Relation of Langmuir Parameters to the Soil Phosphorus Saturation Ratio.

Determination of Langmuir isotherms and subsequent calculation of k, the phosphorus (P) bonding strength, and the equilibrium P concentration (EPC₀) of a soil is a tedious and time-consuming process. We hypothesized that k and EPC₀ should be related to the P saturation ratio (PSR) as determined by P, Fe and Al concentrations in an oxalate or a soil test solution such as Mehlich 1 or Mehlich 3. Langmuir adsorption isotherms were constructed for 52 soil samples collected from the Lake Okeechobee Basin and k and EPC₀ values were computed. The soils were also analyzed for P, Fe and Al in an oxalate solution and PSR calculated as the molar ratio of P to Fe and Al. The threshold PSR value for these soils is 0.05 with a 95% confidence interval from 0 to 0.1.  Results indicate that the k value is high below the threshold PSR and that the value tends to zero after the threshold value. EPC₀ values are minimal below the threshold PSR and increases linearly after the threshold. Therefore it should be possible to obtain EPC₀ and k values at minimal costs by submitting a soil sample to a soil testing laboratory. A simple procedure for determining k would be valuable when such values are needed as input in models for predicting P release from soils.