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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]

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]

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]

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]

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.