Inductively coupled plasma (ICP) optical emission s
pectroscopy and ascorbic acid colorimetry (Murphy R
iley)
are commonly used techniques for P analysis in soil
extracts. DI extracts are generally analyzed for
P by
colorimetry or by ICP, and AmOx extracts are genera
lly analyzed by ICP. In Murphy Riley colorimetric
analysis,
phosphate binds to molybdenum to form a complex tha
t has a blue color. The intensity of the blue colo
r and the
absorption at 882 nm are proportional to the amount
of phosphate available to form the complex. In IC
P the sample
is atomized in a plasma for elemental analysis. An
alysis by ICP typically results in slightly higher
values for STP
than colorimetric methods,
9
because some of the organically bound P can be una
vailable to form the
phosphomolybdate complex necessary for color format
ion, whereas this ability is not an issue in ICP an
alysis as the
plasma releases elemental P in all P forms