『Abstract
　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-P1 (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.』

Abbreviations
(Introduction)
Materials and methods
　Field trial descriptions
　Soil sampling and analyses
　Statistical analyses
Results and discussion
　Relationships between cumulative phosphorus application and soil phosphorus
　　Soil-test phosphorus
　　Total soil phosphorus
　　Soil phosphorus saturation indices
　Relationships between soil phosphorus measurements
　　Total soil phosphorus and soil-test phosphorus
　　Soil phosphorus saturation and soil-test phosphorus
Conclusions
References