Though the initial reactions of P in soils are important with respect to plant growth and efficient fertilizer use, the reactions that occur with time have received less research attention, and may have even more important implications for these aspects. In order to monitor changes in applied P after fertilization, six calcareous soils were incubated up to 200 d with P solutions and periodically extracted with NaHCO3, and inorganically fractioned. Soil batches were equilibrated with P solutions ranging from 0 to 40 mg P L−1 for adsorption isotherm data after 0, 60, and 200‐d incubation. Desorption curves were obtained by water extraction for eight consecutive days. Extractable P values declined rapidly within the first day, and uniformly thereafter. The amounts of P retained against NaHCO3 extraction were closely related to citrate‐dithionite‐bicarbonate (CDB)‐Fe. With the longer incubation period, generally less P was released in the water extraction. Similarly, desorbed P was inversely related to CDB‐Fe. While P additions mainly enriched the NH4Cl‐ and NaOH‐P fractions, the amounts recovered decreased with time. Increasing prior soil‐P contact time also caused a shift in the adsorption isotherms, reflecting changes in retained P and the soils' retention capacity. Thus, free Fe oxides may modify longer‐term P reactions and residual P availability in CaCO3‐rich soils.

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