The reduction current is linearly proportional to orthophosphate concentration. The FI amperometric system for the determination of orthophosphate in the soil extract was modified from the previously reported system [9] by replacing the six port injection valve with the injection valve assembled from three solenoid valves (SV1–SV3) and a three-way connector, as shown in Fig. 1(a). At the loading position (as shown in Fig. 1(a)), a standard/sample was loaded into a sample loop and then by manually switching current, the solenoid valves were actuated to another position to allow the carrier solution (0.1 M KCl) to inject sample into the reagent solution (0.5%, w/v ammonium molybdate in 2.5%, v/v H2SO4). The reaction product, 12-molybdophosphate was reduced at the GCE, at a constant applied potential of 0.20 V vs. Ag/AgCl, producing a FI peak that was recorded by a portable amperometer. Fig. 2 illustrates the FI profiles of phosphate standard solution (1.0–10.0 mg L−1 PO4-P) and some samples. Linear calibration graphs for phosphate in the concentration ranges of 0.1–1.0 (y = 0.2455x + 0.0014, r2 = 0.9996) and 1.0–10.0 (y = 0.2111x + 0.1389, r2 = 0.9966) mg L−1 PO4-P were obtained. The detection limit calculated from three times standard deviation of blank/slope of the calibration graph was found to be 0.02 mg L−1 PO4-P.