Compensation Methods for pH Direct Measurement in Soilless Culture Substrates Using the All-Solid-Stated pH Sensor

pH of soilless culture substrate is an important and frequently measured property in facility agriculture. Compared with laboratory analysis, pH direct measurement (pH-DM) involving the direct insertion of a pH sensor into soil or substrates, is rapid, efficient, and inexpensive. This method requires a reliable pH sensor and exhibits a significant measuring error of 0.8 to 1.1. In this study, all-solid-stated pH sensors were fabricated on alumina substrate through magnetron sputtering. Substrates containing mixed peat and vermiculite were prepared to examine the effects of mass water content (MWC) and mixture ratio on the pH-DM method. Statistical analysis of the measured pH values demonstrated that MWC and mass ratio of peat ( $\boldsymbol {\eta }_{{peat}}$ ) evidently influenced pH-DM. Specifically, in the unsaturated substrate, the errors of pH-DM were highly and negatively correlated with MWC but positively correlated with $\boldsymbol {\eta }_{{peat}}$ . The measured pH values also exhibited wide dispersion in the substrates with high $\boldsymbol {\eta }_{{peat}}$ or extremely low MWC. Furthermore, a two-factor compensation model was established and evaluated. The compensation results indicated that the maximum error with pH-DM method decreased from −1.63 to −0.66 pH unit, and the fluctuation coefficient of pH-DM decreased from 0.796 to 0.264. The influence rules of MWC and $\boldsymbol {\eta }_{{peat}}$ on errors in the pH-DM method, as well as the proposed compensation model, are useful in improving the accuracy and robustness of pH-DM and can contribute to rapid and efficient pH measurements in soilless culture substrate.