Phosphorus is extracted by shaking 1 g of air dried soil in 20 ml of 0.5 M NaHCO3, pH 8.5, for 30 minutes. Phosphorus is determined on the filtrate by the molybdate-blue method using ascorbic acid as a reductant. Color development is measured at 900 nm on a Brinkmann PC 900D probe colorimeter.
(See Method Reference No. 24)