Interlaboratory Comparison of a Standardized Phosphorus Adsorption Procedure

Abstract

Abstract A standard P adsorption procedure was proposed and the ability of four laboratories to produce consistent results over a wide range of soils was determined. For this procedure, 0.5 to 1.0 g of soil were shaken in 0.01 mol L −1 CaCl 2 at a soil/solution ratio of 1:25 in containers allowing a 50% head space for 24 h at 24 to 26°C on an end‐over‐end shaker. Initial dissolved inorganic P concentrations of 0 to 323 µ mol P L −1 (as KH 2 PO 4 or NaH 2 PO 4 ) were used and microbial activity inhibited by 20 g L −1 chloroform. Excellent agreement between the four laboratories was obtained for the amount of P adsorbed by the 12 soils studied, with a mean coefficient of variation (CV) over all P levels and soils of 0.91%. The laboratories also exhibited a high degree of replication of individual treatments with no laboratory showing a strong consistent bias across all soils and P levels in terms of P adsorption. Langmuir, Freundlich, and Tempkin adsorption models were highly correlated with the adsorption data. Respective mean correlations for the 12 soils were 0.98, 0.97, and 0.95. The proposed method, therefore, has the potential to produce consistent results that can be used to predict partitioning of dissolved inorganic P between solid and solution phases in the environment.