A Method to Determine a Phosphorus Adsorption Maximum of Soils as Measured by the Langmuir Isotherm

Adsorption of phosphorus by soils from dilute solutions showed a closer agreement with the Langmuir isotherm than with the Freundlich isotherm. Constants calculated from the Langmuir isotherm and interpretations based upon the meaning of these constants permit a sound theoretical approach to some of the problems of phosphorus retention in soils. The adsorption maximum claculated from the Langmuir isotherm was closely correlated with the surface area of soils as measured by ethylene glycol retention. The correlation coefficients and regression equations were r = 0.98 and y = 0.276x + 3.47 for 10 alkaline soils, and r = 0.96 and y = 0.641x + 5.73 for 12 acid soils, where y = mg. P per 100 g. soil and x = mg. ethylene glycol retained per g. of soil. For a given surface area, i.e., 30 mg. glycol per g. soil, the acid soils held 2.17 times as much phosphorus as the alkaline soils. The average values of a second constant, k, derived from the slope and intercept values, were 0.92 and 4.39 for the alkaline and acid soils, respectively. As the value of this constant increases, the bonding energy of the soil for phosphorus increases. Thus, the acid soils retained more phosphorus per unit of surface area and also held the phosphorus with a greater bonding energy than the alkaline soils