Improving 5TE Measurement for Volcanic Ash Soils: Simple Calibration Method for Volumetric Water Content and Verification of Electrical Conductivity Estimation

Abstract

Currently, the 5TE sensor (Decagon Inc.) is widely used for monitoring soil water content and electrical conductivity. A practical calibration method and validation of the values recorded by the 5TE are required for volcanic ash soils. We investigated a simple correction method to estimate the soil water content (θ) and showed that a calibration approach with a few ( or even one ) points could be used to correct 5TE readings. We also validated estimation method of 1:5 soil to water extract electrical conductivity (EC_1:5) and soil pore water electrical conductivity (EC_p) with the 5TE. EC_1:5 was obtained based on a proportional relation with bulk electrical conductivity (EC_a) , and EC_p could be estimated from EC_a using the Hilhorst model with a simple calibration. However, the measurement accuracy of the 5TE decreased when EC_p>1.0 S/m for Toyoura sand and EC_p>0.8 S/m for Gandai soil. When θ<0.2 for Toyoura sand and θ<0.275 for Gandai soil, the 5TE could not estimate the EC_p. This should be considered when analyzing water content and electrical conductivity values measured in the field with 5TE.