February 16, 2009 by Douglas R. Cobos
Decagon's soil moisture sensors measure the volumetric water content of the soil by measuring the dielectric constant of the soil, which is a strong function of water content. However, not all soils have identical electrical properties. Due to variations in soil texture and salinity, the generic mineral calibration for soil moisture probes results in approximately ± 3-4% accuracy for most medium to fine textured mineral soils, and the accuracy for coarse-textured and high-salinity soils can vary up to ± 10% (see Table 1). However, accuracy increases to ± 1-2% for all soils with soil-specific calibration. Decagon recommends that ECH2O probe users conduct a soil-specific calibration for best possible accuracy in volumetric water content measurements. Recent tests by independent researchers (Czarnomski et al., 2005) indicate that soil-specific calibration of soil moisture probes achieves performance results similar to that of TDR instruments - at a fraction of the price. Note that the resolution, precision, repeatability, and probe to probe agreement of the soil moisture probes are excellent, so the soil specific calibration of one probe can be applied to all other probes of that type in that particular soil. The purpose of this application note is to provide a step-by-step guide for performing a soil specific calibration on soil moisture probes.
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