Relating soil phosphorus indices to potential phosphorus release to water

Relationships between soil test phosphorus (STP) and release of P in surfac e and subsurface runoff are needed to help identify source areas for implem enting management strategies to limit P loss to water. To determine whether soil P release could be predicted either by STP values, sorption-desorptio n indices, or the degree of soil saturation with phosphorus (DSSP), 11 site s with contrasting chemical properties and management histories were sample d From long-term field trials in the UK. Each site offered up to three trea tments, resulting in a total of 29 soil samples. The results showed that th e amount of P desorbed using a successive dilution procedure had no relatio nship with either total soil P content or P sorption capacity. The most sig nificant property was the extent of P saturation. There was little desorpti on for DSSP values below 10%; above this point, the amount of P desorbed in creased linearly with the DSSP, Five STP methods (Olsen, Mehlich-3, acidifi ed ammonium oxalate-oxalic acid, Fe2O3-coated paper strip, and distilled wa ter) were compared to predict their effectiveness in predicting potential P release to water. While STP values obtained using acidified ammonium oxala te proved to be least effective, those extracted with water correlated best with the amount of P desorbed. accounting for 96% of the variability in di fferential P release from the soils.