The Power of a Soil's Threshold Phosphorus Saturation Ratio.

A threshold phosphorus saturation ratio (PSR) has been recognized as a practical means of determining when a soil has reached a level of P loading that constitutes an environmental risk. The ratio was originally calculated as the molar ratio of P to [Fe+Al] in an oxalate extract, and thought to be applicable to sandy soils only. The PSR values can be also be calculated from P, Fe and Al in a Mehlich 1 or Mehlich 3 solution making it easier to obtain PSR values from any analytical laboratory. More recently, threshold PSRs have been obtained for Bh (with organically-complexed Al) and Bt horizons (inorganically-complexed Fe/Al with ~20% clay) as well. Threshold PSR values have also been identified for Oxisols of Uganda and Brazil. When soils are below a threshold PSR value, the equilibrium P concentration (EPC₀) has been shown to be minimal for both upland and wetland soils. Further, the threshold PSR is directly linked to the strength of P bonding (K_L) as determined from Langmuir isotherms, the Freundlich adsorption coefficient (K_F), and K_D, the distribution coefficient related to the strength of sorption thus allowing for “K” predictions from soil test data rather than through the generation of time-consuming isotherms. Predicted K values could be used in chemical equilibrium models for evaluating P release from a soil on a site-specific basis.