Phosphorus is extracted by shaking 1 g of air dried soil in 10 mL of 0.025 M HCl and 0.03 M NH4F for 5 minutes. Phosphorus is determined on the filtrate by the molybdate-blue method using ascorbic acid as a reductant. Color development is measured at 880 nm on a Brinkmann PC 900 probe colorimeter.
(See Method Reference No. 24)