Soil inorganic phosphorus fractions and Olsen-P in phosphorus-responsive calcareous soils: Effects of fertilizer amount and incubation time

Crops grown in calcareous Montana soils with Olsen-P concentrations above critical levels often show responses to phosphorus (P) fertilizers. Objectives of this experiment were to (i) investigate the effects of P fertilization and soil incubation time on soil inorganic P fractions (Pi) and Olsen-P concentrations, and (ii) determine the relationship between each Pi fraction and Olsen-P in P-responsive calcareous soils. Six soils (Ap horizon) were collected from P-responsive locations in Montana based on previous field and greenhouse trials. Each soil was fertilized with 0, 11, 22, and 33 mg P kg− 1-soil and incubated at 20°C for up to 12 weeks. Soils collected after 1, 4, 8, and 12 weeks of incubation were analyzed for Olsen-P, and inorganic P was fractionated into Resin-P, Water-P, NaHCO3-P (bic-P), NaOH-P, and HCl-P. Relative distributions of each Pi fraction followed the order of HCl-P > NaOH-P > Resin-P > bic-P > Water-P, with NaOH-P and HCl-P representing greater than 75% of the total Pi. In general, Resin-P, Water-P, bic-P, their summation (Avail-P), and Olsen-P were significantly correlated with the amount of P added, while NaOH-P and HCl-P were not. Compared to the other Pi fractions, Resin-P was more highly correlated with the amount of P added and increased the most following P fertilization. Olsen-P was significantly correlated with Resin-P at most incubation times, yet, in general, was not significantly correlated with bic-P, NaOH-P, or HCl-P. During the incubation, Resin-P decreased the most of the three available fractions, yet Olsen-P was relatively constant. Combined results suggest that available Pi fractions, especially the Resin-P fraction, may be better indicators than Olsen-P at predicting P availability and P responses in calcareous soils.