Impact of Soil Particle Size on Soil Moisture Measurements through Dielectric and Electrical Resistance Properties

Improved understanding of the interactions of soil properties can reduce calibration and technical complexities related to soil moisture measurement techniques. This can improve technology adoption beyond researchers. This paper investigates a previously unexplored factor: the isolated impact of soil particle size on different moisture measurement techniques. Soil from a semiarid region was sieved to produce 2 sets of samples; a set with a larger fraction of course particles (sand) and a set of samples with a larger fraction of finer particles, both being of similar properties and having controlled moisture levels. Resulting variations in Time Domain Transmission (TDT), capacitance, and resistive measurements were analyzed. Analysis revealed that particle size, even from the same soil, had a significant effect (p < 0.05) on moisture measurements through TDT and resistivity. The interaction between particle size and moisture level significantly affected all measurement types (p < 0.05) but did not affect the repeatability of sensor measurements (p > 0.1). Dielectric measurements (TDT/capacitance) produced stronger correlations with soil moisture (R: 0.85–0.99). TDT produced slightly weaker correlations in course samples but showed higher accuracy (error: 1–10%) than capacitance. Unlike dielectric measurements, resistivity responded mainly to soil water on a mass basis, not volumetric soil moisture. The results add further accumulating evidence that multiple pre-calibration settings for courser and finer soils are needed. Furthermore, accuracy metrics for lower and higher moisture ranges should be reported by manufacturers for improved technology selection by soil scientists and practitioners. This can support monitoring indicators of sustainable development goals relevant to land and water degradation.