Capacitive probe calibration in eutroferric red nitosol cultivated with irrigated forages.

Knowledge of soil water content is important for proper irrigation management because it allows estimating the required amount of water to be applied at the correct time. Capacitance sensors are an alternative of quantification of soil water content (?, cm3 cm-3 ), providing depth readings in a practical and fast way. A capacitive probe (Diviner 2000®) was calibrated in a eutroferric red nitosol cultivated with irrigated forages at the experimental area of ESALQ/USP in the city of Piracicaba - SP, Brazil. Four access tubes were installed and every 2 weeks samples were collected with 3 replicates for each depth level, by the end of 8 weeks starting from ?saturated. Readings were taken with the equipment for every 0.1 m of soil up to 0.7 m of depth, together with the collection of soil samples for the determination of ? in the laboratory. A power equation was developed for each depth studied as well as for the whole soil profile. The proposed general calibration curve (SF = 0,256*?current 0.3422), adjusted by regression analysis, was significantly related to the measurements of the equipment, with high correlation (r 2 = 0.87), and standard error of 0.022 cm3 cm-3 . The calibration for each depth has shown higher coefficients of correlation at the lower depths, minimizing the error of the estimates. On-site calibration allowed better accuracy of soil water monitoring and, thus, provide better irrigation management