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Sequential extraction technique of phosphorus using different extractant solutions were carried out to separate various pools of soil phosphorus such as (I) labile (plant available) Pi, (2) inorganic P bound to Fe and Al, (3) inorganic P bound to Ca and Mg, (4) organic P, and (5) residual P. The procedure were developed for natural amendments applied in this study containing compost (C), phosphate Rock (PR), Citric acid (CA) in different combination treatments, to recognize plant available P in readily available forms (water soluble and bicarbonate form), refractory forms (NaOH-P, Po, HCL Pi, residual-P). Moreover, the rate process of P released from these systems was also determined to evaluate the rate distribution of P in different forms as affected by compost treatments and residence time and subsequently P availability in such systems. The obtained results indicated that application of compost treated with 15%PR (T3), the best treatment in overall experiment, increased the extractable phosphorus extracted with H2O (WS) compared with the other treatments. The acidification of PR by citric acid at different rates of 2 and 4 % treatments applied in sandy soil gradually increased the WS fraction compared with control (PR) through the incubation periods. The higher readily available P was particularly observed after 30 and 120 days of soil incubation period. For these available forms, data showed that the extractable fraction of P by sodium bicarbonate (the exchangeable form) varied significantly for the investigated treatments and time of incubation. The extractable fraction of P by NaOH gave the higher available values particularly for T3 compared with other treatments. In HCl-P pool, results indicated that application of citric acid at rate of 2% was also increased this pool by about 12 % over control. Increasing rate up to 4% gradually increased this fraction by about 25 % particularly after 15 days of incubation. Data observed that the extractable fractions of phosphorus by EDTA for the investigated treatments were varied according to treatment applied or incubation time of treatments; nevertheless higher values were noticed again in T3. The application of kinetic study applied in this experiment showed that the concentration of P through the entire reaction time was varied in different forms into two stages which representing 526 EL-KHERBAWY ET AL different mechanisms controlled P distribution and P availability through the entire reaction time.

EFFECT OF RESIDENCE TIME ON PHOSPHATE DISTRIBUTION IN PHOSPHATE/COMPOST TREATED SOIL