The method of Olsen et al. (1954) has wide international acceptance as an indicator of soil P fertility. Olsen’s method is based on extraction of air-dry soil with 0.5 M NaHCO3, adjusted to pH 8.5 with NaOH. Soil extraction is for 30 min at a soil/solution ratio of 1:20. There is no prior adjustment of soil pH, or any attempt to remove possible interferences such as arsenate and silicate.
Soluble orthophosphate-P in the clarified soil extracts was originally measured using the procedure of Dickman and Bray (1940), with activated C (P-free) included during soil extraction to absorb soluble organic matter and organic P. Subsequently, it was demonstrated (Watanabe and Olsen 1965) that activated C could be excluded from the extraction step when soluble P was measured manually by the Murphy and Riley (1962) procedure. This analytical finish is incorporated in the manual and automated methods described. Soil extracts by this method typically have more organic matter colouring than do corresponding extracts obtained using the Colwell (1963) modification. This can be the cause of analytical interference, especially when soils high in OM are acidified as part of the colorimetric reaction.