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 (P_AL) method and the widely used Olson's sodium bicarbonate (P_Ols) method in 82 topsoil samples taken from Swedish long-term soil fertility field trials. The P_AL-values were usually larger than P_Ols, with a mean P_AL/P_Ols quotient of 2.30±1.04 (0.67–6.58). The P_AL and P_Ols 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 P_AL and P_Ols 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 P_AL and P_Ols, respectively. The mean deviation of predicted compared with measured values was 21.3 and 8.3 mg P kg^?1 dry soil for P_AL and P_Ols, respectively, corresponding to 20 or 19% of the measured values. We conclude that a data set consisting of P_AL-values can be converted into P_Ols-values and vice versa with reasonably high accuracy when accounting for clay content and pH.