Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 27, 2007
Publication Date: January 11, 2008
Citation: Logsdon, S.D. 2008. Electrical spectra of undisturbed soil from a crop rotation study. Soil Science Society of America Journal. 72:11-15. Interpretive Summary: Long-term crop rotations are expected to improve the connection of large (macro) soil pores for moving water and air into the soil and allowing room for root growth, but these changes are not always easy to characterize. We compared soil in a corn-soybean rotation with soil under the long-term rotation of corn-soybean-corn-three years alfalfa, both with no tillage. The long-term rotation did not affect the rate that ponded water infiltrated the soil, nor the density of the soil. The long-term rotation did increase the amount of soil water retained after fast drainage (roughly the field capacity of the soil). Some of the samples from both fields contained continuous earthworm burrows, probably due to no tillage. The samples with earthworm burrows did have a faster rate that the ponded water infiltrated the soil and lower density. There was no difference in the fraction of samples with continuous worm burrows between the soil samples from the short- and long-term crop rotations. Soil electrical properties were affected by the presence or absence of the continuous worm burrows, but not by crop rotation differences. The soil electrical properties were more related to soil differences, especially the presence or absence of naturally-occurring lime (calcium carbonate). In this study, no tillage was beneficial to soil structure, and long-term rotation did not have much additional effect on soil structure. This information is primarily of interest to scientists trying to understand the processes.
Technical Abstract: 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 the 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 two-year rotation and the other with a six-year 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 six-year rotation had significantly higher water content after drainage to 100 cm pressure head, and had significantly lower air-filled porosity (air) after free-core drainage. The cores with continuous macropores had significantly higher natural log of saturated hydraulic conductivity and air after drainage to 100 cm pressure head, and significantly lower bulk density and square root of apparent permittivity at higher frequency than cores without continuous macropores. The cores from soils with carbonates had higher electrical conductivity as a function of air 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.