Modification of Hilhorst model for saturated extract electrical conductivity estimation of coir using frequency domain reflectometry sensors – A laboratory study

Frequency domain reflectometry (FDR) sensors can measure moisture content and electrical conductivity (EC) of growing media, thus providing information for effective water and nutrient management of crops. FDR sensors generally provide the bulk EC (σ_b) value, which is different from the saturated extract EC (σ_e) values that most growers and practical studies generally use. To properly estimate σ_e of media, we modified the previous Hilhorst model that calculates pore water EC (σ_p) from FDR sensor readings. The 16 coir samples with four moisture and four EC levels were prepared, and their changes in the dielectric constant of bulk medium (ε_b), σ_b, and medium temperature from the FDR sensors were recorded and used for analysis. As a result, the ε_σb=0 (ε_b when σ_b is 0) of the coir used in this experiment was 1.64, which was much lower than the ε_σb=0 suggested for mineral soil (4.1), indicating that the medium-specific ε_σb=0 should be adjusted. Although σ_e value estimation through the Hilhorst model with adjusted ε_σb=0 improved, the values wer_e still unreliable to represent the actual σ_e values (root mean square error, RMSE = 2.280, R² = 0.30). To convert the σ_p derived from th_e Hilhorst model with adjusted ε_σb=0 to actual σ_e, the regression equation was applied as σ_e = 0.5381 × σ_p + 1.6493. The adjusted Hilhorst model could provide a more reliable σ_e estimation (RMSE = 0.929, R² = 0.62). These results indicate that proper adjustment of ε_σb=0 for the specific medium is necessary, and adjusting the equation for converting σ_p into σ_e would be helpful to properly interpret the σ_b to practically utilize the FDR sensors for effective water and nutrient management.