Comparison of Two Methods for Calculating the P Sorption Capacity Parameter in Soils

Phosphorus cycling in soils is an important process affecting P movement through the landscape. The P cycling routines in many computer models are based on relationships developed for the EPIC model. An important parameter required for this model is the P sorption capacity parameter (PSP). Using previously published data, we compare two methods for estimating PSP values: (i) measurement of changes in labile inorganic P (P-i) concentrations following 6-mo soil incubation studies, and (ii) calculation of PSP from concentrations of total Pi and labile Pi estimated from commonly used soil test P extraction methods. Depending on how labile P-i was estimated, we either found a very poor correlation or large (>50%) median differences in PSP between these two methods suggesting they are not estimating the same soil parameter. We also found that PSP values calculated from soil incubation studies significantly underpredicted total Pi. It is not clear whether this underprediction is due to limitations in the experimental approach for measuring PSP using 6-mo incubation studies, or whether it is a result of limitations with the model itself. Our results also challenge the validity of the assumption in EPIC that stable P is four-fold the size of the active Pi pool. While calibrating PSP from measured soil P data is less costly and time consuming than long-term soil incubation studies, we show that the fitted values are dependent on how labile Pi is estimated and thus are not representative of an independently measurable physically based parameter.