Effect of compaction on the relationship between electrical resistivity and soil water content in Oxisol

Geophysical methods that allow determination of soil electrical resistivity (ρ) make estimates of the space-time variation of soil water content (θ) viable, as well as of other important physical properties and soil structure. The aims of this study were to test models for the θ-ρ relationship in a Typic Hapludox with clay texture under tropical conditions, and to determine the effect of the degree of compaction on this relationship. Undisturbed samples were collected and, in the laboratory, after saturation, they were artificially compacted to obtain different degrees of compaction (DC): 76 % (control), 84 %, 90 %, and 94 %. Readings of resistivity were made simultaneously to weighing of the samples for determination of θ. The power (θ = aρᵇ), exponential (θ = cexp(ρd)), and logarithmic (θ = e + flog₁₀(ρ)) models were tested. The power model (“Archie’s Law”) best described the θ-ρ relationship (R² = 0.77; p < 0.05), differing from what was expected from the literature for clay soils. The coefficients were different from those reported for temperate climate soils, resulting in lower ρ values as the soil dries. Furthermore, the θ-ρ relationships were affected by the DC of the soil; however, the logarithmic model allowed the best differentiation of these effects. The logarithmic model showed that the θ-ρ relationship has two patterns, depending on θ: pattern A when θ > 0.20 m³ m⁻³ and pattern B for θ < 0.20 m³ m⁻³. Pattern A showed that compaction led to higher values of θ for the same value of ρ; and in pattern B, the opposite occurred. Simultaneously modelling ρ in accordance with the DC and with the degree of saturation (DS) showed that θ has a greater effect on ρ than soil compaction does. Thus, measurement of ρ proved to be a tool of good accuracy for estimation of θ under weathered tropical soil conditions after calibration. However, we suggest that diagnosis of soil compaction be performed under drier soil conditions to eliminate the preponderant effect of θ on ρ.