A Comparison of the Langmuir Freundlich and Temkin Equations to Describe Phosphate Adsorption

Introduction The Langmuir, Temkin and Freundlich equations are often used to describe the quantityhtensity relationships between solid and solution phase phosphate in soils (Olsen and Watanabe 1957; Kaila 1963; Bache and Williams 1971), but normally fit sorption data for only a limited range of solution phosphate concentrations (Syers etal. 1973; Holford el al. 1974; Fitter and Sutton 1975). The lack of agreement between phosphate adsorption data and these equations caused Gunary (1970) to include a square-root term in the Langmuir equation, and both Holford et al. (1974) and Syers et al. (1973) to use a two-surface Langmuir equation. Both Fitter and Sutton (1975) and Barrow (1978) also improved the goodness of fit of the Freundlich equation by including an estimate of native adsorbed phosphate with the sorption data. The quantitative description of soil phosphate interaction is important for predicting the phosphate fertilizer requirements of soils (Ozanne and Shaw 1967; Holford 1976) and critical solution or labile phosphate content of soils for optimum plant growth (Rajan 1973; Holford and Mattingly 1976a). Helyar and Spencer (1977) have also shown that the soil phosphate buffering capacity affected the critical level of the sodium bicarbonate extractable soil phosphate test for responses to applied phosphate on subterranean and white clover pastures. Many parameters have beenused to describe this characteristics of soils (Holford 1974), but themost commonly used are the buffering capacity (Beckett and White 1964), maximum adsorption capacity (Olsenand Watanabe 1957), maximum buffer capacity (Holford andMattingly 1976b), sorption index (Bache and Williams 1971), sorption capacity (Piper 1942), sorption test (Ozanne and Shaw 1967) and buffering index (Holford 1979).
Aust. J. Soil Res., 1981, 19, 33ቤተ መጻሕፍቲ ባይዱ-42
A Comparison of the Langmuir, Freundlich and Temkin Equations to Describe Phosphate Adsorption Properties of Soils
J. A. Mead
New South Wales Department of Agriculture, Agricultural Research Station, Glen Innes, N.S.W. 2370.
Abstract Four adsorption equations were fitted to phosphate isotherm data for 38 soils from northern New South Wales. The two-surface Langmuir equation provided the best fit to the data. The Freundlich equation, however, was almost as effective, provided that an estimate of native adsorbed phosphate was included. It required fewer adsorption points because the transformed data produced a straight line. A regression constant, a, from the Freundlich equation although underestimating adsorption capacity, calculated from the Langmuir I1 equation, was highly correlated with this parameter (r = 0,984) and a buffering index (r = 0.986), calculated from the Langmuir I equation. The other Freundlich regression constant, n, was significantly correlated (P< 1To) with the intensive parameters of adsorption from the one- and two-surface Langmuir equations. Since only two adsorption points are required for fitting the Freundlich equation and the parameters are easier to derive, it is suggested that this equation may be more suitable for commercial soil-testing laboratories than the Langmuir one- and two-surface equations for routine determination of phosphate adsorption characteristics of soils.