Page 24 - Volume 2, Issue 1, January 2009

Giannopolitis & Kati

24

injury to transplanted sensitive crops (3) as

well as for leaching into the soil and con-

tamination of underground waters (2, 13).

Studies conducted so far have indeed es-

tablished that a reduced adsorption may

be observed when glyphosate is applied

to soils that have prior been heavily fertil-

ized with phosphates (4, 8, 11) but the prac-

tical implications of this have not been de-

termined. Furthermore, recent studies show

that the extent to which glyphosate adsorp-

tion is reduced by phosphates can vary dra-

matically in different soils making predic-

tion even more difficult (2).

However, besides the variability of ef-

fects depending on soil type, variable ef-

fects might also be expected depending

on the type of the phosphate fertilizer that

is used. In addition, besides the competi-

tion between phosphorus and glyphosate,

phosphate fertilizers might also affect gly-

phosate adsorption by other means, eg. by

altering the soil pH and this may be true with

a variety of other fertilizers as well. We ex-

amined this possibility by using superphos-

phate fertilizer, which is widely used for the

basal fertilization of many crops at planting,

with typical agricultural surface soils from

Greece. The fertilizer was used at a high rate

to assure maximum possible competition

between phosphorus and glyphosate for

the adsorption sites in all four soils and the

results are presented here.

Materials and Methods

Origin of soil samples

The samples of soil used in these studies

were collected in mid June from the top 10-

cm layer of fields planted to peach orchards

(region of Himathia in Northern Greece, des-

ignated as H1, H2, H3) or vineyards (region

of Korinthia in Southern Greece, designat-

ed as K1, K2) and of uncultivated highland

fields used as pastures (region of Kalavryta

in Southern Greece, designated as KA1). The

fields fromwhich the soil sampleswere taken

had been used as described for many years

and received regular cultivation and fertil-

ization according to the established practic-

es in the respective area. Pasture fields (KA1

soil) were grazed by sheep and had received

no fertilization or other treatment for years.

Some basic characteristics of the soil types

used are presented in Table 1.

Soil treatments

Soil samples were air-dried and sieved

through a 2-mm sieve before use. Soil pH

was determined by preparing 1:1 soil sus-

pensions in deionized water and measuring

with a pH/mV meter equipped with a com-

bined pH electrode and automatic tempera-

ture compensation.

Superphosphate amendment of the

soils was made using granular single super-

phosphate fertilizer (0-20-0) from the Phos-

phoric Fertilizers Industry SA (Greece). To

ensure uniform distribution of the fertilizer

the granules were first ground to a fine pow-

der. The appropriate amount of the powder

was thoroughly mixed with 100 g of soil and

placed in a plastic cup (7-cm height, 6-cm

upper diameter) with 4 holes at the bot-

tom (for watering) covered with a filter pa-

per. The soil was watered (from below) to

the field capacity and kept for 1-4 weeks in

a growth chamber with a light period of 16

hours, day temperature of 25

o

C and night

temperature of 20

o

C.

Liming of the KA1 soil was made by using

Table 1.

Basic characteristics of the soil types used in the experiments.

Soil type

Origin

pH Texture

Other

H1

Peach orchard

7.2 Heavy clay

Dark fertile soil, high Al and Ca

H3

Peach orchard

8.2 Sandy clay

Dark poor soil

K1

Vineyard

7.8 Heavy calcareous

Whitish color, very high CaCO

3

K2

Vineyard

7.6 Heavy calcareous

Whitish color, very high CaCO

3

KA1

Pasture

5.9 Loamy

Red, washed soil, high Fe, low Ca