Accuracies of soil moisture sensors in typical soils in the Hebei Plain

Soil moisture is an essential factor for the growth of crops and plants, and the measurement of soil moisture is the basis of research on agriculture, hydrology, environment, and soil and water conservation. Compared with traditional methods of soil moisture measurement, the main measurement methods currently used are different types of soil moisture sensors. Research on soil moisture sensors is mostly based on the comparative study of foreign soil, and research on the adaptability of sensors by foreign manufacturers in domestic soil is limited. The factors considered in previous studies are usually limited to soil temperature, soil salinity, and soil texture types, and few texture types have been considered. Thus more research is required on more factors and soil texture types. An in-depth understanding of the performance of different types of soil sensors is crucial for improving the accuracy of regional soil water content measurements. Therefore, to explore the measurement performance of different types of sensors in typical soils of the Hebei Plain, five types of soil moisture sensors that are widely used worldwide were selected in this study, and the influence of soil texture and bulk density on their measurement accuracy was investigated. Four typical soil textures (silty clay, silt loam, sandy loam, and sand) were selected to study the accuracy of five common soil moisture sensors (TDR315H, CS655, 5TE, Teros12, and Hydra Probe Ⅱ) under five different bulk density conditions (1.40 g·cm⁻³, 1.45 g·cm⁻³, 1.50 g·cm⁻³, 1.55 g·cm⁻³, and 1.60 g·cm⁻³). The results showed the following: 1) Under the experimental conditions, the measurement accuracy of uncalibrated TDR315H, CS655, Teros12, and Hydra Probe Ⅱ was high, the measurement error was less than 0.03 cm³·cm⁻³, and the performance of TDR315H was the best. 2) Generally speaking, the errors of five types of sensors in coarser soil were greater than those in finer soil, and the influence of soil texture on the measurement accuracy of sensors was far greater than that of soil bulk density. 3) The soil moisture content also had a significant impact on the accuracy of the sensor. With a change in soil moisture content, the measurement error also changed, and there may be a threshold of soil moisture content (such as 0.3 cm³∙cm⁻³ in this study) that made the measurement accuracy of the sensor change significantly. In general, without calibration, TDR315H is expected to be directly applied in field measurements of soil water content in agricultural land in the Hebei Plain. This study provides an important reference for sensor selection in soil moisture monitoring.