Simultaneous Measurement of Soil Penetration Resistance and Water Content with a Combined Penetrometer-TDR Moisture Probe

and nutrient exploration have been obtained (Stelluti et al., 1998), and cone penetrometers have been used Soil mechanical impedance affects root growth and water flow, extensively in soil science studies to identify natural and controls nutrient and contaminant transport below the rooting and induced compacted layers (Henderson, 1989) or to zone. Among the soil parameters affecting soil strength, soil water content and bulk density are the most significant. However, field predict related soil properties (Ayers and Bowen, 1987). water content changes both spatially and temporally, limiting the Among the soil parameters that affect PR, soil water application of cone penetrometers as an indicator of soil strength. content and bulk density are the most significant (VazConsidering the presence of large water content variations within a quez et al., 1991). For example, Stitt et al. (1982) consoil profile and across a field and the large influence of water content ducted a comprehensive study of factors affecting PR on soil strength, there is need for a combined penetrometer‐moisture in coarse-textured soils in the Atlantic Coastal Plain, probe to provide simultaneous field water content and soil resistance and used stepwise regression to relate mechanical immeasurements. Such a probe was developed, which uses the time pedance to various measured soil properties. The highdomain reflectometry (TDR) technique to determine water content est correlation coefficients were found for a regression and its influence on soil penetration resistance. The coiled TDR moismodel that included soil water content, soil particle ture probe consists of two parallel copper wires, each 0.8 mm in roughness and bulk soil density. Shaw et al. (1942) condiameter and 30 cm long, coiled around a 5-cm-long polyvinyl chloride (PVC) core with a 3-mm separation between wires. Calibration curves cluded that soil moisture is the dominant factor influencrelating the soil bulk dielectric constant measured by the coiled probe ing the force required to push a penetrometer into the to water content were obtained in the laboratory for a Columbia soil, with PR increasing as the moisture content define sand loam (coarse-loamy, mixed, superactive, nonacid, thermic creased. In an experimental study by Henderson et al. Oxyaquic Xerofluvent), a Yolo silt clay loam (fine-silty, mixed, non- (1988) it was found that PR was only slightly affected acid, thermic Typic Xerorthent), and washed sand, and data were with a decrease of soil water content to ∪70% of field analyzed based on a mixing model approach. Subsequently, field ex- capacity. However, the PR increased exponentially with periments were conducted to measure simultaneously the penetration a further reduction of the water content of the sandy soil. resistance (PR) and water content along a soil profile. Results showed This study showed that PR increased with an increase of a detailed water content profile with excellent correlation with the bulk density across the whole measured water content gravimetric method, whereas the depth distribution of PR was similar range. However, because soil moisture varies both spato that of dry bulk density as determined from soil cores. tially and temporally and is only one of the soil variables related to PR, the utility of using PR to determine compaction effects is marginal. Moreover, interpretation of