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.