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Extraction and colorimetric methods were evaluated for determining total P in soils. A procedure involving simultaneous digestion of 60 soil samples with perchloric acid and determination of extracted orthophosphate with an ascorbic acid method was evaluated. Analysis of diverse soils indicated that the proposed and conventional HClO4 digestion procedures yielded essentially the same total P values; however, both HClO4 methods underestimated total P by 1–6% when compared to Na2CO3 fusion. A comparison of colorimetric orthophosphate procedures indicated the method of Murphy and Riley (1962) was suitable for determination of orthophosphate following either HClO4 digestion or Na2CO3 fusion. Determination of total P in soils by tube HClO4 digestion and estimation of extracted P by the method of Murphy and Riley (1962) enables rapid and precise estimation of total P in a wide range of soils.

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 mot L CaCI 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 smol P L (as KHPO4 or NaH1`04) were used and microbial activity inhibited by 20 g L 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 01o. 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.