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SOIL PHYSICS

Electrical Spectra of Undisturbed Soil from a Crop Rotation Study

National Soil Tilth Lab., 2110 University Blvd., Ames, IA 50011

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

Soil permittivity can be determined across a range of frequencies, but little is known about how the factors derived from the frequency spectra are related to soil pore structure or crop management. The purpose of this study was to test use of a 12-wire, quasi-coaxial probe for determining soil permittivity spectra, and to see if derived factors could be related to soil pores and crop management practices. Undisturbed soil cores were collected from two management fields, one with a 2-yr rotation and the other with a 6-yr rotation. Comparisons between the fields were based on 95% confidence intervals of the differences in the means for each factor tested. Similar analysis was used to compare cores with and without continuous macropores. The soil from the 6-yr rotation had significantly higher water content () after drainage to 100-cm pressure head, and had significantly lower air-filled porosity (AFP) after free core drainage. The cores with continuous macropores had a significantly higher natural log of saturated hydraulic conductivity and AFP after drainage to 100-cm pressure head, and significantly lower bulk density and square root of apparent permittivity at a higher frequency than cores without continuous macropores. The cores from soils with carbonates had higher electrical conductivity as a function of AFP than those from the soils without carbonates. Overall, soil macropore differences were more pronounced than differences in crop management practices. The 12-wire probe was useful for determining permittivity spectra on undisturbed soil.