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Crop N decision support tools are typically based on either empirical relationships that lack mechanistic underpinnings or simulation models that are too complex to use on farms with limited input data. We developed an N mineralization model for corn that lies between these endpoints; it includes a mechanistic model structure reflecting microbial and texture controls on N mineralization but requires just a few simple inputs: soil texture soil C and N concentration and cover crop N content and carbon to nitgrogen ratio (C/N). We evaluated a previous version of the model with an independent dataset to determine the accuracy in predictions of unfertilized corn (Zea mays L.) yield across a wider range of soil texture, cover crop, and growing season precipitation conditions. We tested three assumptions used in the original model: (1) soil C/N is equal to 10, (2) yield does not need to be adjusted for growing season precipitation, and (3) sand content controls humification efficiency (ε). The best new model used measured values for soil C/N, had a summertime precipitation adjustment, and included both sand and clay content as predictors of ε (root mean square error [RMSE] = 1.43 Mg ha−1; r2 = 0.69). In the new model, clay has a stronger influence than sand on ε, corresponding to lower predicted mineralization rates on fine‐textured soils. The new model had a reasonable validation fit (RMSE = 1.71 Mg ha−1; r2 = 0.56) using an independent dataset. Our results indicate the new model is an improvement over the previous version because it predicts unfertilized corn yield for a wider range of conditions. Core Ideas: An improved N mineralization model predicts unfertilized corn yield for a wide variety of conditions.The new model provides realistic estimates of microbial humification efficiency across a range of soil textures.Humification efficiency is affected more by soil clay content than sand content.The updated coefficients account for the influence of precipitation on corn yield.The improved model provides a foundation for site‐specific N fertilizer recommendations. [ABSTRACT FROM AUTHOR]

In‐depth knowledge about the impact of organic matter (OM) on soil water vapor sorption (SWVS) and associated hysteresis related to various water activity (aw) levels is limited. The objective of this study was to elucidate the effect of OM on SWVS and sorption hysteresis for different aw levels. We measured water sorption isotherms and physicochemical properties of eight soils with varying clay (6%–47%) and OM (2.0%–7.8%) contents before and after OM removal. In general, OM increased SWVS, but the effect of OM on SWVS varied for different aw ranges. The presence of OM generally increased water vapor sorption during the adsorption process. For soils with OM content above 4.6%, the presence of OM decreased water vapor sorption during the condensation process. The local hysteresis (Hw) of soils with OM was higher than that of soils without OM for soils with OM contents above 3.4%. For aw < ∼0.75, the water vapor sorption hysteresis of soils with and without OM showed a significant correlation with cation exchange capacity (CEC), and the Hw values of soils with OM were lower than that of soils without OM for the same CEC. The OM can affect water vapor sorption and the associated hysteresis at low aw levels through adsorbed cations and organic functional groups, and at high aw levels through the structures of pores and OM. Core Ideas: An increase in water vapor sorption due to OM mainly occurred during adsorption.A decrease of water vapor condensation due to OM is caused by a reduced pore volume.The OM content affects water vapor sorption hysteresis.The CEC is a key factor affecting sorption and hysteresis in soils with and without OM. [ABSTRACT FROM AUTHOR]

Soil particle density (ρs) is often assumed as 2.65 g cm−3 (density of quartz). The objectives of this study were to compare the use of different gases for determining ρs in a gas pycnometer and relate measured ρs to soil particle size distributions. The ρs of 36 natural soil samples representing 12 USDA textural classes, fine glass beads, crushed granite rock, kaolinite, and bentonite were measured by a commercial gas pycnometer using He, N2, and dry air. The ρs of 30 of the soil samples, glass beads, and crushed rock were also determined with a water pycnometer. The ρs of 36 soil samples determined by He and 30 samples determined by water had narrow ranges with averages of 2.65 and 2.59 g cm−3, respectively. The ρs determined by air and N2 had much wider ranges with averages of 2.93 and 2.97 g cm−3, respectively. There was a near 1:1 relationship between ρs of all samples determined by air and N2 with a highly significant (p <.001) correlation coefficient (r =.99). The average ratio of ρs determined by He and water was 1.03, but the correlation coefficient for their relationship was only.416. Although the relationship between ρs determined by He and either air or N2 was relatively strong (r <.61), the regression coefficient was <.17. There was a strong relationship between soil clay content and ρs determined by N2 or air but a weaker, yet statistically significant (p <.05) relationship when using He. Core Ideas: Average particle density of soils belonging to 12 textural classes determined by helium was 2.65 g cm−3.Average particle density measured by air and nitrogen were 2.93 and 2.97 g cm‐3, respectively.Helium, nitrogen, or air can be used to measure particle density of coarse‐textured soils.Only helium should be used to measure particle density of medium‐ and fine‐textured soils. [ABSTRACT FROM AUTHOR]

Soil erosion is a major constraint to agricultural sustainability. Land management that protects soil from water erosion can be assessed from the fraction of aggregates stable upon water immersion. Conservation management with minimal soil disturbance is known to enhance water‐stable aggregation, but how annual vs. perennial vegetative cover affects soil biological activity and aggregation from soils varying in texture remains relatively unexplored. This study assessed the effects of long‐term conventional‐till and no‐till cropland, grassland, and woodland on dry‐stable and water‐stable mean‐weight diameter (MWD) from 25 research stations across North Carolina. Soil clay concentration varied from 36 to 279 g kg–1 and sand concentration varied from 392 to 904 g kg–1 (5–95% distribution). Across locations, water‐stable MWD followed the order: conventional‐till cropland (0.64 mm) < no‐till cropland (0.87 mm) < woodland (1.12 mm) = grassland (1.15 mm). Soil stability index (i.e., water‐stable MWD/dry‐stable MWD) followed a similar order: conventional‐till cropland (0.59 mm mm–1) < no‐till cropland (0.76 mm mm–1) < grassland (0.92 mm mm–1) = woodland (0.96 mm mm–1). Although water‐stable MWD was positively associated with clay concentration (r =.50, p <.001, n = 310), soil‐test biological activity had the strongest association with water‐stable MWD (r =.80). Soil‐test biological activity reflected biological manipulation of surface‐concentrated organic matter, as mediated by conservation land uses. This study revealed the large impact of surface organic C and N resources on water‐stable aggregation through soil biological manipulations interacting across a textural gradient. Core Ideas: Conservation land uses of no‐till cropland, grassland, and woodland improved soil aggregation characteristics.Soil stability index discerned conservation land use impacts across a soil texture gradient.Soil aggregation characteristics were not different between grasslands and woodlands.Soil‐test biological activity and sand were strongly associated with soil aggregation characteristics. [ABSTRACT FROM AUTHOR]

This study developed a comprehensive database of soil physical properties to expand the potential applications of soil moisture observations across the Kansas Mesonet environmental monitoring network. The database comprises 14 site‐ and depth‐specific soil hydraulic properties for 40 stations monitoring soil water content. The measured soil hydraulic properties include sand, clay, and silt contents; bulk density; particle density; total porosity; effective saturation; saturated hydraulic conductivity; and water retention at six matric potentials. The soil database spans eight soil textural classes, with silty clay loam, silt loam, and silty clay soils dominating the fine‐textured soils, and sandy loam and sandy clay loam soils comprising the only coarse‐textured soils. All the measured soil hydraulic properties showed low coefficient of variation (CV ≤10%) within soil textural classes, except for the saturated hydraulic conductivity, which varied from a median of 0.520 ± 308 cm d–1 in the clay soils to 47.1 ± 119 cm d–1 in the sandy loam soils. The uncertainty of the Kansas Mesonet soil water content observations was reduced by at least 29% using laboratory sensor calibration equations instead of the factory default sensor equation. Our soil physical property database offers new prospects to use historical and near real‐time in situ soil water content observations across the Kansas Mesonet for diverse agricultural and hydrological applications, including drought monitoring, wildfire preparedness, and estimating potential drainage and groundwater recharge rates. Core Ideas: This study developed a comprehensive soil physical property database for the Kansas Mesonet.The database comprises 14 site‐ and depth‐specific soil hydraulic properties for 40 stations.The soil database is dominated by fine‐textured soils with 92% of the soils having >20% clay content.Laboratory sensor calibration equations reduced the uncertainty of the water content measurements by at least 29%. [ABSTRACT FROM AUTHOR]

Supplemental fertilizer N recommendations for flood‐irrigated rice (Oryza sativa L.) have been developed with tests that quantify plant‐available N within the effective rooting depth. Alkaline hydrolyzable‐nitrogen (AH‐N) soil test methods were recently used to successfully guide supplemental fertilizer‐N management decisions of drill‐seeded, delayed‐flood rice, stimulating interest in evaluating additional indicators of N availability such as soil total carbon (STC) and soil total nitrogen (STN). Our first objective was to evaluate the relationship among STC, STN, and AH‐N to a depth of 60 cm. Our second objective was to correlate STC and STN to rice N uptake and grain yield and develop calibration regression models that identified the yield maximizing N rate (FN 95%RGY). The STC and STN were significantly and positively related to AH‐N when clayey soils were sampled to >15‐cm depth and the coefficient of determination (r2) exhibited a range of.22–.70 for STN and.26–.75 for STC. Overall, STC and STN were significantly related to rice N uptake, unfertilized grain yield (Yield0N), or relative grain yield (RGY) and r2 values tended to be greatest at a 30‐cm depth or only nominally improve when sampling depth exceeded 30 cm. Calibration of STC and STN to predict FN 95%RGY was successfully completed, but all linear regressions had r2 <.40. The influence of sampling depth on the ability of STC and STN to serve as indicators of rice response to supplemental N fertilization demonstrates the utility of sampling within the rice plant's effective rooting depth. Core Ideas: Soil total C and N increased linearly with alkaline hydrolyzable‐N to a 60‐cm depth.Soil total C and N explained <40% of the variability exhibited by rice response parameters.Soil total C and N from >30‐cm depth did not improve prediction of the rice grain yield maximizing fertilizer N rate.Correlation and calibration of soil total C and N to fertilizer N response of rice should consider the extent of rooting depth. [ABSTRACT FROM AUTHOR]

Understanding the variables that affect the anaerobic potentially mineralizable N (PMNan) test should lead to a standard procedure of sample collection and incubation length, improving PMNan as a tool in corn (Zea mays L.) N management. We evaluated the effect of soil sample timing (preplant and V5 corn development stage [V5]), N fertilization (0 and 180 kg ha−1) and incubation length (7, 14, and 28 d) on PMNan (0–30 cm) across a range of soil properties and weather conditions. Soil sample timing, N fertilization, and incubation length affected PMNan differently based on soil and weather conditions. Preplant vs. V5 PMNan tended to be greater at sites that received < 183 mm of precipitation or < 359 growing degree-days (GDD) between preplant and V5, or had soil C/N ratios > 9.7:1; otherwise, V5 PMNan tended to be greater than preplant PMNan. The PMNan tended to be greater in unfertilized vs. fertilized soil in sites with clay content > 9.5%, total C < 24.2 g kg−1, soil organic matter (SOM) < 3.9 g kg−1, or C to N ratios < 11.0:1; otherwise, PMNan tended to be greater in fertilized vs. unfertilized soil. Longer incubation lengths increased PMNan at all sites regardless of sampling methods. Since PMNan is sensitive to many factors (sample timing, N fertilization, incubation length, soil properties, and weather conditions), it is important to follow a consistent protocol to compare PMNan among sites and potentially use PMNan to improve corn N management. [ABSTRACT FROM AUTHOR]

Knowledge about clay fraction is important for soil management. Laboratory analysis is time consuming, however, because information is required at the topsoil (0–0.3 m), subsurface (0.3–0.6 m), and subsoil (0.9–1.2 m). Herein, we developed digital soil mapping (DSM) of clay by coupling limited laboratory datawith proximal sensed data (i.e., elevation, γ-ray, and electromagnetic). Our first aim was to determine whether individual or combined data were better for modeling using various models (i.e., regression kriging [RK], linear mixed model [LMM], and a Bayesian model named integrated nested Laplace approximation [INLA] with stochastic partial differential equation [SPDE]). We also evaluated DSM of clay and associated errors at different depths using various indices (e.g., Lin’s concordance). Digital soil mapping performance was compared by mean square prediction error (MSPE). Considering proximal data, the Akaike information criteria and Log-likelihood showed that RK and LMM performed best by combining proximal data, whereas for INLA-SPDE individual and combined data performed equally well. Considering prediction uncertainty, RK error was larger where predicted clay was larger, whereas LMM and INLA-SPDE had larger errors associated with texture or field boundaries. With increasing depth, model performance decreased, as exemplified by LMM and for topsoil (Lin’s = 0.79), subsurface (0.72), and subsoil (0.71). The MSPE showed that the DSM map for topsoil clay was best for LMM (28), followed by INLA-SPDE (29) and RK (39). The LMM DSM was also best in the subsurface (37) and subsoil (84); however, RK was superior to INLA-SPDE. [ABSTRACT FROM AUTHOR]

Alkaline clayey soil is considered to be a degraded soil, which causes serious issues in engineering and construction projects. The formation and development of desiccation cracks in alkaline clayey soils is a complex process affecting the strength, stability, and permeability of these soils. Therefore, to investigate how soda content can inhibit the development of desiccation cracks in alkaline clayey soils in Ulanhot, Inner Mongolia, China, free desiccation tests were performed to examine the cracks in alkaline clayey soil samples with different soda (Na2CO3) contents in the laboratory. During the cracking process, the changes in water content and the occurrence and evolution of cracks were recorded with a camera. Digital image processing was performed to analyze the structure characteristics of the surface cracks. The results indicated that the characteristics are influenced by the different soda contents. The soda inhibited the development of desiccation cracks in alkaline clayey soils. Three stages were found in the process of desiccation cracking in accordance with the rate of water loss: the residual, normal, and structural shrinkage stages. At <0.4 mol L-1 Na2CO3, the "three straight lines" model could be used to describe the volume shrinkage. However, at >0.4 mol L-1 Na2CO3, the structural shrinkage stage disappeared. Moreover, based on the general soil volume change equation and fractal theory, a model was formulated to quantitatively explain the relationship between the fractal dimension and the volume changes in the soil. The experimental results and equations are important for reducing subgrade deformation and slumps.

There is a need for improved knowledge of the limits to the available water range for root growth in the subsoil. The objective of this study was to recalculate the upper and lower limits of the least limiting water range (LLWR) concept by using respectively the air-filled porosity (εa) at which 0.005 of the relative gas diffusivity (Ds/Do) is reached and readily available water (RAW). The refined upper limit estimates the variation in εa related to pore connectivity and the refined lower limit expresses the boundary at which plants suffer physiological water stress. This study was based on soil sampled in compaction trials on two sandy loam soils. Soil samples were taken from plots with no compaction (Control), and compaction with 78 kN (M8) and 58 kN (M6) wheel loads with multiple wheel passes. The soil cores were analyzed for εa, Ds/Do, bulk density (rb) and penetration resistance (PR). Heavy farm machinery impact of M8 and M6 led to subsoil compaction up to depth of 0.5 to 0.7 m for the soils under study. The subsoil structure was affected by compaction across depths with the decrease in εa (~33-46%) and Ds/Do (~37-61%) and increase in rb (~4-8%) and PR (~40-50%, at -100 hPa at 30-cm depth). The refined LLWR showed a wider water range compared to the original approach. We anticipate that the refined LLWR well reflects the limiting soil physical conditions for root growth for the studied soils, but validation by combined soil physical and plant growth measurements is needed.

Improving mineralizable N estimates from exceptional quality (EQ) biosolids products is important for making more reliable N recommendations to rehabilitate disturbed urban soils for vegetative production. The objectives of this study were to compare the N fertilizer equivalency method and several chemical (NH4–N, NO3–N, total N, and organic N) and biological (7 d anaerobic incubation [7-AI] and Solvita CO2 Burst) tests for quantifying plant available nitrogen (PAN) and organic N mineralization of EQ biosolids used to grow tall fescue (Schedonorus arundinaceus Schreb.) in a clayey urban degraded soil. The EQ biosolids were products of thermal hydrolysis plus anaerobic digestion (Blue Plains exceptional quality biosolids; BLOOM), blending of BLOOM with woody mulch (BM), blending of BLOOM with sand and sawdust (Blue Pains exceptional quality biosolids + sand + sawdust; BSS), composting (Livingston compost; LBC), and heat drying (OCB; OceanGro thermally dried). The EQ biosolids were applied at agronomic N rates, and the inorganic fertilizer was applied at four N rates in a field study. The N fertilizer equivalency method predicted considerably lower organic N mineralization than calculated for BM (7.1%), BSS (–12%), and LBC (4.6%). Treatments BLOOM, BM, and OCB had the highest 7-AI organic N mineralization. Correlations between tall fescue N uptake and soil N tests showed that soil NO3–N and 7-AI were the best indicators of biosolids N availability in our urban soil. However, the relatively low correlations between soil N indicators and tall fescue N uptake was likely due to low residual soil N, high soil clay content, and possible low microbial activity of the low organic matter-containing anthropogenic soil. [ABSTRACT FROM AUTHOR]

To characterize the effect of precipitation events, management practices, and soil type on vineyard carbon (C) dynamics, we monitored CO2 emissions and labile C pools from nine vineyards in Lodi Wine Grape District, California, from April 2011 to December 2012. These commercial vineyards are replicates of three soil series (Redding, San Joaquin, and Tokay), representing a spectrum of soil texture and degree of soil development. We hypothesized that soil characteristics would influence the magnitude of CO2 efflux occurring in response to precipitation and management events in a Mediterranean climate. During each field visit—bimonthly (April–October) and monthly (November–March)—we measured CO2, soil temperature, and gravimetric water content (GWC) from vine and intervine (alleys) rows. Monthly, we collected soil samples for dissolved organic C (DOC), which tended to be greater in the alleys of San Joaquin and Redding than Tokay in summer but decreased after the onset of precipitation. In mid-May and mid-October 2012, CO2 efflux was higher in Tokay than in San Joaquin or Redding. Carbon dioxide efflux across all soils increased as a result of seasonal management practices (i.e., tillage and mowing of cover crops). Management practices distinguished soil DOC between vine rows and alleys from June to October 2012. Soil type or clay content influenced CO2 efflux across these vineyards, as did GWC and soil temperature. This 20-mo study indicated that CO2 efflux responded to soil disturbance from management practices, precipitation, and irrigation. [ABSTRACT FROM AUTHOR]

Coastal Plain soils of the southeastern United States are highly degraded, with low soil organic carbon (C), meager soil fertility, and poor soil structure. We tested the effectiveness of clay soil amendment to promote the formation of soil aggregates, improve organic C stability, and enhance nutrient availability in field. Subsurface clay soils (B horizon, 25% clay) were applied (∼25 T ha–1) to the top of the soil surface, followed by the planting of winter cover crop mixtures. Crop growth and nutrient uptake and relevant soil biogeochemical properties were analyzed at the end of the winter season. Additions of subsurface clay soils prompted the formation of water-stable macroaggregates (250–2000 mm) from 32 to 54% and increased mean weight diameter of the aggregates from 390 to 592 mm. Additions of subsurface clay soils increased clay content from 1.5 to 3.8% but did not change soil texture, pH, water holding capacity, and bulk density. The stock of dissolved organic nitrogen was not affected by the amendment, whereas nitrate leaching from field was reduced by 61% because of clay soil application. Clay soil additions inhibited the activities of C-cycling (N-acetyl-β-glucosaminidase, β-d-cellubiosidase, β-glucosidase, and β-xylosidase) and N-cycling (leucine aminopeptidase) enzymes but did not modify soil microbial community level substrate utilization patterns, resulting in a 63% reduction in C turnover rate and 19% increases in concentrations of labile organic C. Our results indicated that subsurface clay soil amendment can improve soil structure, nutrient availability, and the stability of labile C in the field, at least in the short term, and may be a promising management alternative to enhance the productivity and sustainability of the degraded Coastal plain soils. [ABSTRACT FROM AUTHOR]

The impacts of soils’ heterogeneity due to temporal (ET)- and seasonal (ES) changes from minute variations in precipitation (as rain, ice, and snow) and air temperatures (P+TA) on the ground-penetrating radar (GPR) responses (RGPR) are not rigorously tested. We aimed to understand how environmental changes (ET and ES) can be accountable to alter RGPR and help relate to real-time (TR) soil environmental status. Soil moisture (volumetric water content, VWC), temperature (TS), and electrical conductivity (EC) in every 30 min were recorded by soil sensors daily in different seasons to match the diurnal and seasonal changes. Readings of GPR alerts from six soil types of various GPR-suitability classes were recorded two to three times daily. Changes in P+TA, which changed physicochemical soil properties (PCSP) temporally and impacted RGPR up to 76%. The RGPR, which was negatively corelated with clay (R = –0.89) and organic matter (OM) (R = –0.91) percentages, remained low in soils when cation exchange capacity (CEC) is also high. The RGPR which were pronounced after rain, during fall, in sandy soils, and in case of neutral to basic pH impacted by narrow-range VWC variations on sandy but broad range on clay soils. Soils of same clay percent and similar OM acted differently if they have differential CEC and pH values. Ninety-six percent of RGPR, which is impacted by electric resistances and bonding nature of water and soils, were explainable if interactions of clay, OM, and VWC at TR are considered. Future insights on influences from freeze–thaw cycles and specific silica-mineral types of soils are required. [ABSTRACT FROM AUTHOR]

Understanding the spatial variability of soil texture within field soils is important due to its influence on a large number of soil and plant related processes and for site-specific application of inputs that are crucial to crop production. It remains a problem to obtain a reliable clay content map based on a limited number of sampling locations. The objective of this study was to identify spatial variability of soil clay content and the behavior of the estimation result for different spatial resolutions of measured clay content (0–20 cm depth) in combination with a coregionalization approach using apparent electrical conductivity (ECa). In a silty loam soil, soil clay content was measured at 96 points in a 50-m by 50-m grid within an agricultural field. ECa was measured using a contact sensor Veris 3150. Data were analyzed with ordinary kriging and cokriging while using ECa at a shallow depth. We analyzed different sampling scenarios based on clay subsamples of 48, 24, and 12 data points distributed over the 27-ha field. In all scenarios investigated here, the RMSE stayed in the range of 3 to 4% by using different validations, with cokriging performing constantly better than ordinary kriging. Clay content maps estimated with cokriging maintained a satisfactory precision when the sampling density was reduced to one sample per two hectares, a result that leads to the conclusion that electrical conductivity in combination with spatial coregionalization demonstrated to be a promising tool to estimate the spatial variation of clay content even at a low clay sampling density. [ABSTRACT FROM AUTHOR]

Biochar amendment can change the soil pore architecture and improve soil physical and hydraulic properties. In this study, two clayey soils were selected and treated with or without biochar amendment (four treatments). Synchrotron-based X-ray micro-computed tomography (SR-mCT) was used to characterize pore structures of the selected aggregates from the treatments. Based on the high-resolution soil pore structure information, the three-dimensional lattice Boltzmann (LB) method was used to simulate water flow in pores. The highresolution soil pore structures were also used to define the internal boundary conditions in the LB method that was accelerated by graphics processing unit parallel computing. Results showed that biochar amendment increased soil permeability by at least one order of magnitude, reduced tortuosity by 20 to 30%, and resulted in more uniformly distributed pore water velocities, which was attributed to the change in soil pore geometry. This study is the first physically based modeling effort to assess the effect of biochar amendment on hydraulic properties of soil aggregates. The SR-mCT method and modeling approach developed in the research play an important role in understanding the mechanisms of water and nutrient transport in soil at the pore scale. [ABSTRACT FROM AUTHOR]

The extractable soil-K concentration, used for fertilizer-K recommendations, may be affected by soil drying. Although air or oven drying are the most common soil processing methods, K from field-moist soil has been documented to be a better predictor of soil-K availabilities and fertilizer-K needs for soybean [Glycine max. (L.) Merr.] grown on fertile silt loam to clayey soils. We evaluated the effect of four soil processing methods (field-moist [FM], air-dry [AD], air-dry followed by oven-dry [ADOD], and oven-dry [OD]) in predicting extractable soil-K availability for soybean production on less fertile Mid-Atlantic sandy-textured soils. Twelve soybean field trials were conducted in 2014 on Coastal plain and piedmont soils in Virginia and North Carolina. Soil K was extracted by Mehlich-1 with each soil processing method and correlated with soybean relative yield. Soil-K concentrations from each method were statistically similar and strongly correlated (r² = 0.94-0.98) with each other having intercept and slope coefficients that were not different from zero and one, respectively. Extractable soil-K concentrations from each method were also strongly correlated with soybean relative yield and explained 93 to 95% of the relative yield variation for FM soil, 95 to 96% for AD soil, 83 to 86% for ADOD soil, and 94 to 95% for OD soil. Results suggest that soil-K concentrations from FM, AD, and OD samples are similar in predicting K availability for soybean. Soil drying should not be an issue of concern in extracting soil K and recommending fertilizer-K rate for soybean production on Mid-Atlantic coarse-textured Coastal Plain and Piedmont soils. [ABSTRACT FROM AUTHOR]

Liming represents a common agricultural practice for abating soil acidity. Nevertheless, elevated amounts of agricultural lime in Oxisols, with or without cultural residue addition, could alter soil physicochemical properties and impact soil structure. In this context, the physical, chemical, and structural behaviors of an Oxisol under the addition of lime and straw were assessed. Lime doses (0, 3.9, 7.8, and 15.6 Mg ha-1) were either applied on the surface or incorporated into the 0- to 5-cm soil layer. Straw applications followed the same procedure with quantities of 0, 4, 12, or 16 Mg ha-1. The effects on the soil profile were evaluated through physicochemical (specific surface area, ς potential, and pH), physical (density, penetration resistance, water dispersible clay, and total porosity), and micromorphological properties (surface area, volume, connectivity, size, and pore anisotropy) 1 yr after soil incubation in polyvinyl chloride cylinders 30 cm long and 14.5 cm in diameter. Lime application on the surface or into the 0- to 5-cm layer increased soil pH to values above 7.0 and the electronegative potential of soil colloid surface, promoting clay dispersion. Water-dispersed clay migrated in the soil profile, causing pore obstruction, and higher soil density and penetration resistance. In addition, excessive lime decreased specific surface area, anisotropy degree and the connectivity of the soil pore system. Straw addition promoted a slight increase of colloids electronegative potential but did not alter soil physical properties. [ABSTRACT FROM AUTHOR]

Data of soil thermal conductivity (λ) covering the whole range of soil water content (ϑ) are needed in many environmental disciplines. However, laboratory measurements of λ(ϑ) are time consuming and costly. Therefore the use of pedotransfer functions is promising. In this study, we describe the procedure of deriving such λ(ϑ) pedotransfer functions for the FAO texture groups (TG) of Sand, Silt and Loam. First, high-resolution λ(ϑ) measurements from saturation to dryness of 43 soils were conducted by two laboratory methods: the evaporation approach and single-point measurements of λ at distinct water contents. Measurements were obtained for a set of sandy, silty, loamy, and clayey soils, and for two urban soils and two peat soils. The slope and magnitude of the λ( ϑ) curves show typical dependencies on bulk density (ρb) and soil texture. In a second step, the λ(ϑ) data were used to evaluate the λ(ϑ) model as suggested by Lu et al. (2014). Fitting this model to the complete data set yielded a poor agreement with RMSE = 0.53 W m-1 K-1. However, satisfying results were obtained by dividing the data into TGs Sand, Silt and Loam resulting in RMSEs of 0.18, 0.11 and 0.24 W m-1 K-1, respectively. Estimated λ values at low soil moisture (wilting point) were hardly different among texture classes, but λ differences become highly relevant when soil moisture increased to field capacity. The derived pedotransfer functions are an easy-to-use approach providing reliable λ(ϑ) curves for sandy, silty and loamy soils, when only basic soil information (texture and bulk density) are available. [ABSTRACT FROM AUTHOR]

Models that reθuire inputs of soil texture are often limited by the availability of soil information. While texture class is easier to obtain than particle size, texture classes do not represent the continuum of soil size fractions. Soil texture class and clay percentage are collected as a standard practice for many land management agencies (e.g., NRCS, BLM, FAO) and clay content is freθuently estimated with acceptable accuracy (±5%). When clay and texture class is known, sand and silt can be constrained to a narrow range that may differ from the geometric centroid of a given texture class (gcent). I tested the concept of a modified centroid approach (mcent) using 75,736 soil samples from the National Cooperative Soil Survey pedon database that also had measured hydraulic soil properties. Comparisons were made using the Rosetta pedotransfer function (PTF) to test modeled values from gcent, mcent and measured data in comparison with measured values of water content at field capacity (θ330) and wilting point (θ15000). The mcent approach produced a continuous distribution of values for sand and silt whereas gcent produced a stair-step pattern for each of the 12 texture classes. Rosetta underestimated water content. Error metrics were similar when all texture classes were combined, but differences between models emerged when individual texture classes were compared. Results support the practice of collecting field estimates of clay percentage along with texture class to expand the use of soil profile data for both spatial and non-spatial modeling of soil processes that will advance our understanding of soil contributions to ecosystem function. [ABSTRACT FROM AUTHOR]

The zeta potential at the shear plane is an important electrochemical parameter at solid-liquid interfaces. However, it is difficult to accurately determine the position of the shear plane because a rather important effect- strong ion polarization in the electrical double layer-is neglected. In the present study, a model for estimation of the shear plane position considering ion polarization was developed based on the modified Poisson-Boltzmann equation, and the intensity of polarization on a constantly charged surface was quantified through an ion exchange equilibrium experiment. The polarization effects increased the effective ionic charges of Na+ from 1 to 1.082 and that of K+ from 1 to 1.498 at the montmorillonite-water interface, which resulted in decreasing the thickness of the slipping layer. The theoretical prediction of the swelling pressure was in excellent agreement with the experimental measurements. The theoretical standpoint also sheds new light on one of the oldest puzzles in physical chemistry-specific ion effects. [ABSTRACT FROM AUTHOR]

A rapid and inexpensive method to evaluate soil structural stability via assessment of soil clay dispersion is needed. Such a method would provide critical information for decision-making for soil and agricultural management not currently undertaken due to the expense of testing. The aim of this work was to validate the use of a turbidimeter for quantifying clay released from soil and to relate this back to dispersive parameters. Ten Australian soils with different physicochemical properties were studied. Six-step dilution was used to adjust the clay suspensions sequentially by dilution from a datum concentration of 4000 nephelometric turbidity units (NTU). The linearity of turbidity by dilution was verified, considering the effects of soil color and mean particle size. There was a strong correlation between turbidity (NTU) and dispersed clay (mg L-1, % of soil) for each soil. Clay particle size was related to turbidity response, although the coefficients between turbidity and dispersed clay were very close irrespective of mean particle size differences. Dispersed clay was concluded to be accurately quantified using a turbidimeter after spontaneous and mechanical dispersion. A semi-quantitative dispersion assessment chart was developed for rapid assessment of soil structural stability, the application of which is intended to provide rapid diagnosis and to act as an important index for soil management practitioners. Additionally, when dispersed clay was combined with electrical conductivity and clay content parameters, the dispersive indices (cation ratio of soil structural stability and exchangeable dispersive percentage) were reasonably predicted at a resolution useful for farm management decisions. [ABSTRACT FROM AUTHOR]

See Parker et al., "A database of soil physical properties for the Kansas Mesonet", https://doi.org/10.1002/saj2.20465. B On the cover b : Nathaniel Parker collecting undisturbed soil samples from the top 50-cm soil profile at the Clay Center station of the Kansas Mesonet. [Extracted from the article]

The increased use of soil moisture retrieval from satellites has heightened the need for improved accuracy of point measurements that are used to validate remotely sensed soil moisture products. A wide range of devices can be installed for operational monitoring of soil moisture; however, many of these devices have not been tested in situ in soils with a very high reactive clay content. The objective of this study was to evaluate the accuracy in field performance of five soil moisture sensors: the enviroSCAN probe, the Diviner 2000 (both from Sentek Technologies), the Hydra Probe soil sensor (Stevens Water Monitoring Systems), the Thetaprobe ML2x (Delta-T Devices), and the ECH2O EC-5 (decagon devices) in soils that had about 71% clay content. The instruments' default calibrations were tested against observed soil moisture from core samples using the thermogravimetric method. New calibration equations were developed for each device, which were evaluated using an independent data set. The ThetaProbe had the lowest root mean square error (RMSE) of 0.025 m³ m-3 and mean bias error (MBE) of 0.002 m³ m-3 in the precalibration analysis. Although the Hydra Probe showed the highest precalibration errors, the instrument made the greatest improvement in post-calibration analysis, with an RMSE of 0.129 m³ m-3 using the default equation reduced to 0.014 m³ m-3 using in situ calibration, and the 0.110 m³ m-3 MBE was reduced to 0 after applying in situ calibration. [ABSTRACT FROM AUTHOR]

Research has demonstrated that animal manure may increase soil test P (STP) differently than inorganic fertilizer when applied at the same total P rate. The exact mechanisms controlling the increase in STP after manure application are not well understood. In this study, 40 manure samples [beef and dairy (Bos taurus), goat (Capra aegagrus hircus), sheep (Ovis aries), horse (Equus caballus), turkey (Melleagris galloopavo), chicken (Callus domesticus), and swine (Sus scrofa domesticus)] were applied to 23 agricultural soils at a total P rate of 40 mg kg-1 and incubated for 70 d along with an untreated control. The total P in the manures ranged between 2.8 to 48.7 mg P kg-1 of which 28 to 96% was inorganic P (Plt) and 4 to 28% was enzymatically hydrolyzable P (Pet). The initial soil Bray-1 P ranged from 14 to 69 mg kg-1. For 19 soils, the STP after manure application increased linearly as either the applied manure Plt increased or the applied manure Plt + Pet increased. Hydrolysis of Pet was related to soil clay content. Soils with clay content greater than 121 g kg-1 seemed to provide physical protection against Pet hydrolysis. In contrast, soils with clay content lower than 71 g kg-1 appeared to provide conditions that were suitable for manure Pet hydrolysis. The role that soil clay content may play a role in influencing how manure changes STP has not previously been reported. [ABSTRACT FROM AUTHOR]

Anaerobically digested manure (ADM) is a nutrient-rich coproduct from manure-based biogas plants. The ADM is often separated into a solid fraction, known as solid digestate (SD), and a liquid fraction, which could be applied to cropland at rates equivalent to those for raw manure. Application of SD could result in different nitrogen (N) mineralization patterns compared to raw manure due to biophysicochemical changes occurring during anaerobic digestion. The objective of this laboratory incubation experiment was to characterize changes in the availability of N in a Dark Brown Chernozem (Typic Haploboroll clay loam) and a Black Chernozem (Typic Haplocryoll silty clay) amended with raw beef cattle (Bos taurus) feedlot manure, SD, pelletized SD (PSD), and urea plus monoammonium phosphate (UMAP) at rates equivalent to 400 and 800 mg N kg-1 soil. Across rates, net mineralized N concentrations after 70 d in soils amended with UMAP, manure, SD, and PSD were 44, 18, -5, and -13% of added N, respectively, in the Dark Brown Chernozem and 45, 29, 6, and -18%, respectively, in the Black Chernozem. The lower N mineralization potential of SD and PSD compared with manure implies that anaerobic digestion alters the nutrient release patterns of manure and suggests that these digested manure forms may have to be either applied at higher rates compared with raw manure or should be supplemented with inorganic N fertilizer to ensure adequate N supply for plants. [ABSTRACT FROM AUTHOR]

A review of the McNeal clay swelling model, including its underpinning assumptions and the method for determining its parameters, was undertaken. Limitations of the model include model parameter identification, the fixed threshold exchangeable sodium percentage level at which hydraulic conductivity begins to decline, and the assumption that the expanding clay will always be 10% of the soil. To overcome these problems, an improved form of the model that accommodates a range of clay content is proposed. The modified model was used to estimate the reduction in saturated hydraulic conductivity in the three McNeal groups of soils containing different percentages of expanding clay. The model produced good agreement between the observed and estimated saturated hydraulic conductivities over the entire range of irrigation salinity (3-200 mmolc/L) and sodicity used for the original swelling model. The modified model was also successfully validated against three new sets of relative hydraulic conductivity data. The generalization of interlayer clay swelling in the modified model is more appropriately described as determining the dispersive potential of a given solution, rather than physical interlayer swelling. Consequently, this modified model more accurately predicts the reduction in saturated hydraulic conductivity as a function of the sodicity and salinity of the applied water. [ABSTRACT FROM AUTHOR]

Soil productivity and other soil functions are dependent on processes in the untilled subsoil. Undisturbed soil cores were collected at the 0.3- to 0.4-m depth from a heavy clay soil in Finland subjected to a single heavy traffic event by agricultural machinery three decades before sampling. Untrafficked control plots were used as a reference. Computed tomography (CT) scanning was performed on soil cores at a field-sampled field capacity water content. Gas diffusion and air permeability were measured when the soil cores were drained to -1000 hPa matric potential (air permeability also at -100 and -300 hPa). The air-filled pore space was measured with an air pycnometer and also calculated from mass balance and CT data. Gas diffusion and air permeability were also measured on a straight model tube and on autoclaved aerated concrete. The compaction treatment had not influenced soil total porosity but had significantly lowered the volume fraction of air-filled macropores at the matric potentials investigated. The compacted soil displayed significantly lower air permeability, while gas diffusivity was not affected. Our analyses indicate that this was due to a compaction-induced reduction in the cross-sectional area of vertical, arterial macropores and in the volume of marginal pores branching from these vertical pores. We observed non-Darcian air flow during air permeability measurements and thus suggest the use of a nonlinear regression technique based on measurements at several pressure differences to arrive at true Darcian air permeability. The tests on artificial materials supported the conclusions that the dominating pores in this clayey subsoil are nearly straight, vertical macropores. [ABSTRACT FROM AUTHOR]

This study examined the utility of nonlinear regression as a means of developing a statistically-based function for the estimation of particle density (Dp) of clay-rich mineral soils where the <0.002 mm fraction was mineralogically diverse. Physical and chemical soil property data (including semi-quantitative clay mineralogy) were analyzed for 54 mineral soil horizons sampled from 18 soil profiles at seven different locations across North America. Plausible estimates of particle density for the mineral component (2.72 Mg m-3), and particularly for the humic component (1.48 Mg m-3), were obtained (r² = 0.856, root mean square error [RMSE] = 0.035 Mg m-3 P < 0.0001) even though the calibration data set had a limited range of soil organic matter content (<23%). Particle density was shown to vary with clay content (P < 0.05), and Dp of different mineral particle-size fractions could also he distinguished statistically. The convenient and traditional assumption of Dp = 2.65 Mg m-3 for silicate-based soils in the absence of Dp measurements is challenged, given the considerable observed variability in Dp with changes in soil organic matter content and texture. [ABSTRACT FROM AUTHOR]

Sediments transported from agricultural fields in surface and subsurface waters contain particulate P (PP) that could be partitioned into two pools--one available for aquatic organisms including cyanobacteria, namely bioavailable particulate P (BAPP, determined by 0.1 mol L-1 NaOH extraction), and the remainder not bioavailable (non-BAPP). This study aimed to quantify the PP and BAPP concentrations in surface runoff and tile drain- age water from eight agricultural fields with clay and sandy soils in the Missisquoi Bay region of Quebec, Canada. Particulate P in surface and drainage water varied spatially (among fields) and temporally, with concentrations as high as 3181 µg P L-1 in surface runoff and 1346 µg P L-1 in tile drainage. About 30% of PP was BAPP regardless of the drainage pathway. The PP and BAPP concentrations were related linearly (R² = 0.86) to total suspended solids (TSS) in fine (0.05-1-µm) and coarse (1-100-µm) fractions. About 68% of the PP in clay soils and 50% of the PP in sandy soils were associated with the 0.05- to 1-µm size particles, which had more BAPP, on average 0.46 g P kg-1 than the coarser 1- to 100-µm fraction (0.22 g P kg-1). Soil parameters such as Mehlich-3 extractable (M3) P, M3Fe, and the degree of soil P saturation were related to the PP and BAPP concentrations in TSS and particle size fractions. We concluded that sediments with particle size <1 µm contained more BAPP and their loss from agricultural fields could contribute to the eutrophication downstream. [ABSTRACT FROM AUTHOR]

The velocities of shear waves in three different soils were measured as a function of consolidation using a Bishop and Wesley triaxial cell. We considered the soil deforming plastically on the normal consolidation line as well as elastic deformation on the rebound curve. In both cases, the soils were saturated and subjected to an isotropic stress environment. The relationships between the velocity of a shear wave and effective stress were similar for the sand and silt soils, but for clay soils the velocity of a shear wave was smaller, showing a soil type dependency. Additionally there was no unique relationship between shear wave velocity and effective stress for soils during plastic deformation (on the normal consolidation line) and elastic deformation (on the rebound elastic curve). By combining empirical relationships between shear wave velocity as a function of void ratio and effective stress with the normal consolidation curve, we were able to collapse the data from three soils onto a common curve irrespective of whether the soil was subjected to elastic or plastic deformations. [ABSTRACT FROM AUTHOR]

Synthetic N fertilizers are a major source of N2O emissions from soil. A field experiment was conducted during three growing seasons (2004-2006) on a clay soil (fine, mixed, frigid Typic Humaquept) under corn (Zea mays L.) to evaluate the impact of N fertilizer source and application rate on N2O emissions. Treatments consisted of three sources of N fertilizer (urea-NH4NO3 32% [UAN], Ca-NH4NO3 [CAN], and aqua NH3 [AA]) at four different rates (0, 100, 150, and 200 kg N ha-1). Fertilizers were banded 5 cm below the soil surface between corn rows at the six-leaf stage and N2O emissions were measured weekly. For all 3 yr of this study, cumulative N2O emission decreased in the order UAN = CAN = AA. Averaged across years, fertilizer-induced post-sidedress emissions were greater than the Intergovernmental Panel on Climate Change default factor (0.01 kg N2O-N kg-1 N), with values of 0.038, 0.033, and 0.027 kg N2O-N kg-1 N for UAN, CAN, and AA, respectively. The N2O emissions increased linearly with N rate, even at rates exceeding the optimum level for grain yield. Peaks of N2O flux occurred on the days following fertilizer application and in early fall when the soil was re-wetted. Emissions of N2O were higher at water-filled pore space >0.57 m3 m-3 and were also related to soil inorganic N and water-extractable organic C contents. Our results confirm that N fertilizer source and application rate can impact N2O emissions but these effects are modulated by soil environmental conditions. [ABSTRACT FROM AUTHOR]

Land-use history is often overlooked when assessing soil fertility of intensive cropland production systems. The unusually high organic carbon (OC) content of many sandy cropland soils in Northwestern Europe is unexpected given their general low clay content (3-8%) and organic matter (OM) input typical of cropland, but appears to be related to historical heathland land-use. Clay fraction OM composition was compared between two groups of sandy cropland soils with (HC) or without (CC) a history of heathland/forest land-use. Light (1.6-2.2 g cm-3) and heavy (>2.2 g cm-3) clay fractions in HC soils were nearly twice as rich in OC (on average 199 g kg-1) compared with those of CC soils (on average 109 g kg-1). The hypothesized preferential presence of stable heathland derived OM in light soil fractions, was not supported by our data. Pyrolysis-field ionization mass spectrometry of the clay fractions revealed a more decomposed character of OM in the CC soils and lasting long-term influence of land-use history on SOM composition. This could be concluded from higher proportions of lipids and sterols, a lower thermostability in the HC compared with the CC soils, and enrichment of alkylaromatics and heterocyclic N-containing compounds in the latter. The density fractionation methodology separated organic-mineral particles with similar OM loadings but lower proportions of sterols and medium to long-chained lipids in the heavy compared with the light clay fraction. Given the very high clay OC loadings (6-16 mg C m-2) and low binding capacity of the quartz/kaolinite/mica dominated clays, we hypothesize that OM-OM interactions are involved as an OM stabilization mechanism. However, contrary to our hypothesis high clay OC loading (and hence thick OM layering) were found in all sandy croplands regardless of land-use history or density fraction. [ABSTRACT FROM AUTHOR]

Two standard tests for soil stability in water involve assessment of the readily dispersible clay (RDC) content of predried soil samples. It is shown that this predrying limits the applicability of the tests to very dry soils. Ways to avoid this limitation are developed. Undisturbed soil samples were collected from the 10-to 15-cm depth of arabic topsoils at Villamblain and Faux Perche and from the 0- to 5-cm depth from plowed and direct-drilled plots at Boigneville. These soils in north-central France were collected from the field when moist (near field capacity). A range of water contents and corresponding pore water potentials (or stictions h) was produced by slow drying in air. For moist soils at field capacity (h = 100 hPa) the amount of RDC was large. Readily dispersible clay decreased progressively with increasing antecedent suction, h. For soil samples dried to beyond h > 10 MPa, the RDC was reduced to small values. With such intensely dried soils, rewetting for up to 2 wk did not reverse the effect on RDC. At h = 4500 hPa, re-equilibration to h = 100 hPa reversed the effect of this moderate drying on RDC. Two standard conditions (moist and dry) are proposed for measurement of clay dispersion from soil into water. The moist condition is relevant to soil at the end of along, wet period (e.g., at the end of the winter in temperate climates), whereas the dry condition is relevant to dry surface crusts and dry topsoils. We conjecture that the soil "resets" during long, wet periods to values of stability and RDC at equilibrium with field capacity. Experimental procedures used to determine the content of RDC in moist and dry soils are described. [ABSTRACT FROM AUTHOR]

To understand the hydraulic properties of soils, accurate prediction of the unsaturated hydraulic conductivity is needed, which is commonly derived from the soil water release characteristic. In fine-textured soils, the modified Mualem-van Genuchten (MMVG) function has been used extensively to predict the change in relative conductivity with changes in matric potential, but this assumes that the soil is rigid with a constant porosity (CP), which is unrealistic for shrinking soils. We used a triaxial cell apparatus to accurately monitor soil volume changes during water release in a clay soil. From this we derived relative hydraulic conductivity functions based on either MMVG (CP), or based on geometrically similar shrinkage (GSS) of all pore sizes by a constant factor as calculated from the measured shrinking porosity (SP). The MMVG (CP) function predicted a decline in relative conductivity of up to three orders of magnitude from 0 to -400 kPa matric potential. This was similar to published data on the response of conductivity in the same soil to consolidation of larger pores. Based on SP, however, the soil remained effectively tension saturated (>0.97) down to a matric potential of-85 kPa due to normal shrinkage. The corresponding GSS (SP) function predicted only a modest decrease in relative conductivity from I to 0.85 across this range. In this case the larger pores, although reduced in size, are assumed to be still water filled and conducting. The GSS function was probably the most realistic portrayal of hydraulic functioning in a tensionsaturated shrinking clay soil. [ABSTRACT FROM AUTHOR]

Little is understood about chemical weathering processes in Alaskan arctic soils, where moisture is generally not limited but acidity varies and the average soil temperature is close to or below freezing. Weathering reactions in soil convert primary minerals into secondary clay minerals. Silty loam textured soils from three sites in moist acidic tundra (MAT) and three sites in moist nonacidic tundra (MNT) in the northern Arctic Foothills, Alaska, were characterized with emphasis on the origin of the clay minerals. The MNT soils had a discontinuous and thinner organic layer, which leads to a deeper summer thaw and greater cryoturbation than the MAT soils. The MNT had higher cation exchange capacity and base saturation than MAT. These buffer against acidification and account for the pH differences of MAT and MNT. Other chemical characteristics including C and N content as well as Fe and Al were similar (by horizon) across the MAT/MNT boundary. X-ray diffraction of coarse (0.0002-0.002 ram) and fine clay (<0.0002 mm) fractions indicate that illite, vermiculite, and kaolinite are the predominant clay minerals. Presumably, kaolinite is detrital and vermiculite is weathered from illite. The proportion of vermiculite to illite is higher in MAT and the illite to vermiculite proportion is higher in MNT. This shows that soil acidity does affect weathering processes despite the low soil temperature. [ABSTRACT FROM AUTHOR]

The use of recycled wastewater that has undergone various treatments (effluent) for irrigation is becoming one of the main means by which to mitigate the pressure on freshwater resources. The main objective of this study was to improve the basic understanding of the effects of irrigation with various effluents on the saturated hydraulic conductivity (Ks) of arid and semiarid soils. Clay, loamy, and sandy soils were sampled from experimental plots irrigated with freshwater or secondary effluent for >3 yr, and were used in column experiments. Leaching the loamy and clay soils with reverse osmosis (RO) effluent, which contained low electrolyte concentrations, led to clay swelling and dispersion and thereby decreased the soil Ks. In these soils, the high concentration of suspended solid particles and dissolved organic matter (DOM) in oxidation pond (OP) effluent caused significant pore clogging, which reduced the Ks. Leaching the soils with ultrafiltercd effluent maintained high Ks values due to the low concentration of suspended solid particles and DOM, their small sizes, and the relatively high electrical conductivity of this effluent. The relatively large average porr size in the sandy soil prevented pore clogging and Ks reduction when leached with RO and OP effluent. Secondary effluent irrigation increased the soil exchangeable Na percentage (ESP) compared with freshwater irrigation but did not change the organic matter content in the soil. In the loamy and clay soils, this higher ESP of the effluent-irrigated soils resulted in greater reductions in when leached with deionized water than occurred in freshwater-irrigated soils. [ABSTRACT FROM AUTHOR]

Although mineralogic quantitative phase analysis (QPA) of the soil clay fraction can provide useful information for the improvement of soil management practices, QPA often requires a combination of several analytical techniques, which can be expensive and time consuming. One alternative that involves a single analysis to give accurate QPA of soils is the use of the Rietveld method to analyze powder x-ray diffraction (XRD) data. In this study, we evaluated the accuracy of the XRD-Rietveld approach for mineralogic quantitative analyses of Oxisol clays when observed structure factors [F(hk[)] ofpedogenic minerals (i.e., kaolinite, hematite, and goethite) are used in the Rietveld analyses performed using Siroquant software. The results showed that although the structures of disordered clay minerals are especially difficult to incorporate in standard Rietveld refinement, which relies on ordered three-dimensional structure models, Mineralogic quantification can be accurately done for complex matrices having a large number of phases and various degrees of structural and coinpositional disorders when observed F(hkl) values are used. It is also possible to develop observed F(hkl) values for Al-hematite and Algoethite from impure samples of such oxides to use as starting structure models for Rietveld analysis. We believe that this approach can be successfully extended to other geologic materials. [ABSTRACT FROM AUTHOR]

Comparisons of soil aggregation and aggregate-protected organic matter among soil textures in the same climatic zone and of the same mineralogical composition are somewhat limited. Two undisturbed, semiarid sites in northeastern Wyoming were selected to determine the differences in aggregation and organic matter between loam and sandy clay loans soils. Additionally, data from these two sites were used to illustrate differences between two statistical approaches to data analysis: Bayesian modeling in the context of a two-sample comparison of means and conventional t-tests. Examination of posterior distributions (Bayesian modeling) and confidence intervals (conventional t-tests) resulted in similar conclusions about the influence of soil texture on aggregation and C and N concentrations. Loam soils generally had more structure (as indicated by higher macroaggregation) and aggregate-protected C and N than sandy clay loam soils, which had greater amounts of available C and N not protected by aggregates. For soils in undisturbed semiarid grasslands with similar clay contents, in the range of 11 to 24%, we suspect that aggregate stability was primarily affected by sand content, while aggregate C and N accumulations were influenced by silt. A higher percentage of silt in the loam soils appeared to have provided enough of a difference in small pore space between the two soil types to result in higher C and N storage in loam soils relative to sandy clay loam soils. Soil texture is a major driving force behind aggregation as well as C and N storage in semiarid soils of the same mineralogy. [ABSTRACT FROM AUTHOR]

Sodium-K, Na-Ca, Na-Mg, and Na-Sr exchange isotherms were performed at a total Cl concentration of about 0.005 mol L-1 on the fine fraction (<2 μm) of MX8O montmorillonite, mixed-layer1 illite smectite minerals (IS), and clayey stones from European underground research laboratories. The derived selectivity coefficients for Na-K, Na-Ca, Na-Mg, and Na- Sr were found to vary very significantly as a function of the exchanger composition. A model was built to reproduce the data, taking into account the change of selectivity coefficient as a function the exchanger composition of smectite. Model parameters were fitted using data from the present study and data from the literature obtained on smectite minerals. The addition of parameters for cation-anion pairs was found to improve the fitting results. The model 1.vas then successfully tested to reproduce published cation-exchange features on natural clayey materials and soils suspended in fresh or saline water. Finally, it is shown that the simple combination of this smectite exchange model with a previously published illite exchange model is quite successful although perfectible in reproducing exchange data on IS. [ABSTRACT FROM AUTHOR]

The management of clay-rich soils in agriculture is mainly dependent on their hydromechanical properties that govern their structural evolutions. These structural evolutions are commonly described along the shrinkage curves obtained in a laboratory. Nevertheless, the field survey needs tools of in situ investigations able to characterize the structural profile evolutions of soils through the seasons. This paper presents a method based on the cone penetration and resistivity measurement by penetrometer-salinometer coupling that was gauged in the West Marsh of the French Atlantic Coast. First, the parallel measurements of water and cone resistance (Qd) profiles in clay dominant soils, characterized by very large ranges of water content (W), allows the calculation of a Perdock's-type equation that links the Qd profiles and the shrinkage curve of the clay dominant material. Second, thanks to the homogeneity of mineralogy and cationic exchange capacity of the soil, the structure-resistivity relationship was gauged according to the Archie's law taking into account porosity (Φ), saturation index of the soil (Sat), and the resistivity of the wetting fluid (ρw). The vertical evolutions of structure, resistivity, and cone resistance can be represented in a crossed diagram showing the shrinkage curve-resistivity and Qd profile relationships. Finally, the structural profiles of soils can be modeled from the recorded resistivity and Qd profiles taking into account the ρw. [ABSTRACT FROM AUTHOR]

Time-domain reflectometry (TDR) has, since the early 1980s, become a widely used and robust method for measuring the electromagnetic properties of soils. Although it allows use of relatively simple probes and measurement equipment that free users from complex analysis of small soil samples in the laboratory, it provides little information on the dispersive nature of fine-grained soils. Therefore, we developed a quarter-wavelength analysis (QWA) methodology that allows apparent permittivity spectra to be obtained for soils, while retaining the same range of cells and probes and the simplicity of data analysis associated with TDR. We tested the technique on clay soils across a wide water content range and found that it provided useful data for frequencies ranging from approximately 100 MHz to >1 GHz-the accuracy being considered no worse than two units of apparent permittivity. The results show that the methodology is capable of providing broadband dispersion data that give significantly greater detail on soil electromagnetic properties than can be expected from single-frequency TDR data. This has a number of important advantages including the potential to provide accurate water content measurements in clay soils, a means to extend TDR soil monitoring data, and the opportunity to validate research into the measurement frequency associated with simple TDR measurements and time- to frequency-domain inversions. [ABSTRACT FROM AUTHOR]

The anticipated benefits of increased soil C stocks on net soil-surface greenhouse gas (GHG) emissions after adoption of soil conservation practices can be offset by increases in soil N2O emissions. The objective of this study was to assess the short-term impacts of no-till (NT) on soil N2O emissions. The study was conducted in eastern Canada in the 3rd, 4th, and 5th yr after initiation of NT and fall moldboard plowing (MP) on heavy clay and gravelly loam soils. Annual emissions of N2O were exceptionally high in the heavy clay soil, varying from 12 to 45 kg N2O-N ha-1 during the 3 yr of the study. Such high emissions were probably not associated with fertilizer N inputs but rather with denitrification sustained by the decomposition of large soil organic matter stocks (192 Mg C hat in the top 0.5 m). On average, NT more than doubled N2O emissions compared with MP in the heavy clay soil. The influence of plowing on N2O flux in the heavy clay soil was probably the result of increased soil porosity that maintained soil aeration and water content at levels restricting denitrification and N2O production in the top 0.20 m. In the loam soil, average emissions during the 3 yr were similar in the NT and MP plots. The results of this study indicate that the potential of NT for decreasing net GHG emissions may be limited in fine-textured soils rich in organic matter that are prone to high water content and reduced aeration. [ABSTRACT FROM AUTHOR]

The minerals of the clay fraction in estuarine plains are mainly detrital, being a mixture of marine and continental sediments, but can also be authigenic. Because of the importance of mangrove ecosystems in tropical estuarine areas and the relatively few existing studies of the mineralogical composition of soils in these environments, the aim of this study was to determine the mineralogical assemblage and identify potential contrasts along the coast of the State of São Paulo. Soils from 11 mangroves distributed along the coastal plain of the State of São Paulo were sampled at depths of 0 to 20 and 60 to 80 cm, and samples of suspended sediments from the Ribeira do Iguape River were collected for analysis. Mineralogical analyses were performed on the clay and silt fractions by x-ray diffraction (XRD) and transmission electron microscopy, and fresh soil samples were analyzed by scanning electron microscopy-energy dispersive spectrometry and suspended sediments by XRD. The silt fraction contained quartz, feldspars, gibbsite, kaolinite, illite, and vermiculite, and the clay fraction contained smectite, kaolinite, illite, gibbsite, quartz, and feldspars. Locally, vermiculite, biotite, anatase, halloysite, and goethite may occur because of recent transport of sediments to the system. Pyrite was identified in fresh samples. The allochthonous minerals found either were terrestrial and transported by rivers or had originated from the continental platform by past transgressive events. We suggest that the neoformation of smectite and kaolinite occurs in mangrove soils. Different geomorphological settings along the São Paulo coast appear to regulate mineral distribution in mangrove soils. [ABSTRACT FROM AUTHOR]

Serpentinite-derived soils give rise to botanically distinct systems primarily because of inadequate parent material Ca content. We hypothesized that Ca content varies widely in what have been mapped as soils derived from serpentinite. An exchangeable Ca/Mg ratio <0.7 is often used to relate the imbalance of these nutrient elements in serpentinite-derived soils. We sampled six parent materials and soils from the Coast Ranges of California in Henneke soil series (clayey-skeletal, magnesic, thermic Lithic Argixerolls) modal location map unit polygons. Parent material total CaO content varied from 1.0 to 230 mg kg-1, and CaO/MgO varied from <0.1 to 4. A combination of x-ray diffraction (XRD), polarized light microscopy (PLM), and electron microscopy was used to identify the Ca-bearing accessory minerals diopside, grossularite, andradite, and tremolite. Accessory mineral content was often too low to be detected by XRD or minerals were too finely disseminated and difficult to detect in thin section by PLM. Electron microscopy, in concert with XRD and PLM, was needed to fully characterize the mineral assemblage. Two sites, Napa and Tehama, contained no serpentine minerals, were not serpentinites, and were tectonic inclusions in the serpentinite landscape. Napa rocks contained almost no Ca-bearing minerals and would be identified as a serpentinite if relying on elemental analysis CaO/MgO ratio alone. Tectonic inclusions and Ca-bearing accessory minerals affect Ca distribution and presumably its availability for plants. Careful mineralogical analysis may be required to identify Ca-bearing accessory minerals. [ABSTRACT FROM AUTHOR]

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]