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Solubility products ofsix metal‐glyphosatecomplexes in water andforestry soils, and theirinfluence on glyphosatetoxicity to plantsA. Sundaram a & K.M.S. Sundaram aa Canadian Forest Service , NaturalResources Canada , 1219 Queen Street East,P.O. Box 490, Sault Ste. Marie, Ontario, P6A5M7, CanadaPublished online: 14 Nov 2008.

To cite this article: A. Sundaram & K.M.S. Sundaram (1997) Solubilityproducts of six metal‐glyphosate complexes in water and forestry soils, andtheir influence on glyphosate toxicity to plants, Journal of EnvironmentalScience and Health, Part B: Pesticides, Food Contaminants, and AgriculturalWastes, 32:4, 583-598

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J. ENVIRON. SCI. HEALTH, B32(4), 583-598 (1997)

SOLUBILITY PRODUCTS OF SIX METAL-GLYPHOSATE COMPLEXES

IN WATER AND FORESTRY SOILS, AND THEIR INFLUENCE ON

GLYPHOSATE TOXICITY TO PLANTS

Key words: Metal-ligand complex, sandy loam soil, clay loam soil, tomatoplant bioassay, spruce seedling bioassay, phytotoxicity

A. Sundaram and K.M.S. Sundaram

Natural Resources Canada, Canadian Forest Service, 1219 Queen Street East,P.O. Box 490, Sault Ste. Marie, Ontario, Canada P6A 5M7

ABSTRACT

The solubility products (Ksp) of 1:1 complexes of glyphosate, [N-

(phosphonomethyl)glycine], with Mg2+, Ca 2 + , Mn2+, Zn2+, Cu 2 + and Fe 3 + , were

determined in buffered (pH 7.0) distilled water, moist Ottawa sand, sandy loam

and clay loam soils, each adjusted to 0.02 M with respect to KNO3. The Κsp

values decreased in the order of Mg Ca > Mn > Zn > Cu > Fe, regardless of

the medium in which they were determined. The constants measured in Ottawa

sand were similar to those in water, but those in the forestry soils depended upon

the type of metal ion involved. The values for the Mg, Ca, Mn and Zn complexes

were about 2 to 3 times lower in sandy loam soil than those in water, but those

in clay loam were about 3 to 10 times lower. The Κsp of the Cu and Fe

complexes were similar to those in water regardless of the soil type used.

In a bioassay experiment using tomato plants, immersed in the saturated

solutions of the complexes or planted in the sand and soils containing saturated

solutions of the complexes, no mortality occurred although slight inhibition in

583

Copyright © 1997 by Marcel Dekker, Inc.

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584 SUNDARAM AND SUNDARAM

growth was observed in all cases. The most soluble complexes of Mg and Ca

caused the most reduction in plant height, while the least soluble complex of Fe

caused little growth inhibition. The Zn, Cu and Mn complexes caused some

growth inhibition depending on their Κsp values. The larger the solubility product,

the greater the concentration of glyphosate ion in solution, and the greater the

growth inhibition. In a similar experiment using white spruce seedlings, growth

inhibition was insignificant over the 12-d bioassay period.

INTRODUCTION

The inactivation of glyphosate [N-(phosphonomethyl)glycine], by soil

constituents has been studied for several years. Microbial degradation was

considered as one of the factors contributing to glyphosate inactivation

(Torstensson and Aamisepp, 1977; Hoagland and Duke, 1982). While this

process occurred gradually over a period of weeks or months (Rueppel et al.,

1977), the herbicidal activity disappeared in a much shorter period in soils

(Hensley et al., 1978). Adsorption to organic matter and clay minerals in soils

occurred much more rapidly than microbial degradation (Sprankle et al., 1975a),

and it was postulated that the loss in activity was primarily due to the formation

of metal-glyphosate complexes (Hanson and Rieck, 1976).

Glyphosate has three functional groups (amine, carboxylate and

phosphonate) which are responsible for strong coordination with metal ions

(Pearson, 1963). Madsen et al. (1978) prepared some metal-glyphosate

complexes and determined their dissociation constants in water by potentiometric

pH titration. Subramaniam and Hoggard (1988) prepared several complexes of

glyphosate with metal ions and determined their structures by X-ray powder

diffraction spectra. However, information is sparse on the solubility products of

metal-glyphosate complexes in water and forestry soils, and their influence on

biological processes in plants grown in the soils, is not known.

The objectives of the present study were (i) to prepare six sparingly soluble

metal-glyphosate complexes, (ii) to determine their solubility products in

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SIX METAL-GLYPHOSATE COMPLEXES 585

phosphate buffered water and in buffered sandy loam and clay loam soils by

using a high-performance liquid Chromatographie (HPLC) method (Sundaram and

Curry, 1996), and (¡ii) to evaluate the relationships between the solubility products

obtained and bioavailability of glyphosate to plants.

MATERIALS AND METHODS

Preparation of Metal-Glyphosate Complexes

All solutions were prepared in distilled water. A buffer solution (pH 7.0) was

prepared by adding 61.3 mL of 0.1 M K2HPO4 to 38.7 mL of the same molar

KH2PO4. Potassium hydroxide, 0.05 M, was prepared from May and Baker

concentrated solution. Potassium nitrate, 0.1 M, was prepared from analytical

grade. Glyphosate was prepared from the commercial formulation concentrate,

Vision®, by ion-exchange chromatography on Dowex® 1 (Madsen et al., 1978).

Formic acid (2 M) was used for elution, and the crude product was recrystallized

three times from 1:1 v/v water/ethanol mixture. The purity of the final product was

99.1% w/w, as determined by HPLC (Sundaram and Curry, 1996).

Six complexes were prepared using Mg2+, Ca2+, Cu2+, Zn2+, Mn2+ and

Fe3+. The procedures used for Mg2+, Ca2+, Cu2+ and Fe3 + complexes were the

same as described by Subramaniam and Hoggard (1988), except that KOH was

used instead of NaOH. The procedures used for Zn2+ and Mn2+were similar to

those of Cu2+ complex. Briefly, the method involved mixing 0.010 mole of metal

salts with 0.010 mole of glyphosate in an KOH-based alkaline medium for the five

divalent ions, and with 0.020 mole of glyphosate for the trivalent ion, Fe3+. The

mixtures were allowed to stand at room temperature until crystalline precipitates

were formed because of their low solubility in water. The structures of Mg2+,

Ca2+, Cu2 and Fe3 + complexes were established by Subramaniam and Hoggard

(1988) as Mg(Hglyp).2.25H2O, Ca(Hglyp).2.25H2O, [Cu(Hglyp). H2O].0.4CuO and

[Fe(glyp) (H2O)2].1.25 H2O respectively. However, the structures of Zn2+ and

Mn2+ complexes were assumed to be similar to that of Cu2+.

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586 SUNDARAM AND SUNDARAM

Solubility Products of Metal-Glyphosate Complexes in Phosphate BufferedDistilled Water fpH 7.0)

All solutions, except the phosphate buffer, were adjusted to a concentration

of 0.02 M with respect to KNO3. Three replicate samples (1.0 g) of each metal-

glyphosate complex were taken in 23.75 mL of distilled water (adjusted to 0.02

M with respect to KNO3) in a Pyrex® graduated cylinder, and an aliquot of 1.25

mL of the buffer was added. The cylinders were placed in a thermostatic bath

(20°C), and the contents were stirred at 2000 rpm. Fifteen minutes later, the

stirring was discontinued and the crystals were allowed to settle at the bottom.

The aqueous phase was transferred and centrifuged at 4000 rpm. The

supernatant was stored at -20°C. Prior to analysis, the samples were thawed at

20°C and a 20-mL aliquot was subjected to glyphosate extraction following the

principle of Wigfield and Lanouette (1990), but with some modifications. The 20-

mL aliquots (pH adjusted to 9.0 by adding drops of 0.2 M NaOH) were passed

through an anion-exchange column (Bio-Rad, Richmond, California) containing

the AG 1-X8 resins in the hydroxide (top 1 mL) and chloride (bottom 2 mL) forms.

The column was washed with 20 mL of distilled water and the glyphosate was

eluted using 10 mL of 0.4 M sodium citrate buffer at pH 5.O. The control water

samples fortified with glyphosate standards, were also extracted similarly.

The concentrations of glyphosate present in the samples were determined

by the HPLC method of Sundaram and Curry (1996). Briefly, the method involved

injection of samples containing glyphosate into a cation exchange column and

then oxidation with calcium hypochlorite in a post-column reactor coil at 43°C to

form glycine. The glycine formed was treated with O-phthalaldehyde in presence

of mercaptoethanol to form a fluorophore and was detected with a fluorescence

detector ( λ β χ = 230 nm, kem = 445 nm). The recovery level (mean ± SD) of

glyphosate from fortified water was 97 ± 7%, and the detection limit was 3

ng/mL. The solubility products of the six metal-glyphosate complexes in buffered

distilled water were calculated and are presented in Table 1.

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Table 1. Solubility products of metal-glyphosate complexesOttawa sand and forestry soils [K (S)]

Complex

Mg (Hglyp).[2.25 H2O]

Ca (Hglyp).[2.25 H2O]

[Cu (Hglyp).H2O]. 0.4 CuO

Zn (Hglyp)3

Mn (Hglyp)3

[Fe (glyp)(H2O)2].1.25H2O

Water

0.933 χ 10"4

1.07 χ 10"4

2.69 χ 10"1 3

5.62 x10" 1 0

0.955 χ 10'6

5.62 x10" 1 5

Ottawa sand

0.989 x10"4

(0.9434)b

1.33 χ 10"4

(0.8045)

2.93 χ 10"1 3

(0.9181)

5.31 χ 10"1 0

(1.058)

0.977 χ 10"6

(0.9775)

5.77 χ 10"15

(0.9740)

in phosphate buffered water

Sandy loam

0.311 x10' 4

(3.000)

0.426 χ 10"4

(2.512)

2.55 χ 10"1 3

(1.055)

3.03 x10" 1 0

(1.855)

0.411 x10"6

(2.324)

5.51 χ 10"15

(1.020)

[Ks (aq)] and in buffered

Clay loam

0.0885 χ 10"4

(10.54)

0.115 χ 10"4

(9.304)

2.42 x 1 0 ' 1 3

(1.112)

1.91 x10" 1 0

(2.942)

0.0924 χ 10"6

(10.34)

5.42 χ 10' 1 5

(1.037)

SIX M

EHI - 1-G

LY

PH

C

CO

mη)M

PL

EX

ES

a: These are 1:1 metal-glyphosate complexes (Madsen et al., 1978), but their exact molecular structures are not known.

b: Values in parenthesis refer to the ratio calculated as (solubility product in buffered water) / (solubility product inbuffered sand or soils), or [K (aq)] / [K (S)].

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588 SUNDARAM AND SUNDARAM

Solubility Products of Metal-Glyphosate Complexes in Buffered Ottawa Sandand Forestry Soils

The solubility products of the complexes in Ottawa sand and two types of

forestry soils were determined. Ottawa sand (20 to 30 mesh) was purchased from

Fisher Scientific, Toronto, ON. Sandy loam soil was collected from an area about

80 km northwest (46°53'N, 84°03'W) of Sault Ste. Marie, and clay loam soil from

a location about 30 km southeast (46°23'N, 84°01'W) of Sault Ste. Marie. The

ground vegetation was uprooted, and the litter layers were removed to expose

the soil underneath. Soil samples were then collected as single cores by driving

a cylindrical metal tube (5 cm in diameter) to a depth of 2 cm. Each sample was

packed separately in aluminum foil, and brought to the laboratory. In the

laboratory, the samples were sieved (2-mm openings) to remove stones, twigs,

roots etc., and stored at -20°C. Prior to use, the soils were thawed at room

temperature, rehydrated to a constant moisture level of 45% w/w, and sterilized

in an Amasco 2022 isothermal autoclave (Eagle Series, American Sterilizer

Company, Erie, PA) for 1 h at 121 °C.

The characteristics of Ottawa sand and the two soils are given in Table 2.

The percentage of organic matter was determined by using the loss-on-ignition

method (Davies, 1974). The amount of sand, silt and clay was determined as

described by Bouyoucos (1962). The cation exchange capacity (CEC) was

measured according to the method of Cappo et al. (1987). The moisture content

was determined by taking 5-g aliquots of the samples and drying them in a

thermostatlc oven at 120°C (Anon., 1955) for 16 h.

Concentrations of extractable metal ions (Table 2) were determined by using

a new procedure developed in our laboratory. The principle involved complexation

of metal ions with ethylenediaminetetra-acetic acid (EDTA). An aliquot of 0.3 g

of soil was taken in a 50-mL centrifuge tube (eight replicates) and 30 mL of 0.1

M Na4P2O7.10H2O was added. The contents were mixed for 16 h in a

mechanical shaker and centrifuged at 9000 rpm for 10 min. The supernatant was

decanted and adjusted to pH 9.0 by adding an ammonia buffer (prepared by

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SIX METAL-GLYPHOSATE COMPLEXES 589

Table 2. Characteristics of Ottawa sand and forestry soils used for determiningsolubility products of metal-glyphosate complexes

Parameter

Organicmatter8

Sanda

Silta

Claya

Moisturea

CECa

pH

Feb

Alb

Znb

Cub

Cab

Mgb

Ottawa sand

0.00

99.982

0.00

0.00

0.018

0.242

6.5

NDC

ND

ND

ND

ND

ND

Sandy loam

ca4

51

42

7

24.3

72.2

5.9

4.5

6.3

2.2

4.4

11.2

5.6

Clay loam

ca 3

18

40

42

25.7

29.6

5.3

9.2

11.4

3.2

2.8

24.4

4.5

a: Values are presented in percentages.

b: The data are expressed in pg/g wet weight.

c: ND = none detected.

mixing concentrated ammonia solution to 17.5 g of analytical grade NH4CI, and

diluting to 250 mL). The individual metal ions present in the supernatant were

determined by complexometric titration with EDTA, after selecting suitable

masking agents to prevent interference from the remaining metal ions and using

selective metal ion indicators to obtain sharp colour changes (Vogel, 1964).

Prior to mixing the metal-glyphosate complexes with Ottawa sand and

forestry soils, the metal ions present in the soils were removed by extraction with

Na4P2O7.10H2O as described above. This was done because the metal ions

would likely affect the solubility products of the complexes. After extraction, the

soils were dried to constant weight at 80°C. Aliquots (1.0 g) of each complex

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590 SUNDARAM AND SUNDARAM

were thoroughly mixed with 25 g of Ottawa sand, sandy loam or clay loam soil.

The moisture level was brought up to 64.3% w/w by adding calculated amounts

of distilled water (adjusted to 0.02 M with respect to KNO3) containing 5% v/v of

the phosphate buffer (pH 7.0). The soils were taken in centrifuge tubes and

shaken in a mechanical shaker for 16 h. Aliquots (4.0 g each) were extracted with

2 χ 10 mL of aqueous NaOH (pH 9.0), each time using a vertical sample shaker.

The contents were centrifuged at 5000 rpm for 20 min, the supernatant was

filtered through a 0.2 μιτι Nylaflo® filter (Gelman Sciences, Rexdale, ON). The

filtrate was partitioned with dichloromethane and the aqueous phase was cleaned

on the Bio-Rad anion-exchange column as described above. Control and fortified

soil samples were also extracted similarly. Further purification of the soil eluates

was done using the AG 50W-X8 cation exchange column (Wigfield and

Lanouette, 1991), and glyphosate concentrations were determined using the

HPLC method (Sundaram and Curry, 1996) as described above. The recovery

level (mean ± SD) of glyphosate from fortified Ottawa sand or the forestry soils

was 92 ± 11%, and the detection limit was 12 ng/g wet weight. The solubility

products of the six metal-glyphosate complexes in buffered Ottawa sand, sandy

loam and clay loam soils were calculated and are presented in Table 1.

Bioassay Experiments in Aqueous Solutions of the Metal Complexes

Distilled water (adjusted to 0.02 M with respect to KNOg) containing the

phosphate buffer (5% v/v) was saturated with each metal-glyphosate complex as

above. Tomato (Lycopersicon esculentum) seeds were germinated in soils in

styrofoam cups and the seedlings were allowed to grow until they were ca 12 cm

high above the soil. The seedlings were removed from the cups. The roots were

washed with water to remove the soil and placed in 25 mL of phosphate buffered

distilled water (saturated with the metal complexes) in a conical flask at the rate

of one seedling per flask. A split cork was used to support the stem of the plant

at the desired height. Eighteen seedlings (divided into three groups of six) were

used per complex. Control seedlings were placed in similar flasks in phosphate

buffered distilled water that did not contain the metal-glyphosate complexes. The

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SIX METAL-GLYPHOSATE COMPLEXES 591

*-> 30

υ

~ 2 5

JZσ» 20

1 15ο.

10

~ 30Εα

~ 2 5

αϊΟ»

'* 20

1 15Ο-

ΙΟ

Water

-

*' . · ' ' ! • •

ι ι ι

Control, Zn,

Μη

Ug, Ce

Ottawa sand

-

" "Λν;;·;;»·

' . - " ' .·<•'•·

•;:£•'•'••*""

Cu.

ι

σ'"

.τ**

!

Fe

.Ο

,.ν..τ

. . .

ι

Ζη,

Μη

Mg,

Cu

Ca

Sandy soll

o ^

. g ; ? ' : : : : 5 :

I I 1

Clay soil

-

o o ^

«: * '. v I

I 1 1

,.··•*

• ' ' " ' . .

::'-'o—

:"::f::

.•V*

. - · • - '

1

--•τ ·*

t

- - • ' * "

. . V

1

1

Zn,

Mn

Mg.

I

Zn,Mn

Mg.

1

1

Cu

Ca

Cu

Ca

2 4 6 8 10 12 14

Days after incubation

2 4 6 8 10 12 14 16

Days after incubation

Figure 1. Height of tomato plants after Immersing In buffered distilled water, Ottawa sand,

sandy loam soil and clay loam soil saturated with metal-glyphosote complexes

flasks were placed in an environmental chamber (temperature 20°C, relative

humidity 75%, and a photoperiod of 16 h light and 8 h darkness) for four days,

and plant growth was monitored every day by measuring the height above water

level. Any growth inhibition caused by the giyphosate ion in solution is presented

in Figure 1 along with the heights of appropriate control plants.

In a similar bioassay experiment, green-house grown white spruce seedlings

[Picea glauca (Moench) Voss], 12 cm high, were placed in phosphate buffered

distilled water saturated with metal complexes, and plant growth was monitored

for a 12-d period. The data were expressed in changes in height (± Ah in cm) of

treated plants compared to that of controls, and are presented in Table 3.

Bioassay Experiments in Moist Ottawa Sand and Forestry Soils ContainingSolutions of the Metal Complexes

The two forestry soils were extracted with Na4P2O7.10H2O to remove the

metal ions present in them, and dried to constant weight at 80°C. Aliquots of the

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592 SUNDARAM AND SUNDARAM

Table 3. Changes in heighta of white spruce seedlings after incubating for 12 d inbuffered distilled water, Ottawa sand, sandy loam soil and clay loam soilcontaining saturated solutions of metal-glyphosate complexes (valueswere expressed in comparison to the heights of control seedlings)

Metal-glyphosate

complex

Mg (Hglyp).[2.25 H2O]

Ca (Hglyp).[2.25 H2O]

[Cu (Hglyp).H2O]. 0.4 CuO

Zn (Hglyp)b

Mn (Hglyp)b

[Fe (glyp).(H2O)2] 1.25 H2O

Mg (Hglyp).[2.25 H2O]

Ca (Hglyp).[2.25 H2O]

[Cu (Hglyp).H2O]. 0.4 CuO

Zn (Hglyp)b

Mn (Hglyp)b

[Fe (glyp).(H2O)2] 1.25 H2O

Changes in height

2 d

-1.1

- 1.0

<

0.0

0.0

< —

< _

4 d

(± Ah in

6 d

cm) over a period of

8 d

Distilled water and Ottawa

-2.0

- 1.5

Sandv loam

0.0

0.0

0.0

0.0

None -

None -

None -

None -

and clay

0.0

0.0

- None

- None

- None

- None

0.5

0.5

loam

0.5

0.5

10d

sand

+ 1.2

+ 1.5

soils

+ 1.3

+ 2.2

12d

+ 2.1

+ 1.5

>

+ 2.1

+ 2.3

a: Values represent the mean of three sets of seedlings, each set consisting ofsix.

b: These are 1:1 metal-glyphosate complexes (Madsen etal., 1978), but their exactstructures are not known.

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SIX METAL-GLYPHOSATE COMPLEXES 593

Ottawa sand and the dried forestry soils were taken in plastic pots and mixed

with the saturated solutions of each metal-glyphosate complex (pH 7.0, and

adjusted to 0.02 M with respect to KNOg), to provide a ratio of 18 g solution to

10 g soil. Tomato plants and white spruce seedlings were planted in the soil (one

plant per pot). The pots were placed in the environmental chamber at the same

temperature, relative humidity and photoperiod as above, and plant growth was

monitored for 12 days. During this period, the moisture content of the soil (64.3%

w/w) in the pots was maintained constant by periodic addition of required

quantities of phosphate buffered distilled water. Any growth inhibition caused by

glyphosate formed by solubilization of the metal-glyphosate complexes is

presented in Figure 1 for the tomato plants, and in Table 3 for the conifers.

RESULTS AND DISCUSSION

Solubility Products of Metal-Glyphosate Complexes in Phosphate BufferedWater (pH 7.0 and 0.02 M with respect to KNO3)

The solubility product (K ) of a 1:1 metal-ligand complex that is sparingly

soluble in a medium can be defined by equation (1),

tM Solution N- solution = Ksp C)

where [Mn+]so|ut¡on a n c ' [^solution r e*e r t 0 t n e concentrations (in moles/L) of the

solubilized metal and ligand (glyphosate) ions respectively.

From equation (1), the concentrations of glyphosate ion (measured by

HPLC) in water would be identical to the concentration of the metal ion (not

measured). Therefore, the measured glyphosate concentration (moles/L) when

multiplied by the same value refers to the solubility product (KSD) of each complex

in water, as shown by equations (2) and (3).

For a divalent metal ion,

{[HglyP2-]aq}

2 = [M2+]aq [Hglyp2-]aq = Ksp (aq) (2)

For a trivalent metal ion,

{[giyp3']aq}2 = [M3+]aq [giyp3-]aq = Ksp(aq) (3)

Accordingly, the Κ (aq) values for the six complexes in the buffered water were

calculated using equations (2) and (3) and are given in Table 1.

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594 SUNDARAM AND SUNDARAM

Solubility Products of Metal-Glyphosate Complexes in Phosphate BufferedOttawa Sand and Forestry Soils

To calculate the solubility products, Κ (S), in Ottawa sand and forestry

soils, the metal ion concentrations were assumed to be the same as those

obtained in the buffered water (i.e., same as the concentrations of glyphosate

measured). Then using the measured concentrations of glyphosate in the sand

and soils, values of Κ (S) were calculated using equations (4) and (5):

For M2+, [Hglyp2-]aq = [M2 +]s; but, [M2 +]s * [Hglyp2-]s

Therefore, Ksp (S) = [M2 +]s [Hglyp2-]s (4)

For M3+, [glyp3\q = [M3+]s; but [M3 +]s * [glyp3-]s

It follows then, Ksp (S) = [M3 +]s [glyp3"]s (5)

The Κ (S) values in the Ottawa sand and the two forestry soils are given

in Table 1. The data indicate that, for each complex, the Κ (aq) was about the

same as Κ (S) in Ottawa sand, evident from the ratio [K (aq)] / [K (S)] of

about 1.0. However, K (S) values were smaller in the sandy loam soil, and the

ratios ranged from 1.020 for the least soluble Fe-glyp complex to 3.000 for the

most soluble Mg-Hglyp complex. The K (S) values in the clay loam soil were

generally much smaller than the Κ (aq), and the ratios ranged from 1.037 for

the Fe-glyp complex to 10.54 for the Mg-Hglyp complex. The greater the solubility

of the complex, the higher the glyphosate concentration in solution, and the

higher the ratio. Thus, the least soluble complexes appear to provide similar

glyphosate concentration in buffered water, Ottawa sand and in the two forestry

soils. In contrast, the complexes with high solubility appear to provide less

glyphosate concentration in the forestry soils than in the buffered water. This

apparent discrepancy can be explained on the basis of interaction between the

solubilized glyphosate ion and soil constituents.

It is worth noting that solubilization of these complexes is expected to occur

similarly in all aqueous media, implying that the Κ (aq) would probably be equal

to K (S). However, the concentration of the solubilized glyphosate ¡on must

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SIX METAL-GLYPHOSATE COMPLEXES 595

have disappeared in soils soon after dissolution. Microbial decomposition is

unlikely because the soils were autoclaved prior to the experiment. Adsorption to

the soil constituents occurs invariably (Muljadi et al., 1966; Levesque and

Schnitzer, 1967; Tisdale and Werner, 1968), although the mechanism of

adsorption is unlikely due to the formation of metal-glyphosate complexes,

because the metal ions present in the soils were extracted prior to the

experiment. However, it is possible that not all types of metal ions might have

been extracted. Especially, aluminum ion concentration is known to be high in

clay loam soils, and consequently, any unextracted Al3+ could have complexed

with the dissolved glyphosate ion (Sprankle et al., 1975b). On the other hand,

glyphosate was also known to be strongly adsorbed to organic matter in soils

(Nomura and Hilton, 1977). Conversion to the metabolite, aminomethylphos-

phonic acid, is unlikely because of the short duration of the experiment.

Tomato Plant Bioassay

Figure 1 presents the heights of tomato plants after incubating in the

buffered distilled water, Ottawa sand, sandy loam and clay loam soils (each

containing the metal-glyphosate complexes). No mortality occurred in plants due

to the presence of solubilized glyphosate, although growth inhibition was noted

in all cases. The highly soluble complexes of Mg and Ca provided the highest

concentration of glyphosate ¡on in solution, and caused the most reduction in

plant height [ANOVA P<0.1, Ryan et al. (1985)], regardless of the medium used.

Similarly, the least soluble complex of Fe provided the lowest amount of

glyphosate ion in solution, and caused little reduction in plant heights (Figure 1).

The Zn, Cu and Mn complexes caused some growth inhibition depending upon

their solubility products. The larger the solubility product, the greater the

concentration of glyphosate ion in solution, and the greater the growth inhibition.

In the buffered water, however, complexes of Zn and Cu behaved similar to that

of Fe, and failed to cause any growth inhibition.

Differences in growth patterns were also observed between the sandy loam

and clay loam soils. This was particularly observed in the case of Mn-glyphosate

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596 SUNDARAM AND SUNDARAM

complex. Growth inhibition occurred significantly more in sandy loam soil than in

clay loam (ANOVA Ρ < 0.07). Once again this behaviour is in agreement with the

solubility products of this complex in the two media (Table 1).

Spruce Seedling Bioassay

Table 3 presents changes in heights of spruce seedlings (± Ah in cm) after

incubating for a 12-d period in buffered distilled water, Ottawa sand, sandy loam

and clay loam soils (each containing the metal-glyphosate complexes). It is

apparent that, during the initial 4-d period, slight growth inhibition was noted in

seedlings incubated in distilled water and Ottawa sand, but none was observed

in seedlings planted in the sandy and clay loam soils (ANOVA Ρ < 0.07).

Surprisingly, a slight increase in growth was noted beyond the 6-d post-incubation

period in all seedlings regardless of the medium used (ANOVA Ρ < 0.08). The

reason for this behaviour is not known, and requires further investigation.

CONCLUSIONS

The present study showed that the solubility products (K ) of metal-

glyphosate complexes decreased in the order of Mg ~ Ca > Mn > Zn > Cu > Fe,

whether they were measured in buffered (pH 7.0) distilled water, moist Ottawa

sand, sandy loam soil, or clay loam soil (each adjusted to 0.02 M with respect to

KNO3). The Κ (S) constants measured in Ottawa sand were similar to those

in water Κ (aq) , but those in the forestry soils depended on the type of metal

¡on involved. The greater the solubility of a complex, the higher the glyphosate

ion in solution, and the greater the difference in the two values, Κ (aq) and Κ

(S). These findings have been explained on the basis of interaction between the

dissociated glyphosate ion and the soil constituents.

In the bioassay experiments using tomato seedlings, the most soluble

complexes caused the most reduction in plant height, while the least soluble

complex caused little growth inhibition. In a similar experiment using white spruce

seedlings, slight growth inhibition was noted during the initial 4-d period in

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SIX METAL-GLYPHOSATE COMPLEXES 597

seedlings incubated in distilled water and Ottawa sand, but no growth inhibition

was observed in the seedlings planted in the two forestry soils.

ACKNOWLEDGEMENTS

The authors thank John Leung, John Hatherley, Reg Nott, Johanna Curry and

Linda Sloane for their technical assistance in this investigation.

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Received: November 27, 1996

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