Relationships between extractable soil phosphorus and phosphorus saturation after long-term fertilizer or manure application

Total soil P (TP), soil-test P (STP), and the degree of soil P saturation are affected by long-term P application but relationships between these measurements need to be established for grain production cropping systems to improve P management guidelines. This research studied these relationships from samples collected from 11 long-term (4-23 yr) Iowa P trials. Mean soil clay content and pH (0-to 15-cm depth) ranged from 171 to 375 g kg(-1) and 6.1. to 6.8, respectively, and maximum cumulative P application was 192 to 1098 kg P ha(-1). Soil was analyzed for Bray-P, (BP), Mehlich-3 P (M3P), Olsen P (OP), TP, P sorption index (PSI), and P saturation by STP/ PSI and Mehlich-3 extractable P, Al, and Fe (M3sat) indices. Soil-test P increased as P applied increased and declined when P was not applied. Total P increased linearly with increasing BP, M3P, and OP (r = 0.52-0.55), and increases were 1.8, 1.7, and 3.5 mg TP kg(-1) per mg STP kg-1 for BP, M3P, and OP, respectively. Usually STP was linearly correlated to M3sat and STP/PSI (r = 0.80-0.94), and M3sat was linearly correlated to STP/PSI (r = 0.86-0.92). Results indicate that STP can approximately estimate long-term effects of P application on TP, and soil P saturation for conditions similar to those in this study, but TP estimates are improved by grouping similar soil series. Further research for a wider range of soils and STP would be useful to better describe relationships between these measurements.