In acid forest soils (pH < 4.5; Zn total < 10 mg kg -1 ; Cd total < 0.3 mg kg -1 ), 80% of all measured Zn and Cd solution concentrations exceeded current ground water quality standards as a result of the high metal solubility at low pH. Taking into account the low acid buffering capacities in these soils, continuous acidification may cause further increase in the soil solution concentration of the metals considered in this paper.

Modeling Vertical Distribution of Carbon in Oxisols of the Western Brazilian Amazon (rondonia)

Abstract: Oxisols have great ecological significance in tropical soils because they are the dominant soil type of the Brazilian Amazon ecosystem, comprising more than 40% of its total. To estimate carbon (C) stocks and changes requires knowledge of the vertical distribution of C in profiles. The objective of this study was to determine if specific patterns occur in C profiles of Oxisols in the Western Brazilian Amazon so that total C storage can be assessed down to any given depth by simple models with low input data requirements. Two models, a power-based model and an exponential-based model, were tested using nonlinear regression analysis on a soil database made up of 129 Oxisol profiles corresponding to 519 soil horizons. These models, as judged by the coefficient of determination (R 2 ) value, explained more than 55% of the total variance for all of the horizons, whether or not segregated by taxonomic unit. The models were then tested with individual profiles. The power model exhibited a tendency to overestimate C stocks when integration was done for the 0-20-cm and the 0-100-cm layers. Results from the exponential model were better than those from the power model. The R 2 values were greater than 0.82, and the associated standard error was reduced. In a validation procedure, the mean error (ME) was close to zero for the exponential model, with a systematic ME of only 0.06 kg C.m -2 for the 0-100-cm layer.

The micellar model of humic acid: evidence from pyrene fluorescence measurements

Abstract: A series of studies has shown that the fluorescence intensity of pyrene dissolved in certain humic acid (HA) solutions increased significantly when salts were added, even those of highly quenching anions. The phenomenon was found to depend on the cation of the salt, generally varying with its charge

Partitioning soil phosphorus into three discrete pools of differing availability1

Abstract: The Hedley phosphorus (P) fractionation procedure provides an alternative for characterizing soil P availability without qualifying the component P species. Interpretation of results involving all fractions is complicated and offers little practical utility. The objective of this study was to group

Freeze-thaw cycles increase near-surface aggregate stability

Abstract: Soils with stable surface aggregates resist water and wind erosion better than soils with unstable aggregates. From earlier studies, we had preliminary evidence that one to three freeze-thaw cycles (FTCs) increased soil aggregate stability when measured by wet sieving field-moist aggregates. In this study, we measured the stability of aggregates vapor-wetted to field capacity from the Ap horizons of four soils after undergoing either zero, one, two, or four FTCs, and we determined the number of FTCs at which aggregate stability would be greatest for each soil. Moist soil was packed to a dry bulk density of 1.15 Mg M-3 by tapping it into 28-mm-diameter, 50-mm-tall brass cylinders. Each cylinder was then sealed in a polyethylene bag and inserted into a polystyrene foam tray. The soil in each cylinder was frozen convectively at —5°C for 48 h and then thawed at +6°C for 48 h for each FTC. Aggregate stability increased with the first one to two FTCs but changed little thereafter. Trend analysis revealed that aggregate stability would be greatest after two or three FTCs. When averaged across the four soils, FTCs stabilized aggregates more at 0 to 15 mm than at 15 to 30 mm. Near the surface of wet soils, two or three FTCs may be beneficial rather than detrimental to soil structure.

Gas Permeability in Undisturbed Soils: Measurements and Predictive Models

Abstract: Accurate prediction of changes in the gas permeability during variable soil-moisture conditions is a prerequisite for improved simulation and design of soil-venting systems for removal of volatile organic chemicals in polluted soils. Air permeability, k, as a function of soil air-filled porosity,

Amino sugar signature of particle-size fractions in soils of the native prairie as affected by climate

Abstract: Characterizing the amino sugar signature in particle-size fractions allows elucidation of the fate of microbially derived compounds during the alteration and turnover of soil organic matter (SOM) in soils of different climate regimes. The purpose of this study was to evaluate amino sugar pools an

Macroporosity and initial moisture effects on infiltration rates in vertisols and vertic intergrades

Abstract: In Vertisols and vertic intergrades, spatial and temporal variability remains a challenge in water flow and chemical transport studies. Infiltration measurements were made with tension infiltrometers operating at supply potentials ≥ −0.24 m in 42 clay-textured horizons from seven Vertisols and three

Lithologic discontinuities in some soils on drumlins : Theory, detection, and application

Abstract: This paper discusses the importance of lithologic discontinuities in pedologic and geologic research, reviews the primary methods used to detect them, and examines some soils in northern Michigan that exhibit varying degrees of evidence for lithologic discontinuities. Although many different parameters have been used successfully to detect discontinuities in soils, those involving immobile and inert components offer the best likelihood of success, and these data are best reported on a clay-free basis. Parameters involving acquired (pedogenic) characteristics or the mobile element (plasma) of soils should be avoided. Six Typic Eutroboralfpedons, formed on drumlins, were the primary focus of this study. Obvious-to-subtle evidence exists for lithologic discontinuities within the lower sola of these soils. Frequently, a weakly expressed stone line exists at or near the discontinuity. In this geologically young landscape, the origin of the discontinuity is presumed to have been glaciosedimentologic rather than pedologic (i.e., formed by bioturbation, surface creep, or eolian additions to near-surface horizons). Depth functions involving clay-free particle-size data, especially for coarser (coarse sand and fine gravel) fractions, were the most consistent indicators of the discontinuity. Mean particle-size data and heavy versus light minerals were also somewhat useful in discriminating between the two materials. This study may be the first of its kind to use measures of sand grain sphericity (e.g., mean feret diameter, compactness, and shape factor) to identify discontinuities in soils, although the utility of these indices in detecting discontinuities was mixed. The data underscore the need for multiple lines of evidence in the detection oflithologic discontinuities in soils and cautions that they are not all geologic/sedimentologic in origin.

Estimating the soil water retention from particle-size distributions: a fractal approach

Abstract: Soil water retention is an important hydraulic property in the study of water flow and solute transport in soils. However, soil water retention measurements are costly and time-consuming. In this study, a procedure was developed to estimate this function based on soil particle-size distribution

Intensive Tillage Effects on Spatial Variability of Soil Physical Properties

Abstract: Studies evaluating intensive tillage effects on the spatial variability and sample size requirements for assessing soil physical properties have been limited. Research to address this need was conducted on a Bertie silt loam (fine-loamy, mixed, mesic Aquic Hapludult) at Wye Research and Education Center near Wye Mills, Maryland. The field was subsoiled twice at a 38-cm depth, chisel plowed to a 20-cm depth, and disked at a 15-cm depth using controlled wheel traffic patterns. Rye grass (Secale cereale L.) was planted in October 1987. Undisturbed soil samples were taken at 1-m intervals in April 1988 along two parallel 45-m transects in the East-West direction and along two parallel 37-m transects in the North-South direction. Soil samples (3.0-cm length × 7.6-cm diameter) were taken from soil depths of 6 to 9 and 27 to 30 cm for determination of particle size, bulk density (Pb), soil strength (SS), mean pore size (MPS), and saturated hydraulic conductivity (Ksat). Coefficients of variation were 4.17 and 7.14 for Pb, 48.00 and 36.42 for SS, 21.54 and 23.65 for MPS, and 172.9 and 231.4 for Ksat, for 6- to 9-cm and 27- to 36-cm depths, respectively. All of the soil physical properties exhibited spatial structure at both depths. All soil physical properties except Ksat were weakly spatially dependent for the 6- to 9-cm depth. For the 27- to 30-cm depth, the properties exhibited moderate spatial dependence. The number of samples needed to estimate the true mean for Ksat was large; however, properties that exhibited low to moderate variability required fewer samples. More intense tillage at the 6- to 9-cm depth reduced the number of samples required to estimate true mean when compared with the 27- to 30-cm depth. Knowing the intensity of tillage in a given field is critical when sampling for soil physical properties.

Long-term tillage effects on physical properties of eroded soil

Abstract: Conservation tillage protects the soil from erosion, but it can result in soil compaction and higher bulk density. This corn-soybean rotation study was conducted to determine the effect of 8 years of continuous no-till (NT), chisel plow (CP), and moldboard plow (MP) treatments on physical properties

Degradation rates of eight pesticides in surface and subsurface soils under laboratory and field conditions

Abstract: The rate at which a pesticide is degraded in both the surface and subsurface soils is an important factor in determining the groundwater contamination potential of the pesticide. Degradation rates of eight pesticides used commonly in Western Australia-chlorpyriphos, chlorthal dimethyl, fenamiphos, linuron, metalaxyl, metribuzin, prometryne and propyzamidm-in the surface (0-25 cm) and subsurface (25-50 cm) layers of a sandy soil, were studied under controlled incubation conditions and were compared with those under field conditions. Field degradation rates were also simulated on the basis of degradation rates measured in the laboratory, taking into account the effects of temporal variations in field moisture and temperature. The incubation study showed that the eight pesticides had widely different degradation rates in both the surface and subsurface soils, with half-lives ranging from 23 to 142 days. Four of the pesticides had lower degradation rates in the subsoil than in the surface soil, but the reverse was observed for the other four pesticides. It is postulated that the nonuniform changes in degradation rates with soil depth were probably caused by interactive effects of changes in soil microbial activities and in organic matter content (and thus pesticide sorption) in the different soil layers. No consistent relationship was found between field degradation rates and those measured in the laboratory. The simulated field half-lives were all longer than those measured in the laboratory and did not match all the field-measured degradation rates.

Advanced tensiometer for shallow or deep soil water potential measurements

Abstract: Tensiometers are required for measuring soil-water potential at depths greater than several meters to quantify the direction and rate of soil-water movement. This paper describes a permanently installed tensiometer (advanced tensiometer) designed to measure soil-water potentials at any depth below land surface. This advanced tensiometer was designed with a removable pressure transducer to allow for field calibration and servicing and has two parts, a permanently installed outer tensiometer assembly and a removable transducer assembly. The permanently installed portion has a porous cup, an adapter containing a reservoir of water, and an outer guide pipe that extends to the land surface. The removable electronic pressure transducer assembly has a stopper on the bottom, a connector to attach the stopper to a pressure transducer, and an inner guide tube to raise and lower the assembly. The transducer assembly is lowered into the outer tensiometer assembly until the stopper connects and seals into the permanently installed adapter. This configuration of the advanced tensiometer allows it to be installed at any depth. Advanced tensiometers were operated with no maintenance at depths of 2 to 4.8 meters for periods of more than 3 months.The nearly constant temperature condition in boreholes provided stable, long-term water potential values and reduced field maintenance.

Formation of ferric hydroxide-silica coatings on pyrite and its oxidation behavior1

Abstract: The approach used most commonly to prevent pyrite oxidation in the field is the use of limestone. This approach, however, has a short span of effectiveness because the pyrite particle surfaces remain exposed to the atmospheric O2 after treatment. The objective of this study was to examine the feasib

Soil chemical changes after nine years of differential n fertilization in a no-till dryland wheat-corn-fallow rotation

Abstract: Intensively cropped dryland systems in the central Great Plains require adequate N fertilization for optimum residue and grain production. However, this N fertilization could be slowly changing the chemistry of the surface soil because of a decrease in soil pH and an increase in soil organic matter (SOM) and basic cations, even in previously well buffered calcareous soil systems. We investigated the effects of five increasing ammonium-N fertilizer rates in a Platner loam, on physical and chemical changes at the 0 to 5, and 0 to 15-cm depths after three cycles of no-till wheat (Triticum aestivum L.)-corn (Zea mays L.)-fallow rotation. We measured soil pH, texture, bulk density, cation exchange capacity (CEC), total P, soluble and total soil organic carbon (SOC), nitrate-N to a depth of 60 cm, and grain yields. No significant changes were found with soil texture, bulk densities, CEC, and total P. The data showed a significant reduction in surface (0-5 cm) soil pH (6.5 to 5.1) with the highest N rate (112 kg/ha), but this was accompanied by a 40% increase in SOC. Although there were significant increases in Al and Mn and decreases in Ca concentrations in the surface 0 to 5 cm at the highest N rate, no reduction in grain yields occurred relative to lower N levels with near neutral pHs. Because only a shallow depth of the soil was affected, residue, SOM, and rapid root growth could be compensating for surface acidity. Over the longer term, we need to monitor the effects of ammoniacal-N on downward soil acidity and yield trends under these new intensive cropping systems.

Combined Chemical and 31P-NMR Spectroscopic Analysis of Phosphorus in Wetland Organic SOILS1

Abstract: The organic P (Po) status of wetland soils has a large influence on the internal P cycling, and therefore on the biological productivity and water quality, of wetland ecosystems. Chemical extraction and solution-state 31 P-nuclear magnetic resonance (NMR) spectroscopy were used to identify and quantify the distribution of the major forms of P in organic soils of three created wetland sites-Apopka Marsh, Eustis Muck Farm, and Sunny Hill Farm (SHF). Spectra were obtained on 0.25 M NaOH-0.05 M EDTA extracts of soils with and without previous chemical removal of labile soil P. 31 P-NMR spectral analysis of the NaOH-EDTA extracts revealed the presence of inorganic ortho P (Pi), ortho-P monoesters, and ortho-P diesters. Chemically determined proportions of total P (TP) as Pi in the NaOH-EDTA extracts were similar to those obtained by 31 P-NMR. P monoester was the predominant P form in Apopka Marsh (51% TP) and SHF (59% TP) soils. The Eustis soil contained a high proportion of Pi (65% TP), and the SHF soil contained significant (10% TP) P diester. Chemical removal of labile Pi, using 1 M KCl and 0.5 M NaHCO 3 , enhanced the quality of all NMR signatures and demonstrated that not all P diesters are removed in the NaHCO 3 extract. NMR spectra were obtained with acquisition times ranging from 0.5 to 1.2 h, which represents a significant reduction in data acquisition times reported in the previous 31 P NMR studies of soil extracts.

Intensive tillage effects on spatial variability of soil test, plant growth, and nutrient uptake measurements

Abstract: Spatial variability and relationships between soil chemical properties, plant growth, and nutrient uptake in an intensively tilled field have not been well documented Our objective was to examine the spatial variability and positional similarities between soil chemical and plant properties and to determine sample size requirements for assessing these relationshipsThe research was conducted on a Bertie silt loam (fine-loamy, mixed, mesic Aquic Hapludult) near Wye Research and Education Center at Wye Mills, MarylandTillage-related operations are provided in the accompanying manuscript Rye grass (Secale cereale L ) was planted in October 1987 Samples were taken at 1-m intervals in April 1988 along two parallel 45-m transects in the East-West direction and two parallel 37-m transects in the North-South direction Plant tissue was hand-clipped from an area of 01 m 2 and subsequently analyzed for biomass, plant nitrogen (N), phosphorus (P), and potassium (K) Disturbed soil samples were taken from the 0- to 15-cm depth and analyzed for soil nitrate nitrogen (NO 3 -N), P, K, calcium (Ca), magnesium (Mg), pH, and organic matter (OM) Plant tissue macronutrients exhibited minimal differences in the coefficients of variation (CV) with values near 25% Soil N, P, K, and Ca generally showed similar CV values of 15 to 20% Magnesium, pH, and OM concentrations generally exhibited CVs of less than 10% Plant N and P exhibited strong spatial dependence,whereas biomass and plant K exhibited moderate spatial dependenceWith the exception of P and Ca, all soil chemical properties exhibited moderate spatial dependence There were few positional similarities between biomass production and plant macronutrient concentrations Biomass exhibited strong positional similarities with soil NO 3 -N and pH, whereas soil NO 3 -N and plant N indicated positional similarities with increasing values Soil OM and NO 3 -N had similar patterns for high measured values Soil and plant P exhibited strong positional similarities, but this was not true for soil K or plant K uptake, indicating other factors may be involved After intensive tillage, the sample size requirements were lower for soil properties than for plant properties We conclude that it is inappropriate to make recommendations on equal sample size for plant and soil properties unless the properties exhibit the same degree of variability

Release Of Potassium From The Silt And Sand Fraction Of Loess-derived Soils

Abstract: The objective of this investigation was to examine whether the silt in loess-derived soils represents a major K + source for plants. The experiment was carried out with 14 arable loess-derived soils (Alfisols) from which, in one series, the clay was removed and in which, in another series, complete soils were used. In this work, the latter are denoted as whole soils, whereas the soils without clay are labeled silt + sand fractions, of which the silt proportion was about 80%. Potassium was extracted from whole soils and from silt + sand fractions by electroultrafiltration (EUF). Potassium quantities extracted from the silt + sand fractions were about half the K + quantities extracted from whole soils. Potassium rates extracted and plotted on extraction time yielded curves that fitted the Elovich equation. Dry matter yield of ryegrass (Lolium perenne) grown in pots on whole soils did not differ significantly from dry matter yields obtained from grass grown in pots on silt + sand fractions. Thus, it is suggested that the K + present in the silt + sand fraction was readily available to the grass. Close correlations between K + uptake of the grass and K + soil parameters were obtained with the EUF extraction at 400 V and 80°C, with which the interlayer K + was mainly obtained. The b values of the Elovich equation reflect the K + release rates of interlayer K + . The data provide evidence that in loess-derived soils, the silt fraction, presumably because of its high mica concentration, is of great importance for the supply of K + to plants.

Water saturation and surfactant effects on bacterial transport in sand columns

Abstract: Bacterial breakthrough curves from clean sand columns were used in two related studies of the effects of unsaturated flow and added surfactant on cell transport. In the first study, step-inputs of bacteria were used to evaluate the effects of water saturation (saturated and unsaturated condition), surfactant concentration (0 and 20 μmol/L sodium dodecylbenzene sulfonate (DDBS)), and bacterial strain (aquifer isolates E3W7 and W31) on the kinetics of cell deposition and entrainment. These surfactant concentrations and bacterial strains did not have significant effects on the rate coefficients. Unsaturated columns, however, always had delayed cell breakthrough compared with saturated columns, and the deposition rate coefficient was significantly greater in unsaturated conditions compared with saturated conditions. After the outflow concentration of E3W7 from the saturated columns reached the inflow concentration, the columns were partially desaturated. This resulted in a rapid drop in outflow concentration-a decline of 29% with surfactant and 78% without surfactant. A second study was conducted to investigate the mechanism of surfactant-enhancement of bacterial transport by applying 100 μmol/L DDBS before a pulse of W31, concurrent with the pulse, following the pulse, at all times, or at no time. Only the presence of surfactant at all times and concurrently with the cell pulse resulted in significantly greater cell recovery compared with the no-surfactant control. This suggests that 100 μmol/L DDBS interacted with the cells to reduce their adsorption to gas-water interfaces in unsaturated sand.

Effect of cultivation on hydraulic properties of an iowa soil using tension infiltrometers1

Abstract: The potential for groundwater contamination at a given location is influenced strongly by the hydraulic properties (K) of the soils. The objective ofthis study was to investigate the effect offield cultivation in a no-till soil on the variability of infiltration rates, fractional porosity distribution, and soil hydraulic conductivity during the growing season. Infiltration rates were measured at different tensions using tension infiltrometers in two no-till plots with cultivation (cultivated) and two no-till plots without cultivation (uncultivated) at two different depths (soil surface and 0.15-m depth). Tension infiltrometer readings were taken at four soil water tensions (0-, 30-, 60-, and 90-mm) and at three different times during the growing season july, August, and September). Results of this study showed that infiltration rates at 0-mm water tension were significantly larger than infiltration rates at the three other tensions for all plots. The data on infiltration rates show that under saturated flow conditions, macropore flow is a significant pathway for water infiltration. For the cultivated plots, 70 to 80% of the saturated flux at the soil surface occurred though macropores. At 0.15-m depth for the same plots, there was a decrease in macropore flow during the growing season (from 69% in July to 44% in September). However, in the uncultivated plots, macropore flow increased (by almost 52%) at the soil surface during the growing season. In the uncultivated plots, macroporosity increased with time at the soil surface. During the latter part of the growing season, macroporosity values for no-till plots with cultivation were very similar to those of no-till plots without cultivation. The effect of cultivation disappeared with time during the growing season. Although there was great variability in the saturated K values, K values at the surface remained constant in the cultivated plots at the beginning ofthe growing season and increased during the latter part of the growing season. In the plots without cultivation, there was an increase in the surface-saturated K values from July through September.

Management Discriminant Properties in Semiarid Soils

Abstract: The physical properties of coarse-textured soils in semiarid regions often deteriorate with use. We hypothesized that the changes in the physical properties of the soil were related to the cropping systems employed. Surface samples of 52 Entic Haplustolls under three different uses (24 under continuous cultivation (C), 18 under rotation with grass leys (R), and 10 virgin soils (V)) were analyzed for clay, silt, organic matter and water content, bulk density, compaction and aggregate stability. Data were analyzed statistically using principal components, canonical variables, and discriminant functions. A satisfactory segregation of the soils according to discriminant properties (coarse organic matter, aggregate stability, and susceptibility to compaction) was obtained. The model developed satisfactorily classified the soils under different uses (100% R, 83% C and 88% V). Principal component analysis also showed that bulk density, compaction, and wet aggregate stability are related to organic matter content. We conclude that, in the studied region, the lower the ratio of organic matter to clay + silt content, the more severe the physical deterioration of the soils.