Maataloustieteellinen Aikakauskirja
Vol. 59: 67—72, 1987

Determination of soil specific surface area by water vapor adsorption

II Dependence of soil specific surface area on clay and organic carbon content

RAINA NISKANEN 1 and VÄINÖ MÄNTYLAHTI 2

1 Department of Agricultural Chemistry, University of Helsinki,
SF-00710 Helsinki, Finland

2 Viljavuuspalvelu Oy (Soil Analysis Service Ltd.), Vellikellontie 4,
SF-00410 Helsinki, Finland

Abstract. The specific surface area of 60 mineral soil samples estimated by water vapor
adsorption at 20 % relative humidity ranged from 12.1 +3.6 to225.1 ±18.4 mVg. Clay (range
1—72 %) and organic carbon content (0.7 —14.6 %) together explained 84 % of the variation
in the surface area. The regression equation predicting the specific surface area of soil was
surface area (mVg)=2.69+ 1.23c1ay-% +8.690rg.C-%.

Index words: water vapor adsorption, relative humidity, mineral soils

Introduction

Physical and chemical properties of soil are
largely related to the specific surface area of
soil. The ability to reserve available plant
nutrients and the cation-exchange and buffer
capacity of soil are ultimately derived from the
surface area of soil particles. The surface area
of soil is dependent on the content and mineral
composition of clay fraction and on the con-
tent of organic matter and amorphous oxides.

Many methods used for the measurement
of specific surface area are based on gas or
liquid mono- or multilayer adsorption on soil
surface. The number of molecules of gas

which will sorb onto surface at equilibrium
depends on the partial pressure of the gas and
the area of the surface. Water vapor adsorp-
tion isotherms are used in the estimation of
the total surface area of soil. It is possible by
the BET equation (Brunauer et al. 1938) to
determine the amount of water at monolayer
coverage, which gives the surface area when
a standard value for the area covered by a
molecule of water is known. If only com-
parative values of area are required and there
is to be no isotherm analysis, use can be made
of the fact that the water monolayer is com-
plete for many soils and clays at the relative
humidity, p/pD, corresponding approximately

67

JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND

https://www.c-info.fi/en/info/?token=xtKVzcnh913YpPRa.zhjR7-uzm-UznWnNvyMTwQ.mvdzJcAb3uYFf6VPn4mEpngD4Zcf_oCmpINTXASIKrKEfGpIioVHYH-d1t7kQmuTXG3OIE5FQnHPyYRWI6dTHi1gqsJlBVIJ_ls55Saa-C-m4ohLAG8uVd5VgIZaSzaU4oFzc-w5xTuvCPyrdL8dN7DpbS2Bv20-tXBbByiBEp6uOR3ADLm26wbl_boSlHYI_FCiktEF4GhYga_Ijq4Z-w


to 0.20 (Quirk 1955). It is therefore possible
to obtain a rough estimate of the BET water
area by a one-point determination at this
relative humidity (Greenland and Mott
1978). This one-point method has been ap-
plied eg. by Pritchard (1971), BASCOMBand
Thanigasalam(1978) and Borggaard(l9B2).

In many countries, the measurement of
specific surface area is an essential soil anal-
ysis. In Finland, surface area measurement is
mainly used for classification of till fractions
by engineering geologists (Lindroos 1976,
Nieminen and Kellomäki 1982). The aim of
this study was to apply the one-point method
of surface area measurement for arable soils
and to study the relationship between water
surface area and soil organic carbon and clay
content.

Material and methods

The material was collected at 43 sampling
sites, mainly locating in the southern part of
Finland, and it consisted of 31 surface soil
samples and 29 samples from deeper soil layer
(Table 1). At 17 sampling sites, both surface
and deeper layer samples were taken. The
samples were air-dried and ground to pass a
2-mm sieve. The particle-size distribution of
the inorganic matter in the soil was determined
by the pipette method (Elonen 1971). The
organic carbon content was determined by a
modified (Graham 1948) Alten wet combus-
tion method.

For estimation of the specific surface area
of soil, 1 g of soil in a tared weighing bottle
was placed in a desiccator over a saturated
CHjCOOK solution at 20 % relative humidity.
After 2 weeks of equilibration at + 20°C the
soil + weighing bottle was weighed. The water
content of soil was determined by drying for
4 hours at + 105°C (Niskanen and Mänty-
lahti 1987). In calculation of the specific sur-
face area of soil, the cross-sectional area of
0.106 nm2 (Gal 1967) was assigned for a
water molecule. Considering the water mono-
layer on soil surface complete at p/p Q

0.20,

the soil water content of 1 °7« corresponds to
the surface area of 35.45 mVg dry soil. The
surface area measurement was carried out in
quadruplicate.

Results and discussion

Soil water retention properties are largely
related to the soil clay content. The surface
adsorptive forces are effective on the water
retention especially at high water tensions.
Under these circumstances, the clay fraction
with its large surface area is the principal con-
tributing factor capable of water adsorption.
Accordingly, clay has been reported to have
strong influence on soil water retention espe-
cially at the wilting point pF 4.2 (Petersen
et al. 1968), which is largely a function of the
permanent negative charge of clay mineral
particles. In the material of Kivisaari (1971),
the soil water content at pF 4.2 and clay con-
tent were highly correlated (r = o.9B***, n = 90).

In this study, the water tension was essen-
tially higher thanpF 4.2. The pF value of water
corresponding to 20 % relative humidity
(R.H.) obtained from the equation (Bolt and
Frissel 1960) pF = 6.5 + log(2—log R.H.) is
6.3. In the present material (Table 1), which
included 25 clay soils (clay-% > 30) and 35
non-clay soils, the mean content of adsorp-
tion water was 2.45 % in the clay soil group
and 1.46 % in the non-clay group.

The total surface area of soil determined by
means of water vapor as an adsorbent not
only depends on the clay content but to a great
extent on the content of humus as well
(DECHNiKand Stawinski 1970). The total sur-
face area of soil increases with increasing clay
and humus content (Curlik 1973). According
to BuRFORoet al. (1964), organic matter pre-
sent even in small amounts greatly affects the
surface area. The values of the total soil sur-
face area are higher in the presence of organic
substances than after their removal (Doer-
zanski et al. 1972).

In the present material (Table 1), the effect
of organic matter on soil water adsorption and

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69

Table I, Soil samples.

Soil Locality Sampling Org.C, Particle-size HjO adsorbed Surface
sample depth % distribution % of dry soil area
No. cm (gm), % *) mVg dry soil

<2 2—20 >2O
*)

la Vaala o—2o 8.4 3 4 93 1.62 + 0.10 57.4±3.6
lb » 20—40 1.3 1 3 96 0.85 ±0.06 30.1+2.1
2a Viikki o—2o 9.2 3 7 90 1.96 + 0.27 69.5±9.6
2b » 20—40 2.7 2 5 93 0.94±0.21 33.3 + 7.5
3a Hyvinkää o—2o 12.5 6 11 83 3.3710.33 119.5111.7
3b » 20—40 1.6 4 8 88 0.5010.08 17.712.8
4a » o—2o 3.7 15 24 61 1.7610.08 62.412.8
4b » 20—40 0.7 8 II 81 0.7510.08 26,612.8
5a Salo o—3o 3.1 20 31 49 1.8610.18 65.916.4
5b » 30—60 2.3 32 17 51 1.8410.06 65.212.1
6a Rajamäki o—2o 3.0 23 25 52 1.61 10.13 57.1 14.6
6b » 20—40 1.0 49 19 32 2.6110.16 92.515.7
7a Imatra o—2o 5.5 24 43 33 2.0310,26 72.019.2
7b » 20—40 2.2 29 46 25 1.3110.22 46.417.8
8a Säkylä o—3o 3.2 25 41 34 1.8810.24 66.718.5
8b » 30—60 1.4 29 45 26 1.7110.19 60.616.7
9a » o—3o 3.1 27 42 31 1.7410.14 61.715.0
9b » 30—60 1.5 30 45 25 1.5810,27 56.019.6

10a Imatra o—2o 3.2 28 41 31 1.9710.24 69.818.5
10b » 20—40 1.1 33 39 28 1.8110.18 64.216.4
11a Viikki o—2o 2.4 29 30 41 1.4610.16 51.815.7
lib » 20—40 1.4 49 20 31 1.5410.19 54.616.7
12a Laukaa o—3o 2.6 32 55 13 1.4610.16 51.815.7
12b » 30—60 2.1 34 57 9 1.4410.08 51.112.8
13a Viikki o—2o 3.3 37 13 50 1.1610.21 41.117.5
13b » 20—40 0.8 39 6 55 0.7610.11 26.913.9
14a Hyvinkää o—2o 8.7 42 47 11 4.0010.22 141.817,8
14b » 20—40 6.5 38 52 10 3.3910.19 120.216.7
15a Viikki o—2o 3.4 43 33 24 2.5510.06 90.412.1
15b » 20—40 2.6 47 30 23 2.6110.37 92.5113.1
16a Mietoinen o—3o 2.7 50 21 29 1.7510.21 62.017.5
16b » 30—60 2.4 54 21 25 1.6710.13 59.214.6
17a Imatra o—2o 10.7 70 18 12 5.9110.59 209.5120.9
17b » 20—40 11.5 72 17 II 6.3510.52 225.1 118.4
18 Hyvinkää o—2o 3.9 4 10 86 1.2010.21 42.517.5
19 » s—lo 4.5 4 13 83 1.1110.18 39.416.4
20 Viikki o—2o 1.5 4 16 80 0.5110.05 18.111.8
21 Imatra o—2o 6.0 5 6 89 1.5410.11 54.613.9
22 Tohmajärvi o—2o 3.7 6 19 75 1.5510.18 55.016.4
23 Naantali o—3o 1.6 10 7 83 0.6710.13 23.814.6
24 Viikki o—2o 4.4 10 7 83 1.4910.19 52.816.7
25 Imatra o—2o 3.6 13 20 67 1.8010.16 63.815.7
26 Hyvinkää o—2o 4.3 20 47 33 2.2110.25 78.418.9
27 Imatra o—2o 3.8 22 31 47 1.6710.22 59.217.8
28 Säkylä o—3o 14.6 25 43 32 3.7610.22 133.317.8
29 Imatra o—2o 3.2 31 43 26 2.0310.29 72.0110.3
30 Viikki o—2o 4.7 36 8 56 1.5910.11 56.413.9
31 Imatra o—2o 2.7 51 27 22 3.8810.33 137.6111.7
32 Vaala 20—40 1.0 1 2 97 0.3410.10 12.113.6
33 Viikki 20—40 0.8 2 1 97 0.6310.10 22.313.6
34 Tohmajärvi 30—50 LI 2 20 78 1.0110.14 35.815.0
35 Hyvinkää 20—40 2.3 3 4 93 0.6510.05 23.011.8
36 Turenki 30—60 0.9 5 15 80 0.8410.11 29.813.9
37 Imatra 20—40 3.7 14 19 67 1.6210.41 57.4114.5
38 Naantali 30—60 1.7 28 20 52 1.2410.33 44.0111.7
39 Hyvinkää 20—40 1.4 31 39 30 1.66+ 0.21 58.917.5
40 Viikki 20—40 1.7 45 19 36 1.9610.22 69.517.8
41 Salo 30—60 1.3 45 27 28 1.9310.40 68.4114.2
42 Nurmijärvi 20—40 5.2 56 36 8 3.5210.16 124.815.7
43 Mietoinen 30—60 2.0 70 23 7 2.1610.25 76.618.9

*) means with the confidence limits at the 95 °/o level.



Table 2. Soil characteristics.

All soils
(n = 60)

Surface soils Deeper layers
(n = 3 1) (n = 29)

X s range X s range X s range

Org.C, % 3.6 3.0 0.7—14.6 4.6 3.0 1.5—14.6 2.3 2.2 0.7—11.5
23 16 3—70 29 22 1—72Clay (<2 pin), % 26 19 1—72 1—72

Silt (2—20 gm), % 24 15 1—57 25 15 4—55 23 16 1—57
Coarser fractions
(>2O (ira), % 50 29 1—97 51 26 I—B6 48 32 7—97
H 2O adsorbed,
%of dry soil 1.87 1.15 0.34—6.35 2.04 1.10 0.51—5.91 1.70 1.20 0.34—6.35
Surface area.
mVg dry soil 66.340.8 12.1—225.1 72.3 39.0 18.1—209.5 60.3 42.5 12.1—225.1

surface area was particularly clear when top-
soil and corresponding deeper layer samples
of non-clay soils were compared. The values
of water adsorption and surface area of top-
soil samples la-4a and 7a were much higher
than those of the corresponding deeper layer
soils. The mean values of water adsorption
and surface area of all deeper layer soils were
83 % of those in the topsoil group (Table 2).

The relationship between water adsorption
at p/p0 0.20 and clay and organic carbon
content was more accurately studied by the
regression analysis. When clay and organic
carbon contents were used as independent vari-
ables, together they explained 84 °7o of the var-
iation in the water content of the whole ma-
terial (n = 60), the regression equation being
H2O-<Vo = 0.076+ 0.035c1ay-»/o + 0.2450rg.C-%.
Using the silt content as an additional in-
dependent variable in the regression analysis
showed that silt content was an insignificant
explainer. The partial correlation coefficients
for the relation between water content (1), clay
content (2) and organic carbon content (3)
were as follows: r 123 o.B2***

r,3.2 o.B4***

Expressing the water adsorption as surface
area, the regression equation takes the form:
surface area (mVg) = 2.69-1- 1.23c1ay-% +

8.690rg.C-%.
Sillanpää (1982) has used for the expres-

sion of soil texture as a single figure a texture
index TI = 1.0x °/o of fraction < 2 /un + 0.3 x %

of fraction 2—60 /rm + 0.1x% of fraction
60—200 /tm. When this texture index and or-
ganic carbon content were used as independent
variables in the regression analysis, together
they explained 84 °7o of the variation in the
surface area, the regression equation being
surface area (mVg) = —14.69+ 1.22T1 + 8.89
org.C-%.

The disadvantage of using a polar molecule
like water in the determination of surface area
is that it is more strongly adsorbed on specific
sites on the surface, and thus not uniformly
distributed (Greenland and Mott 1978).
Water molecules are attracted to the bare ex-
changeable cations and are clustered around
them, which implies overlapping of the mo-
nolayer and multilayer processes. In addition,
the geometry of clustering depends on the ex-
changeable ion, so that e.g. Ca2+ ion attracts
water more strongly than K + (Stawinski 1978).
However, exchangeable cations in Finnish
arable soils largely consist of divalent cations
Ca2+ and Mg 2+ (Kaila 1972, Niskanen and
Jaakkola 1986), and thus exchangeable cation
composition is probably not a very important
source of uncertainty in the determinationof
surface area.

Despite various uncertainties involved in the
measurement of surface area, we still agree
with Hillel(1971) who claims that the mea-
surement of soil specific surface area may
eventually prove a more significant and per-
tinent index for characterizing a soil than the
particle-size distribution.

70



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—& Mäntylahti, V. 1987. Determination of soil
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Ms received March 18, 1987

SELOSTUS

Maan ominaispinta-alan määrittäminen
vesihöyryn adsorption avulla

Il Maan ominaispinta-alan riippuvuus
saveksen ja orgaanisen hiilen pitoisuudesta

Raina Niskanen 1 ja Väinö Mäntylahti 2

1 Maanvitjelyskemian laitos, Helsingin yliopisto.
00710 Helsinki

1 Viljavuuspalvelu Oy, Vellikellontie 4, 00410 Helsinki

Maan ominaispinta-alan määrittämistä 20 % suhteel-
lisessa kosteudessa tapahtuvan vesihöyryn adsorption

avulla tutkittiin 60 kivennäismaanäytteellä. Ominaispinta-
alan vaihteluväli oli 12.1 ±33.225.1 + 18.4mVg. Savek-

71



72

sen (1—72 %) ja orgaanisen hiilen pitoisuus (0.7—14.6 %)

selittivät yhdessä 84 % maan ominaispinta-alan vaihte-
lusta. Maan ominaispinta-ala riippui saveksen ja orgaa-

nisen hiilen pitoisuudesta seuraavan regressioyhtälön mu-
kaisesti: ominaispinta-ala (mVg)=2.69+ I.23saves-% +

8.690rg.C-%.