Soil bulk density (ρb) is a key indicator of soil compaction and soil health that relates to water infiltration, plant rooting depth, nutrient availability, and soil microbial activity. Under field conditions, ρb usually varies with time and depth because of agronomic practices, root growth, and environmental processes (e.g., rainfall events, wetting/drying, and freezing/thawing). The traditional technique (i.e., the coring method) for determining ρb has the problems of destructive sampling, labor intensive, and is unable to capture the spatial and temporal variations. In a chapter of the recent Methods of Soil Analysis book, we present a review of the theory, instrumentation, and procedures of the thermo–time domain reflectometry (thermo-TDR) technique for monitoring in situ ρb (Lu et al., 2017).
A thermo-TDR sensor (Fig. 1) measures soil thermal properties and water content (θ) concurrently by integrating the functions of the heat-pulse and TDR sensors. The method employs available models that relate heat capacity (C) or thermal conductivity (λ) to soil texture, θ, and ρb. With the prior information of sand/clay fractions and specific heat of soil solids, ρb is estimated inversely from θ and C or λ measurements made with thermo-TDR sensors. Laboratory and field tests have shown that the relative errors in ρb estimates are generally within 10%. The new method provides in situ and continuous ρb measurements with no calibration requirement, thus offers the potential for studying coupled heat and water processes in deformable soils where ρb changes with time and depth.