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Extractable Phosphorus - Olsen Method

Fertility: Olsen-P


This method estimates the relative bioavailability of inorganic ortho-phosphate (PO4-P) in soils with neutral to alkaline pH.  It is not appropriate for soils which are mild to strongly acidic (pH <6.5).  The method is based on the extraction of phosphate from the soil by 0.5 N sodium bicarbonate solution adjusted to pH 8.5.  In the process of extraction, hydroxide and bicarbonate competitively desorb phosphate from soil particles and secondary absorption is minimized because of high pH.  The orthophosphate ion reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions to form a complex.  This complex is reduced with ascorbic acid to form a blue complex which absorbs light at 880 nm.  The absorbance is proportional to the concentration of orthophosphate in the sample. The method has shown to be well correlated to crop response to phosphorus fertilization on neutral to alkaline soils.  The method has a detection limit of 1.0 ppm (soil basis).

Sample amount requested:  5 g
Questions concerning limited sample size can be answered by the UC Davis Analytical Laboratory.

Olsen, S. R. and Sommers, L. E. 1982. Phosphorus. pp. 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.

Prokopy, W. R.  1995. Phosphorus in 0.5 M sodium bicarbonate soil extracts. QuikChem Method 12-115-01-1-B.  Lachat Instruments, Milwaukee, WI.