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Compatibility of the ammonium lactate (AL) and sodium bicarbonate (Olsen) methods for determining available phosphorus in Swedish soils

Abstract Plant-available P was first extracted in soils 114 years ago and a number of different analytical methods have since been developed, but for good reasons none of these methods has been adopted as a standard for all soils. With increasing cooperation within research, there is a need to harmonise the interpretation of analytical data for fertiliser recommendations, research, and environmental control. This paper evaluates the compatibility of the Swedish standard ammonium lactate (PAL) method and the widely used Olson's sodium bicarbonate (POls) method in 82 topsoil samples taken from Swedish long-term soil fertility field trials. The PAL-values were usually larger than POls, with a mean PAL/POls quotient of 2.30±1.04 (0.67–6.58). The PAL and POls means and ranges were 126±89 (5–360) and 55.1±33 (3.1–122.8) mg P kg−1 dry soil, respectively. Stepwise multiple linear regression analysis was used to evaluate the relationship between PAL and POls and how this relationship was affected by clay content, pH, and soil organic carbon content (SOC). After statistical transformation, it emerged that the square root of clay content (17.4%±13.82, range 1–54.4) and pH (6.45±0.54, range 5.5–7.7) significantly affected the relationship at partial R 2-values of 2 and 12%, respectively, while ln(SOC) (2.54%±1.21, range 1–6.03) did not, apparently due the narrow range. The regressions of predicted vs. measured values explained 95 and 94% of the variation in PAL and POls, respectively. The mean deviation of predicted compared with measured values was 21.3 and 8.3 mg P kg−1 dry soil for PAL and POls, respectively, corresponding to 20 or 19% of the measured values. We conclude that a data set consisting of PAL-values can be converted into POls-values and vice versa with reasonably high accuracy when accounting for clay content and pH.