THE EFFECT OF VARIOUS CATIONS ON THE WATERSTABILITY OF SOIL AGGREGATES Mikko Sillanpää Department of Soil Science, Agricultural Research Centre, Helsinki Received July 20, 1960 Formation of the structure in different soils depends largely on the organic matter content and texture of soils. However, the chemical properties of soils, especially the role of cations has been found to be important in stabilizing soil aggre- gates. Because of the electronegative character of soil colloids the effect of anions seems to be less important in aggregate formation. Improvements of soil structure produced by N and P have been noticed which, however, may be indirectly due to their favourable effect on herbage yield (10). The increase of aggregate stability that was caused by the NH 3 treatment, however, was found to be only for a short duration emphasizing the dynamic nature of soil aggregate stability (5). Katschinski (9) has summarized the results of several investigators concerning the ability of cations to coagulate soil colloids as follows: Fe +++ >A1 + ++ >Ba++ > Sr ++ > Ca ++ > Mg++ >K + >Na + > Li+ . Hydrogen, even though monovalent, comes after calcium or it may even be more effective. The order of coagulation effect of Ca++ , Mg ++ , K + and Na+ in Finnish heavy clay soils was the same as above (21). In the study of Mazurak (12) the order of the geometric mean diameters of Hesperia soil aggregates formed with different cations was found to be associated with the hydrat- ion of ions: H + > Cs + > Rb + >NH , + >K + > Na+ > Li+ . In this soil the pre- sence of Ca ++ in the aggregates decreased the water stability progressively with increasing concentrations of Ca++ within the aggregates. It was concluded that chan- ges in salt composition and salt concentration of irrigation water affect the disper- sion of aggregates. In many soils, however, liming has been found to improve the structural stability of aggregates (15, 17 etc.) Czeratzki (3) assumed that the effect of lime is a direct coagulationg effect rather than a stimulation of biologial proc- esses. Homrighausen (7) states that because increasing amounts of CaC03 increased the stability of aggregates in very fine soils but not in coarser soils it was possibly due to bridge formation through fine soil and lime particles. According to Filippovich (4) the good structure of calcareous soils, such as chernozems and rendzinas, is a result of their high Fe content. The favourable effect https://www.c-info.fi/en/info/?token=I_GEb6xlW6JsFw1a.Q7Hi3AOD0aKKTduADdUtzg.YTQsw_5JR0Ev8TvTBxFZ4Z1ohmJCZmPpTkBaJ4sWukkJyGM4JL8lW-P_PrtwGOEpFwNihWK4dbhmaRf55UU4i_i8qhufq6Di7UJN5Hb5ykzCABpaoe2R37aIGhP-P74z0F2q59mDcAFJEEwdA4elvNEJX1AXTZ4lsbzxgkh3ivhTmo50nIJ8 212 of calcium on soil structure is based on its converting effect of soluble Fe salts into colloidal insoluble hydrated Fe203 . Further, the stable structure of acid soils unsat- urated with bases but rich in hydrated Fe203 and humus is not due to humus accu- mulation unless accompanied by accumulation of colloidal hydrated Fe203 . In many tropical and subtropical soils the silt and clay are cemented into »pseudo sand» by iron hydroxide (5). According to Levin (11) liming decreases the stability of acid podsolic soil aggregates soon after application by decreasing the content of mobile sesquioxides but later, owing to Ca-humate formation, the stability increases. Chesters et al. (2) found iron as an important factor in aggregate formation in all of the four Wisconsin soils studied. They also found that at pH range 4—B an optimum pH occurred for soil aggregation at approximately pH 6.5. In some Sardim ian soils an inverse correlation between pH and structural stability was noticed (13). In the investigation of Robinson and Page (16) hydrogen ion proved to be much more effective in aggregate stabilization than the other cations tested. Sodium caused the most slaking. There are different opinions about the role of exchangeable magnesium in aggregate formation. The poor physical properties of some Havaiian soils rich in exchangeable magnesium were believed to be a result of the hydration of the exchangeable magnesium ion in the presence of certain humates resulting in a dispersion of the clay and organic matter (6). Joffe and Zimmerman (8) have concluded that magnesium has an effect similar to that of sodium but in the study of Brooks et al. (1) magnesium acted more like potassium and calcium. From the literature it is evident that the effect of various cations is not similar in all soils but depends upon the conditions and soil constituents present. Materials and methods Muddy clay (or gyttja clay) soils are generally low in bases, high in sulphur and very acid. The average content of CaCO„ K2 O and S are 1,5 to/ha, 486 kg/ha and 3872 kg/ha respectively and average pH 3.9 (14). The physical properties of muddy clays are good; the average mean weight diameter of water stable aggregates and also the average hydraulic conductivity was highest among the Finnish soil types studied bySillanpää (19,20). To make these soils more productive they require very large amounts of lime and fertilizers. Thus their chemical properties will be essentially changed. Because of this muddy clay was selected for investigating the effect of cations on aggregation. Also analyses with a heavy, loamy and silty clay soils are included in this study for comparative purposes. Large homogeneous soil samples were spread as a thin layer, largerclods were crushed gently by hand and the soils were allowed to become air dry. In experiments with heavy, silty and loamy clays the soils were previously dry-sieved with 2 mm and 1.3 mm screens to obtain an aggregate fraction of 1.3 2.0 mm for analyses. In the case of the better aggregated muddy clay soil, however, 2.2 —3.5 mm agg- regates were selected. Samples of 25 grams were weighed in 50 ml dishes over a 2 cm wide cloth (Fig. 1). The samples were wetted by submerging one end of the cloth in the wetting solution as shown in Fig. 1, A, and allowing the solution to pass along the cloth into the other dish to moisten the soil sample slowly by capillarity. The samples were left in a submerged condition for two days, after which the excess solution above the soil was poured away and the rest of the solution was drained from the soil by raising the dish containing the sample on a stage (Fig. 1, B). The samples were air dried in open dishes. The forma- tion of fungus vegetation on soil was prevented and the drying process accelerated by placing lamps above the dishes. The temperature during the dryingprocess was kept 28 31° C. 213 The solutions used in the experiments with heavy, silty and loamy clays were 0.01-N HCI, Na, K, Mg and Ca chlorides and distilled water as a check. In the case of the muddy clay soil the experiments were extended to include ferric and aluminium chlorides as well; all solutions in concentrations of 0.003, 0,01, 0.03, 0.1 and 0.3-N. On the seventh day after drying the samples were rewet by the above mentioned method with distilled water, poured into 500 ml shaking bottles containing 400 ml distilled water and shaken for 5 minutes at 30 r.p.m. Then the soil samples were poured on the uppermost sieves of the wet sieving apparatus (18) and sieved for 30 minutes at 30 oscillations per minute. Results and discussion The results of aggregate analyses after different treatments are given in tables 1 and 3 and the regressions concerning muddy clay soil in table 4 and in Fig. 2. Statis- tical significances of the differences between the treatments are given in table 2 (heavy clay; 0.01-N solutions and destilled water) and in table 5 (muddy clay 0.01 and 0.03-N solutions and destilled water). In the heavy clay soil hydrogen and calcium have improved the structural stability of soil most effectively. Their effect exceeds significantly those of all other cations except that of magnesium (table 2). Also the influence of magnesium is relatively clear with significantly better aggregation than with the check soil (dist. Table 1. Mean weight diameters of aggregates (mm; mean and standard deviation) of three clay soils after treatments with distilled water and various 0.01-N chloride solutions. Soil type Dist. water Nad KCI MgCI 2 CaCl2 HCI (0.01-N) (0.01-N) (0.01-N) (0.01-N) (0.01-N) Heavy clay 0.993 1.027 1.060 1.087 1.105 1.114 ±0.074 ±O.OlO ±0.028 ±O.Oll ±0.019 ±0,040 Silty clay 1.022 1.019 1.045 1.045 1.049 1.046 ±0.044 ±0.033 ±0.031 ±0.028 ±0.037 ±0.031 Loamy clay 0.881 0.885 0.939 0.939 0.960 0.958 ±0.058 ±0.069 ±0.085 ±0.054 ±0.043 ±0.065 Fig. 1. Wetting a soil sample by capillarity along a ribbon of cloth (A) and removing of excess liquid from the sample (B). 214 Table 2. Statistical significance of the differences (t-values) in aggregate stability of a heavy clay soli after treatments with distilled water and various 0.01-N solution (significances at 10‘, s*, 1 *� and o.l*** per cent levels). NaCl KCI MgCl2 CaCl 2 HCI f H 2O 0.92 1.68 2.54» 2.95» 2.88* NaCl . 2,20- 8.57»*» 7.09*»» 4.14»» KCI - 1.80 2.65* 2.25- MgCl 2 - 1.64 1,29 CaCl 2 - 0.41 Table 3. Mean weight diameters of the muddy clay soil aggregates (mm; mean and standard deviation) after treatments with different solutions. Normality of solution Solution 0.000 0.003 0.01 0.03 0.1 0.3 (H,O) NaCl 1.725 1.805 1.835 1.703 1.630 1.470 ±0.060 ±0.192 ±0,152 ±0.147 ±0.059 ±0.042 KCI 1.717 1.736 1.790 1.800 1.896 1.753 ±0.090 ±0.137 ±0.168 ±0.142 ±0.091 ±O.lOO MgCl2 1.728 1.838 1.933 1.930 1.823 1.583 ±0.090 ±0.139 ±0.116 ±O.llO ±0.076 ±0.057 CaCI2 1.720 1.853 1.855 1.900 1.873 1.733 ±0.129 ±0.206 ±0.281 ±0.220 ±0.182 ±0.094 AlClj 1.660 1.910 2.016 2.093 2.206 2.060 ±O.OlO ±0.017 ±0.035 ±O.lOl ±0.106 ±0.161 FeCl, 1.663 1.793 2.096 2.103 2.193 1.943 ±0.067 ±0.162 ±0.107 ±0.182 ±0.120 ±0.160 HCI 1.745 1.848 1.853 1.775 1.658 1.57 3 ±0.267 ±0.065 ±0,094 ±0.120 ±0.063 ±0.093 Average 1.712 ±0.120 _____ water) and sodium. The significance, however, may be more due to the small stan- dard deviation rather than the difference between the mean values of mwda. In the silty clay and loamy clay soils the order of the effects of the cations is mainly the same as in the heavy clay. In these soils the similar effects of potassium and magnesium should be noted. This is similar to the results of Brooks et al. (1). 215 Table 4. Regressions of the mean weight diameter of aggregates (Y, mm) in the muddly clay soil on the normality of the treatment solution (X), (R = correlation coefficient). Solution Regressions R NaCl KCI Y = 1.22-0.523 log X-0.1171 (log X)2 0.60*» Y = 1.74-0.123 log X —0.0461 (log X)2 0.68»* MgCl 2 CaCl 2 AICI, Fed, HCI Y = 1.28-0.702 log X—0.1866 (log X)3 0.62** Y = 1.58-0.369 log X-0.1073 (log X) 2 0.53** Y = 1.96-0.327 log X-0.1413 (log X)2 0.78»** Y = 1.75-0.557 log X-0.2007 (log X)2 0.81*»* Y = 1.34-0.454 log X-0.1041 (log X) 2 0.59** Table 5. Statistical significance of the differences (t-values) between the results of aggregate analyses from the muddy clay soil alter treatments with distilled water, 0.01-N and 0.03-N solutions. The t-values for the last mentioned concentration are given within parantheses. (Significances at 10*, s*, I**, and o.l*** per cent levels). H 2 O NaCl KCI MgCl2 CaCl2 AICI, FeCI, HCI HaO 1.54 0.78 3.52** 1.00 9.6B*** 5.79*** 2.67* NaCl (0.12) - 0.37 1.02 0.40 2.30' 2.66* 0.20 KCI (1.03) (0.88) - 1.27 0.38 2.28' 2.66* 0.58 MgCl 2 (3.63**) (2.47*) (1.32) - 0.51 1.35 1.92 1.07 CaCl 2 (1.67) (1.49) (0.73) (0.24) - 1.14 1.57 0.13 Aid, (6.o4***) (4.16**) (2.91*) (2.03’) (1.55) - 0.92 3.19* FeCI, (3.63**) (3.12*) (2.27‘) (1.46) (1.33) (0.08) - 3.13* HCI (0.97) (0.76) (0.25) (1.90) (1.00) (3.80*) (2.71*) Fig. 2. Regressions of the mean weight diameter of aggregates (MWDA, mm) in the muddy clay soil on the concentration of various treatment solutions. In the case of distilled water treatment the mean and standard deviation are given. 216 In the case of silty clay the differences between treatments did not reach any statis- tical significance and in loamy clay there were statistically significant differences only between H2O—CaCl2 (t = 2.47*), H,,O—HCI (t = 1.97-) and NaCl—CaCl2 (t = 2.08-). Generally, with increasing solution concentrations in the muddy day soil (table 3, and Fig. 2) all cations initially improved the structural stability. When the concentration of solutions was further increased the effect of all cations gradually changed to dispersion. The highest solution concentrations used in treatments appar- ently caused changes in soil cation composition to such a degree that it produced a dispersion. Mazurak (12) similarly concluded that changes in salt composition and concentration of irrigation water may affect the dispersion of aggregates. Different cations seem toreach their optimum effects in different concentrations. For NaCl, HCI and MgCl2 the optimum normality seems to lie around 0.01-N, for CaCl 2 from 0.01 to 0.03-N and for KCI, AIC13 and FeCl 3 the optimum aggregation is first reached with approximately 0.03—0.1-N solutions. The shape of the regres- sion curve of potassium differs from other curves because of its milder curvilinearity; i.e. it seems that potassium affects aggregation less than the other cations under study. Aluminium and iron have had the strongest stabilizing effect on the muddy clay aggregates. The effect of magnesium seems to be stronger than that of calcium in milder concentrations but with increasing concentration the regression line of magne- sium turns more steeply toward dispersion. As an interesting feature it can be seen that in this soil the regression line of HCI corresponds closely to that of sodium, which has the weakest stabilizing capacity. In this respect the influence of HCI differs considerably from that in other clay soils studied. This shows, as already seen from the literature, that a cation may play different roles in different soils thus limiting considerably the application of results from one soil to another. Summary The effects of various cations on the aggregation of four clay soils were studied. To avoid errors and variation and to obtain clear differences between the treatments only certain limited size fractions of aggregates were used. The samples were wetted slowly by capillarity with the treatment solutions, allowed to stand in a submerged condition, air-dried, rewetted and analyzed by the wet sieving method. The results bring out rather distinct differences among the effects of various cations. The water stability of aggregates was a function of the concentration of the treatment solutions. REFERENCES (1) Brooks, R. H., Bower, C. A. & Reeve, R, C. 1956. The effect of various exchangeable cations upon the physical condition of soils. Soil Sei. Soc. Amer. Proc. 20: 325 327. (2) Chesters, G., Attoe, O. J. & Allen, O. N. 1957. Soil aggregation in relation to various soil con- stituents. Ibid. 21: 272 277. 217 (3) Czeratzki, W. 1957. Untersuchungen über Krümelstabilität an einem Kalkversuch. Z. Pfl. Ern. u. Düng. 78: 121-135. (4) Filippovich, Z. S. 1956. [Absorption of colloids by soils and the formation of structure]. Pochvo- vedenie N:o 2: 16—26 (Ref. Soils Fert. 19: p. 333). J5) Gifford, R. O. & Strickling, E. 1958. The effect of anhydrous ammonia on water stability of soil aggregates. Soil Sei. Soc. Amer. Proc. 22: 209 212. (6) Gill, W. R. & Sherman, G. D. 1952. Properties of the gray hydromorphic soils of the Hawaiian Islands, Pac. Sei. 6: 137 (Ref. Soils Fert. 15: p. 297). (7) Homrighausen, E. 1958. Untersuchungen über die Aggregation von Böden und ihre Kennzeich- nung durch Schlagfestigkeitsmessungen. Z. Acker-u. Pfl. bau 105: 61 88. (8) Joffe, J. S. & Zimmerman, M. 1945. Sodium, calcium, and magnesium ratios in the exchange complex. Soil Sei. Soc. Amer. Proc. 9: 51 55. (9) Katschinski, N. A. 1957. Die Natur der mechanischen Stabilität und Wasserstabilität der Boden- struktur. Probleme der Krümelstabilitätsmessung und der Krümelbildung. Wiss. Arbeits- tagung, Berlin 10—11. Okt. 1957. Tagungsberichte 13: 139—149. (10) Koslekov, P. N., Osipova, Z. M. & Tanin, K. E. 1952. [The change of structure of heavy peat- podzolized soils in long-term experiments with fertilizers], Pochvovedenie 820 828 (Ref. Soils Fert. 16: p. 29). (11) Levin, F. I. 1957. Water stability of the structure of sod-podzolic soils with application of lime and organic fertilizers. Ibid. 10: 98—104 (Ref. Soils Fert. 21: p. 100). (12) Mazurak, A. P. 1953. Aggregation of colloidal clay from Hesperia sandy loam as affected by univa- lent and calcium ions. Soil Sei. 76: 181 191. (13) Pallotta, U. 1957. La stabilita di struttura nei terreni della Sardegna in rapporto con alcune carat- teristiche fisicochimiche. Agrochimica 1: 268 287 (Ref. Soils Fert. 20: p. 265). (14) Purokoski, P. 1959. Rannikkoseudun rikkipitoisista maista. Referat: Über die schwefelhaltigen Böden an der Küste Finnlands. Agrogeol. pubi. 74: 27 p. (15) Ravikovitch, S. & Hagin, J. 1957.The state of aggregation in various soil types in Israel. Ktavim 7: 107-122 (Ref. Soils Fert. 20: p. 328). (16) Robinson, D. O. & Page, J. B. 1950. Soil aggregate stability. Soil Sei. Soc. Amer. Proc. 15: 25 29. (17) Schachtschaeel, P. & Hartge, K. 1958. Die Verbesserung der Strukturstabilität von Ackerböden durch eine Kalkung. Z. Pfl. Ern. u. Düng. 83: 193 202. (18) Sillanpää, M. 1958. Soil Aggregation as determined by wet sieving method after different sample treatments. Selostus: Muruanalyysistä märkäseulontamenetelmällä. Acta agr. fenn. 94,16: 20 p. (19) —*— 1959. Hydraulic conductivity of Finnish subsoils as related to some other soil physical properties. Selostus: Eräiden maan fysikaalisten ominaisuuksien vaikutuksesta pohjamaan vedenläpäisevyyteen. Agrogeol. pubi. 73: 28 p. (20) —*— 1959. The influence of some physical soil properties on subsoil structure. Selostus: Eräiden maan fysikaalisten ominaisuuksien vaikutuksesta pohjanmaan rakenteeseen. Ibid. 75: 24 p. (21) Vuorinen. J. 1939. Untersuchungen über die Koagulation des schweren Glazialtons. Selostus; Jäykänglasiaalisaven koagulaatiota koskevia tutkimuksia. Ibid. 50: 114 s. SELOSTUS: KATIONIEN VAIKUTUKSESTA MAAN RAKENTEESEEN Mikko Sillanpää Maantutkimuslaitos , Maatalouden tutkimuskeskus, Helsinki Maan rakenteen ominaisuuksien on todettu riippuvan paitsi maan lajitekoostumuksesta ja orgaa- nisesta aineksesta myös sen kemiallisista ominaisuuksista. Erityisesti kationien laatuun on kiinnitetty huomiota. Maakolloidien elcktronegatiivisesta luonteesta johtuen anionien merkitys lienee vähemmän 218 merkityksellinen, ja mm. N- ja P- käsittelyillä saadut muruisuuden lisäykset on selitetty johtuvan epä- suorasti niiden aiheuttamien sadonlisäysten edullisesta vaikutuksesta maan orgaanisen aineksen mää- rään. Nykyisin käytettävät suuret kalkki- ja lannoitemäärät voivat melkoisesti muuttaa maan kemial- lisia ominaisuuksia, mistä voi olla seurauksena muutoksia myös sen fysikaalisissa ominaisuuksissa mm. rakenteessa. Kationien vaikutusten tutkimiseen käytettiin pääasiassa liejusavinäytteitä, mutta analyysejä suo- ritettiin vertailumielessä myös aitosavi-, hiesusavi- ja lietosavinäytteistä. Ilmakuiva maa esiseulottiin, sekä virheiden ja hajonnan pienentämiseksi kokeissa käytettiin vain tiettyjä murufraktioita (2.3 3.5 mm liejusavesta ja 1.3—2.0 mm muista savista). Ilmakuivat näytteet (25 g) kostutettiin kapillaari- sesti käsittelyliuoksilla (kuva 1, A), annettiin seisoa liuoksiin upotettuna 2 vrk. ja kuivattiin samaa menetelmää käyttäen (kuva 1, B). Kuivatusta jatkettiin antamalla näytteiden seisoa avonaisissa as- tioissa. Homeenmuodostus ehkäistiin asettamalla voimakkaita lamppuja näytteiden yläpuolelle. Lämpötila kuivatuksen aikana pidettiin 28 31° C:na. Näytteiden kostutuksessa käytettiin 0.01-n, NaCl-, KCI-, MgCl 2 -, CaCl 2 - ja HCI-liuoksia sekä tis- lattua vettä. Liejusavinäytteiden käsittelyssä olivat edellisten lisäksi mukana myös FeCI3 ja AICI 3 sekä kaikki liuokset 0.003-, 0.01-, 0.03-, 0.1: ja 0.3-normaalisina. Viikon kuluttua näytteiden käsittelyn päättymisestä suoritettiin niistä muruanalyysit märkä- seulontamentelmällä. Niiden tulokset on esitetty taulukoissa 1 ja 3 sekä liejusavea koskevat regressiot taulukossa 4 ja kuvassa 2. Eri käsittelyjen vaikutusten erojen tilastolliset merkitsevyydet on esitetty taulukossa 2 (AS) ja taulukossa 5 (LjS). Aitosavessa ovat HCI- ja CaCl 2 -käsittelyt parantaneet murujen vedenkestävyyttä eniten. Niiden vaikutus on merkitsevästi parempi kuin muiden lukuunottamatta MgCl 2 -käsittelyjä (taulukko 2). Myös Mg-käsittelyn vaikutus on parantanut murujen vedenkestävyyttä merkitsevästi NaCl- ja H2O-käsitte- lyihin verrattuna. Hiesu- ja Helosavissa ovat eri kationien vaikutukset samansuuntaiset kuin aitosavessa. Hiesu- savessa eri käsittelyjen vaikutus ei saavuttanut tilastollista merkitsevyyttä ja lietosavessa se rajoittui vain H 2 O —CaCI2 - (t = 2.47*), H 2 —HCI- (t = 1.97") ja NaCl —CaCI2 - (t = 2.08") käsittelyihin. Liejusavella suoritetuissa kokeissa todettiin yleispiirteenä kaikkien kationien auksi parantavan murujen vedenkestävyyttä käsittelyliuosten konsentraatioiden kasvaessa tiettyyn optimiin asti, jonka jälkeen niiden vaikutus alkoi pienentyä (taulukko 3 ja kuva 2). Suurimpien liuoskonsentraatioiden ai- heuttamassa murujen vedenkestävyyden huononemisessa lienee kysymyksessä sama ilmiö, jonka mazurak (12) havaitsi aiheuttavan murujen heikkenemistä silloin, kun keinokasteluveden suolakokoo- muksessa ja konsentraatiossa tapahtui muutoksia. Eri kationien vaikutus näyttää saavuttavan optiminsa tässä maalajissa jonkinverran eri konsen- traatioissa. NaChn, HCl:n ja MgCl 2:n optimikonsentraatiot ovat 0.01-normaalisuuden paikkeilla, CaCl2 :n 0.01 0.03:n ja KCl:n, AICl 3:n sekä FeCl3 :n 0.03—O.l-n. Alumiinin ja raudan vaikutus liejusaven murujen vedenkestävyyteen on selvästi voimakkaampi kuin muiden kationien. Kalsium ja magnesium poikkeavat toisistaan melko vähän, joskin magnesium näyttää alhaisemmissa konsentraatioissa parantavan murujen vedenkestävyyttä voimakkaammin, kun taas suuremmissa konsentraatioissa kalsiumin regressiokäyrä osoittaa vähemmän dispersiotaipumusta. Kaliumkloridikäsittely näyttää aiheuttaneen vähiten muutoksia murujen vedenkestävyydessä, mikä ilmenee KCI-käyrän huomattavasti muita lievemmästä kaarevuudesta. Mielenkiintoisena piirteenä voidaan todeta, että HCI:n vaikutus liejusavessa on lähinnä saman ta- painen kuin natriumin, joka kaikissa neljässä maalajissa on osoittautunut kationeista epäedullisimmaksi murujen vedenkestävyydelle. Tässä suhteessa HCl:n vaikutus muihin käsittelyihin verrattuna on olen- naisesti erilainen kuin muissa savimaalajeissa. Todennäköistä onkin, että eri kationien vaikutus maan rakenteeseen riippuu suuresti muiden kationien määristä sekä käsittelyjen aiheuttamista kationinvaihto- prosesseista eri maissa. Tämä rajoittaa olennaisesti jollakin maalla saatujen tulosten yleistämistä ja soveltamista muihin maalajeihin.