Page 23 - Volume 2, Issue 1, January 2009

Hellenic Plant Protection Journal

2:

23-32, 2009

Effect of superphosphate fertilizer on glyphosate adsorption

by four Greek agricultural soils

C.N. Giannopolitis

1

and V. Kati

2

Summary

Single superphosphate fertilizer (0-20-0) applied to four distinct surface soils from Greek

agricultural fields, at a rate that provided an elevated phosphorus supply (220-260 ppm P), increased

glyphosate and aminomethylphosphonic acid (AMPA) adsorption by the two soils and did not have

any effect on their adsorption by the other two soils. These effects of superphosphate are contrary

to the expected reduced adsorption if phosphorus had competed glyphosate for the same adsorp-

tion sites in the soils. The superphosphate-induced increase of adsorption was associated with a par-

allel decrease of the soil pH which was caused by the fertilizer in the neutral or slightly acidic soils but

not to the alkaline and calcareous soils. Further evidence that the effect of superphosphate on gly-

phosate soil adsorption is brought about by its effect on soil pH was obtained by measuring adsorp-

tion after liming of an acidic soil and after strong acidification (using sulfuric acid) of an alkaline soil.

The increased glyphosate adsorption in one of the soils amended with superphosphate resulted in

an apparent retardation of glyphosate decomposition and AMPA accumulation, indicating that it was

sufficient to reduce availability of glyphosate to soil microorganisms. These results provide good evi-

dence that superphosphate fertilizer applied to Greek agricultural soils can affect glyphosate adsorp-

tion more positively (by reducing the soil pH) than negatively (by a possible competition for adsorp-

tion sites between phosphorus and glyphosate) and thus cannot contribute to an increased risk of gly-

phosate leaching.

Additional keywords: AMPA, calcareous soils, herbicide leaching, herbicide persistence, liming, soil pH

and iron oxides to play an important role (9,

12). At present, it is generally accepted that

the phosphonic moiety of the glyphosate

molecule controls the adsorption by com-

plexation through hydrogen bonding. Soil

organic matter seems to have only an indi-

rect effect by a blockage of adsorption sites

while the pH of the soil solution is the most

important factor for adsorption because

it affects the electrical charge of both gly-

phosate and the soil hydrous oxides (2).

Since glyphosate is adsorbed to soil in a

manner similar to phosphorus, phosphate

fertilization has been suspected as able

to negatively affect the adsorption of gly-

phosate through competition for similar ad-

sorption sites (7). There has been concern

that applying glyphosate on soils rich in in-

organic phosphate or in soils with a low un-

occupied P-adsorption capacity may result

in free glyphosate in the soil solution which

can be available for plant root uptake and

Introduction

Glyphosate has been one of the world’s

most applied herbicides since it came into

the market in 1974 and its current use is fur-

ther expanded with the incorporation of re-

sistance genes into genetically modified

crops grown in large acreage. It is a non se-

lective foliar-appliedherbicidewhich is read-

ily absorbed through foliage and shoots and

translocated throughtout the entire plant.

Root absorption does not normally seem to

contribute to herbicide uptake by plants, as

glyphosate is quickly adsorbed to soil be-

coming unavailable to roots.

Glyphosate is adsorbed mainly by the

mineral phase of the soil, with aluminium

Laboratory of Chemical Weed Management (1) and

Laboratory of Weed Biology (2), Department of Weed

Science, Benaki Phytopathological Institute, 8 St. Delta

str., GR-145 61 Kifissia (Athens), Greece

Corresponding author:

C.Giannopolitis@bpi.gr