This method estimates the relative bioavailability of inorganic ortho-phosphate (PO4-P) in soils with acid to neutral pH, using a dilute acid solution of hydrochloric acid containing ammonium fluoride. The orthophosphate ion reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions to form a complex. This complex is reduced with abscorbic acid to form a blue complex which absorbs light at 880 nm. The method is shown to be well correlated to crop response on neutral to acid soils. The absorbance is proportional to the concentration of orthophosphate in the sample. The method has a detection limit of approximately 0.5 mg kg-1 (soils basis) and is generally reproducible within 8%.
Diamond, D. 1995. Phosphorus in soil extracts. QuikChem Method 10-115-01-1-A. Lachat Instruments, Milwaukee, WI.
Olsen, S. R. and Sommers, L. E. 1982. Phosphorus. p. 403-430. In: A. L. Page, et al. (eds.) Methods of soil analysis: Part 2. Chemical and microbiological properties. Agron. Mongr. 9. 2nd ed. ASA and SSSA, Madison, WI.