Relating “Freundlich k” to the Soil Phosphorus Saturation Ratio.

The strength of phosphorus bonding “k” in a Fe- and Al-dominated soil is obtained by developing isotherms. The Freundlich k is often used in chemical equilibrium models to predict P release from soils. We hypothesized that Freundlich k will be related to the P saturation ratio (PSR) as determined from P, Fe and Al concentrations in an oxalate or a soil test solution such as Mehlich 1 or Mehlich 3. If the k value can be obtained from a soil test solution, it would offer an easy means of obtaining k for modeling and other purposes. Isotherms were constructed for 42 soil samples with a range of Fe and Al contents and Freundlich k values computed.  All soils were analyzed for P, Fe and Al in an oxalate extract. The threshold PSR for the soils (i.e., the PSR at which P release is high) is 0.05 with an upper 95% confidence interval of 0.1. Based on the threshold PSR value, the soil P storage capacity (SPSC) can be calculated using the equation SPSC = (0.05 – Soil PSR) * [Fe+Al] * 31. The Freundlich k was found to be variable below the threshold value and to trend toward zero with increasing PSR values above the threshold. However, Freundlich k increased with increase in positive SPSC (i.e., increased with SPSC when the soil PSR was below the threshold value).  Ongoing research on the Freundlich k/PSR and the Freundlich k/SPSC relationships will confirm if a simple procedure for determining Freundlich k is possible for all soils.