Phosphorus determination in plant materials and foods is important. For accessible inorganic P assessment, solid-liquid extraction (SLE) is required. In this work, a flow-based analytical procedure employing zones penetration approach was developed aiming at the soluble inorganic phosphorus (PI) determination in plants and foods. The analyte was quantified by the spectrophotometric molybdenum blue method. Linear response was observed between 2.0 and 30 mg L-1 PI, and detection limit was estimated at 0.50 mg L-1 PI (99.7% confidence level, n = 20). Determination rate was estimated at 20 h-1 using in-line extraction with 20 mg of sample. Precise timing of the flow system allowed the use of incomplete SLE for the first time prior to PI determination. The online extraction was carried out for 60 s, leading to an extraction efficiency of 65 ± 3% in comparison to batch quantitative extraction. The results obtained by the proposed procedure were in agreement with those obtained with batch extractions (95% confidence level).