Page 29 - Volume 2, Issue 1, January 2009

Effect of superphosphate on glyphosate adsorption in soil

29

Figure 4.

Effect of the superphosphate amendment (at three

levels equivalent to 220-880 ppm P) on the pH of the four

soils.

superphosphate, which is a good evidence

that the superphosphate-induced decrease

of the soil pH leads to the increased gly-

phosate adsorption.

To acidify soil H3, samples were equili-

brated with various sulphuric acid concenta-

tions which at saturation decreased the pH

from 8.2 to 7.6. The pH of the same samples,

after a subsequent superphosphate amend-

ment, was even lower, ranging between 7.3

and 6.8 (Figure 6A). Acidification of the H3

soil increased glyphosate adsorption as ex-

pected (Figure 6B). The increase of adsorp-

tion brought about by increasing the acid-

ification level was parabolic in the control

soil samples (not amended with superphos-

phate) but almost linear in the superphos-

phate amended samples. As it can be noted

in Figure 6B, regarding soil H3, at a certain

Figure 5.

Effect of 0,3 g/100 g of superphosphate, added

to KA1 soil which had previously been limed with increas-

ing amounts of CaCO3 (0, 0.5, 1 and 2), on soil pH (A) and gly-

phosate adsorption (B).

vourable for adsorption. Many researchers

have already shown that glyphosate adsorp-

tion is stronger in acidic soils, with a pH well

below 7.0, where electrical charge of both

glyphosate and the soil aluminium and iron

oxides are most favorable for complex for-

mation (4, 6, 9).

To obtain a better insight of the possible

correlation of the two superphosphate ef-

fects (a decrease of pH against an increase

of glyphosate adsorption), the most acidic

soil (KA1) was limed and the most alkaline

and least adsorptive soil (H3) was acidified.

Glyphosate adsorption was then compared

with soil samples that had been limed or

acidified at various levels and subsequently

amended or not with superphosphate.

Liming of the KA1 soil with 0,5% CaCO

3

caused a sharp increase of pH and liming

with 1 or 2% of CaCO

3

caused a slight fur-

ther increase to the saturation pH of about

7.7 (Figure 5A). Glyphosate adsorption on

these soil samples followed a pattern that

mirrored that of the pH (Figure 5B). Sam-

ples amended with superphosphate had

the same pH and glyphosate adsorption

patterns but shifted to lower pH values and

higher adsorption values, as it would be ex-

pected from a CaCO

3

neutralization by the