Sorption curves can be established by the addition to soil of graded amounts of P in 0.01 M CaCl2 or 0.02 M potassium chloride (KCl). The former solution is preferred here because: (i) it has been more commonly used in Australia and elsewhere (Ozanne and Shaw 1967; Fox 1978; Probert 1983), and (ii) it is less aggressive than KCl. The supernatant P concentration (C) is measured following an equilibration period of 17 h. Calculation of the amount of P sorbed can then be made.
The P sorption curve for a given soil is constructed by plotting P sorbed against log10C for each addition of P. The plot is usually linear for most soils up to a supernatant P concentration of at least 0.1 mg P/L.
The equilibrium P concentration (EPC) is derived by extrapolation of the P sorption curve to the point when P sorbed = 0 (zero). The PBC, which is a reflection of the P sorbing or P fixing ability of the soil, corresponds to the slope of the regression line fitted to the P sorption curve. The higher the slope the higher the PBC.
Manual and automated options for measuring the supernatant P concentration are included, both based on the reagents of Murphy and Riley (1962). It is chemically challenging to include soils of widely different PBCs in the same batch of samples, unless there is prior knowledge sufficient to ‘tailor’ the solution concentrations of the graded P additions.