Soil Inorganic Phosphorus Fractions and Their Uptake Relationships in Calcareous Soils

Abstract Crop responses to recommended P‐fertilizer applications are erratic in high pH, calcareous soils of Montana. The objective of this research was to evaluate relationships between soil inorganic P (P 1 ) fractions, Olsen‐P, and plant uptake. Soil‐incubation and plant‐growth experiments were conducted on samples of six calcareous soils from P‐responsive locations in Montana. Rates of P were 0, 11, 22, and 33 mg P kg −1 applied to soils in a randomized complete‐block design with four replications. Spring wheat ( Triticum aestivum L. cv. Newana) was grown successively during three, 4‐wk growing periods. Fertilized soils were incubated without plants during the same period. Soils were subjected to a sequential‐fractionation procedure (resin‐P, NaHCO 3 ‐P, NaOH‐P, and HCl‐P). Soil P 1 quantities were HCl‐P > NaOH‐P > NaHCO 3 ‐P (bic‐P) > resin‐P. Higher Olsen‐P soils had higher summation of P 1 fractions than lower Olsen‐P soils. Changes in resin‐P accounted for most of P 1 ‐fraction differences induced by P application and plant uptake. Cropped soils generally contained lower amounts of P 1 in each fraction than noncropped soils. Highly significant ( P > 0.01) correlations were found for Olsen‐P vs. resin‐P, P uptake vs. resin‐P, and P uptake vs. Olsen‐P, but not for Olsen‐P vs. bic‐P, resin‐P vs. bic‐P, and P uptake vs. bic‐P. Resin‐P was the best indicator of P‐uptake responses on these soils to P applications and should be considered in the prediction of crop responses.