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Increasing precipitation storage as plant-available soil water increases dryland crop yields on the semiarid southern Great Plains. The dominant soil in this region is a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll), which features dense subsoil layers that limit infiltration and root proliferation. Deep (∼0.7 m) plowing may eliminate these layers, but the duration of treatment efficacy must be sufficient to recoup costs. Our objective was to quantify the long-term effects of deep plowing on soil bulk density (BD), penetration resistance (PR), ponded infiltration, and crop yield. In fall 1971, paired 24- by 460-m level conservation bench terrace plots at the USDA-ARS Conservation and Production Research Laboratory, Bushland, TX (35°11′ N, 102°5′ W) were moldboard plowed to 0.7 m or untreated and, subsequently, maintained with conventional sweep tillage. Grain sorghum [Sorghum bicolor (L.) Moench] was grown intermittently during the next 34 yr for 15 paired yield comparisons. Compared with the control, deep plowing significantly decreased BD and PR and increased water infiltration. Long-term mean grain yield of 2.86 Mg ha-1 with deep plowing was greater (P < 0.05) than the 2.61 Mg ha-1 yield in control plots. Approximately 89% of the 15-yr 3.75 Mg ha-1 cumulative yield increase with deep plowing occurred during 4 yr with large early growing season rains. We attributed this yield increase with deep plowing to greater internal drainage that reduced crop injury during infrequent floods and not greater root growth through the dense subsoil. Thirty years after deep plowing, high water intake suggests that flow-limiting subsoil layers had not yet redeveloped. The sustained deep-plowing benefit extends the period to recoup the 1971 plowing costs of US$160 ha-1. [ABSTRACT FROM AUTHOR]

The objective of this work was to construct and validate a model that shows how the clay shrinkage curve, under the influence of a silt-sand admixture as well as an inter- and intraaggregate structure, is transformed to the soil shrinkage curve. To meet this objective, we investigated (i) the reference shrinkage curve, that is, one without cracks, because cracks lead to a multivalued shrinkage curve; (ii) the rigid superficial (interface) layer of aggregates, with changed pore-size range and distribution, compared with the intraaggregate matrix; and (iii) soils with sufficiently high clay content when large pores inside the intraaggregate clay (so-called lacunar pores) are nonexistent. The methodology is based on detail accounting for contributions of the interface aggregate layer and intraaggregate matrix to the soil volume and water content during shrinkage. The reference shrinkage curve is determined by six physical soil parameters: oven-dried specific volume; maximum swelling water content; mean solid density; soil clay content; oven-dried structural porosity; and the ratio of aggregate solid mass to solid mass of the intraaggregate matrix. Only the last parameter was fitted for lack of data and compared with an unfitted estimate. The model was validated using data for eight soils. The important new conclusion is that the mere existence of the rigid superficial aggregate layer leads to a soil reference shrinkage curve that is convex upward in the structural shrinkage area, unlike the shrinkage curve of the clay contributing to the soil. The model can have numerous applications. [ABSTRACT FROM AUTHOR]

Management scenarios aimed at optimizing irrigation in a Sicilian vineyard characterized by a cracking clay soil irrigated with saline water were explored for seven soil profiles (Baglio 1-Baglio 7), by using the simulation model soil-water-atmosphere-plant environment (SWAP), which accounts for shrinkage and cracking. Accurate pre- diction of water content, θ, was obtained for the seven profiles by expressing the soil hydraulic properties according to the Brutsaert retention equation coupled with the hydraulic conductivity model proposed by Gardner (B-G model). A satisfactory prediction of the electrical conductivity of saturated extract (ECsat) was obtained using for the dispersivity (Ldis), a calibration value of 20 cm. Different irrigation schedulings and alternating waters of different quality were then explored as viable management options. The results showed that bypass flow determined a favorable water distribution, and that the best irrigation strategy was to make a minimum number of irrigations, by maximizing at the same time the amount of water supplied at each irrigation. Water storage in cracks was found to promote salt-leaching; neglecting cracks and bypass flow was shown to overestimate salinization. Alternating two different irrigation waters proved to be the best strategy, which could be adopted to reduce soil salinization and enhance crop transpiration. Findings concerning the role of cracks in the process of salt-leaching suggested that, under field conditions, application of a leaching solution was more efficient if the soil presented a considerable degree of cracking. [ABSTRACT FROM AUTHOR]

Land leveling is a government-subsidized, water-conserving agricultural practice common in the Mississippi River Delta region of the midsouthern United States. Though practiced to more uniformly deliver irrigation water, land leveling is a severe soil disturbance that alters the quasi-equilibrium among near-surface soil biogeochemical properties. The objectives of this study were to characterize the short-term impacts of land leveling on the magnitude, variance, and spatial variability and distributions of soil physical and biological properties and to evaluate the impact of land leveling on the relationships among soil properties in a Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquert) used for irrigated soybean [GIycine max (L.) Merr.] and rice (Oryza saliva L.) production in the Mississippi Delta region of northeast Arkansas. Bulk density and clay increased (P < 0.001), while sand, silt, fungal biomass, and fungal/bacterial biomass ratio decreased (P <0.001) as a result of land leveling. Despite the effect on the fungal/bacterial biomass ratio, hence a significant alteration of the soil microbial community structure, soil bacterial biomass was unaffected by land leveling. The variance associated with silt increased (P < 0.01), while the variance for fungal biomass and the fungal/bacterial biomass ratio increased (P < 0.001) as a result of land leveling. Spatial variability and distributions of soil physical and biological properties and relationships among them were noticeably altered by land leveling. Increased variations in soil physical and biological properties as a result of any soil disturbance will make uniform field management difficult. Compared to a similar study, the magnitudes of near-surface soil physical and biological property change as a result of land leveling were lower on a clayey Vertisol than on a silt-loam Alfisol. [ABSTRACT FROM AUTHOR]

The shrinkage geometry factor connects the changes of subsidence and the total crack volume at the shrinkage of a soil layer matrix. An available approach for estimating the factor does not account for the crack volume in samples, the stretching of a shrinking soil layer, and a possible change of the factor with water content. A recent new approach for estimation of the factor enables one to account for the factor's dependence on water content and the impact of cracks in a sample. The present work also accounts for the stretching of a shrinking soil layer. Corrected value of the factor is described in terms of the shrinkage curves of: a layer including unconnected solid cubes from the available approach, a stretched layer with cracks, a sample with cracks, and the soil matrix without cracks. This correction leads to changes in the soil subsidence and total crack volume compared with those obtained from the available approach. Two known physical conditions are used: the specific soil matrix volume without cracks is the same for the sample and the layer, and, the matrix deformations of an unlimited shrinking soil layer are longitudinal those of a thin quasi-elastic plate. Two available experimental examples are considered. The former relates to clay paste samples. The latter relates to an aggregated clay soil in situ and in samples. The geometry factor from the available approach, and that obtained after correction, differ significantly. The corrected factor also shows the appreciable change with water content. [ABSTRACT FROM AUTHOR]

The introduction of no-tillage farming stabilized soybean [Glycine max. (L.) Men.] yields on "Terra Rossa" soils of eastern Paraguay at a yield level that is among the highest in the world. The objective of this study was to clarify the effect of long-term no-tillage in a soybean-based farming system on physical properties of "Terra Rossa" soils with different texture. Four farmers' fields of different soil texture were studied in 1990 and 2000. Two fields had fine-textured soil and other two had coarse-textured soil. A 1-m deep pit was prepared at each site at the exact same location in 1990 and 2000, and clay contents, bulk density, cone index, saturated hydraulic conductivity (Ksat,), aggregate-size distribution, and Ap horizon thickness were investigated. To evaluate changes in Ap horizon thickness over time, data from several additional pits were included. Effects of long-term no-tillage on physical soil properties differed with soil texture. High erosion rates were associated with coarser soil texture, low Ksat, poor aggregate stability, and high compaction. Coarse-textured soils lost 34% (about 7 cm) of their Ap horizon in 10 yr compared with 11% in fine-textured soils. Cone index readings in the subsoil of a coarse-textured soil became extremely high (4.72 MPa). Soil compaction was furthermore associated with rapid soil drying and on the sixth day after rain reached a level that could restrict vertical root development of germinating seedlings. These results suggest that the no-tillage system as currently practiced in eastern Paraguay may not be suitable for coarse-textured "Terra Rossa" soils. [ABSTRACT FROM AUTHOR]

Sodium affected rice (Oryza sativa L.)-cropped fields are very common. Due to their high exchangeable Na percentage, clay dispersion is one of the major risks for these soils when they are flooded. During flooding, Fe2+ may become a major cation due to reductive dissolution of Fe oxides, but the flocculation effect of Fe2+ is not known. In this paper, the effect of Fe2+ on clay flocculation is studied by establishing critical coagulation concentration (CCC) curves of clays extracted from a paddy soil of Northeast Thailand. The effect of Na-Fe2+ and Na-Ca electrolytes on the CCC values is compared for sodium adsorption ratio (SAR) values ranging from 0 to 40 and total electrolyte concentration (TEC) ranging from 0.5 to 10 mmol L-1. The extracted day is a mixture of kaolinite and smectite but only the smectite and some poorly ordered kaolinite could be dispersed. The CCC values largely reflected the behavior of smectite, in agreement with previous studies. The CCC values were equal for electrolytes with Fe2+ or Ca2+ cations, suggesting that Fe2+ strongly adsorbs on smectite exchange sites and behaves similarly to Ca2+. The Fe2+ concentration needed to flocculate the clays was at maximum 0.6 mmol L-1 for the most dispersive electrolytes, which is a common concentration in flooded rice fields. Reductive dissolution of Fe can protect sodic soils from clay dispersion upon flooded rice cropping. [ABSTRACT FROM AUTHOR]

Capacitance probe sensors are a popular electromagnetic method of measuring soil water content. However, there is concern about the influence of soil salinity on the sensor readings. In this study capacitance sensors are calibrated for a saline silty clay soil. An electric circuit model is used to relate the sensor's resonant frequency F to the permittivity (ε) of the soil. The circuit model is able to account for the effect of dielectric losses on the resonant frequency. Dielectric mixing models and empirical models are used to relate the permittivity to the soil water content (θ). The results show that the electric circuit model does not fit the F-ε(θ) data if the calibrated bulk electrical conductivity (EC) model is used. The dielectric losses are overestimated. Increasing the exponent c in the tortuosity factor of the bulk EC model and thereby lowering the bulk EC and the dielectric losses improves the performance of the model. Measured and calculated volumetric water contents compare reasonably well (R² = 0.884). However, only 73 out of 88 data points can be described. The rejected points are invariably at high water contents where the high dielectric losses result in the sensor frequency being insensitive to ε(θ). [ABSTRACT FROM AUTHOR]

The fate of pig (Sus scrofa) slurry labeled with 15N was investigated when applied on a clay soil (fine, mixed, frigid, Typic Humaquept) and a sandy loam (loamy, mixed, frigid, Typic Dystrochrept) cropped to maize (Zea mays L.) in 2000 and to barley (Hordeum vulgare L.) in 2001. The slurry was applied in spring 2000, and plant and soil samples were collected at 6 h (Day 1) and at Days 14, 42, 96, and 413 after application. The samples were analyzed for 15N content in plant, in whole soil, and in soil NH4+-N, NO3--N, organic N, and clayfixed N pools. Percentage 15N recovery was >93% in both soils at Day 1 and decreased slowly thereafter. Rapid clay fixation of slurry 15 occurred at Day 1, and was greater in the clay soil (34% of applied 15N) than in the sandy loam (11%). At Day 96, less of the applied slurry 15N was recovered in maize grown on the clay soil (29%), as compared with the sandy loam (50%). At the same period, the residual soil 15N was mostly present as organic N and NO3-N in the sandy loam, and as organic and clay-fixed N in the clay soil. At Day 413, 15N recovery in barley was about 3% of the initially applied N in both soils. The 15N recovery was generally higher in the clay soil than in the sandy loam, but total 15N recovery from the soil-plant system was similar for both soils. We conclude that soil type had little influence on the total 15 recovery from the soil-plant system, but significantly influenced the fate of slurry N in the various soil pools and plant N uptake on the year of application. [ABSTRACT FROM AUTHOR]

Examines the effects of litter quality and soil clay content on carbon (C) and net nitrogen (N) mineralization rates in mineral soils. Comparison of aspen litter quality and pine litter quality; Release of soil C in pine and aspen soils; Microbial biomass under pine soil; Relationship between net N mineralization rates and species or soil mineralization rates.

Analyzes crystalline and amorphous phases of the clay fraction of agriculturally important Hawaiian surface soils using x-ray diffraction and Rietveld method. Quantification of the phyllosilicates; Estimation of amorphicity; Mineral phase quantification; Optimal spike addition; Rietveld refinement of soil diffraction patterns.

Investigates the tortuosity of crack networks in swelling clay soils. Derivation of the dependency of the tortuosity on the connectedness of a statistically isotropic crack network; Characterization of the crack network; Method for processing two-dimensional images of crack networks.

Investigates the small-scale solute transport in clay loam soil. Variability in convective-dispersive transport processes; Measurement of transport parameters; Variability in dispersion and mass recovery.

Investigates bromide leaching on a Piedmont toposequence. Soil properties associated with increasing clay content; Modification of the soil-based leaching classification system; Susceptibility of footslopes to leaching losses; Comparison of groundwater depth between foot and linear slopes.

Focuses on the development of a physically based probabilistic model which is used in predicting the crack network geometry, relating to the water content profile and shrinkage curve of the clay soil matrix. Importance of modeling the crack network geometry in Vertisols; What the simple changes in the two-dimensional case are converted to.

Presents a study which determined how clay water content might control the enhanced survival of bacterial Pseudomonas fluorescens (Migula) 2-79RN10 in the presence of various clays. Methodology employed to conduct study; Suspension of the bacteria in phosphate buffer solution; Determination of the bacterial colony-forming units, gravimetric water, and water activity; Implications of results.

Presents a study of the morphological and mineralogical modifications of clay particles which were observed in A horizons of soils developed under arid Sahelian conditions. Methods and materials used in the study; Results of the study; Details on clay mineralogy and clay chemistry; Discussion of the results.

Presents a model that describes physical protection explicitly as a function of the capacity of clay particles and aggregates to hold organic matter. Incubation experiment; Description of the model; Comparison between models of physical protection; Sensitivity analysis of models b and c.

Examines the measurement techniques for hydraulic conductivity in a clay soil. Spatial characterization; Effect of the magnitude and spatial distribution of the field-saturated soil hydraulic conductivity (Kfs); Relevance of the Guelph permeameter method for field measurement of Kfs.

A letter to the editor is presented in response to the article "Application and Evaluation of the SWAP Model for Simulating Water and Solute Transport in a Cracking Clay Soil."