Classification of acid sulphate soils of Finland according to Soil taxonomy and the FAO/Unesco legend

Suomessa luokitellaan happamiksi sulfaattimaiksi sellaiset maat, joissa kuivaamattomasta naytteesta mitattu maaprofiilin minimi-pH on alle 5 ja maan hapetus- pelkistyspotentiaali ensin nousee ja sitten laskee mentaessa maaprofiilissa alaspain. Kansainvalisia maaperan luokitusjarjestelmia ovat FAOn/Unescon jarjestelma ja Yhdysvalloissa kehitetty Soil Taxonomy -jarjestelma. Jotta maa olisi sulfaattimaa, sen pH:n on oltava Soil Taxonomy-jarjestelmassa alle 4,0 ja FAOn/Unescon jarjestelmassa alle 3,5. Vaihtoehtoisesti pelkistyneessa maassa on oltava niin paljon sulfidia, etta hapettuvan maan pH laskee alle e.m. raja-arvojen. Luokittelurajojen tarkoituksena on, etta (happamiksi) sulfaattimaiksi nimettaisiin vain sellaisia maita, joissa on tai joissa voi kehittya runsaasti happamuutta. Tallaiset maat voivat tuottaa ymparistoonsa merkittavaa happamoittavaa kuormitusta.Tassa tutkimuksessa Limingasta, Ylistarosta ja Laitilasta otettuja happamia sulfaattimaita luokiteltiin kansainvalisten kriteerien mukaan. Maaprofiilien tutkimus osoitti, etta maamme lansirannikolla esiintyy yleisesti Soil Taxononomy -jarjestelman Typic Sulfaquepts- ja Sulfic Cryaquepts- luokkiin ja FAOn/Unescon luokituksen Thionic Gleysols- luokkiin kuuluvia viljelymaita. Sellaiset Suomessa happamiksi sulfaattimaiksi nimitetyt maat, joiden pH on 4-5, eivat kansainvalisten kriteerien mukaan kuitenkaan ole happamia sulfaattimaita.

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 Taxonomy (Fanning and Witty 1993) seeks to distinguish the 'raw acid sulphate soils' as Sulfaquepts 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 Gulf of Bothnia, have developed in sulfidic sedimentaccumulated during theLitorina 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 of Finland have not been classified according to international systems, but local 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 S0 4 -S content >lOO mg I 1 of soil, determined in air-dried samples.
More recently Saari 1987, Palko et al. 1987), the use of S0 4 -S content as the criterion 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 redox potential was a secondary variable.
The purpose of this paper is to place the a.s. soils ofFinland in an international context. Five representative pedons from three a.s. soil areas on the western coast of Finland are classified according to Soil Taxonomy and the revised legend to the FAO/Unesco Soil Map of the World. Acid sulphate soils in Soil Taxonomy 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 environments. 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-  (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, Inceptisols and Histosols, e.g. Sulfaquents, Sulfaquepts and Sulfohemists, respectively, and Sulfic subgroups in Entisols and Inceptisols, e.g. Sulfic Endoaquents and Sulfic Cryaquepts, respectively. In that system, soils in which sulfide is oxidizing and reduced soils which contain sulfidic materials but not an acidic horizon are classified 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 relevant 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 inspection. 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-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) represents the major acid sulphate soil area of Finland (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 cultivated 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 October 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 encountered on September 24, 1996. The Laitila 2 pedon is about 200 m away from the Laitila 1 pedon. 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 analysis. 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. A small 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 drying. Particle size distribution of mineral material was determined by a pipette method. The total contents of C, N and S were determined using 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.

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 pedon 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:   According to Soil Taxonomy, the pedons belong to the aquic suborder of Inceptisols, Aquepts. Throughout Finland, the mean annual soil temperature at the depth of 50 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). 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 Survey Staff 1996) and medium-textured Thionic Gleysols (FAO 1988). Both Liminka pedons also have sulfidic materials below the acidic horizons (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.  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 leaching 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 100 cm 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 identify 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 little or no jarosite was observed. At 50-150 cm there were sulfidic materials according to the requirements ofboth classification systems (Table 2). The pedon classifies as a fine, cryic Typic 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: neither jarosite nor sulfidic materials occurred within the sampling depth, and the content of S0 4 -S was too low. However, at 85-100 cm there was a S0 4 -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 materials can be expected below this depth, supporting 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 consequent decrease of pH and 2) leaching of watersoluble 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 saturation and decreasing contents of sulfidic materials 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 materials and sulfuric horizons with high contents of easily soluble S0 4 -S. The Laitila 1 pedon is placed first because it is the only pedon which contained sulfidic materials also according to the criteria of the FAO/Unesco legend. The sulfuric horizon was closer to the soil surface in the Liminka 2 pedon than in the Liminka I pedon and therefore it has more pronounced a.s. characteristics than the Liminka 1 pedon. The Ylistaro and Laitila 2 pedons (Sulfic Cryaquepts/ Thionic Gleysols) were oxidized and leached to greater depths, resulting in lower base saturation. 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 landscape while the Laitila I pedon was taken from the more poorly drained deepest point of the polder. However, owing to the lower base saturation 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 commonly qualify as Typic Sulfaquepts (Soil Survey Staff 1996). The soils of Mälsor (Koivulahti), Bäckby (Ähtävä) and Vaasa in Southern and Central Ostrobothnia (Hartikainen and Yli-Halla 1986, Palko et ai. 1987, and Erviö 1991 probably also meet the requirements of Typic Sulfaquepts. The present study and previous investigations suggest that the soils of Storsjö (Lapväärtti) in Southern Ostrobothnia Yli-Halla 1986, Palko andSaari 1987) and many soils in the Sirppujoki catchment in Southwestern Finland (Palko et al. 1985) can be classified as Sulfic Cryaquepts. The occurrence of Sulfic Cryaquepts in Finland, in addition to the Swedish soils (Öborn 1989), justifies the recognition of this taxon which was added only in 1992.
Pedogenesis results in gradual changes in the soils formed in sulfidic sediments. Besides artificial 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 of Finland. The present pedons constitute a sequence exemplifying dif-ferent stages ofpedogenesis of a.s. soils. Initially, all of them were probably Sulfaquents, developing into Sulfaquepts when drained for agriculture. 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 100 cm. That soil probably once had a sulfuric horizon below 50 cm because there was still abundant jarosite at that depth. The Ylistaro pedon resembles 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 further 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 system, Thionic Gleysols become Dystric Gleysols or Gleyic Cambisols. The Härkmeri soil (Lapväärtti) in Southern Ostrobothnia (Yli-Halla and Hartikainen 1984) represents this final stage. It had a pH(CaCl 2 ) of 3.5 in aerobic subsoil, 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 S0 4 -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 certainly 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 questioned 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 contain actively oxidising sulfide or appreciable 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 values 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 environmental problems caused by a.s. soils in Finland 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.