The NaOH-EDTA technique provides a simple and rapid measure of total
inorganic and organic P in pasture soils
Caroline Johnston1, Tim McLaren2, Mike McLaughlin1, 2, Ron Smernik2, Therese McBeath1, Chris Guppy3,
Richard Simpson4, Alan Richardson4
1
CSIRO Sustainable Agriculture Flagship, Glen Osmond South Australia 5064, [email protected]
2
The University of Adelaide, School of Agriculture, Food and Wine, Urrbrae South Australia 5064, [email protected]
3
University of New England, School of Environmental and Rural Science, Armidale New South Wales 2350, [email protected]
4
SIRO Sustainable Agriculture Flagship/CSIRO Plant Industry, GPO Box 1600 Canberra Australian Capital Territory 2601,
C
[email protected]
Chemical fractionation is the most commonly used technique to measure the bioavailability of soil P
fractions in pasture soils, which can then be summed to estimate total inorganic and organic P content. The
ignition-H2SO4 and NaOH-EDTA techniques are primarily used to estimate total organic P but have also
been used as an estimate of total inorganic P in some studies. Little is known of how the measures of total
inorganic and organic P determined by the ignition-H2SO4 and NaOH-EDTA techniques compare with that
of chemical fractionation, which has implications for accurately assessing inorganic-organic P dynamics
in pasture soils. This study found that all measures of total inorganic P were strongly correlated between
all techniques. On average 102 and 84 % of total inorganic P measured by chemical fractionation was
accounted for by the ignition-H2SO4 and NaOH-EDTA techniques, respectively. Similarly, all measures
of total organic P were strongly correlated between all techniques. However, total organic P determined
by the ignition-H2SO4 and NaOH-EDTA techniques was on average 149 and 78 % of that measured by
chemical fractionation, which suggests the former is overestimating total organic P. We recommend that the
NaOH-EDTA technique be used as a simple and rapid measure for total inorganic and organic P in pasture
soils, as it has the added advantage of providing a suitable matrix for 31P NMR spectroscopy for detailed
characterisation of organic P forms if significant differences are detected.