In Situ Comparison of Three Dielectric Soil Moisture Sensors in Drip Irrigated Sandy Soils

The number of different sensors available for measuring soil water content has increased since the introduction of time domain reflectometry (TDR). In this study the performances of the CS616 (Campbell Scientific, Ltd., Shepshed, UK) sensor and the Aquaflex (Streat Instruments., Ltd, Christchurch, NZ) sensor were compared with TDR using both vertically and horizontally installed sensors. It was found that the CS616 manufacturer's standard calibration needed to be linearly transformed, y = 0.59 x + 0.01 (m3 m−3), to obtain accurate measurements in a sandy soil with horizontally installed probes. In two different soils the standard calibration performed better, and smaller corrections were found, y = 0.87 x − 0.01 for a sandy loam and y = 0.96 x − 0.001 (m3 m−3) for a sandy loam with a larger clay content, respectively. The CS616 sensor was most likely affected by electrical conductivity at 1.6 dS m−1 in the soil solution when measuring in drip-fertigated potatoes ( Solanum tuberosum L.). In the period of fertigation, the sensor overestimated the soil water content in the sandy soil by 0.01 to 0.11 m3 m−3 compared with TDR measurements. The dynamic response of the vertically installed sensor to changes in soil water content was shown to be good, and the sensor may be useful for assessing threshold values in water content for the start and end of irrigation. The performance of the Aquaflex sensor was investigated in the sandy soil only, and the sensor was found to reflect the dynamics of soil water content well. However, the manufacturer's standard calibration underestimated the soil water content in the order of 0.10 m3 m−3 and even showed negative values.