Page 31 - Volume 2, Issue 1, January 2009
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Effect of superphosphate on glyphosate adsorption in soil
31
ing contrasting Danish surface soils, have re-
vealed that adsorption of glyphosate and
phosphate can be both competitive and
additive, with the competition not always
been as pronounced. Adsorption of the two
anions seems actually to be only partially
competitive.
Increased glyphosate and AMPA adsorp-
tion following the addition of superphos-
phate fertilizer was observed with the two
most acidic soils (KA1 and H1) and the fact
that this superphosphate amendment de-
creased the pH of these two soils to even
lower values in the acidic range seems to
contribute to that. It is well established that
glyphosate and phosphate adsorption is
mostly contributed to aluminium and iron
in acid soils and to calcium in alkaline soils.
Changing the pH of the soil from the alka-
line to the acidic range would increase ad-
sorption since higher charged cations (Al
3+
,
Fe
3+
) are capable of complexing more gly-
phosate than lower charged cations (Ca
2+
).
Furthermore, with decreasing pH both the
clay and glyphosate become less negative-
ly charged and thus more interactive (more
adsorption). The pH and glyphosate adsorp-
tion patterns of KA1 limed soil samples (Fig-
ure 5) and of the H3 acidified soil samples
(Figure 6) in this study further document the
validity of theses statements.
The strong acidifying action of the su-
perphosphate fertilizers in the soil is already
known (1). When granules of the fertilizer
are incorporated into the soil, the sparing-
ly soluble calcium dihydrogen phosphate
[Ca(H
2
PO
4
)
2
], which they contain, absorbs
water and is hydrolyzed to calcium hydro-
gen phosphate [CaHPO
4
] and ortho-phos-
phoric acid (H
3
PO
4
). The three phosphate
compounds coexist in an equilibrium, form-
ing the so called “triple point solution”, with
a pH of 1.0-1.5, which diffuses around the
granules in the soil. It seems, therefore, that
in acidic and neutral soils the ortho-phos-
phoric acid, during diffusion of the triple
point solution, lowers the soil pH and sol-
ubilizes aluminium and iron oxides thus in-
creasing the adsorption capacity of the soil.
In alkaline calcareous soils, however, the or-
tho-phosphoric acid is more quickly neutral-
ized by calcium carbonate and precipitates
as insoluble tricalcium phosphate, thus be-
ing unable to affect glyphosate adsorption.
Many other fertilizers are known to alter
soil pH and it would be of interest to exam-
ine how they affect glyphosate and AMPA
adsorption.
Literature Cited
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1.
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2.
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3.
dy soil affect tomato transplants.
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