Twelve Sri Lankan Alfisols were characterized for P. Soils
were incubated for three weeks with sufficient P applied
(P2) to elevate soil solution P to 0.2 ug P/g solution as
determined by sorption curves, with 75 percent of P2 (P1),
and without P (P0). For PO treatments, (a) organic P,
(b) 0.5 M NaHCO₃ extractable "labile" inorganic P, (c) 0.1 M
NaOH extractable "moderately labile" inorganic P, and
(d) concentrated HC1 extractable residual P fractions
contained 44, 10, 13, and 22 % of total P respectively.
Fifty five percent of applied P (P1 and P2) was recovered in
the inorganic 0.5 M NaHCO₃ fraction and 25 percent in the
inorganic 0.1 M NaOH fraction. Significant amounts of
applied P were recovered in the organic fractions of some
soils. Factor analysis of PO treatment fractionation data
suggested that inorganic and organic fractions were separate
equilibrium systems. Both inorganic and organic systems,
however, contributed significantly to P uptake in a Neubauer
experiment. Moreover, applied fertilizer P appeared to
increase contributions of organic P to P uptake.
Correlation of Neubauer plant uptake data to Olsen, Bray 1,
Bray 2, CAL, and Double acid extractable soil P showed that
P uptake was most highly correlated to Double acid
extractable P. Regression analysis suggested that the
higher correlation was due to the Double acid extractant's
ability to measure the organic P contribution to P uptake.
Phosphorus sorption by the soils ranged from low to medium
and was described satisfactorily by either the Langmuir two
surface or the Freundlich equation but not by the Langmuir
one surface equation. Correlation analysis indicated that
oxalate extractable Fe was the soil component most active in
P sorption. Although organic matter and pH were correlated
to sorption capacity and may have been directly involved in
the P sorption mechanisms they could also have covaried with
other components responsible for sorption. Although
citrate-dithionite extractable Fe did not appear to be
involved in P sorption its high correlation with residual P
fractions suggests an involvement with "slow" sorption.
Phosphorous sorption had no apparent influence on P uptake
by foxtail millet from the 12 soils in a Neubauer
experiment
