of soils
E. de Jong
Department of Soil Science, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A8, E-mail: [email protected]. Received 21 September 1998, accepted 10 February 1999.
de Jong, E. 1999. Comparison of three methods of measuring surface area of soils. Can. J. Soil Sci. 79: 345–351. Surface area (SA) is an important property of soils, but different methods can give widely different estimates of SA, and of the contribution of organic matter to SA. This study was undertaken to compare two common methods of measuring SA (EGME [ethylene glycol monoethyl ether] and N2sorption) with SA estimates using H2O sorption on selected Saskatchewan soils; some soils from Kenya were included to show the impact of clay mineralogy. For the Saskatchewan soils, the three estimates of SA were highly correlat-ed to each other and to clay content, but SA EGME was 2 to 3 times SA H2O and 7 to 52 times SA N2. Organic matter did not appear to contribute to SA EGME, increased SA H2O and decreased SA N2. Clearly the three methods differ in their access to internal and external surface area and this should affect their utility as indices of the sorptive capacity of field soils.
Key words: Surface area, EGME, N2sorption, water sorption
de Jong, E. 1999. Comparaison de trois méthodes de mesure de la surface spécifique.Can. J. Soil Sci. 79: 345–351. La surface spécifique (SS) est une importante propriété des sols mais diverses méthodes de mesures peuvent toutefois donner des valeurs très différentes de la SS ainsi que de l’effet de la matière organique sur cette propriété. Nous avons comparé deux méthodes courantes de mesure de la SS (EGME et sorption de N2) à la méthode basée sur la sorption de H2O. La comparaison portait sur certains sols de la province de Saskatchewan et incluait aussi des sols du Kenya pour démontrer l’impact de la minéralogie de l’argile. Pour les sols de la Saskatchewan, les trois valeurs de la SS étaient étroitement corrélées l’une à l’autre ainsi qu’à la teneur en argile, mais les valeurs obtenues par la méthode EGME étaient 2 à 3 fois supérieures à celles obtenues par sorption de H2O et de 7 à 52 fois celles obtenues par sorption de N2. La matière organique ne semble pas avoir eu d’impact sur les valeurs EGME, augmenté les valeurs par sorption de H2O ou diminué les valeurs par sorption de N2. Il est clair que les trois méthodes diffèrent par leur mode d’accès à la surface spécifique interne et externe, ce qui devrait restreindre leur utilité comme indicateur de la capacité de sorption des sols agricoles.
Mots clés: Surface spécifique, EGME, sorption de N2, sorption de l’eau
The SA of soil particles plays an important role in many processes. Water retention (Puckett et al. 1985; Petersen et al. 1996), shrink-swell properties (Ross 1978; de Jong et al. 1992) and sorption of organic and inorganic substances (Greenland 1965; Scheidegger and Sparks 1996) in soils are affected by the extent and nature of the solid surfaces. Surface area is related to clay content and type, but the con-tribution of amorphous organic (Pennell et al. 1995) and mineral compounds to the surface area of soils is not clear.
Surface area of soils is usually determined from the sorp-tion of selected chemicals. The most common method involves the sorption of EGME under conditions designed to produce a monolayer of EGME (Carter et al. 1986). Other “one point” estimates of surface area rely on the sorption of dyes such as methylene blue (Hang and Brindley 1970). Alternatively, several points on the sorption isotherms of N2 or H2O are used to estimate surface area using the theory of Brunauer, Emmett and Teller (BET). The weight or vol-ume of gas sorbed at relative vapor pressures (p/po) less than 0.5 is used; the BET theory is invalid at higher relative vapor pressures because of “capillary condensation” (Joyner et al. 1945). Polar sorbates such as EGME and H2O give higher estimates of SA than non-polar substances such as N2 due to the inability of N2to penetrate interlamellar spaces
and micropores (the “internal” surface area). Different methods result in different estimates of the contribution of organic matter to the surface area of soils (Pennell et al. 1995).
Pretreatment of the soil samples varies from none, oven drying or drying over P2O5, to removal of amorphous sub-stances, and is likely to affect the measured SA. Removal of organic matter, saturating the exchange complex with spe-cific cations and minor variations in technique appear to have no great impact on SA measured with EGME (Cihacek and Bremner 1979; Tiller and Smith 1990). However, the conversion factor from weight of EGME sorbed to area cov-ered by the monolayer can vary several fold between differ-ent soil minerals (Tiller and Smith 1990). Sorption of water vapor by soils is known to be higher for soils dried over con-centrated H2SO4than oven dried at 105°C (Hutcheon 1941). Thus, surface area measurements that do not require elabo-rate pretreatments or oven drying of the soil sample, e.g., sorption of water vapor or methylene blue, may be prefer-able.
The objective of this paper was to compare surface area measurements by EGME (SA EGME), N2 (SA N2) and water vapor (SA H2O) and to assess which technique is most relevant to soils under field conditions.
346 CANADIAN JOURNAL OF SOIL SCIENCE
MATERIALS AND METHODS
The soil samples used for this study had been used in other projects. Unless indicated differently, the soil samples had been air dried and ground to pass through a 2-mm sieve. The samples fall into three categories: 1) Twenty-seven soil horizons from eight Chernozemic profiles used in a study of the shrink-swell potential of Saskatchewan soils (de Jong et al. 1992). SA EGME had been measured without pretreat-ment to remove organic C or saturation of the exchange complex with Ca. 2) Coarse and fine clay separates of five Ck horizons used in a study of till and modified till parent materials in southwestern Saskatchewan (Dubbin 1991). Organic and inorganic C were removed during the size-frac-tionation, and Dubbin (1991) measured SA EGME after the separates were Ca saturated. 3) Six soil samples from Kenya (three depths from an Oxisol and three depths of an Alfisol) used by Onyatta (1997) who determined SA EGME and SA N2without pretreating the soils. In addition, a Ca montmo-rillonite clay sample was used for a reference.
In addition to the SA EGME measurements, the previous studies also provided data on texture (percent sand, silt and clay on a mineral basis), organic and inorganic C, and cation exchange capacity (CEC) of the samples. For this study SA N2of the samples in categories 1 and 2 and the Ca montmorillonite was determined with the same Autosorb 1 (Quantachrome Corp., NY) used for the samples from Kenya. Samples were run in duplicate or triplicate.
For this study, the surface area by water sorption (SA H2O) was determined on all samples by placing 5 g (if enough soil was available) air-dried soil over saturated solu-tions of K C2H3O2, CaCl2.6H2O and LiNO3in dessicators at 20°C. The relative vapor pressures (p/po) of these solu-tions are 0.20 and 0.323 (Weast 1979), and 0.47 (Newman 1983), respectively. The samples were weighed twice a week until the weights changed by less than 0.0005 g; they were then dried at 105°C for 48 h and weighed again. The dessicators were evacuated after each weighing, but often had lost their vacuum by the next weighing. Samples were run at least in duplicate. The clay separates and Ca mont-morillonite were run at two relative humidities because of the small original sample.
SA H2O was calculated using the approximation of the BET equation for unlimited sorption:
(1)
where X = p/po= relative vapor pressure, Wx = weight of water sorbed at different p/po (g g–1oven-dried soil), Wm = weight of a monolayer of water (g g–1oven-dried soil), C = constant.
Figure 1 shows two examples of the H2O sorption isotherm and the corresponding plots according to Eq. 1. Wm was calculated from the intercept (i) and slope (s):
(2) Values for i and s were found by linear regression. It
was assumed that a monolayer of water covered an area of 3600 m2 g–1; Facek (1966) uses a value of 3610 m2 g–1 and Newman (1983) uses 3500 m2g–1. The constant
varied from –2330 to 1102 (a similar large variation in C was found in the N2sorption), but the estimate of Wmis reasonably precise as s>> i and the effect of an error in i on Wmis small.
RESULTS Soil Horizons
Figure 2 shows the dependance of SA EGME on clay con-tent. The Ca montmorillonite sample showed the expected SA EGME of approximately 800 m2 g–1. SA EGME of the Saskatchewan soils was considerably larger than that of the Kenyan samples, reflecting the difference in clay mineralo-gy. The Saskatchewan clays are mainly smectites and micas (Kodama 1979) while the soils from Kenya were dominated by kaolinite and micas (Onyatta 1997). Topsoils, subsoils and parent materials appeared to have a similar dependance on clay content, suggesting that organic matter had little effect on SA EGME.
SA H2O of Ca-montmorillonite and the Saskatchewan samples was about 1/3 to 1/2 of SA EGME (Fig. 3 vs. Fig. 2), but SA EGME and SA H2O were quite similar for the soils from Kenya. The attraction of organic matter for water is evident from the fact that all topsoils are located on or above the relevent regression lines (Fig. 3). For all soils SA N2was less than SA EGME (Fig. 4 vs. Fig. 2). The differ-ence is most pronounced for the Saskatchewan soils and Ca montmorillonite, and reflects the large contribution of the internal surface area to the total surface area as measured by EGME. In the Saskatchewan soils, most topsoils fall below the regression line (Fig. 4).
The correlation matrix (Table 1) for the Saskatchewan soils confirms that clay content is the dominant factor con-trolling the SA of these soils. Clay content accounts for 86% of the observed variation in SA EGME, 76% of the variation in SA H2O and 51% of the variation in SA N2. Organic C had no affect on SA EGME, increased SA H2O and decreased SA N2. All three measures of SA were correlated (Table 1), but the correlation was weakest between SA H2O and SA N2because of the opposite effect of organic C on these two SA estimates.
Soil Separates
The five parent materials all had similar clay mineralogy (Dubbin 1991). The coarse clays had an SA EGME of about 300 m2g–1, while the SA EGME of the fine clays ranged from 610 to 710 m2g–1(Table 2). The SA EGME of the fine clays indicates the dominance of smectites in this separate. SA H2O was about 35 to 55% of SA EGME, and the SA H2O of the fine clays is similar to that of the Ca montmoril-lonite (Fig. 3) and to the value quoted by Quirk (1955) for the < 0.3µm fraction of a montmorillonitic Barnes loam.
The SA N2for Ca montmorillonite (Fig. 4) indicates that the internal surface area is about 87% of the total surface
T F LAI LA = × × 1000 W i s m= + 1 C i s i = +
area (Fig. 2), which agrees with other data in the literature. The measured SA N2for the coarse and especially the fine clay fractions are unbelievably low, but were obtained dur-ing the same run that the Ca montmorillonite was measured. Visual inspection of the BET plots (Eq. 1) showed good fits for the coarse clays, but often poor fits for the fine clays. The fine clays had a dark color, but the organic C was sim-ilar for both fractions and averaged 2 mg g–1. The clays were Mg-saturated with Mg(OAc)2 and this organic C may repre-sent leftover acetate or may be due to incomplete removal of organic C during the clay fractionation.
DISCUSSION
In the Kenyan soils, SA EGME and SA H2O are of similar
magnitude (Table 1) indicating that both EGME and H2O measure the same surface area. SA N2 indicated that the external surface area was approximately 60% the total sur-face area. In the Saskatchewan soil samples (Tables 1 and 2) and the Ca montmorillonite SA H2O is about 1/3 to 1/2 of SA EGME, and SA N2is about 10% of SA EGME. The reduction in SA measured with H2O vs. EGME in the smec-tite-dominated Saskatchewan soils and Ca-montmorillonite is believed to be due to limited water uptake in the interlay-er spaces of smectites. Karathanasis and Hajek (1982) and Puri and Murari (1963) multiplied the water-sorption BET estimate of the surface area by a factor of 2 to correct for the observation (Mooney et al. 1952) that only a single layer of
Fig. 1. Water sorption isotherms for a Chernozemic A horizon (the point at p/po = 0.98 corresponds to the 1.5 MPa water content) and an
Oxisol topsoil, and the corresponding BET plots.
Fig. 2. Dependance of surface area measured by EGME sorption
(SA EGME) on clay content.
Fig. 3. Dependance of surface area measured by water vapor
348 CANADIAN JOURNAL OF SOIL SCIENCE
water is present in the interlayer spaces of montmorillonite when the external surfaces are covered by a monolayer. If interlayer water uptake is limited, Eq. 1 cannot be used to estimate SA. In the case of a single monolayer of water sorbed on all surfaces, the general BET equation reduces the Langmuir equation (Joyner et al. 1945; Facek 1966):
(3)
(4) .
Equations 3 and 4 predicted SA H2O Langmuir values slightly above SA EGME for the Saskatchewan soil hori-zons (Fig. 5) and much higher values than SA EGME for the soils from Kenya. Clearly the multiple layer BET equation (Eq. 1) is preferred for the kaolinitic soils, while the mono-layer Langmuir form (Eq. 3) gives better estimates of total surface area for soils dominated by smectites. Compared to the EGME estimates, the Langmiur water sorption SA seems to include a contribution from organic matter as the topsoil samples fall above the regression lines.
The ratio of 1/3 to 1/2 between SA H2O and SA EGME is slightly lower than the 1/2 reported by others (Puri and Murari 1963, 1964), possibly due to hysteresis in the H2O sorption isotherms or differences in micro-structure and consequent accessibility of the surfaces to EGME and water. SA H2O was determined on air dry samples that were equi-librated at p/po= 0.20, 0.323 and 0.47. The water content of the 27 soil horizons from Saskatchewan at p/po= 0.20 was on average 0.4% (range 0.1 to 0.6%) lower than that at air
dryness: thus the samples must have continued on the major drying curve. The samples equilibrated at p/po= 0.323 and 0.47 gained water from air dryness, and must have been on the wetting curve. Thus Wxis too high at p/po= 0.20 rela-tive to Wxat p/po= 0.323 and 0.47 (Orchiston 1954) and s is too high and SA H2O too low. Microstructure was proba-bly different between the water and EGME sorption experi-ments; in both cases the sample had been ground to pass a 2-mm sieve, but the oven-dried EGME samples were slur-ried. When the 27 Saskatchewan soil samples were slurried with water and then dried to p/po= 0.20 the average water content was 0.3% (range 0.1 to 0.7%) higher than when they were dried from airdryness to p/po= 0.20. This suggests that microstructure may have limited water uptake in the airdry samples during the H2O sorption experiment; Hutchon’s (1941) work indicated that method of drying also affects water uptake of these types of soil.
Organic C strongly affected SA N2 and SA H2O of the Saskatchewan soil horizons, but did not affect SA EGME (Table 1). Multiple linear regression gave the following equations: SA EGME = (86 ± 13) + (0.40 ± 0.03) clay, r2= 0.86 SA N2= (6 ± 3) + (0.056 ± 0.006) clay – (0.37 ± 0.06) organic C, R2= 0.78 SA H2O = (18 ± 6) + (0.15 ± 0.01) clay + (0.54 ± 0.14) organic C, R2= 0.85
where SA is in m2 g–1, and clay and organic C in mg g–1. The negative effect of organic C on SA N2resulted in lower ratios of SA N2/SA EGME (external/total SA) for topsoils (0.05 ± 0.03) than for B and C horizons (0.10 ± 0.02). Kaiser et al. (1996) have reported a similar negative affect of organic C on SA N2, and Pennell et al. (1995) found that removal of humus increased SA N2. Thus, humus appears to reduce the access of N2to particle surfaces. Perhaps organ-ic coatings on morgan-icroaggregates are also responsible for the very low SA N2measured on the clay separates (Table 2). This problem did not arise in the Ca montmorillonite, which had only 0.6 mg organic C g–1, and was freeze-dried after Ca saturation. The drying history of the Mg-saturated clay separates is not known, but may have led to a different microstructure.
It has been suggested (Ross 1978; Hillel 1982) that SA measurements may provide more information on physical and chemical properties of soils than textural analysis. Table 3 shows the correlations between the three SA measure-ments, and CEC and the 1.5 MPa water content. For com-parative purposes clay and organic C are also included. SA H2O shows the highest correlation with CEC and 1.5 MPa water content and appears a useful predictor of some soil properties; however, its determination is laborious. It has been suggested that the water content at p/po = 0.20 (Orchiston 1953; Facek 1966) or at p/ponear 0.50 (Newman 1993; Puri and Mahari 1964) might be used to estimate SA. For the Saskatchewan soil horizons the water contents at p/po= 0.20 (% W20) and at p/po= 0.47 (% W47) predicted SA EGME and SA H2O equally well:
Fig. 4. Dependance of surface area measured by N2sorption (SA N2) on clay content. X W CW X W X m m = 1 + and W s m = 1
wheresis the slope of X vs. WX X
SA EGME = (44 ± 17) + (73 ± 6) % W20, r2= 0.84 SA H2O = (–1 ± 4) + (32 ± 2) % W20, r2= 0.94 SA EGME = (52 ± 17) + (44 ± 4) % W47, r2= 0.84 SA H2O = (–0.2 ± 1.4) + (19.6 ± 0.3) % W47, r2= 0.99 The slopes of the linear regression equations, however, do not necessarily show the value corresponding to 3600 m2 g–1water, and this suggests that there is no single p/p
oat which a monolayer is present on soil surfaces. The percent W20 and percent W47 were also excellent predictors of CEC and water retained at 1.5 MPa.
The surface areas measured by the EGME, N2and H2O sorption methods generally do not indicate the soils ability to transmit water or gases (Puckett et al. 1985; Petersen et al. 1996), but hopefully will indicate the sorbing power of the soil for constituents in these fluids. SA EGME is believed to represent the total SA of the soil minerals, but it is not clear if it includes the contribution of the amorphous substances. This study and others suggest that the internal surface area is only partly accessible to water and, therefore, to compounds dissolved in it. Access of salts (and other dis-solved substances) to interlamelar spaces of montmoril-lonites may be limited until at least two water layers are present (Quirk and Murray 1991). Thus SA EGME presents an upper estimate of the area available for sorption of polar substances, and SA H2O may be a more realistic estimate. SA N2represents the sorption capacity for non-polar
mole-cules in the absence of water (Pennell et al. 1995) and seems to include a contribution of amorphous mineral compounds (Padmanabhan and Mermut 1995). In the field, soil surfaces will always be covered by water layers. For example, for the Elstow Ap horizon in Fig. 1, the water contents at airdryness
Table 1. Correlation matrices between clay and organic C content and surface areas measured by EGME, water vapor and N2sorption (bottom left: Saskatchewan soils; top right Kenyan soils)
Clay Organic C SA EGME SA H2O SA N2
380-710z 10-50 45-133 43-122 25-93 Clay 0.51 0.99*** 0.94** 0.99*** 179–763z Organic C 0.17 0.55 0.75 0.45 1–49 SA EGME 0.93*** 0.15 0.96** 0.99*** 130–379 SA H2O 0.88*** 0.45* 0.91*** 0.93** 29–147 SA N2 0.73*** -0.39* 0.78*** 0.53** 3–41
zRange of data: mg g–1for clay and organic C, and m2g–1for SA.
*, **, *** Significant at P = 0.05, P = 0.01 and P = 0.001, respectively.
Table 2. Surface area (m2g–1) of coarse and fine clay separates of five parent materials measured by EGME, and water vapor and N 2sorption
Coarse clay fraction Fine clay fraction
(2–0.2 µm) (<0.2 µm)
SA SA SA SA SA SA
Soil association EGME H2O N2 EGME H2O N2
Scotsguard 302 102 12.6 714 317 0.4
Ardill 300 121 14.5 690 331 0.2
Ardill 323 NDz 15.0 691 270 0.1
Ardill 273 96 19.4 614 221 0.6
Fife Lake 224 122 12.2 664 327 0.1
zNot determined due to insufficient sample.
Fig. 5. Comparison between surface areas measured by the
Langmuir equation applied to water sorption data and the EGME method.
350 CANADIAN JOURNAL OF SOIL SCIENCE
(3.1%) and 1.5 MPa matric suction (16.5%) are equivalent to about 1/2 and 2 1/2layers of water based on SA EGME (235 m2g–1) or 5 and 28 layers of water based on SA N2 (22 m2 g–1). If one assumes that the internal surfaces (SA EGME - SA N2) share a single layer of water, then 0.2 water layers would be present on the external surfaces at air dry-ness and about 22 layers of water would cover the external surfaces at 1.5 MPa. If two layers of water were present in the interlamelar spaces, the external surfaces would be cov-ered by about 17 layers of water at 1.5 MPa. Layers this thick would severely limit the partitioning of volatile com-pounds to the solid phase.
CONCLUSION
Surface area of Saskatchewan soils (dominant clay minerals smectites and mica) and Kenyan soils (dominantly kaolinite and micas) were measured by EGME, N2and H2O sorption. The three surface area measurements of each group of soils were highly correlated with each other and depended strong-ly on clay content. Differences between the Saskatchewan and Kenyan soils in total surface area (SA EGME) and internal surface area (SA EGME-SA N2) reflected differ-ences in clay mineralogy. Organic C had no significant effect on SA EGME, increased SA H2O, and had no effect on SA N2of the soils from Kenya, but decreased SA N2of the Saskatchewan soil horizons. It is suggested that organic C content may also have caused the extremely low SA N2 measured on coarse and fine clay separates of five parent materials from Saskatchewan.
The utility of the three surface area measurements as indices for the sorptive capacity of field soils needs further investigation. Attention should be paid to the accessibility of the interlamelar spaces to the sorbents, and possible effects of microstrucutre, method of drying and organic matter con-tent of the soil.
ACKNOWLEDGMENTS
The N2sorption measurements were carried out by Ms. Chen Liu. Funding was through an NSERC research grant. Publication No. R841 of the Saskatchewan Centre for Soil Research.
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SA H
2O Organic H2O at p/po
EGME H2O N2 CEC 15 MPa Clay C 0.20 0.47
SA EGME 1.0 SA H2O 0.91*** 1.0 SA N2 0.78*** 0.53** 1.0 CEC 0.66*** 0.87*** 0.13 1.0 H2O 1.5 MPaz 0.81*** 0.85*** 0.42* 0.76*** 1.0 Clay 0.93*** 0.88*** 0.73*** 0.65*** 0.81*** 1.0 Organic C 0.15 0.45* -0.39* 0.82*** 0.42* 0.17 1.0 H2O aty p/po= 0.20 0.92*** 0.97*** 0.52** 0.87*** 0.85*** 0.87*** 0.47* 1.0 p/po= 0.47 0.92*** 1.00*** 0.53** 0.88*** 0.86*** 0.88*** 0.46* 0.99*** 1.0
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