Control of the moisture content of growing media by time domain reflectometry (TDR)

The irrigation of pot crops requires considerable follow-up because the autonomy of the water reserve of the growing medium is low in relation to plant water requirements. Computerised systems for irrigation management are therefore under investigation. Time domain reflectometry (TDR) was used on different peat-based mixtures to which perlite or composted bark were added in volumetric ratios of 0, 25, 50, 75 and 100%. Calibration equations (linear or polynomial) established between the water content and dielectric constant were obtained for each mixture. Their precision is highly satisfactory (R2 > 0.95), except for the mixture with 100% perlite. However, this method requires an equation per growing medium. We therefore attempted to establish a general formula that could be applied to binary peat-based mixtures. We tested second-order polynomial calibration equations, a function of the dielectric constant measured as well as the volume percent of complementary material added to the peat (bark or perlite). The precision of these equations is excellent for all mixtures (R2 > 0.96). For all practical purposes, this approach would considerably simplify the use of TDR for computerised irrigation management because it would introduce only a limited number of parametric equations into the irrigation system