Simultaneous measurement of soil penetration resistance and water content with a combined penetrometer-TDR moisture probe

Soil mechanical impedance affects root growth and water flow, and controls nutrient and contaminant transport below the rooting zone. Among the soil parameters affecting soil strength, soil water content and bulk density are the most significant. However, field water content changes both spatially and temporally, limiting the application of cone penetrometers as an indicator of soil strength. Considering the presence of large water content variations within a soil profile and across a field and the large influence of water content on soil strength, there is need for a combined penetrometer-moisture probe to provide simultaneous field water content and soil resistance measurements. Such a probe was developed, which uses the time domain reflectometry (TDR) technique to determine water content and its influence on soil penetration resistance. The coiled TDR moisture probe consists of two parallel copper wires, each 0.8 mm in diameter and 30 cm long, coiled around a 5-cm-long polyvinyl chloride (PVC) core with a 3-mm separation between wires. Calibration curves relating the soil bulk dielectric constant measured by the coiled probe to water content were obtained in the laboratory for a Columbia fine sand loam (coarse-loamy, mixed, superactive, nonacid, thermic Oxyaquic Xerofluvent), a Yolo silt clay loam (fine-silty, mixed, nonacid, thermic Typic Xerorthent), and washed sand, and data were analyzed based on a mixing model approach. Subsequently, field experiments were conducted to measure simultaneously the penetration resistance (PR) and water content along a soil profile. Results showed a detailed water content profile with excellent correlation with the gravimetric method, whereas the depth distribution of PR was similar to that of dry bulk density as determined from soil cores.