Determining Soil Bulk Density with Thermo‐Time Domain Reflectometry: A Thermal Conductivity‐Based Approach

Core Ideas A method for monitoring soil bulk density from thermal conductivity and water content is provided. The new approach provides reliable bulk density data in both laboratory and field conditions. The new approach performs better than the heat capacity‐based method. The bulk density (ρ b ) of a tilled soil layer usually varies with depth and time. It is difficult to measure in situ ρ b with available instruments. Here we present a new approach that is capable of monitoring in situ ρ b based on thermal conductivity (λ) measurements. The proposed approach relies on an empirical model that relates λ to soil particle‐size distribution, ρ b , and volumetric water content (θ). The thermo‐time domain reflectometry (thermo‐TDR) sensor measures λ and θ simultaneously, and knowing soil texture a priori, ρ b is estimated using the empirical model. The approach was tested using datasets obtained on repacked and intact soil samples with a wide range of ρ b and θ. Further evaluations were made using in situ ρ b measurements with thermo‐TDR sensors in four tillage treatments. The laboratory results showed that the λ‐based thermo‐TDR method was able to provide reliable ρ b with root mean square errors (RMSE) within 0.17 g cm ‐3 . Under field conditions, the λ‐based thermo‐TDR method also provided accurate ρ b estimates with relative error generally within 10%. Under both laboratory and field conditions, the λ‐based thermo‐TDR method performed better than the heat capacity (C)‐based thermo‐TDR method. Differences were attributed mainly to C errors resulting from probe deflections, while λ measurements with the thermo‐TDR method were not affected by probe deflections. The λ‐based approach provides a way to obtain nondestructive, repeated, and in situ ρ b estimates with the thermo‐TDR method both in laboratory and field settings.