Classification of acid sulphate soils of Finland according to Soil Taxonomy and the FAO/Unesco legend Markku Yli-Halla Agricultural Research Centre ofFinland, Institute ofResource Management, FIN-31600 Jokioinen, Finland, e-mail: markku.yli-halla@mtt.fi In order to place acid sulphate soils (a.s. soils) of Finland in an international context, five pedons from cultivated a.s. soils from Liminka, Ylistaro and Laitila on the western coast of Finland were analyzed and classified according to Soil Taxonomy and the revised legend of the FAO/Unesco Soil Map of the World. Three of the pedons (Liminka 1 and 2, Laitila 1) had sulfuric horizons within 50 cm of soil surface and qualify as Typic Sulfaquepts. One pedon (Ylistaro) had a sulfuric horizon at the depth of 100-150 cm and was classified as a Sulfic Cryaquept. The fifth pedon (Laitila 2) did not have either a sulfuric horizon or sulfidic materials, but it had a pFl<4.o and enough S0 4-S to be classified as a Sulfic Cryaquept. According to the FAO/Unesco legend, all pedons were classified as Thionic Gleysols. Thus Typic Sulfaquepts and Sulfic Cryaquepts (Soil Taxonomy) and Thionic Gley- sols (FAO/Unesco classification) occur commonly on the western coast of Finland. As a result of artificial drainage and leaching, they gradually fail to meet the requirements of a.s. soil classes and will be classified as Typic Cryaquepts. In terms of the FAO/Unesco nomenclature, Thionic Gleysols become Dystric Gleysols or Gleyic Cambisols over time. Key words: FAO/Unesco Soil Map of the World, pedogenesis, soil morphology, soil pH, soil survey, Soil Taxonomy, sulphur content ntroduction Acid sulphate soils (a.s. soils) are soils in which, “as a result of processes of soil formation, sul- phuric acid either will be produced, is being pro- duced or has been produced in amounts that have a lasting effect on main soil characteristics” (Pons 1973).A more quantitative criterion, need- ed for soil classification, has been presented, e.g. by van Mensvoort and Dent (1997): In a.s. soils, enough sulphuric acid is produced to bring soil pH below 4. In the revised legend to the FAO/ Unesco Soil Map of the World (FAO 1988), these soils are called Thionic Gleysols, Thionic Flu- visols or Thionic Histosols. In Soil Taxonomy (Soil Survey Staff 1996), a.s. soils are dispersed through several taxa. The most coherent and de- tailed classification of a.s. soils by Dent (1986) makes the important distinction between ‘sul- phidic soils’ (or potential a.s. soils), ‘raw a.s. soils’ that are actively generating sulphuric acid © Agricultural and Food Science in Finland Manuscript received May 1997 247 Voi 6 (1997): 247-258. AGRICULTURAL AND FOOD SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=Y_2JmVJRIV2acL05.Pr-c0zpaI1CgsO35dBolyw.f0BTBEpzgN64qS0WGylVM2EkUNeHLxOC-4tbc-07bWOuU3gDDFgby2TJwuMd2vSa_LW3iOkUubfy_RFHkjM-wxktRJPn-fm5utaiSUy_VrwuXHsAA5kNnIizj_VtJIlPNVTqvPDMqhQk7v3DEu5SbGKKVW9nnBur1e7oL0fmVLCqeByLKadwpxTyhiP7G3AOZeRoJDfH-_Is6LBHLFbGoZzXZ5qAesRpO-KXnQfptqAC2TXrH6lSTyBTGupd9C0WnoA5K5GO8g and which, therefore, pose an environmental hazard, and ‘ripe acid sulphate soils’ that are still severely acid (pH 4 or less) but are no longer exporting acidity. The 1994 revision ofSoil Tax- onomy (Fanning and Witty 1993) seeks to dis- tinguish the ‘raw acid sulphate soils’ as Sulf- aquepts using a severe pH criterion (pH<3.5) for the diagnostic sulfuric horizon. The a.s. soils of Finland, mainly located on the coast of the GulfofBothnia, have developed in sulfidic sedimentaccumulated during theLito- rina period of the Baltic sea (7000 - 3000 yr 8.P.). These soils were earlier studied mainly from the agricultural point of view (Kivinen 1938, Purokoski 1958, 1959, Erviö 1975,Erviö and Palko 1984). Since the 1980’s, the emphasis has been in environmental aspects, particularly acid loading from these soils to surface waters (Palko et al. 1985, 1987, Palko and Saari 1987, Hartikainen and Yli-Halla 1986, Palko 1988, 1994, Erviö 1991, Palko and Yli-Halla 1993). Acid sulphate soils ofFinland have not been clas- sified according to internationalsystems, but lo- cal criteria have been applied to identify a.s. soils in national soil surveys. In Finland, Erviö (1975) and Erviö and Palko (1984) called a soil an a.s. soil if the subsoil (40-60 cm or 50-70 cm) had a pH(H,G, 1:2.5) <5.0 and/or a S04-S content >lOO mg I 1 of soil, determined in air-dried samples. More recently (Palko and Saari 1987, Palko et al. 1987), the use of S0 4-S content as the criteri- on for classification was discontinued and the pH was determined in the field in a moist soil. The minimum pH (<5.0) of the profile was the principal criterion in the classification, and re- dox potential was a secondary variable. The purpose of this paper is to place the a.s. soils ofFinland in an internationalcontext. Five representative pedons from three a.s. soil areas on the western coast of Finland are classified according to Soil Taxonomy and the revised leg- end to the FAO/Unesco Soil Map of the World. Acid sulphate soils in Soil Tax- onomy and FAO/Unesco legend A sulfuric horizon and sulfidic materials (Table 1) are essential in the classification of a.s. soils. In Soil Taxonomy (Soil Survey Staff 1996), the determination of sulfidic materials is based on soil pH before and after aerobic incubation. Sulfidic materials are stable in anaerobic envi- ronments. A sulfuric horizon develops when sulfidic materials are oxidized to sulfuric acid, resulting in a sufficient decrease of pH. In Soil Taxonomy, evidence that the acidity is caused by sulfuric acid is required. Accepted evidence is at least one of the following: 1) presence of jarosite, 2) high content of water-soluble S0 4-S in the horizon concerned and 3) sulfidic materi- Table 1. Characteristics of sulfidic materials and the sulfuric horizon. FAO/Unesco Soil Map of the World Taxonomic Soil Taxonomy feature Sulfidic materials pH > 3.5. After incubation pH decreases Total S >0.75% mostly in the form of sulfide by at least 0.5 units and reaches a pH<4. and a pH > 3.5. After drainage pH decreases below 3.5. Sulfuric horizon pH < 3.5 and thickness at least 15 cm pH < 3.5 and thickness at least 15 cm and and one or more of the following: generally jarosite mottles -jarosite concentrations - directly underlying sulfidic materials - water-soluble SO-S >0.05%4 248 Yli-Halla, M. Acid sulphate soils ofFinland AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 2. General requirements of some acid sulphate soil units according to Soil Taxonomy (Soil Survey Staff 1996) and the legend of the FAO/Unesco Soil Map of the World (FAO 1988). Soil unit Description Soil Taxonomy: Sulfaquents Entisols with aquic soil moisture regimes and sulfidic materials within 50 cm of soil surface Sulfaquepts Inceptisols with aquic soil moisture regimes and sulfuric horizons within 50 cm of soil surface Sulfic Cryaquepts Inceptisols with aquic soil moisture regimes and cryic temperature regimes and with one or more of the following: - a sulfuric horizonbetween 50 and 150 cm from soil surface - sulfidic materials within 150 cm of soil surface - within 150 cm ofsoil surface a horizon which has all the characteristics of a sulfurichorizon except pH which is between 3.5 and 4. FAO/UNESCO Soil Map of the World: Thionic Gleysols Gleyic properties within 0.5 m of soil surface. Sulfuric horizons or sulfidic materials within 125 cm ofsoil surface als below the acidic horizon. A sulfuric horizon must have a pH < 3.5. This requirement attempts to include horizons where sulfide is presently oxidizing (raw acid sulphate soils in Dent’s clas- sification) and to exclude horizons where all sulfide has already disappeared (Fanning and Witty 1993). The legend of the FAO/Unesco Soil Map of the World (FAO 1988) uses merely the oxidized pH value which must be <3.5. The essence of the taxonomic definitions of the a.s. soils is to include soils which have enough sulfuric acid to exhibit a pH <3.5 (FAO 1988) or <4.0 (Soil Survey Staff 1996), or enough sulfide to produce, upon oxidation, a pH below these values. In Soil Taxonomy, a.s. soil units include great groups in Entisols, Incepti- sols and Histosols, e.g. Sulfaquents, Sulfaquepts and Sulfohemists, respectively, and Sulfic sub- groups in Entisols and Inceptisols, e.g. Sulfic Endoaquents and Sulfic Cryaquepts, respective- ly. In that system, soils in which sulfide is oxi- dizing and reduced soils which contain sulfidic materials but not an acidic horizon are classi- fied into different units. In the revised legend of the Soil Map of the World (FAO 1988), thionic units are recognized in Histosols, Fluvisols, Gleysols and soils with a sulfuric horizon and those with only sulfidic materials are classified in the same taxa. The requirements of some rel- evant a.s. soil units are summarized in Table 2. Material and methods Five pedons representative of the range of a.s. soils in Finland were described from Liminka (two pedons), Ylistaro and Laitila (two pedons) in Northern and Southern Ostrobothnia and Southwestern Finland, respectively (Fig. 1). In Liminka, the study area (64°50’N, 25°24’E) is flat and about I m above the level of the Gulf of Bothnia. It rose above the sea level 100-150 years ago. The Liminka 1 pedon (Fig. 2) has been cultivated for grass and small grains for about 15 years, but was fallowed at the time of inspection. Initially, the field had open drains, but it was pipe-drained 8 years prior to inspec- tion. The ground water was not encountered within 1 m of soil surface on August 13, 1992 (sampling). The Liminka 2 pedon comes from an experimental polder (described in detail by Palko 1988) which was constructed 8 years be- 249 Vol. 6 (1997): 247-258. AGRICULTURAL AND FOOD SCIENCE IN FINLAND 2 fore sampling. At the same time, the sampled plot was pipe-drained to the depth of 113 cm and limed (15 tons dolomitic limestone ha 1). Oats had been grown in the field for three years, after which cropping was abandoned and the area was now covered with a grassy vegetation. Ground water was at 95 cm below the soil surface on August 14, 1992. The Ylistaro pedon (62°55’N, 22°29’E) re- presents the major acid sulphate soil area ofFin- land (Purokoski 1958, Erviö 1975). The pedon (Fig. 3) is located on a flat coastal plain with an elevation of 26 m asl. The area had been culti- vated for about 100 years but has been fallow since 1991. Different grasses grow on the area now. Pipe drainage has been installed. No water table was encountered above 150 cm on Octo- ber 3, 1996. The Laitila pedons (60°53’N, 21°4I’E) are located in a cultivated Valkojärvi polder (1000 ha) 10 m asl. The a.s. area, described by Palko et al. (1985), is drained to the River Sirppujoki by pumping. During the Litorina period the area was a gulf of the sea and later it became a lake. Lake Valkojärvi was drained for agricultural use piecemeal between the 1930’s and 1966. Small grain and sugarbeet have been grown. The area of the Laitila 1 pedon was drainedabout 30 years ago. The site is the lowest-lying spot of the Valkojärvi polder. No groundwater was encoun- tered on September 24, 1996. The Laitila 2 pe- don is about 200 m away from the Laitila 1 pe- don. Ground water (pH 3.5) was 95 cm below the soil surface on September 15, 1992. In all pedons, moist Munsell colours were recorded. Where mollic/umbric epipedons were considered, also dry colours were determined. Each horizon was sampled, and samples air-dried and ground to pass a 2-mm sieve prior to analy- sis. In the Liminka 1 and 2 and Laitila 2 pedons, redox potential and soil pH were determined in the field in the soil pit at intervals of 10 cm by inserting electrodes directly into the soil. Asmall amount of deionized water was added to allow proper contact between the soil and the electrode (Puustinen et al. 1994). The samples from the Laitila 1 and Ylistaro pedons were transported to the laboratory in plastic bags, and the pH( 1:1) of the moist samples was determined the day after sampling. The pH of all soil samples was also measured after aerobic incubation and dry- ing. Particle size distribution of mineral materi- al was determined by a pipette method. The to- tal contents of C, N and S were determined us- ing a Leco dry combustion apparatus (CNS- -1000). Because the samples were acid, all C was assumed to be organic. After incubation for 2 months, SO-S was extracted with 0.01 M CaCl, ’ 4 2 and determined by Inductively Coupled Plasma Emission Spectrometry (ICP). Exchangeable cations (Ca, Mg, K, Na) were extracted with I M NH 4-acetate at pH 7. Acidity released from the soil was determined by titrating the soil ex- tract back to the original pH with 0.02 M NaOH. The sum ofK, Na, Ca, Mg and acidity was taken as the cation exchange capacity (CEC) and the base saturation was calculated as the percentage of Ca+Mg+K+Na of CEC. Free acid, present Fig. I. Locations of the pedons. 250 Yli-Halla, M. Acidsulphate soils ofFinland AGRICULTURAL AND FOOD SCIENCE IN FINLAND originally in the soil or formed upon drying and partial oxidation of sulfide, probably results in a slight overestimation of CEC and underesti- mation of base saturation in the most acid (pH<3.5) horizons and in the horizons contain- ing sulfidic materials. Possible presence of free sulphate salts also adds to the overestimation of CEC. Poorly crystalline Fe and A 1 (hydr)oxides were extracted with 0.2 M NH 4-oxalate (pH 3.0) in the dark (McKeague and Day 1966). Iron and A 1 were determined by ICP. Results All the soils were mineral soils. The organic C content was usually highest in the Ap horizons (Table 3), but the deeper layers were also richer in organic C than mineral soils in general, par- ticularly in the Laitila pedons. The Laitila 1 pe- don had the highest C content at the depth of 35-50 cm, presumably inherited from the initial sediment. The pedons represent the following particle size classes: Pedon Soil FAO/ Taxonomy Unesco legend Liminka 1 and 2 coarse silty medium textured Ylistaro fine silty medium textured Laitila I and 2 fine fine textured Based on the morphological descriptions (Ta- ble 4), it can be concluded that all pedons had ochric epipedons. Relatively high pH values in the Ap horizons of all pedons except Ylistaro (Table 5) are attributable to liming. All pedons had cambic B horizons (as indicated by the de- velopment of structure) and aquic moisture re- Fig. 2, The Liminka 1 pedon. Fig. 3. The Ylistaro pedon from 0.80 m below. 251 Vol. 6 (1997): 247-258. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 3. Selected physical and chemical properties of the pedons." Depth Clay Silt C S m N M CEC Base satu- Fe21 Al 21 cm % % % % % cmol(+)/kg ration, % g/kg g/kg Liminka 1 0-25 10 82 2.9" 0.19" 0.19" 17.5 89 10.1d 1.7 J 25-35 8 86 0.4" 0.17» 0.04a 7.1 53 4.8'b 0.5» 10 86 0.4a 0.27b 0.04" 8.8 46 3.8" 0.5" 45-65 10 85 0.6" 0.26" 0.06" 13.1 44 6.0 b 0.5" 65-85 12 86 0.7" 0.45d 0.06" 15.5 48 6.0 b I.l' 85-100 13 86 0.7" 0.36c 0.06" 13.7 80 8.0 C 0.6 b Liminka 2 0-25 11 83 6.0b 0.29" 0.28b 29.6 81 8.5b 1.6' 25-65 9 85 0.5» 0.24" 0.05" 11.6 41 5.7» 0.6" 65-72 10 80 0.6» 0.66b 0.06" 18.2 32 6.0» 1.2 b 72-85 11 86 0.6» 0.73" 0.06» 14.1 59 7.9" 1.1" Ylistaro 0-30 24 64 4.6 d 0.17» 0.40" 20.5 9 10.5" 3.4 b 30-52 28 64 1.1 » b 0.19» 0.19 ab 12.3 13 7.9"b 1.1» 52-70 27 67 1.0» 0.17 a 0.18» 9.5 13 6.1 a 0.9" 70-100 26 67 1.0» 0.21" o.lB' 11.1 17 5.7a 0.9 a 100-125 28 65 0.22" 0.22 h: 12.1 16 8.5 1* 1.0» 125-150 26 65 13' 0.31 b 0.24 c 13.2 29 7.8"b 1.0" Laitila 1 0-25 43 53 6.6' 0.22» 0.72' 27.3 75 7.3 c 3.0 ' 25-35 51 46 6.6' 0.53" 0.87 d 30.4 13 17.8 f 1.5" 35-50 48 46 7.6 d 0.77 c 1.08 e 34.2 15 16.5' 1.6° 50-90 49 49 3.0" 1.72' 0.44» 49.7 25 8.6 d 3.5 C 90-125 50 48 3.0» 1.54 b 0.44" 51.2 55 7.1" 2.4 d 125-150 49 49 3.7" 1.68c 0.54" 51.2 61 5.9" 1.4b 150-180 53 46 3.6b 1.45 d 0.58 b 34.5 84 2.6" 1.2" Laitila 2 0-25 48 50 4.3b 0.24» 0.39c 23.6 53 8.7b 1.6' 25-35 53 44 2.6» 0.38" 0.33» 19.4 18 10.7" 1.2» 35-55 48 49 2.7» 0.43b 0.35" 20.2 16 12.0 de 1.2» 55-85 48 50 2.5" 0.44" 0.32» 20.1 18 13.5* 1.3 b 85-100 52 46 2.7» 0.82" 0.35b 23.6 25 4.0» 2.7 d "Results of each pedon and soil characteristics were tested separately. Means marked with the same superscript do not differ at P = 0.05. Results without superscripts were not tested, owing to too few replicates. 21 NH4-oxalate extraction, pH 3.0 gimes. Gleyic properties, as indicated by iron (hydr)oxide cutans on ped faces, were exhibited within 50 cm of soil surface, meeting the require- ments of Gleysols of the FAO/Unesco legend. The high contents of oxalate extractable Fe (Ta- ble 3) are in accordance with the thick Fe hy- droxide coatings on ped faces occurring in most investigated pedons. According to Soil Taxonomy, the pedons be- long to the aquic suborder of Inceptisols, Aquepts. Throughout Finland, the mean annual soil temperature at the depth of50 cm is between 0 and B°C, and the mean summer temperature is below 15°C (Table 6). Thus the entire country has a cryic temperature regime (Soil Survey Staff 1996). 252 Yli-Halla, M. Acid sulphate soils ofFinland AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 4. Morphological characteristics of the pedons. Hor- Depth Matrix Mottles and cutans" Text- Struct- izon cm colour colour ure2) ure 3) Liminka I Ap 0-25 5 Y 4/2 c 7.5YR 3/4 si gr Bgj I 25-35 5Y5/2 c 7.5YR 3/44 ', f 2.5 Y 7/641 si Ifsbk Bgj2 35—45 5Y5/2 m 2.5 Y 7/641 , m 7.5YR 3/4 4>, si 2fsbk c 7.5YR 4/6 5 ' Bg 45-65 SYRS/2 c 7.5YR 3/4s >, f 2.5 Y 7/6 si Imsbk CB 65-85 IOYR4/1 f 10YR 3/6 51 si 2m sbk Cg 85-100 2.5 Y 2.5/1 fIOYR3/6! > si Liminka 2 Ap 0-25 IOYR2/2 6 ’, c 7.5YR 3/4 si gr 5Y 5/2 71 Bgj 1 25-65 5Y5/2 c 7.5YR 3/44 ', c 2.5 Y 7/64 ', si Imsbk c 10YR 3/6 5 ' Bgj2 65-72 IOYR4/I c 2.5 Y 7/6”, c 7.5YR 3/4” si Imsbk CBg 72-85 2.5 Y 2.5/1 si 2c sbk Ylistaro Ap 0-30 7.5YR 2/2, IOYR 6/2 8> sil Imsbk Bw 30-52 5Y5/1 c 7.5YR 4/4 4 '- 5 > sil 2m sbk Bgj 1 52-70 5Y5/1 m 7.5YR 3/4 4> s', c 2.5 Y 8/44 ' 5 > sil 2m sbk Bgj2 70-100 5Y5/1 m SYR 3/34l -5) , 2.5 Y 8/44| -5) sil 2c pr Bgj3 100-150 5 Y 5/1 m 7.5YR 3/4 4> s>, m 2.5 Y 8/44l 5 > sil 2vc pr Laitila! Ap 0-25 2.5 Y 3/2,2.5Y 6/281 sic Imsbk AB 25-35 2.5Y4/2 m 10YR 3/4 sic 2m pi Bgl 35-50 2.5Y4/2 m 7.5YR 4/6 51, m SYR 3/4 51 sic 3m pr Bg2 50-90 2.5 Y 4/2 m 7.5YR 4/6 s ', m SYR 2.5/2 51 sic 3c pr BCg 90-125 2.5Y4/I C7.5YR4/65 ' sic 2c pr C 125-180 2.5Y4/1 sic massive Laitila 2 Ap 0-25 2.5 Y 3/2,2.5Y 6/28 ' sic Imsbk AB 25-35 2.5Y4/2 m 10YR 3/4 4,5) , vf 2.5 Y 7/6 sic 2m sbk Bgl 35-55 2.5 Y 4/2 m 7.5YR 4/6 s ', m SYR 3/4s >, sic 3m pr vf 2.5 Y 7/6 Bg2 55-85 2.5Y4/2 m 7.5YR 4/6”, m SYR 2.5/2 51 sic 3c pr BCg 85-100 2.5Y4/1 c 7.5YR 4/65) , f 2.5 Y 2.5/19' sic 3c pr 11 m=many, >20%; c=common, 2-20%; f=few, <2% 2) si=silt, sil=silt loam, sic=silty clay 3) I =weak, 2=moderate, 3= strong, f=fine, m=medium, c=coarse, vc=very coarse gr=granular, sbk=subangular blocky, pl=platy, pr=prismatic 41 around previous root channels, yellow zone between dark brown zone 51 on ped faces 61 organic matter and 7)mineral material not completely mixed 81 dry color 91 interiors in peds Both Liminka pedons had horizons that had a pH<3.5 (Fig. 4), a S0 4-S content above 0.05% (Table 5) and yellow jarosite mottles at the 30- 65 cm depth (Table 4), which are the criteria of sulfuric horizons. Because they occurred within 50 cm of soil surface, these pedons classify as coarse silty, cryic Typic Sulfaquepts (Soil Sur- vey Staff 1996) and medium-textured Thionic Gleysols (FAO 1988). Both Liminka pedons also have sulfidic materialsbelow the acidic horizons 253 Vol. 6 (1997): 247-258. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 5. Soil pH and S0 4-S content of the pedons. " Depth pH pH S0 4-S 21 cm moist dried rng/kg Liminka 1 0-25 6.5' 6.1" 623' 25-35 4.7** 4.0" 271» 35-45 3.9"b 3.6 d 323" 45-65 3.4" 3.4= 655 d 65-85 5.3ta 3.5' 1325f 85-100 6.5' llO6c Liminka 2 0-25 5.6" 5.7" 854" 25-65 3.4" 3.4" 661" 65-72 4.6"" 2.9 C 2442" 72-85 5.4" 3.4 b 1496' 90 6.0" n.d. n.d. Ylistaro 0-30 4.3' 4.2 d 29" 30-52 3.9" 3.7' 42" 52-70 3.8' 3.5' 60 70-100 3.5b 3.3* 112" 100-125 3.4" 3.3" 156' 125-150 3.4" 3.4"* 480f Laitila 1 0-25 5.9' 5.7' 222" 25-35 3.5"b 3.3' 413" 35-50 3.3" 3.0" 1410= 50-90 3.8" 2.8" 5777 f 90-125 6.0 3.1"* 4329' 125-150 7.6 d 3.0*" 4250» 150-180 7.5 d 3.7d 3394d Laitila 2 0-25 5.3' 5.1" 61" 25-35 3.7" 3.8" 156" 35-55 3.4"b 3.5C 227' 55-85 3.2" 3.4' 462d 85-100 3.6" 3.3' 2069' 11 Results of each soil and soil characteristics were tested separately. Means marked with the same superscript do not differ at P = 0.05. 21 Incubated samples, 0.01 M CaCl2 extraction n.d. = not determined (below 65 cm), as indicated by the drop in pH upon drying (Table 5). These black layers had negative redox potentials (Fig. 2) and the colour ofiron monosulfide although these layers did not meet the total S requirement (0.75%) of sulfidic materials in the FAO/Unesco legend. The Ylistaro pedon was oxidized throughout the sampling depth as indicated by the low pH Fig. 4. Soil pH and redox potential of the Liminka I and 2 and Laitila 2 pedons, measured in the soil pit. 254 Yli-Halla, M. Acid sulphate soils ofFinland AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 6. Soil temperatures in Finland, measured at the depth of 50 cm. Location Period Mean Mean annual summer °C °C Jokioinen 60°49'N, 23o3O'E 1957-1970 5.9" 12.7" Ylistaro 62°57 , N, 220 3TE 1968-1980 5.5 21 13.1 2 ' Sodankylä 67°22'N,260 39'E 1963-1970 3.0" 11.8" Utsjoki 69°45'N, 27°02'E 1964-1970 1.9" 6.2" " Finnish Meteorological Institute (1979) 2) Agricultural Research Centre of Finland, unpublished data which did not change upon aerobic incubation (Table 5). Long-term drainage has lead to leach- ing of S0 4 -S to below the requirement for the sulfuric horizon in Soil Taxonomy. However, below 52 cm, jarosite was present (Table 4), an alternative criterion of the sulfuric horizon, and below 100cm the pH was <3.5 (Table 5). Thus a sulfuric horizon was present at 100-150cm. The Ylistaro pedon is classified as a fine silty Sulfic Cryaquept (Soil Survey Staff 1996). In the FAO/ Unesco legend, the pH<3.5 is sufficient to iden- tify a sulfuric horizon, and this pedon classifies as a medium-textured Thionic Gleysol (FAO 1988). The Laitila 1 pedon had a pH<3.5 and a high content of S0 4-S in the 35-50 cm layer, which qualifies as a sulfuric horizon (Table 5). Very lit- tle or no jarosite was observed. At 50-150 cm there were sulfidic materials according to the requirements ofboth classification systems (Ta- ble 2). The pedon classifies as a fine, cryic Typ- ic Sulfaquept (Soil Survey Staff 1996)and a fine textured Thionic Gleysol (FAO 1988). In the Laitila 2 pedon there was a layer at 40-85 cm which had a pH<3.5 in moist soil (Fig. 4). None of the other requirements of a sulfuric horizon of Soil Taxonomy were fulfilled: nei- ther jarosite nor sulfidic materials occurred with- in the sampling depth, and the content of S04-S was too low. However, at 85-100 cm there was a S04-S content >0.05% and a pH<4.O (Table 5), allowing the soil to be classified as a fine Sulfic Cryaquept (Soil Survey Staff 1996). Moreover, the redox potential reached a value of 0 mV at approximately 100 cm (Fig. 2). Sulfidic materi- als can be expected below this depth, support- ing the above classification. According to the FAO/Unesco legend, a sulfuric horizon occurs at 40-85 cm in this soil. Therefore, Laitila 2 is a fine-textured Thionic Gleysol. Discussion Major processes after drainage of soils formed in sulfidic sediments include 1) oxidation of sulfidic materials to sulfuric acid with a conse- quent decrease of pH and 2) leaching of water- soluble components (sulphate accompanied by acid and base cations), resulting in the decrease of base saturation. The pedons of this study can be arranged according to decreasing base satu- ration and decreasing contents of sulfidic mate- rials and sulphate as follows: Laitila 1 > Liminka 2 > Liminka 1 > Laitila 2 > Ylistaro The Liminka 1 and 2 and Laitila 1 pedons (Typic Sulfaquepts/Thionic Gleysols) contained sulfidic materialsand sulfuric horizons with high contents of easily soluble S04-S. The Laitila 1 255 Voi 6 (1997): 247-258. AGRICULTURAL AND FOOD SCIENCE IN FINLAND pedon is placed first because it is the only pedon which contained sulfidic materials also accord- ing to the criteria of theFAO/Unesco legend. The sulfuric horizon was closer to the soil surface in the Liminka 2 pedon than in the Liminka I pe- don and therefore it has more pronounced a.s. characteristics than the Liminka 1 pedon. The Ylistaro andLaitila 2 pedons (Sulfic Cryaquepts/ Thionic Gleysols) were oxidized and leached to greater depths, resulting in lower base satura- tion. The Laitila 2 pedon had a sulfuric horizon only according to the criteria of the FAO/Unesco legend. It was much more oxidized and leached than the Laitila 1 pedon nearby, because the Laitila 2 pedon was slightly higher in the land- scape while the Laitila I pedon was taken from the more poorly drained deepest point of the polder. However, owing to the lower base satu- ration and S0 4 -S content, the Ylistaro pedon can be considered the most leached one and is placed last in the sequence. According to this investigation, cultivated a.s. soils along the western coast of Finland com- monly qualify as Typic Sulfaquepts (Soil Sur- vey Staff 1996).The soils of Mälsor (Koivulah- ti), Bäckby (Ähtävä) and Vaasa in Southern and Central Ostrobothnia (Hartikainen and Yli-Hal- la 1986, Palko et ai. 1987, and Erviö 1991, re- spectively) probably also meet therequirements of Typic Sulfaquepts. The present study and pre- vious investigations suggest that the soils of Storsjö (Lapväärtti) in Southern Ostrobothnia (Hartikainen and Yli-Halla 1986, Palko and Saari 1987) and many soils in the Sirppujoki catch- ment in Southwestern Finland (Palko et al. 1985) can be classified as Sulfic Cryaquepts. The oc- currence of Sulfic Cryaquepts in Finland, in ad- dition to the Swedish soils (Öborn 1989), justi- fies the recognition of this taxon which was add- ed only in 1992. Pedogenesis results in gradual changes in the soils formed in sulfidic sediments. Besides arti- ficial drainage, pedogenic transformations in a.s. soils are promoted also by the postglacial land uplift which continues at the annual rate of 4-8 mm on the western coast ofFinland. The present pedons constitute a sequence exemplifying dif- ferent stages ofpedogenesis of a.s. soils. Initial- ly, all of them were probably Sulfaquents, de- veloping into Sulfaquepts when drained for ag- riculture. Over time the sulfuric horizon moves downwards and a Sulfaquept is transformed into a Sulfic Cryaquept like in the Ylistaro pedon which exhibits a sulfuric horizon below 100cm. That soil probably once had a sulfuric horizon below 50 cm because there was still abundant jarosite at that depth. The Ylistaro pedon resem- bles the two Sulfic Cryaquepts (Ängesby and Ersnäs) from Northern Sweden (Öborn 1989) which contained jarosite but were low in S0 4-S and had the most acidic horizons at the depth of 70-1 10 cm. Principally, a similar sequence as described above was presented by Fanning and Fanning (1989) in an East-Texas environment where a Sulfaquent developed into a Sulfaquept and fur- ther into a Sulfic Endoaquept and finally into a Haplustalf and a Vertic Albaqualf. In the cool and humid climate of Finland, cultivated a.s. soils are eventually transformed from Sulfic Cryaquepts to Typic Cryaquepts (Soil Survey Staff 1996). In terms of the FAO/Unesco sys- tem, Thionic Gleysols become Dystric Gleysols or Gleyic Cambisols. The Härkmeri soil (Lapväärtti) in Southern Ostrobothnia (Yli-Hal- la and Hartikainen 1984) represents this final stage. It had a pH(CaCl 2 ) of 3.5 in aerobic sub- soil, but none of the other requirements of the a.s. soils were met. Typically, low pH is a more persistent soil characteristic than a high S04-S content. In a recent survey (Puustinen et al. 1994), approximately 300,000 ha of cultivated soils in Finland with a minimum pH<5 in subsoil were recognized as a.s. soils. This estimate is certain- ly much greater than the area of soils meeting the requirements of a.s. soils as stated in Soil Taxonomy or the FAO/Unesco Soil Map of the World. The national criteria can also be ques- tioned from the environmental point of view. Based on the above results, it can be concluded that soils with subsoil pH values of4-5 (a.s. soils according to the national criteria) do not con- tain actively oxidising sulfide or appreciable 256 Yli-Halla, M. Acid sulphate soils ofFinland AGRICULTURAL AND FOOD SCIENCE IN FINLAND quantities of acidic solutes. These soils may have passed the raw, acid-generating phase or their sulfide content was too low in the first place to produce sufficient sulfuric acid to lower the pH below 4.0. Therefore, soils with subsoil pH val- ues between 4 and 5 do not present a hazard of acid loading to the recipient water courses. If these soils are included in a.s. soils, the envi- ronmental problems caused by a.s. soils in Fin- land are apparently overemphasized and may draw attention of authorities away from the re- mediation of the acid loading in areas with the highest loading potential. Acknowledgements.The author thanks Dr. Jukka Palko from the North Ostrobothnia Regional Environment Centre in Oulu for helping locate and sample the representative pe- dons in Liminka and Laitila, Mr. Antti Raittila and Mrs. Mirja Palttila,M.Sc., for helping sample the Laitila pedons and Dr. Delvin S. Fanning from the University of Mary- land, USA, Dr. Delbert L. Mokma from Michigan State University, USA, and Dr. David Dent from the University of East Anglia, England, for reviewing this manuscript. References Dent, D. 1986. Acid sulphate soils: a baseline for research and development. ILRI Publication 39. Wageningen, The Netherlands, 204 p. Erviö, R. 1975. Kyrönjoen vesistöalueen rikkipitoiset viljelysmaat. Summary: Cultivated sulphate soils in the drainage basin of river Kyrönjoki. Journal of the Scientific Agricultural SocietyofFinland 47: 550-561. - 1991. Chemical properties of air-dried samples from an unlimed and limed acid sulphate soil profile and leaching of elements from the profiles. Annales Agri- culture Fenniae 30: 221-229. - & Palko, J. 1984. Macronulrient and micronutrient sta- tus of cultivated acid sulphate soils at Liminka, Fin- land. Annates AgricultureFenniae 23: 121-134. Fanning, D.S, & Fanning, M.C.B. 1989. Soil: morpholo- gy, genesis and classification. John Wiley and Sons. 395 p. New York. - & Witty, J.E. 1993. Revisions of Soil Taxonomy for acid sulphate soils. In; Dent, D.L. & Mensvoort, M.E.F. van (eds.). Selected papers of the Ho Chi Minh City symposium on acid sulphate soils. ILRI Publication 53: 61-69, FAO 1988. FAO/Unesco Soil Map of the World. Revised legend, with corrections. World Resources Report 60, FAO, Rome. Reprinted as Technical Paper 20, ISRIC, Wageningen, The Netherlands, 1994. 140 p. Finnish Meteorological Institute. 1979. Results of soil tem- perature measurements in Finland 1961-1970. Soil temperature measurements 3. 59 p. Hartikainen, H. & Yli-Halla, M. 1986. Oxidation-induced leaching of sulphate and cations from acid sulphate soils. Water, Air and Soil Pollution 27: 1-13. Kivinen, E. 1938. Ober die Eigenschaften der Gyttja- böden. Bodenkunde und Pflanzenernährung 9-10: 122-134. McKeague, J.A. & Day, J.H. 1966. Dithionite- and oxalate- extractable Fe and Al as aids in differentiating vari- ous classes of soils. Canadian Journal of Soil Sci- ence 46: 13-22. Mensvoort, M.E.F. van & Dent, D. 1997. Acid sulphate soils. Advances in Soil Science 22: 301-335. Öborn, I. 1989. Properties and classification of some acid sulphate soils in Sweden. Geoderma 45: 197- 219. - 1994. Morphology, chemistry, mineralogy and fertili- ty of some acid sulfate soils in Sweden. Swedish Uni- versity of Agricultural Sciences. Department of Soil Science. Reports and dissertations 18. 65 p. Ph D. Thesis. Palko, J. 1988. Happamien sulfaattimaiden kuivatus ja kalkitus Limingan koekentällä 1984-87. Summary: Drainage and liming of acid sulphate soils in the pol- der in Liminka, Finland 1984-87. National Board of Waters and Environment. Publications 19. 86 p. - 1994. Acid sulphate soils and their agricultural and environmental problems in Finland. Acta Universita- tis Ouluensis C. Technica 75. 58 p. Ph.D Thesis. - , Räsänen, M. & Alasaarela, E. 1985. Happamien sul- faattimaiden esiintyminen ja vaikutus veden laatuun Sirppujoen vesistöalueella. National Board of Wa- ters. Report 260, 95 p. Räsänen, M. & Alasaarela, E. 1987. Luodon-Öjan- järven valuma-alueen maaperän ja vesistön happa- muuskartoitus. Summary: The survey of the soil and water acidity in the catchment area of Luodon-Öjan- järvi. National Board of Waters and Environment. Publications 11: 61-100. - & Saari, M, 1987. Lapväärtin-lsojoen vesistöalueel- la sijaitsevan Storsjön järvikuivion happamat sulfaat- timaat. Summary: Acid sulphate soils in the drained lake area or Storsjö in the drainage basin of river Lapväärtti-lsojoki. National Board of Waters and En- vironment. Publications 11: 3-22. - & Yli-Halla, M. 1993. Assessment and managament of acidity release upon drainage of acid sulphate soils in Finland. In: Dent, D.L. & Mensvoort, M.E.F. van (eds.). Selected papers of the Ho Chi Minh City sym- posium on acid sulphate soils. ILRI Publication 53: 411-418. 257 Vol. 6 (1997): 247-258. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Pons, L.J. 1973. Outline of the genesis, characteristics, classification and improvement of acid sulfate soils. In: Dost, H. (ed.). Proceedings of the international symposium on acid sulphate soils. ILRI Publications 18, I: 3-27. Purokoski, P. 1958. Die schwefelhaltigenTonsedimente in dem flachlandgebiet von Liminka im Lichte che- mischer Forschung. AgrogeologischePublicationen 70. 85 p. Helsinki. - 1959. Rannikkoseudun rikkipitoisista maista. Refe- rat: Über die schwefelhaltigen Boden an der Kuste Finnlands. Agrogeotogische Publicationen 74. 27 p. Helsinki. Puustinen, M., Merilä, E., Palko, J. & Seuna, P. 1994. Kuivatustila, viljelykäytäntö ja vesistökuormitukseen vaikuttavat ominaisuudet Suomen pelloilla. Summa- ry: Drainage level, cultivation practices and factors affecting load on waterways in Finnish farmland. National Board of Waters and Environment. Report 198. 323 p. Soil Survey Staff 1996. Keys to Soil Taxonomy. 7th ed. USDA. Natural Resources Conservation Service. U.S. Government Printing Office. 644 p. Yli-Halla, M. & Hartikainen, H. 1984. Rikin, raudan, alu- miinin ja mangaanin huuhtoutuminen kolmen hap- paman sulfaattimaaprofiilin kerrosnäytteistä. Vesihal- lituksen monistesarja 35. Helsinki. 40 p. SELOSTUS Suomen happamien sulfaattimaiden kansainvälinen luokittelu Markku Yli-Halla Maatalouden tutkimuskeskus Suomessa luokitellaan happamiksi sulfaattimaiksi sellaiset maat, joissa kuivaamattomasta näytteestä mitattu maaprofiilin minimi-pH on alle 5 ja maan ha- petus-pelkistyspotentiaali ensin nousee ja sitten las- kee mentäessä maaprofiilissa alaspäin. Kansainväli- siä maaperän luokitusjärjestelmiä ovat FAOn/Unes- con järjestelmä ja Yhdysvalloissa kehitetty Soil Tax- onomy -järjestelmä. Jotta maa olisi sulfaattimaa, sen pH:n on oltava Soil Taxonomy-järjestelmässä alle 4,0 ja FAOn/Unescon järjestelmässä alle 3,5. Vaih- toehtoisesti pelkistyneessä maassa on oltava niin pal- jon sulfidia, että hapettuvan maan pH laskee alle e.m. raja-arvojen. Luokittelurajojen tarkoituksena on, että (happamiksi) sulfaattimaiksi nimettäisiin vain sellai- siä maita, joissa on tai joissa voi kehittyä runsaasti happamuutta. Tällaiset maat voivat tuottaa ympäris- töönsä merkittävää happamoittavaa kuormitusta. Tässä tutkimuksessa Limingasta, Ylistarosta ja Laitilasta otettuja happamia sulfaattimaita luokitel- tiin kansainvälisten kriteerien mukaan. Maaprofii- lien tutkimus osoitti, että maamme länsirannikolla esiintyy yleisesti Soil Taxononomy -järjestelmän Ty- pic Sulfaquepts- ja Sulfic Cryaquepts- luokkiin ja FAOn/Unescon luokituksen Thionic Gleysols- luok- kiin kuuluvia viljelymaita. Sellaiset Suomessa hap- pamiksi sulfaattimaiksi nimitetyt maat, joiden pH on 4-5, eivät kansainvälisten kriteerien mukaan kuiten- kaan ole happamia sulfaattimaita. 258 Yli-Halla, M. Acid sulphate soils ofFinland AGRICULTURAL AND FOOD SCIENCE IN FINLAND