The calibration of wet-sensor for volumetric water content and pore water electrical conductivity in different horticultural substrates.

A frequency domain dielectric sensor (WET ® ), which measures permittivity (e), temperature (T, °C) and bulk electrical conductivity (σ, dS/m) simultaneously in the same soil volume, was calibrated for the volumetric water content (θ, m 3 m -3 ) and the salinity of both pore water (σP) and water extract (σE) in different horticultural substrates: peat, pumice, perlite and mixtures (1:1, v:v) of peat and perlite or peat and pumice. The experiment was conducted under laboratory conditions over a T range between 22 and 28°C using plastic pots filled with each substrate, irrigated to fully container capacity with nutrient solutions of known concentrations and left to dry in air to θ ranging from approx. 0.20 and 0.50 m 3 m -3 . In order to avoid the development of significant gradients in substrate moisture and salinity, the pots did not host plants and the evaporation from the top surface was prevented by means of a plastic wrap. Pore water was collected by centrifugation, whereas water extract was obtained by means of 1 substrate: 2 water suspension method. A significant linear relationship was found between σE and σP, with the slope dependent on the type of substrate. θ calibration was faintly dependent on the nature of substrate and it was not affected by the salinity of irrigation water, at least in the peat-pumice mixture. In this substrate, the linear regression of σP against σ was markedly affected by θ and hence e.