ALUMINIUM. EXTRACTABLE FROM SOIL SAMPLES BY THE
ACID AMMONIUM ACETATE SOIL-TESTING METHOD

Osmo Mäkitie

Agricultural Research Centre, Department of Soil Science, Tikkurila, Finland

Received December 28, 1967

A salt solution of strong acid is commonly used for the extraction of the exchangeable
forms of soil aluminium, while the determination of soluble aluminium in soils is based on
the extraction treatment by a salt solution of a weak acid (Pratt & Bair 1961). Molar
ammonium acetate solution adjusted to pH 4.8 with acetic acid is often used for this pur-
pose (Black etal., 1965).

Our method of soil-testing uses an 0.5 molar ammonium acetate solution at pH 4.65 as
extractant which is thus a half-neutralized, molar acetic acid solution. This extraction
solution is nowadays in extensive use as a »universal» extractant for the estimation of the
quantities of available plant nutrients in soil samples in Finland (Vuorinen & Mäkitie
1955).

Aluminium is the most important soil acidity component in acid soils where the content
of soluble aluminium is also considerably high. The toxic effect of extraneous aluminium in
soluble form cannot be underestimated in acid soils. The extractability of soil aluminium,
particularly by the soil-testing extractant, and the relations to the exchange characteristics
in our soils, have been studied in the present work.

Experimental

Sample material. The sample material consisted of 30 pre-treated soil samples which had
been air-dried, homogenized and sieved through a 2 mm round-holed sieve. The soil types
and some general characteristics are listed in Table 1.

Extractions. A modification of the original soil-testing procedure was used. The extrac-
tant used was the solution of 0.5 M ammonium acetate 0.5 M acetic acid, at pH 4.65.
The extraction was carried out not by volume but by weight basis of the soil sample and
by repeated shaking and centrifugation of alO g lot of the sample with a total volume of
150 ml of the extractant. The procedure was similar to the common method for extraction

of the total-exchangeable bases from soil samples (Schollenberger & Simon, 1945). The
samples were also pre-treated by washing with 60 % ethanol/water solution.



55

The total-exchangeable cations were leached with molar ammonium acetate solution
and centrifuged according to the common procedure (Mäkitie & Virri, 1965).

The extractions with molar potassium chloride solution were similary carried out. The
acidity of the leachate was also titrated with sodium hydroxide solution against phenol-
phthalein (Table 1, column f).

The exchange acidity was determined with the molar ammonium acetate solution in
accordance with the modification ofBrown’s method (Brown 1943, Mäkitie 1965).

The extractions with molar acetic acid, and with the various ammonium acetate solu-
itons were similarly carried out as with the molar ammonium acetate solution (Mäkitie
1956).

Determinations. Aluminium was determined spectrophotometrically as aluminon (aurin-
tricarboxylic acid) complex (Black et al. 1965, p. 988, Rolfe et al. 1951, Frink & Peech
1962, Hsu 1963). Iron was complexed with thioglycollic acid (Chenery 1948, 1955).

A Beckman Quartz spectrophotometer with 10-mm cells was used for the measurements of
absorption at 525 m|x.

The determinations of the individual metallic cations were carried out by complexometric
titration procedure and by flame photometric methods (Table 1, column k). Brown’s
method was used for estimation of the »S »-value of the permanent exchange complex of soil
(Table 1, column g).

The pH-measurements were taken by means of a Radiometer PHM 4c potentiometer
with a glass electrode and an open bridge reference electrode filled with saturated potassium
chloride solution.

Results and discussion

Four different extraction solutions were at first used; M KCI solution, M acetic acid
solution, the soil-testing extractant 0.5 acetic acid 0.5 M ammonium acetate solution
(pH 4.65), and M ammonium acetate solution. Table 1 shows the data of the different
determinations.

Fig. 1. Average percentages of aluminium in ammonium acetate extracts (molarity of acetate = 1.0)
1 = Coarse soils, 2 = Silts and clay soils, 3 = Organogenic soils. Soil-testing extractant = 100 %.



Table
1.

Data
of
the
samples.
AI,
H
+

and
»S»

concentrations
in

milliequivalents
per
100
g
of
soil.

O.M.
Clay

pH

M
KCI
M
CH

3
COOH
Soil-testing

Exchangeable
cations

%

(2
[i.)

susp.
extraction
extraction
method

(M
CH
3

COONH
4
)

%(1:2-5)

Soil
tyP
e

H
2

O
M
KCI
Al

H+
»S»

AI

»S»

AI

»S»
AI

H+

pH

value

value

value

equil.

abcdefghijkl
mn

10
HHk

(Sand)

4.8

14

5.5

4.8

0.04
0.65

13.5
1.51
12.1

0.56
11.0
0

5.9

6.72

6

KHt

(Finesand)

3.1

4

5.3

4.3

0.53
1.15

2.5

4.93
2.5

2.34
2.2

0.32
5.5

6.75

16
KHt
(

»

)

4.0

8

5.2

4.2

0.87
3.75

3.8

4.45
3.8

1.98

3.4

0.04
7.8

6.67

21
HHt
(Fine
finesand)

8.3

4

5.3

4.6

0.34
0.95
8.5

7.48
7.2

2.30
7.1

0.25
4.7

6.81

22
HHt
(

»

)

6.7

4

5.9

5.0

0.11
0.60

10.5
7.41
9.0

2.57

7.9

0.20
3.6

6.86

11

hsHHt
(Silty
»

)

3.8

30

5.1

3.9

0.51
1.00
11.2
1.51
12.4
1.05
11.3
0

9.6

6.60

4

sHHt
(Clayey
»

)

8.5

28

6.0

5.0

0.05
0.48

15.8
1.73
16.1

1.04
15.7

0.56
7.7

6.67

18
Hs

(Silt

)

4.4

30

6.5

5.6

0.01
0.65
17.2

2.03
15.8

0.58
14.2

0.06
4.9

6.80

17
Hs

(

»

)

5.3

28

6.0

5.1

0.06
1.25
14.2

2.01
13.9

0.78
12.2

0.06
7.3

6.71

2

sHs

(Clayey
silt
)

2.8

42

6.1

5.0

0.06
0.50

10.1
1.96

8.5

1.04
9.1

0.38
4.3

6.80

23
HtS

(Sandy
clay
)

6.7

36

5.1

4.3

0.35
3.40
13.6

2.09
11.8
1.06
11.9

0.06
5.9

6.77

24
HtS

(

»

)

6.0

36

5.7

4.5

0.18
0.53

13.4
1.82
13.9
1.10
13.1

0.10
9.8

6.63

19
HsS

(Silty
clay
)

5.9

42

6.3

5.3

0.06
1.60
16.7

3.22
19.4
1.07
17.3

0.07
7.8

6.69

20
HsS

(

»

)

5.5

42

6.0

4.9

0.05
0.48

16.4
3.18
17.6
1.09

16.3
0.03
8.6

6.67

1

AS

(Heavy
clay
)

7.5

84

5.4

4.2

0.77
1.75
14.1

2.69
16.5

2.78
15.4

0.34
14.6

6.46

13
AS

(

»

)

7.3

78

7.3

6.5
0.02
0.45
59.7
1.47

67.9
0.40
48.9
0

0.3

7.02

12
AS

(

»

)

4.0

48

6.3

5.3

0.03
1.30
18.1

0.85
20.0
0.29
20.0
0

4.1

6.81

9

AS

(

»

)

6.9

42

5.5

4.5

0.10
0.85

18.1

1.20
22.3
0.71
21.1
0

10.3
6.58

5

LjS

(Gyttjaclay
)

20.7

58

5.4

4.3

0.64
1.65
14.3

5.56
15.5

3.50
14.9

0.58
16.8

6.41

3

Lj

(Gyttja
)

11.6

5.0

4.0

1.71
3.05

10.4
4.34
11.4

2.22
12.4

0.19
17.3

6.40

14
Mm

(Mould
)

11.7

5.4

4.1

0.35
1.00

29.9
1.78

30.8
1.13
29.1
0

15.6
6.46

25
Mm
(

»

)

20.8

5.3

4.5

0.35
0.75

16.0
4.56
15.4

2.22
14.3

0.32
17.9

6.42

26
Mm
(

»

)

19.4

5.7

4.7

0.19
0.62
20.1
3.87
20.6
1.76
18.9

0.21
15.0

6.48

8

Ct

(Carexpeat
)

4.4

3.7

1.78
3.85
11.0

9.48
9.4

4.00
8.5

1.60
39.9
6.43

15
Ct

(

»

)

5.6

4.9

0.07
1.22

41.8
1.20

48.9
0.67
49.7
0.07

18.2
6.41

27
Ct

(

»

)

5.1

4.5

0.29
1.05

23.0
3.01
25.0
1.48

24.4
0.22
40.2
6.46

28
Ct

(

»

)

5.4

4.6

0.21
0.74
28.0
2.54
31.4
1.36

29.6
0.31
21.2
6.35

29
Ct

(

»

)

4.3

3.8

1.52
4.20

10.9
11.83
12.1

4.08
11.8

0.68
43.0
6.43

30
Ct

(

»

)

5.0

4.2

0.58
1.10

24.8
5.50
27.1
3.21
24.4
0.49
39.3
6.47

7

LSt

(Ligno Snhaemum
neat

5.1

3.7

0.34
Rl5

135
3
90

153
9
99

HS
133

«8(1

fi
97

56



57

Additional aluminium determinations were carried out from extracts where the acetic
acid and ammonium acetate concentrations varied from 0.1 to 0.9 and where the total
molarity was 1.0. The extractions were thus carried out at different pH ranges of the
acetic acid ammonium acetate system (Mäkitie 1956). All these extractions (at seven
different pH, totally) are summarized in Fig. 1, where the extractability of aluminium is
correlated with the extraction-pH.

The aluminium values show that relatively high amounts of soluble aluminium are
extracted from acid soils when the hydrogen ion concentration in the extractant is high.
When the extraction pH is over 5, only very little aluminium is extracted, particularly
with the M ammonium acetate solution where the equilibrium pH of the extractant soil
suspension is mainly between 6.0 and 6.5.

The potassium chloride extraction represents the leaching of exchangeable aluminium of
soils. When the aluminium values of this M KCI extraction are compared with the pH Kcl
of soil suspension, a proper correlation is obtained (Fig. 2). In the figure, the upper limit
of possible aluminium content in solution, decreasing with increasing pH, is shown.

These aluminium values obtained by potassium chloride-extractions are relatively low
when compared with acetate-extractions at low pH, but are of about the same level as
when extracted with ammonium acetate at a pH near 6.

At pH 4.65 the ammonium acetate extraction includes the exchangeable aluminium and
some of the soluble »hydroxy-aluminium». In acid soils this soluble fraction should be very
small (McLean & al. 1959). No correlation is, however, observable in the present data.
The exchangeable amounts of aluminium are only 24 % (mean value) of the soluble
amounts extracted at pH 4.65, even when the soils are rather acid. It has been observed
in several connections that sodium acetate at pH 4.8 gives low results, but when buffered
to pH ~ 4, high amounts of aluminium are extracted (mainly from sesquioxidic soils.
Little 1964).

Fig. 2. Correlation of M KCL extractable aluminium with pHKCI -values of soils.
A = Coarse soils
0 = Clay soils
Q = Organogenic Soils



58

Table 2. Percentage extractability of aluminium by different extractants.

Method Coarse
soils

Clay- Organogenic All
soils soils soilssoils soils

(9) (10) (11) (30)

M CH 3 COOH extraction 242 227 199 222
0.5 M CHgCOOH, 0.5 M CH3 COONH4,

pH = 4.65 100 100 100 100
M CH 3GOONH4 extraction
M KCI extraction

4 6 19 13
18 18 30 24

Summary

The extractant, 0.5 M acetic acid 0.5 M ammonium acetate at pH 4.65, which is used
in soil-testing, extracts relatively high amounts of aluminium from acid soils. The mean
values of acetate-extractable aluminium at pH 4.65, 1.75 meq Al/100 g of soil, and of
exchangeable aluminium (M KCI extraction), 0.41 meq Al were obtained from a material
of 30 samples of acid soils (Table 2). Several other acetic acid ammonium acetate ex-
tractants, from M acetic acid to M ammonium acetate solution were also used for stu-
dying the extractability of soil aluminium.

The soil-testing extractant can be used for the estimation of the soluble amounts of
aluminium in acid soils, however, further studies are needed for a better interpretation of
the ammonium acetate extractable (at pH 4.65) aluminium in our soils.

REFERENCES

Black, C. A. (Editor) et al. 1965. Methods of Soil Analysis, Part 2. Chemical and Microbiological Pro-
perties. Amer. Soc. Agron., Inc. Publisher, Madison, USA.

Brown, I. C. 1943. A rapid method of determining exchangeable hydrogen and total exchangeable bases
of soils. Soil Sci. 56: 353—357.

Chenery, E. M. 1948. Thioglycollic acid as an inhibitor for iron in the colorimetric determination of A 1 by
means of »aluminon». Analyst, 73: 501—502.

—»—- 1955. A preliminary study of A 1 in the tea bush. Plant and Soil, 6: 174—200.
Frink, C. R., and Peech, M. 1962. Determination of aluminium in soil extracts. Soil Sci. 93; 317 —324.
Hsu, P. H. 1963. Effect of initial pH, phosphate, and silicate on the determination of aluminium with

aluminon. Ibid. 96: 230—238.
Little, I. 1964. The determination of exchangeable aluminium in soils. Austr. J. Soil Res. 2: 76—82.
McLean, E. 0., Heddleson, M. R., and Post, G. J. 1959. Aluminium in soils; 111. A comparison of

extraction methods in soils and clays. Soil Sei. Soc. Amer. Proc. 23: 289—293.
Mäkitie, O. 1956. Uuttamisesta viljavuusanalyysissa, Summary; Studies on the acid ammonium acetate

extraction method in soil testing. Agrogeol. pubi. 66: 1—22.
» 1965. On determination of lime reguirement of soils. Ann. Agric. Fenn. 4: 238—252.
» - & Virri, K. 1965. On the exchange characteristics of some clay soils in the Middle-Uusimaa. Ibid.

4: 277—289.
Pratt, P. F., and Bair, F. L. 1961. A comparison of three reagents for the extraction of aluminum from

soils. Soil Sci. 91: 357—359.



59

Rolfe, A. C., Russell, F. R., and Wilkinson, N. T. 1951. The colorimetric determination of aluminium
in water using ammonium aurintricarboxylate, J. Appi. Chem. 1: 170—178.

Schollenberoer, C. J., and Simon, R. H. 1945. Determination of exchangeable bases in soil ammonium
acetate method. Soil Sci. 59: 13—24.

Vuorinen, J., and Mäkitie, O. 1955. The method of soil testing in use in Finland. Agrogeol. pubi. 63:
1—44.

SELOSTUS

MAAN ALUMIININ LIUKENEMISESTA VILJAVUUSANALYYSIN HÄPEÄMÄÄN
AMMONIUMASETAATTILIU OKSE EN

Osmo Mäkitie

Maatalouden tutkimuskeskus, Maantutkimuslaitos, Tikkurila

Happamista maista saadaan suhteellisen runsaasti helppoliukoista alumiinia uutetuksi 0.5 M etikka-
happo 0.5 M ammoniumasetaattiliuokseen jota käytetään maanäytteiden viljavuusanalyysissa. pH
4.65 :ssa asetaattiliuokseen uuttuneiden alumiinin määrien keskiarvo, 1.75 mekv. Al/100 g maata, edustaa
liukoisen alumiinin määrääjä se on verrattavissa 1 M kaliumkloridiliuoksella saatuun vaihtuvan alumiinin
keskiarvolukuun 0.41 mekv. Al/100 g maata.

Näyteaineistona on ollut 30 maanäytteen aineisto, koottuna happamista maanäytteistä (Taulukko 1).
Alumiinin uuttumista on myöskin tutkittu erilaisiin etikkahappo ammoniumasetaatti liuoksiin ja
todettu että pH:n osuus on ratkaiseva maan alumiinin liukenemisessa.

Viljavuustutkimuksen uuttoliuosta voidaan käyttää alumiinin uuttamiseen, joskin lisätutkimukset ovat
tarpeen helppoliukoisen alumiinin määrien tulkitsemiseksi ja vertaamiseksi maanäytteen muihin kemialli-
siin ominaisuuksiin.