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Availability and Transformation of Applied Phosphorus In Calcareous Lebanese Soils¹

ABSTRACT

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.

¹ Contribution from the Faculty of Agricultural and Food Sciences, American University of Beirut, Beirut, Lebanon. Journal no. 622B.

² Professor of Soil Science; Graduate Student, currently Lecturer, Faculty of Sciences, Juba Univ., Sudan; Assistant Professor of Irrigation; and Instructor, respectively.

Received for publication June 8, 1984. Accepted for publication March 8, 1985.

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