In order to monitor nitrate leaching from non-point source pollution, this study used tensiometers to measure insitu nitrate concentration and soil-moisture potential. Instead of filling the tensiometers with pure water, the study filled the tensiometers with nitrate ionic strength adjuster (ISA, 1 M (NH⁴)²SO⁴). After the installation of the tensiometers at various depths along soil profiles, a portable pressure transducer was used to measure the soil moisture potential, and a nitrate electrode attached to an ion analyzer was used to measure the nitrate concentration insitu. The measurement was continuous and non-destructive. To test this method in the laboratory, eight bottles filled with pure sand were treated with known nitrate solutions, and a tensiometer was placed in each bottle. Measurements were taken every day for 30 days. Laboratory test showed a linear relationship between the known nitrate concentration and the tensiometer readings (R² = 0.9990). Then, a field test was conducted in a watermelon field with green manure mulch. Field data indicated a potential of nitrate leaching below the soil depth of 100 cm when crop uptake of nutrients was low.