Page 25 - Volume 2, Issue 1, January 2009

Effect of superphosphate on glyphosate adsorption in soil

25

analytical grade CaCO

3

from Acros Organics

(Belgium) and acidification of the H3 soil by

using a 2 N solution of sulfuric acid (Merck,

pro-analysis, 95-97%). After thorough mix-

ing, 100 g samples of the treated soils were

placed in the above plastic cups, watered to

field capacity and left to equilibrate for 4-7

days in the growth chamber.

All the treated soil samples along with

the respective controls were dried in the

oven (40

o

C) for 4 hours and sieved through

a 2-mm sieve before used for batch equili-

bration tests to determine glyphosate and

AMPA adsorption.

Measurement of glyphosate and AMPA

adsorption

The capacity of the various soil samples

to adsorb glyphosate and AMPA was deter-

mined by conducting batch equilibration

tests using aqueous glyphosate and AMPA

solutions of various concentrations. The

tests were performed by placing 1 or 1.5 g of

the soil sample and 10 ml of the glyphosate

+ AMPA solution in 25-ml glass test tubes.

The tubes were kept in an orbital shaker for

2 hours to equilibrate (12), then centrifuged

at 5000 rpm for 10 min and the superna-

tants collected and analysed. The amount of

glyphosate and AMPA adsorbed by the soil

was calculated by substracting the amount

found in the supernatant from that in the

initial solution.

The glyphosate and AMPA solutions

used in these studies were prepared using

analytical reference standards (Monsanto,

certified as 99.8 and 99.5% respectively). An

aqueous stock solution containing 500 μg/

ml of each, of the two compounds, was pre-

pared in HPLC-grade water and working so-

lutions of various concentrations were pre-

pared by diluting with de-ionized water.

Glyphosate and AMPA were quantita-

tively determined using cation exchange

HPLC and fluorescence detection following

post-column derivatization with hypochlo-

ride and o-phthalaldehyde (OPA), which is

an improved version of the US EPA method

547 (10, 14.). The instrumentation was as de-

scribed before (5). Each sample solution was

first diluted with de-ionized water as need-

ed and filtered through a 0.22 μm dispos-

able syringe filter with a PTFE membrane,

into a 2 ml amber borosilicate glass vial and

then directly injected into the HPLC system

at 20-50 μl.

All tests were set in a completely ran-

domized design with the treatments repli-

cated three times. The combined data from

each test were subjected to ANOVA and in

most cases to an LSD comparison of the

treatment means. Most of the conducted

tests were repeated three times and the re-

sults obtained from a typical run of each test

are presented here.

Results and Discussion

Addition of superphosphate fertilizer to

the soil increased the amount of glyphosate

adsorbed by the soils KA1 and H1 but did

not affect glyphosate adsorption by the

soils K2 and H3 (Table 2). The increased gly-

phosate adsorption by the soils KA1 and H1

was evident one week after the addition of

superphosphate and lasted for at least one

month. In further experiments with short-

er time intervals after the addition of su-

perphosphate, it was realized that the gly-

phosate adsorption started to increase as

soon as two days after its addition (data not

presented).

The addition of superphosphate also

caused a decrease in the pH of all four soils

(Table 2), although to a varying extent de-

pending on the soil. This pH-decreasing ef-

fect of the superphosphate seems to be ac-

companied by an increase of glyphosate

adsorption only in the two soils (KA1 and H1)

in which the pH was lowered to a value be-

low about 7.0.

The increased glyphosate adsorption,

after the addition of superphosphate, by the

soils KA1 and H1 but not by the soils K2 and

H3, was further confirmed by comparing the

adsorption isotherms obtained with three

glyphosate concentrations and soils amend-

ed or not with superphosphate (Figure 1).

The applied rate of superphosphate fer-