Page 30 - Volume 2, Issue 1, January 2009

© Benaki Phytopathological Institute
Giannopolitis & Kati 30
level of acidification (the resulting with the
addition of 12.5 ml 2 N H2SO4/100 g of soil)
superphosphate amendment seems not to
differentiate glyphosate adsorption from
that in the unamended control. At lower
acidification levels superphosphate seems
to have a slight negative effect and at high-
er levels a positive effect on glyphosate ad-
sorption. It appears therefore that even in
this alkaline soil, glyphosate adsorption can
be increased by the addition of superphos-
phate if combined with an acidifying agent
of sufficient strength to reduce the soil pH
below 7.0.
The increased glyphosate adsorption in-
duced by superphosphate in soil KA1 was
further reflected in a slower decomposition
of glyphosate to AMPA when this soil was
amended with superphosphate. As indicat-
ed in Figure 7, glyphosate dissipated quick-
ly in this soil and within two weeks most of
the applied herbicide (3.1 μg/g) was decom-
posed with a parallel accumulation of AMPA.
Addition of superphosphate to the soil (one
week before glyphosate application), which
has been shown to increase glyphosate ad-
sorption, caused an apparent decrease of
the initial rate of glyphosate decomposition
and AMPA accumulation, i.e. a slight retar-
dation of both processes. This is a reason-
able effect to expect since more adsorption
means less herbicide available to soil micro-
organisms for decomposition. Not exam-
ined in theses studies but already well doc-
umented by others, increasing glyphosate
adsorption in the soil may also mean reduc-
ing the risk of leaching and of underground
water contamination. Increasing adsorption
may also mean reducing the risk of root up-
take and toxicity to crop plants transplant-
ed to the soil soon after glyphosate applica-
tion.
The results presented above clear-
ly demonstrate that superphosphate fertil-
izer, even when applied at high rates, can
not lead to any significant reduction of gly-
phosate adsorption to soil as it would be ex-
pected from a competition between gly-
phosate and phosphorus for the available
adsorption sites. Contrary to that, excessive
superphosphate fertilization of certain ag-
ricultural soils from Greece significantly in-
creases glyphosate adsorption and this in-
crease seems to depend more on the rate of
glyphosate than on the rate of superphos-
phate. A study by Gimsing et al. (2004), us-
Figure 7. Dissipation of glyphosate and accumulation of
AMPA in soil KA1 amended or not with 0,3 g/100 g of super-
phosphate fertilizer.
Figure 6. Effect of 0.3 g/100 g of superphosphate, added to
H3 soil samples which had previously been acidified with in-
creasing volumes of 2N H2SO4 (0, 6.25, 12.5 and 25 ml/100g),
on soil pH (A) and glyphosate adsorption (B).