Maataloustieteellinen Aikakauskirja Vol. 57: 279—283, 1985 RESEARCH NOTE Acid-neutralizing capacity of Finnish mineral soils HELINÄ HARTIKAINEN Department of Agricultural Chemistry, University of Helsinki, SF-00710 HELSINKI, Finland Abstract. The acid-neutralizing capacity (ANC) was determined graphically from curves obtained in HCI titration (at a constant ionic strength I = 0.1) and was expressed as a quanti- ty ofacid (meq kg-1 ) needed to reduce the soil pH to 3.8. The relationship between ANC3 g and soil characteristics was studied statistically. In 84 soil samples, ANCJ g ranged from 12to 184 meq kg-1 . The average ANC, g was highest in the heavy clay soils and lowest in the non-clay soils, but the differences between the various textural soil groups were not significant. In all soil groups the initial pHCaC | 2 was relatively the most important factor explaining the variation in ANC3 s . Organic C was also a significant variable; this was considered to indicate the importance of cation exchangereac- tions of organic matter in acid-buffering. With the exception of heavy clay soils, oxalate-soluble Al significantly explained the variation in ANC38 , suggesting that dissolution of Al hydrox- ides acted as a sink for H + ions and contributed to the neutralizing capacity at the reference pH of 3.8. Index words: acid-neutralizing capacity, soil acidity, titration, pH-buffering Introduction From the agricultural and ecological point of view soil pH is a very enlightening attribute of a soil. In addition to intensity of acidity it indicates the chemical and biological condi- tion of a soil. Addition of H+ ions to the edaphic system generally, but not always, de- creases soil pH; any alterations depend on the buffering properties of the respective soil. Soil acidification is actually defined as a decrease in acid-neutralizing capacity rather than as a decrease in pH (Van Breemhn et al. 1983). The intensity of buffering depends on the type of buffer system present, whereas the capacity is determined by its size. In a study of Hartikainen (1985) on the intensity of acid- and base-buffering, the acid quantities needed to reduce soil pH by 0.5 units were the higher the lower the initial soil pH was. It was further observed that in soils of different initial pH levels the variation in buffer values was explained by different soil factors. In the present study on the acid-neutralizing capac- ity and related soil characteristics, attention was paid to the capacity of soils in various 279 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=y1APxfC7wTWFCNC_.J_46nmu_FaqkAAWwpRJXnw.Ro38aCIXvSo4MP88gvOpyDn_d87FfYeRG5YubGjXZ66f_v4u3HTWbIYYI_qeLRQSFEUWvsals9FHn0WfWRuwPkF_6RIIDDYd9PipTlLdowAIL-btIKKMzvjY2NIABYTJaedFTtl928ZLbiAAPIn4earwvoiEAGxk8rkJY7MgH7FYygM6ZQ textural classes to counterbalance the effect of acidifying factors. Materials and methods a) Soil samples The experimental material, collected from southern and central Finland, consisted of 15 heavy clay soils (60 % or more clay fraction < 2 /un), 41 coarser clay soils (30—59 % clay), 20 silt soils (main fraction 2—20 /tm) and 8 fine sand soils (main fraction 20—200 /un). The characteristics of the soils are presented in Table 1. The air-dried 2-mm sieved samples were analysed for pH in a 1:2,5 0.01 M CaCl2 suspension and for organic C by the wet com- bustion method (Graham 1948). Exchangeable basic cations displaced with 1 M NH 4OAc (pH 7.0) were determined by AAS (Ca and Mg) or by flame photometry (K and Na). Al, Fe, and Mn extracted with 0.05 M NH4 - oxalate (pH 3.3) (1:20 WA) and Al extracted with 1 M NH 4OAc (pH 4.8) (according to McLean 1965) were determined by AAS. b) Determination of acid-neutralizing capacity Analogously to aqueous systems, the acid- neutralizing capacity (ANC) of the soils can be determined by titration with a strong acid to a given reference pH. In thepresent study, a batch titration method was used: 5 g of soil was treated with 50-ml volumes of solutions containing 0, 0.3, 0.6, 0.9, 1.2 or 1.5 meq HCI at an ionic strength of I = 0.1 (adjusted by KCI). After a 4-day equilibration (stirred once), the pH of the suspensions was measured with an analogous pH-meter, using a separate reference electrode. The titration graphs were drawn by expressing the measured pH as a function of acid added. The ANC was deter- mined graphically from the curve and ex- pressed as a quantity of acid (meq kg ‘) needed to reduce the soil pH to 3.8. In other words, the ANC stands for H + consumption between the pH of zero point of titration (ZPT), i.e. pH in 0.1 M KCI, and pHKCI 3.8. The subindex of ANC denotes the reference pH. The titration was carried out in duplicate. The precision of the method is described in details elsewhere (Hartikainen 1985). Results and discussion The magnitude of ANC depends on the reference pH chosen. According to Vanßree- men et al. (1983), a pH of 5 might be appro- priate for agricultural soils and that of 3 more reasonable for forest soils. The reference pH of 3.8 used in the present study is inter- mediate, but from an ecological point of view it may be universal for soils of undefined utilization. The ANC38 ranged from 12 to 184 meq kg -1 , the average and median being 68.8 and 58.0 meq kg -1 , respectively. It decreased with increasing initial soil acidity; the corre- lation of ANC 38 vs. soil pH CaC, 2 was r = o.77*** (n = 84). The correlation between log Table 1. Characteristics of soil samples. Means with confidence limits at 95 per cent, w = range. pH (CaCl 2) Org. C Oxal. extr. Acet. extr. Basic % of D.M. AI Fe AI mmol kg-1 cations mmol kg-' meq kg - ' Heavy 5.1 ±0.3 5.0 ± 1.3 97±28 91 ±l5 10.5 ±5.8 229±68 clays w 4.2—6.0 1.0—9.0 45—255 30—149 2.2—39.4 103—613 Coarser 5.2±0.2 4.2±0.6 62±6 72±9 6.4±1.4 128 ±l3 clays w 4.3—6.5 0.5—11.9 28—111 31—171 1.3—18.9 61—219 Non-clay 5.2±0.3 3.5± 0.7 60±12 62±6 7.1 ±2.3 98 ± 17 soils w 3.8—6.4 0.7—7.7 17—141 33—112 1.4—21.2 24—206 280 Table 2. ANC3 g (meq kg-1 ) of soils in different tex- tural groups. Means with confidence limits at 95 per cent level. ANCj, g Range Heavy days Coarser clays Non-clay soils 76.1 ±17.9 69.3 ±9.4 64.1 ±12.7 22—134 40—158 12—184 ANC 3 8 and pH was not significantly closer (r = o.7B***). Furthermore, the neutralizing capacity tended to increase with the increase in exchangeable basic cations, the correlation coefficient being r = o.46***. It should be pointed out that when a heavy clay sample ex- ceptionally rich in NH4OAc soluble cations was excluded, the value of r rose to o.64***. On the other hand, ANC 38 did not cor- relate with the clay content of soils (r = 0.12). In fact, there were great variations in the neu- tralizing capacity within the textural groups, but the differences between the various soil classes were not noticeable (Table 2). Cer- tainly, the average ANC3 8 was highest in the heavy clay soils and lowest in the non-clay soils. When the dependence of ANC 3 g (y) on the soil characteristics was studied by the regres- sion analysis, only soil pH CaC, 2> the content of organic C (%) and oxalate-extractable Al (mmol kg-1 ) were statistically significant variables (P = 0.05). In various textural soil groups therelationship conformed to the fol- lowing equations: Heavy clay soils: y = 65.68 pH + 10.29 org. C 307.59 R 2 = o.B2*** Standard error of estimate S = 14.66 Coarser clay soils: y = 55.91 pH + 2.12 org. C + 0.45 oxal. AI—- -257.81 R 2 = o.BB*** S = 10.61 Non-clay soils: y = 51.91 pH + 7.95 org. C + 0.26 oxal. AI 246.97 R 2 = o.Bl*** S = 14.94 On the basis of /3-coefficients the initial soil pH was the most decisive factor in all soil groups. In the non-clay soils the relative importance of organic C content was greater than that of oxalate soluble Al, whereas a reverse rank was found in the coarser clay soils. More detailed studies are needed to clarify the causes for different buffer capacities of different soils and the mechanisms responsible for buffer action, but some interpretations can be discussed. Generally, in all textural groups the same factors explained the variation in the acid-neutralizing capacity. However, in the heavy clay soils, where the organic C and oxalate-soluble Al were highly correlated (r = o.77***), the oxalate-soluble Al was ex- cluded from the equation. Although it ex- plained 17 % of the variation it was insignifi- cant owing to the small number of samples. The relationship between ANC and pH is consequential, because a higher activity of H + ions (lower pH) can be considered a result of a reduced inactivation ability of soil. The other factors explaining the variation in ANC depend on the reference pH chosen. Vir- tually, the reference pH determines which buffer systems are involved. Ulrich (1981) has demonstrated the characteristic chemical soil state for various buffer ranges and cal- culated the pH of 3.8 (in equilibrium soil solu- tion) to represent the upper limit of the iron buffer range. In the present study, the oxalate- soluble Fe was insignificant in explaining the variation in ANC 3 8 , which suggests that the iron buffer range was not reached. The oxa- late-soluble Al, on the contrary, was a signi- ficant variable, infering that the dissolution of Al hydroxides might act as a sink for H + ions. The buffering by this mechanism can be expected to be ample but ecologically harm- ful. The contribution of organic C may be at- tributable to the significance of organic mat- ter as cation exchanger and indicate the role of exchange reactions in acid-buffering. The H + ions exchange cations directly only on slightly acid (variable) charge sites (Veith and Schwertmann 1972) the main source of 281 which in Finnish soils is generally organic matter. Thus, an influx of H + to soil implies a reduction in effective cation exchange capac- ity even though not necessarily in pH. How- ever, especially at higher reference pHs, the organic C may be a poor measure of the buff- ering capacity due to organic matter. The pKa values of organic constituents range from 3.8 to 6.2 (Martin and Reeve 1958, Hargrove and Thomas 1982), wherefore also the efficiency of organic matter as proton ac- ceptor can be concluded to vary. Further studies on theANC values at vari- ous reference pHs are needed to give estimates on the susceptibility of our soils to various acidifying factors. On the other hand, also studies on the type and kinetics ofproton con- suming reactions are necessary in order to infer the ecological consequences of acid- buffering reactions. Acknowledgement.The author wishes to thank the Maj and Tor Nessling Foundation for the grant which made it possible to complete this study. References Hargrove, W.L. & Thomas, G.W. 1982. Titration prop- erties of Al-organic matter. Soil Sci. 134: 216—225. Hartikainen, H. 1985. Acid- and base-titration behav- iour of Finnish mineral soils. In preparation. (Manu- script available at Department of Agricultural Chem- istry, University of Helsinki). Graham, E.R. 1948. Determination of soil organic mat- ter by means of a photoelectric colorimeter. Soil Sci. 65: 181—183. Martin, A.E. & Reeve, R. 1958. Chemical studies of podzolic illuvial horizons. 11l Titration curves of organic-matter suspensions. J. Soil Sci. 9: 89—100. McLean, E.O. 1965. Aluminium, pp. 978—998 in Meth- ods of soil analysis. Agronomy 9 (2). Van Breemen, N., Mulder, J. & Driscoll, C.T. 1983. Acidification and alkalinization of soils. Plant and Soil 75; 283—308. Veith, J. & Schwertmann, U. 1972. Reaktionen von Ca- Montmorillonit und Ca-Vermiculit mit Kohlensäure. Z. Pflanzenern. u. Bodenkde 131: 21—37. Ms received September 6, 1985 SELOSTUS Suomalaisten kivennäismaiden haponneutralointikapasiteetti Helinä Hartikainen Helsingin yliopisto, maanviljelyskemian laitos, 00710 Helsinki Laboratoriokoe tehtiin Etelä- jaKeski-Suomesta kerä- tyillä 84 maanäytteellä, joista 15 luokiteltiin aitosaveksi, 41 hiesu- tai hietasaveksi, 20 hiesuksi ja 8 hiedaksi. Il- makuivista maista otettiin 5 g:n eriä, joihin lisättiin 50 ml titrausliuosta, jossa oli 0, 0.3,0.6,0.9, 1.2 tai 1.5 mekv HCl;ää. Jokaisen titrausliuoksen ionivahvuus (1) oli sää- detty KC1:llä O.lrksi. Neljän päivän reaktioajan jälkeen suspensioiden pH mitattiin ja mittaustuloksista piirret- tiin käyrä, jossapH esitettiin happolisäyksen (mekv kg- 1 maata) funktiona. Haponneutralointikapasiteetti (ANC) ratkaistiin graafisesti ja ilmoitettiin happomääränä(mekv kg-1), joka tarvittiin laskemaan maan pH 3.B:aan. Tutkituissa näytteissä ANC3 8 vaihteli 12—184 mekv kg-'. Keskimääräinen neutralointikapasiteetti oli suurin (76 mekv kg-') aitosavissa, seuraavaksi suurin (69 mekv kg- 1) hiesu- ja hietasavissa ja pienin (64 mekv kg-1) hiesu- ja hietamaiden muodostamassa ryhmässä. Maala- jiryhmien väliset erot eivät kuitenkaan olleet tilastollisesti merkitseviä. Koko aineistossa neutralointikapasiteetti ei korreloinut saveksen pitoisuuden kanssa. Saves-% ei myöskään ollut tilastollisesti merkittävä selittäjä regres- sioyhtälöissä, joilla pyrittiin kuvaamaan ANCJg :n ja maan ominaisuuksien välistä suhdetta. Voimakkaimmin ANCj g riippui maan alkuperäisestä pH:sta. Orgaanisen 282 hiilen pitoisuus oli merkittävä selittäjä kaikissa maalaji- ryhmissä, minkä katsottiin olevan osoitus orgaanisen ai- neksen kationinvaihtoreaktioiden merkityksestä hapon puskuroinnissa. Aitosavien ryhmää lukuunottamatta ok- salaattiuuttoinen AI oli kolmas merkitsevästi ANC, g:n vaihtelua selittävä tekijä. Tämä viittaa siihen, että Al- hydroksidien liukenemiseen perustuva puskurointimeka- nismi alkaa tuntuvasti vaikuttaa neutralointikapasiteetin arvoon, kun referenssi-pH:ksi on valittu 3.8. 283