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Nutrient Management & Soil & Plant Analysis

Relationships between Extractable Soil Phosphorus and Phosphorus Saturation after Long-Term Fertilizer or Manure Application

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

^* Corresponding author (apmallar{at}iastate.edu)

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: BP, Bray-P₁ • DPS_ox, soil P saturation index based on ammonium-oxalate extractable P, Fe, and Al molar ratio • M3, Mehlich-3 • M3P, Mehlich-3 P • M3sat, soil P saturation index based on Mehlich-3 extractable P, Fe, and Al molar ratio • OM, organic matter • OP, Olsen P • PSI, P sorption index • STP, soil-test P • TP, total P