Soil response to acid input in a titration experiment HelinäHartikainen Hartikainen, H. 1992. Soil response to acid input in a titration experiment. Agric. Sei. Finl. 1: 577-585. (Dept. Applied Chem. Microbiol., SF-00014 University of Helsinki,Finland.) Cultivated surface soil samples of an acid Gleysol (soil 1,pH 4.9) and a slightly acid Podzol (soil 2, pH 6.7) were equilibrated for 48 h with oto 144 meq H + kg' 1 by a batch technique designed to simulate reactions of acid load with soil constituents. The pH of the titration suspensions ranged in soil 1 from 5.6 to 3.3, in soil 2 from 7.2 to 4.7. The exchange reaction with base cations on variable charge sites was an important mechanism for H + inactivation. The quantities of cation equivalents released were, however, lower than the proton equivalents added. Calcium dominated the supernatant solutions, but as related to exchangeable reserves. Mg seemed to be more susceptible to acidification at high soil pH. Protons were also consumed in the mobilization of divalent base cations from a non-exchangeable pool to an exchangeable one. The experimental soils differed in their response of acid cation fractions to proton loading. In the rather neutral soil 2, the quantities of soluble and exchangeable acid cations were very low and not affected by acidification. The A 1 dissolved by proton attack was immobilized by complexation reactions. This mechanism did not operate in the acid soil 1 where the proton loading markedly increased the exchangeable A 1 pool and, consequently, the soluble A 1 in the supernatant solution. This was associated with a simultaneous reduction in the complexed Al and a small increase in complexed Fe. Furthermore, acidification diminished the effective cation exchange capacity (ECEC) decisively less in soil 1 than in soil 2, because the increase in exchangeable Al markedly compensated the reduction in the exchangeable base cations. As compared to freely- drained systems, the batch titration overestimated the release of Al to solution phase. Key words: pH-buffering, acidification, cation exchange, complexation Introduction Soil acidification is characterized by intensity and capacity factors. Intensity factors are determinedby chemical properties and are independent of the size of the system considered, whereas capacity factors are a function of the size of the system (Van Bree- men et al. 1983). Soil pH is an attribute indicating the intensity of acidity as well as the chemical and biological conditions of a soil. Its alteration in re- sponse to acid loading is determined by the buffer- ing properties of the respective soil. The impact of acid precipitation in a special edaphic ecosystem is dependent on the type of buffering reactions in volved. Buffering by certain mechanisms can be ample but ecologically harmful (e.g. Ulrich 1981, SCHWERTMANN et al. 1987). In Finland, the soil factors responsible for the 577 Agric. Sei. Fin!. 1 (1992) https://www.c-info.fi/en/info/?token=LfA3aK5SWQG9OuLC.0SFROD9GDHBoCDC6jzs1CQ.aNvUEB2qcnDdLEF170Q9PU9GWAno4VmmenEX-0a83z6fC8FN3vcbSAgD1TF_psEVXg0Rgiv4p8tVfbmda296Utr_glnzQeQFezKjHyTjiV8Ifw5xN46i0RbQaikRHY-AIhm-BY5qxkAl6JkfpaH6VxYCjAERvjVmU5bP_eRkEi7yIFCIcUufRmNQFbwRG1WpGBG_3iU9G_vNmU3uwCmMHInwZKmI5Rl3Imbt_wyUSDj2B38xibRS8u3X6HCBYUuwzYUGicqzsmeb08j4KHc Table 1. Characteristics of the test soils. Soil Clay Silt pH Org. C CECpoi Fe ox Alox Mn ox % % (CaCh) % meq kg” 1 mmol kg” 1 ~~ 1 27 22 4.9 4.6 268 88 95 1 2 13 15 6.7 3.6 168 61 178 1 buffer action have been previously investigated statistically in a study carried out with 84 non-cal- careous mineral soils (Hartikainen 1986). The present paper reports the first part of a series of experiments aimed to monitor experimentally the acid-induced changes in soils and soil extracts as well as the role of various soil components in buf- fering reactions. In this study, the effect of acidific- ation on soil elements and on theirmobilizationwas investigated by a titration procedure. A titration curve for soil combines the two soil acidification characteristics: acid addition refers to the capacity and pH shows the intensity factor. In addition, it integrates these factors to describe buffering reac- tions by soil. Material and methods The titration experiment was carried out with two cultivated fine sand soils of very different pH. The samples taken from the surface layers are described in Table 1. Soil 1, a Gleysol from the postglacial sediment, was taken from the Viikki Experimental Farm (University of Helsinki) near the GulfofFin- land, and soil 2, a Podzol from the glacial till, from Northern Karelia. The clay fraction of the soils in both regions are dominated by illite, the other clay minerals being chlorite and vermiculite (Carlson and Hartikainen, unpublished). Soil samples were analyzed for pH in a 1:2.5 0.01 M CaCU suspension, organic carbon by a CHN analyzer and soil texture according to Elonen (1971). Potential CEC was determined at pH 7.0 by using NHaOAc solution (four extractions). Adsorbed NH4 + re- placed by KCI was determined by distillation. Se- miamorphous Alox, Feo x and Mno x were extracted according to a modified Tamm’s method (Niska- nen 1989) with 0.05 M NHa-oxalate (pH 3.3) at a soil to solution ratio of 1:20 and determined by atomic absorption spectrophotometry (AAS). A set of 5 g soil samples (three replicates) was weighed into centrifuge tubes and 50 ml of water or a H2SO4 solution of a concentration of 0.0012, 0.0024, 0.0036, 0.0048, 0.0060 or 0.0072 M was added. The suspensions were shaken for one min and allowed to stand for 48 h. The pH of the suspen- sions was measured after manual reshaking. The supernatant solution obtained after centrifugation was filtered through a hard filter paper (Schleicher & Schuell 5893 ) and analyzed for Ca, Mg, Fe and Mn by AAS, for K and Na by flame photometry and for A 1 by the Aluminon method (Yuan and Fis- KELL 1959). The soil samples treated with water or acid were washed with 30 ml of ethanol and ana- lyzed for exchangeable cations extracted with four 25 ml portions of I M NH4CI. Another set of soil samples was treated similarly with water and acid. After washing with ethanol the soil samples were analyzed for exchangeable and complexed cations according to a modified Juo and Kamprath’s method (1979) by extracting with four 25 ml por- tions of 0.33 M CuCl2. Similarly to the study of NÄTSCHER(1987), the CuCb solution was adjusted to the same ionic strength as the NH4CI solution. The soil cations were determined as described above, except for A 1 which was determined by AAS. 578 Agric. Sei. Finl. 1 (1992) Results Titration solutions The effect of increasing acid load on the pH of the soil suspensions and on the release of cations from soil to solution is described in Figure la-b. The cation species non-hydrolyzable at pH values pre- vailing in soils (Ca, Mg, K, Na) are referred to as basic cations, the hydrolyzable species (Al, Fe, Mn) as acid cations. When calculating the equivalents of the acid species, Fe and Mn were assumed to appear as divalent ions and Al as a trivalent one. The reciprocal of the slope of the titration graph stands for the buffer capacity (BC), defined as the number of mmols or meq of H + that must be added to 1 kg of soil to lower pH by one unit. When using pH of the zero point of titration (i.e. pH in H2O) as a reference pH, the buffer capacity was higher for soil 2 (53 meq) than for soil I (33 meq). The ex- perimental soils differed also in the shape of the titration curves. At high pH’s obtained for soil 2 (range 7.2-4.7) the graph was linear implying the BC to be rather independent ofpH. In soil 1, on the contrary, the pH was lower (range 5.6 - 3.3) and BC increased with decreasing pH. However, pH being a logarithmic measure the BC values of various soils are comparable only at the same pH level. The comparison of the graphs at a coincident pH range (5.6-4.7) revealed the slope to be steeper for soil 1 than for soil 2. This suggests that, at this pH range, soil 2 was more effectively buffered against acid. In both soils, the basic cations dominated the titration solutions, Ca being the main cation, fol- lowed by Mg and K. The difference between the cation quantities dissolved in the acid and water treatments was taken to describe the acid-induced release into supernatant. Similarly, the differences calculated for each acid increment of 24 meq kg’ 1 were considered to measure the gradual dissolution as response to progressing acidification. The results in Table 2 (only the statistically significant differ- ences are recorded) reveal that with increasing acid load the differential release of Mg decreased pro- portionately most. The release of Ca dimished clearly in soil 1 but remained rather constant in soil 2. Na was not affected. The portion of acid cations in the solutions, mainly Al, distinctly increased with progressing acidification in soil 1, but very slightly in soil 2 where the release of Al was of the same magnitude as that of Mn (on the equivalent basis). No Fe was dissolved. Fig. la-b. Suspension pH and the release of the base and acid cations from the soil to solution in the titration experiment. 579 Agric. Sei. Finl. 1 (1992) 580 Table 2. Differential dissolution of cations (meq kg' l ) calculated for each acid increment of 24 meq kg' 1 Increment of acid meq kg' l Ca Mg K Na A 1 Fe Mn 2 Soil 1 o—>24 16.7 2.1 0.7 - -0.3 -1.2 0.2 18.2 24—>48 18.4 2.1 0.7 - 0.5 0.2 0.1 22.0 48—>72 16.0 1.7 0.5 - 1.5 -0.1 0.2 19.8 72—>96 14.2 1.3 0.5 - 3.6 0.2 0.1 19.9 96—>120 11.2 1.1 0.4 0.2 5.2 0.1 0.1 18.3 120-M44 7.9 0.9 0.3 0.1 9.1 0.4 0.2 18.9 Soil 2 o—>24 15.0 3.9 0.7 - - -0.1 0.2 19.7 24—>48 16.3 3.0 0.5 0.2 - - 0.3 20.3 48—>72 20.2 2.7 0.6 0.4 0.1 - 0.3 24.3 72—>96 18.4 2.0 0.4 -0.1 0.3 - 0.3 21.3 96—>120 18.4 2.0 0.2 0.1 0.3 - 0.2 21.2 120—>144 18.2 0.7 0.4 0.2 0.5 - 0.5 20.5 Table 3. Cations (meq kg' l ) replaced by NH4CI in soil samples after titration treatment. Acid added meq kg' l Ca Mg K Na Al Fe Mn Z Soil 1 0 101.8 11.7 7.4 2.3 4,1 2.2 1.1 130.6 24 88.5 9.5 6.4 2.5 9,2 1.7 0.9 118.7 48 75.4 7.6 5.8 2.1 20.6 0.9 0.8 113.2 72 64.3 6.5 5.4 2.3 31.1 0.9 0.7 111.2 96 53.6 5.2 5.0 2.2 40.8 0.8 0.6 108.2 120 45.6 4.6 4.8 2.3 51.4 0.9 0.5 110.1 144 41.5 4.0 4.6 2.3 58.5 0.9 0.5 112.3 Soil 2 0 124.0 16.6 5.0 2.3 0.0 0.1 1.2 149.2 24 117.9 14.2 4.4 2.2 0.0 0.1 1.6 140.4 48 108.3 11.5 3.9 1.9 0.1 0.1 2.0 127.8 72 104.0 9.8 3.7 2.3 0.2 0.1 2.0 122.1 96 95.0 7.9 3.5 2.2 0.3 0.2 2.3 111.4 120 86.9 6.8 3.1 2.1 0.7 0.1 2.1 101.8 144 74.1 5.7 2.9 2.2 1.4 0.2 2.1 88.6 Soil samples Acidification markedly depleted the reserves of the basic exchangeable cations (Table 3). Magnesium was reduced proportionately more than Ca and K, but no changes were found forNa. The total quant- ity ofNHqCI-replaceable cations, taken to represent effective cation exchange capacity (ECEC), dimin- ished decisively more in soil 2 than in soil I. This was attributable to the fact that in soil 1 the decrease in the exchangeable basic cations was to a marked degree compensated by an increase in the exchange- able Al. The exchangeable Fe and Mn, on the other hand, were lower in the acidified subsamples than Table 4. The sums (meq kg )of base cations dissolved in the supernatant solution and left in the exchangeable form in soil after titration. Acid added meq kg-1 Ca Mg K Na Z Soil 1 0 105.8 13.6 9.4 5.1 133.9 72 119.5 14.2 9.2 5.0 147,9 144 130.4 15.1 9.6 5.2 160.3 Soil 2 0 132.0 19.2 7.9 4.7 163.8 72 163.5 21.9 8.4 5.4 199.0 144 188.6 22.5 8.5 5.3 224.9 Table 5. Cations in complexed form (meq kg *) in the soil samples after titration. Acid added meq kg ' Ca Mg Al Fe Mn Soil 1 0 10.2 0.8 84.1 -0.7 0.1 24 9.1 1.4 84.2 0.2 0.1 48 5.1 0.7 82.4 1.6 0.1 72 4.3 0.3 80.9 2.1 0.1 96 3.3 0.5 77.8 2.4 0.1 120 1.0 0.5 70.9 2.9 0.1 144 -0.3 0.3 66.1 3.3 0.1 Soil 2 0 32.4 1.0 37.7 0.0 4.4 24 31.4 1.0 39.8 0.0 4.2 48 30.0 0.9 45.1 0.0 3.8 72 21.2 0.7 52.3 0.1 3.7 96 22.5 1.3 51.6 0.0 3.4 120 13.7 0.7 57.3 0.1 3.5 144 12.4 0.8 66.7 0.1 3.5 in the water-treated ones. In soil 2, the exchange- able Al was found in very small quantities and only at the highest acid doses. In this soil, the highest acid additionalmost doubled the exchangeable Mn but had no effect on Fe. For quantitative estimation of the acid-derived changes in the basic cations, the cation equivalents dissolved in the supernatant solutions were sum- med upto theresidual exchangeable reserves (com- piled data given in Table 4). The lower sums for the water-treated subsamples as compared to those for the acid-loaded ones indicate that proton additions evoked mobilizationfrom a non-exchangeable pool to the exchangeable and/or soluble fraction. For Ca, and to a lesser extent for Mg, release from these reserves was enhanced with progressing acidifica- tion. For the monovalent species, the phenomenon was less pronounced. Furthermore, the cation supply from non-exchangeable reserves appeared to be of greater significance in soil 2. The differences between the CuCl2- and NH4Cl- extractable cations were considered to represent non-exchangeable reactive reserves, mainly organ- ically bound ones. Acidification gradually exhaust- ed Ca in this fraction in soil I and markedly de- creased it in soil 2. ComplexationofMg was imma- terial and very slightly influenced by acid additions. For acidic cations, Al dominated the complexed reserves. In soil 1, acidification reduced this Al pool but increased the corresponding Fe pool. In soil 2, on the contrary, increasing proton load re- sulted in an accumulation of Al in a complexed form and a slight decrease in complexed Mn. Discussion In order to monitor acid-evoked changes in the soil and solution cations, the titration was performed without background electrolyte. This technique, previously used e.g. by Wells and Davey (1966) and Federer and Hornbeck (1985), obviously un- derestimates the buffer capacity (BC). The present experiment produced lower values than obtained in 581 Agric. Sei. Finl. 1 (1992) a parallel percolation experiment with the same soils (Hartikainen 1992a) where soil pH was me- asured in a CaCh suspension after elution. Obvi- ously, in the titration experiment without back- ground electrolyte the matrix solution affected dis- similarly soil pH in the various treatments, the pH being erroneously high in the water and maybe in the most dilute acid solution suspensions. This hy- pothesis is supported by the finding that the differ- ence between the titration and percolation experi- ment was more pronounced for soil 1 in which the portion of salt replaceable acid cations was high. Also the higher BC for soil 2 of higher pH is in contradiction with general response of non-calcar- eous soils, confirmed e.g. in the earlier titration study at a constant ionic strength (Hartikainen 1986), that the acid buffering is lowest in soils with the highest pH. This behaviour is attributable to the logarithmic nature of pH. The marked reduction in the exchangeable base cations in the acid-treated soils evidences inactiva- tion of H+ by cation exchange. Furthermore, the decrease in ECEC indicates the buffering to have taken place by protonation of variable charge sites. The reduction in ECEC was mainly attributable to the depletion in the divalent base cations. The buf- fering by exchange reactions can be concluded to be mainly attributable to humic material. It is known that added H+ ions will associate first with the conjugate base of the weakest acid in the soil. Owing to the weak-acid nature, organic matter with variable charge has a preference as a proton ac- ceptor. The buffering by the exchange on the permanent charges can be concluded to be rather ineffective because these sites act like strongly acidic anions so that H+ ions linked to them are strongly ionized. The H+ ion has to compete with other cations pres- ent in ambient solution for thepermanently charged exchange sites. Its preference for mineral sites has been found to be between K and Na (Gilbert and Laudelout 1965. Talibudeen 1981). Actually, H+ ions are weakly adsorbed on the permanent charge sites and will remain in a salt-replaceable form and, thus, are not really buffered. Therefore, base cations on the permanently charged sites are hardly exchanged directly by H + ions but by lattice cations (mainly Al) (Veith and Schwertmann 1972) or oxide cations (Hartikainen 1986) re- leased by proton attack. Extraction with NH4CI is known to replace Al only from the permanent charge surfaces. Thus, the significant increase in NHrCI-replaceablc Al upon progressing acidification in soil 1 evidences that, in this soil, exchange occurred markedly also on the mineral surfaces. It is noteworthy that with increas- ing Al saturation the exchangeable divalent base cations were highly reduced, whereas the monova- lent species were only slightly affected. This sug- gests that, on permanent charges, exchangeable Al replaced mainly divalent species dominating the cation composition. The increase in the Al satura- tion was reflected as a marked increase in solution Al 3+ . In the parallel percolation experiment (Har- tikainen 1992b) the increased Al saturation en- hanced the Al leaching immaterially. This differ- ence in the reaction patterns is due to a higher increase in the ionic strength (due to H2SO4) in the titration solutions, which is shown to enhance the displacement of Al3+ from exchange sites (Reuss 1983, Bruce et al. 1989). This reveals that the results obtained for cation exchange in a batch tit- ration are not applicable to freely-drained systems. The titration results imply that in addition to exchange reactions also other buffering mecha- nisms were involved. Firstly, the quantities of ca- tions released by acid were lower than the proton equivalents added. Secondly, the depletion in the exchangeable basic cations was smaller than a re- spective mobilization to solution. The contribution by other buffering reactions in mineral soils of Finland can be concluded also from the titration data published by Mäntylahti and Niskanen (1986) showing the H+ consumption to be greater than the corresponding reduction in CEC. In fact, it can be calculated from the data in Table 4 that the highest acid load dissolved 24.6 and 56.6 582 Agric. Sei. Finl. 1 (1992) meq kg 1 non-exchangeable Ca from soil 1 and 2, respectively. A concomitant increase in P mobiliza- tion (6.7 mg kg’ 1 in soil 1 and 39.0 mg kg' 1 in soil 2) observed in a parallel titration study (Hartikai- nen 1992, unpublished data) gives reason to sup- pose that acid dissolved some Ca from primary or secondary Ca-phosphates, especially in soil 2. Magnesium and monovalent cations were re- leased mainly from exchangeable reserves. As found also in earlier studies (Jeffrey and Weber 1982, Haun et al. 1988), Na did not respond to acidification. When related to the exchangeable pool, the mobilization of Mg in soil 2 was propor- tinately more pronounced than that of Ca. The re- sult suggests that in a soil of high pH Mg may be more susceptible to proton load, and in the first phase of acidification it will be lost proportionately more effectively than Ca. The acid-induced impo- verishment of Mg has been reported in numerous studies on forest soils (e.g. Abrahamsen 1980, Jeffrey and Weber 1982, Bosch et al. 1983, Zech and Popp 1983) and also on acid sulphate soils (Hartikainen and Yli-halla 1986). The difference between CuCb- and NIUCI-ex- tractable cations was assumed to represent non- exchangeable reactive reserves, mainly organically bound ones. Cu2+ has a high affinity for functional groups of humic compounds and is able to replace complexed cations (Bloom et al. 1979). Being acidic, the CuCh solution may extract also some inorganic polymerized metals, but according to Oates and Kamprath (1983), it hardly markedly enhances the replacement of Al from the mineral fraction. Furthermore, because CuCk determines the pH of the extraction mixture (Oates and Kamprath 1983), it can be concluded that in the present study the same Al pool was affected in all treatments. The role of organic matter in regulating the acid- derived changes in soil cations was dependent on the soil pH. In both soils, the acid loading resulted in replacement of the complexed Ca by Al3+ or H + . The reduction in the complexed Ca explained a small part of the total mobilization from non- exchangeable reserves. In the slightly acid soil 2 (pH 6.7), acidification enhanced the accumulation of A 1 into the non-exchangeable complexed frac- tion. At the same time some dissolution of weaker Mn complexes appeared to take place. In the acid soil 1 (pH 4.9), on the contrary, complexed Al began to decrease and exchangeable Al to increase from the second acid increment on (suspension pH 4.3). This response, demonstrating a gradual shift from a non-labilecomplexed pool to a labile one, was similar to that found by JAMESand Riha (1984) in soil extracts. The reduction in the complexed reserves was, however, lower than the concomitant increase in the exchangeable and dis- solved fractions, which indicates that Al was mobil- ized also from mineral components of soil. The decrease in complexed Al coincided with an accu- mulation ofFe in the complexed form, which gives reason to suppose that Fe dissolved by acid began to replace Al from the complexation sites. This hypothesis is supported e.g. by the results of Schnitzer and Skinner (1965) showing that orga- nic matter has a higher affinity for Fe than for Al, even though the retention of both metals decreases when pH is lowered. The replacement of Al by Fe did not, however, quantitatively explain the deple- tion in the complexed Al. This indicates that with increasing acid load, also H+ began to compete more effectively with Al '+ for ligand binding sites. Also Bloom et al. (1979) concluded the Al re- placement by H + on organic matter exchange sites to be an important source of pH buffering. The experimental soils differed in theirbuffering mechanisms, as concluded from the dissimilar shapes of their titration graphs. The results demon- strate that the role of humic material as H + buffer- ing agent is limited at low pH’s. Also the complex- ation of detrimental metals alleviates the effects of proton loading more effectively in soils of high pH, because the stability of the metal complexes de- creases with lowering pH. A common feature was that the release of soil elements to supernatant due to acidification did not quantitatively explain the changes in soil chemistry. More detailed studies are 583 Agric. Sei. Finl. 1 (1992) needed, e.g. to qualify and quantify the role of Acknowledgements. The author wishes to thank Ms. Marjatta Koivisto, B. Sc., for her skillful technical assistance. This study was financially supported by the Finnish Academy, which is gratefully acknowledged. organic matter in buffering. References Abrahamsen, G. 1980. Acid precipitation, plant nutrients and forest growth. Proc. int. conf. ecol. impact acid precip., Norway 1980:58-63. Bloom, P. R., Mcßride, M. B. & Weaver, R. M. 1979. Aluminum organic matter in acid soils: Buffering and solution aluminum activity. Soil Sei. Soc. Am. J. 43: 488-493. Bosch, C., Pfannkuch.E., Baum.U. & Rehfuess.K. E. 1983. fiber die Erkrankung der Fichte (Picea ahies Karst.) in der Flochlagen des Bayerischen Waldes. Forstwissensch. Centralblatt 102: 167-181. Bruce, R. C., Bell, L. C, Edwards, D.G. & Warrell, L. A. 1989. Chemical attributes of some Queensland acid soils. 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Magnesiummangel, einer der Griinde fiir das Fichten- und Tannensterben in NO-Bay- em, Forstwissensch. Centralblatt 102: 50-55. Helinä Hartikainen Department of Applied Chemistry and Microbiology SF-00014 University ofHelsinki, Finland SELOSTUS Maa-aineksen reaktiot happotitrauksessa HelinäHartikainen Helsingin yliopisto Laboratoriossa tehdyssä titrauskokeessa seurattiin kasvavien happolisäysten aiheuttamia muutoksia maa-aineksessa ja sitä ympäröivässä liuoksessa. Kokeessa käytettiin kahta viljely- maan muokkauskerroksesta otettuahietanäytettä, joista toinen (maa I) oli selvästi hapan (CaCb-pH 4,9) ja toinen (maa 2) vain heikosti hapan (pH 6,7). Ilmakuivaa maata (5 g) punnit- tiin sentrifugiputkiin, joihin lisättiin 50 ml vettä tai rikkihap- poliuosta (0,0012 - 0,0072 M). Suspensioiden pH mitattiin 48 tunnin kuluttua, minkä jälkeen maa-aines ja liuosfaasi erotet- tiin sentrifugoimalla ja analysoitiin erikseen. Vety-ionien sitoutuminen pH:sta riippuville varauspaikoil- le (pääasiassa humukseen) emäskationeja syrjäyttämällä oli tärkeä puskurointimekanismi, minkä seurauksena efektiivi- nen kationinvaihtokapasiteetti (EKVK) pieneni, EKVK:n lasku jäi kuitenkin pienemmäksi alunperin happamassa maanäytteessä 1, jossavaihtoreaktioita tapahtui merkittävässä määrin myös mineraaliaineksen pysyvän negatiivisen varauk- sen omaavilla vaihtopaikoilla. Niihin sitoutuneita vaihtuvia emäskationeja korvautui alumiinilla, jota vapautui maasta vaihtuvaan muotoon happamoitumisen seurauksena. Titraus- liuokseen liukeni eniten kalsiumia, mutta vaihtuviin varoihin suhteutettuna magnesiumia näytti vapautuvan maasta her- kemmin etenkin pH:n ollessa korkea. Happamoituminen edis- ti myös vaihtumattomana olevien emäskationien (lähinnä 2- arvoisten) mobilisoitumista vaihtuvaan muotoon. Happamien kationien osalta maanäytteet poikkesivat selvästi toisistaan. Maanäytteessä 2, joka oli alunperin melko neutraali, happa- moitumisen seurauksena liuennut alumiini näytti sitoutuvan orgaanisiksi komplekseiksi ja sitä vapautui suurillakin happo- kuormilla liuokseen erittäin vähän. Sen sijaan maanäytteessä 1 liukoisen alumiinin määrä kasvoi happamoitumisen myötä samanaikaisesti kun vaihtuvan alumiinin määrä maassa li- sääntyi merkittävästi ja kompleksoituneen alumiinin määrä pyrki laskemaan. Eri kationien vapautuminen liuosfaasiin ei kuitenkaan kvantitatiivisesti selittänyt maan kemiassa havait- tuja muutoksia. Kun titrauskokeen tuloksia verrattiin huuhto- miskokeessa vastaavilla happokäsittelyillä saatuihin tulok- siin, havaittiin mm., että erätitraus yliarvioi alumiinin vapau- tumista liousfaasiin. 585 Agric. Sei. Finl. 1 (1992)