Fertilization management of intensive crops requires control methods to adjust the mineral fertilizer input to the actual needs of the crop. This paper outlines a method to estimate phosphorus (P) nutrient status of maize (Zea mays hybrid c.v. Volga) by analyzing the plant sap extracted from vascular tissue of the stem base. Phosphate concentration is analyzed with phosphomolybdic complex reaction, using test strip with a small autonomous pocket reflectometer. The experimental conditions for maize diagnosis P nutrition are: stem base harvested between 10 and 12 a.m., phosphate concentration measured in sap squeezed by a hand hydraulic press. The critical level of P nutrition should be between 20 and 25 ppm. The validation of this method by comparison of colorimetry and chromatography measurement on different plant P status indicates a reverse correlation between weight production and P lower stem sap concentration.