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SOIL PHYSICS

Determination of Soil Bulk Density with Thermo-Time Domain Reflectometry Sensors

^a Dep. of Soil and Water, China Agriculture Univ., Beijing, China 100094
^b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

^* Corresponding author (tsren{at}cau.edu.cn).

Surface soil bulk density varies with time and location. In most cases, destructive soil sampling has been used to monitor soil bulk density, _b. Recently the thermo-time domain reflectometry (TDR) technique was shown in principle to be able to estimate _b. Thermo-TDR sensors can measure soil volumetric heat capacity (c) and soil water content (). Since c depends on and _b, measurements of c and can be used to estimate _b. Previous studies indicated, however, that there were large deviations between thermo-TDR estimates and gravimetric measures of _b. Deviations were attributed mainly to the change of thermo-TDR needle-to-needle spacing during sensor insertion into the soil. The objective of this study was to improve the thermo-TDR sensor design to improve estimation of _b. The ability of three new prototype thermo-TDR sensors, along with an existing thermo-TDR sensor, to estimate _b was investigated. Evaluation results indicated that a three-needle sensor design with 2-mm needle diameter, 40-mm needle length, and 8-mm needle-to-needle spacing provided the most accurate estimation of soil _b. For this sensor, the RMSEs of _b estimates compared with gravimetric measures of _b in laboratory evaluations were 0.055, 0.051, and 0.046 Mg m^–3 for a silt loam, a clay loam, and a sand soil, respectively, and was 0.095 Mg m^–3 in a field evaluation. In terms of relative error, this specific design was generally within 5% under laboratory conditions and within 10% under field conditions. The improved thermo-TDR sensor provides an opportunity for obtaining accurate, nondestructive, repeated estimates of soil _b.

Abbreviations: OM, organic matter • RE, relative error • TDR, time domain reflectometry