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SOIL PHYSICS

CS616 Calibration: Field versus Laboratory

NSTL, 2110 University Blvd., Ames, IA 50011

^* Corresponding author (sally.logsdon{at}ars.usda.gov).

Recently developed permittivity probes operate at lower frequencies (MHz range). Soils with large amounts of high-charge clays (superactive) often have higher measured permittivity values for a given water content than do factory calibrations of the newer probes. The purpose of this study was to determine site-specific field and laboratory calibrations of CS616's (water content reflectometers) for soils with "superactive" mineralogy. Field calibration of CS616 (readings converted to square root of apparent permittivity or _a^1/2) was determined as a function of neutron-probe water content data, . Laboratory calibrations used undisturbed columns taken from the same depths where the CS616 probes had been in the field. Another laboratory study compared data for two packed columns, one with wet and dry soil zones and the other with homogeneous water content. The laboratory calibration was linear with _a^1/2. The field calibration was nonlinear, and the _a^1/2 values were even higher than the laboratory values, emphasized more in the intermediate range. The column with wet and dry soil had higher _a^1/2 than the homogeneous column, probably due to the electromagnetic field (EMF) preferentially responding to the wet zones. Heterogeneous field soil water content could have contributed to the higher _a^1/2 than for laboratory calibration, and the nonlinear _a^1/2() relation for field data.

Abbreviations: CEC, cation-exchange capacity • EMF, electromagnetic field • PVC, polyvinyl chloride • TDR, time domain reflectometry • _a, apparent permittivity • _dc, direct current electrical conductivity

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