Estimating soil bulk density with combined commercial soil water content and thermal property sensors

Abstract

Accurate information of soil bulk density (ρb) is essential for many models that predict soil water, gas, and heat transfer processes and for estimating soil carbon pools. Several indirect methods have been used to estimate ρb as a derivative of various soil properties. One approach is to estimate ρb from soil thermal conductivity (λ) and volumetric water content (θw) measured with a custom fabricated sensor (thermo-TDR). In this study, we introduce a new approach to determine ρb with a combination of commercially available θw and thermal property sensors. Repacked samples of four differently-textured soils and a field experiment on a clay soil were used to evaluate the ability of four available sensors from METER Group, Inc. (Pullman, WA, USA) to estimate ρb. The θw was measured with the GS3 and EC-5 sensors, and soil λ was determined with the TR-1 and SH-1 thermal property sensors. The θw and λ measurements were used to determine ρb inversely from a λ-model. Compared with the GS3 sensor, the EC-5 sensor provided more accurate measurements of θw for the investigated soils, and thus, the EC-5 sensor was used with the new ρb estimation approach. Both TR-1 and SH-1 sensors gave accurate λ estimates when compared to modeled values. The ρb estimates with the TR-1/EC-5 and SH-1/EC-5 sensor combination methods agreed well with independent gravimetrically-derived ρb values of the repacked samples and the in-situ measurements, with average root mean square errors of 0.12 and 0.13 Mg m−3, respectively. Thus, the commercial multi-sensor combinations can provide accurate ρb estimates similar to those with custom fabricated thermo-TDR sensors, and they are simpler to operate than the custom fabricated sensors.