Problems encountered when classifying the soils of Finland

Transcription

1 Problems encountered when classifying the soils of Finland YLI-HALLA 1 Markku and MOKMA 2 Delbert L. 1 MTT Agrifood Research Finland, FIN JOKIOINEN, Finland 2 Michigan State University, Department of Crop and Soil Sciences, East Lansing,48824 Michigan, USA Abstract The World Reference Base for Soil Resources (WRB) system was tested in classification of pedons from Finland and compared to classifications according to the FAO/Unesco system (FAO) and Soil Taxonomy. Soils of Finland are rather young (<10000 yr), and because of the weathering-resistant felsic parent material and cool climate, most of them are weakly developed. As a result soils are often at the border of units at the highest level of classification. In the Soil Geographical Database of Europe at scale 1:1,000,000, Finland is dominated by Podzols (56) and Histosols (29), with some Cambisols, Gleysols and Leptosols. Recently also Regosols, Arenosols and man-made Phaeozems (WRB) have been identified. Podzolization is the major pedogenic process in sandy soils, with well-developed Podzols occurring in fine sands. Soils developed on sandy and loamy glacial till are often weakly podzolized and marginally meet the requirements of Podzols of the WRB system. Since these soils primarily exhibit podzolic morphology, it should be expressed in soil classification. Artificial drainage of agricultural soils has created cambic horizons, which are probably permanent and have to be taken into account in soil classification. Sandy loamy agricultural soils with less than 8 of clay can be referred to Cambisols in the WRB system but not in the FAO system. It is a matter of discussion whether the B horizon in these soils is altered enough to be qualified for a cambic horizon. Some clayey agricultural soils exhibit vertic properties that should be recognized in their classification. Acid sulphate soils occur along the coasts of Finland producing hazardous acidity when drained. Sulphidic materials and sulphuric horizons often occur below a depth of 100 cm. These features (occur within 200 cm of soil surface in artificially drained soils) should be accepted as diagnostic in the WRB system. Keywords: WRB, FAO/UNESCO system, Soil Taxonomy, soil mapping, Finland Introduction The FAO/UNESCO (FAO) and the World Reference Base for Soil Resources (WRB) systems and Soil Taxonomy have been developed with little input from Finland and other Nordic countries. Recently, comments were made relative to the WRB system (Greve et al., 2000) and to the criteria of the frigid and cryic soil temperature regimes (Yli-Halla et al., 2001). The subgroup Sulfic Cryaquept was added to the Soil Taxonomy according to a request from Sweden (Öborn, 1989). Minor contributions from Finland are attributable to the fact that pedogenic characteristics of soils of the country have not been comprehensively described or reported. However, during the past few years, the descriptions of forested (Mount et al., 1995) and agricultural soils (Yli-Halla and Mokma, 1999; Greve et al., 2000) and those of two agricultural toposequences (Mokma et al., 2000; Yli-Halla and Mokma, 2001) were published and interpreted in terms of WRB, FAO and Soil Taxonomy. Therefore, some advantages and weak points of different classification systems can be tentatively assessed. Soils of Finland are presented quite broadly in the Soil Geographical Database of Europe at Scale 1:1,000,000. The distribution of soils has mostly been derived from the interpretation of topographic maps. The soils are quite young (<10,000 yr), occur on acidic parent material, and have experienced relatively little pedogenic development in the cool climate. The soils are often intergrades between units at the highest level of classification (Mokma et al., 2000). In many instances, slight variation in personal Problems encountered when classifying the soils of Finland. Yli Halla & Mokma. 183

2 judgement on soil profile or diagnostic horizons may substantially influence the classification and, consequently, the outcome of soil mapping. Therefore, making a consistent soil database for Finland at a scale of 1: (European Soil Bureau, 1998) requires that typical soils of the country be correlated internationally. In this paper we discuss the classification of some soils of Finland according to the WRB system (FAO, 1998; Driessen et al., 2001), the FAO system (FAO, 1990), and Soil Taxonomy (Soil Survey Staff, 1998). We have also revealed a few problems in the use of the WRB system, which may require some modification of the criteria for the Nordic environment. Materials and methods Selected soil pedons were investigated in The pedons represent primarily cultivated soils of southern and central Finland, with some examples of forest soils, one from the very northern part of the country. The pedons were morphologically described and chemically analyzed. Finally, we have compared the presentation of soils of Finland in the Soil Geographical Database of Europe at Scale 1:1,000,000 with the existing knowledge of Quaternary deposits and land use. Results and discussion Anthropogenic impact on soils Even although cultivated soils occupy only 8 of land area, nearly all soils that can be cultivated have been cleared for agricultural use in Finland during the last 400 years. Stony glacial till and deep peatlands are usually not used for agriculture. Few data are available for native soils to compare with similar soils that are now in agricultural use. The cultivation-induced changes in these soils include structure formation after drainage, change of cationic chemistry of the plough layer by liming and addition of fertilizers and manure, and mixing of the top horizons of the soil. It is not known whether these cultivation-induced changes are temporary or sufficiently permanent to be used in classification. Structure formation In the cool (mean annual soil temperature in southern and central Finland 4-6 C; Yli-Halla and Mokma, 1998) and humid (mean annual precipitation mm) climate, drainage is required. Most agricultural soils of Finland have subsurface drainage systems installed at the depth of cm. Drainage produces drying of the soil, followed by structure formation. Artificial drainage has aided in the formation of medium-sized prisms and angular blocky peds to depths below the drainage pipes. Thus, drained soils have cambic horizons, whereas, undrained soils may have no diagnostic subsurface horizon. Iron oxide coatings on ped faces and other abundant redoximorphic features, stabilizing the aggregates, have formed as a result of the lowered water table. The structure formed by drainage-induced processes may be at least partly irreversible. It is unlikely that these soils would, if abandoned, revert structurally to the same stage as they were before drainage. We believe that the cambic horizon developed after intensive drainage will be permanent, and therefore, these soils should be classified as Cambisols/ Inceptisols. Their uncultivated counterparts may not have a cambic horizon and they would be classified as Regosols/ Entisols. However, it is difficult to prove this assumption since almost all land, which can be cultivated, has been cleared for agriculture. Few clayey or silty soils exist under forest vegetation but are reclaimed for agriculture and have artificial drainage. Impact of soil amendments As a result of acidic parent materials, acidic litter from coniferous trees and leaching, the native A horizons have low values. Manuring and liming, applied repeatedly to most agricultural soils, changed the chemistry of these soils substantially. As a result, plough layers are dominated by exchangeable Ca, and their range is commonly Many soils (Jokioinen 09, Table 1) have sandy Ap horizons over clayey subsoils. The Ap horizons are commonly dark, cm thick and have base saturation exceeding 50 as a result of liming and manuring, thereby meeting the criteria of a mollic horizon. These horizons have more than 250 ppm P 2 O 5 extractable with citric acid, which was the earlier upper limit for mollic and umbric horizons in the FAO system and Soil Taxonomy. With fimic A horizons, these soils were Eutric Cambisols in the FAO system. 184 Problems encountered when classifying the soils of Finland. Yli Halla & Mokma.

3 However, they have less than 1500 ppm P 2 O 5 and after 1998 these soils are classified as Haplocryolls in Soil Taxonomy, instead of Eutrocryepts until If they have less than 100 ppm bicarbonate-extractable P 2 O 5, they also have a mollic horizon and are classified as Haplic Phaeozems in the WRB system. If the P content is higher, it may be a Eutric Cambisol with a hortic horizon (WRB). The man-made mollic epipedon may not be permanent in sandy soils that naturally have low base saturation throughout the profile. But it may be more persistent in sandy-over clayey soils, where some bases of Ap horizons may be replenished from subsoils that are enriched in bases. In the FAO system, some sandy cultivated soils were not possible to classify. Many of these soils had B horizons that were too coarse-textured to be cambic and they couldn t be classified as Regosols because they had fimic A horizons and some had albic E horizons, neither of which are allowed in Regosols. In the WRB system, all of these soils could be classified; most of them seem to be Eutric or Dystric Regosols. Thus, Regosols in WRB appear to include soils with E and fimic horizons, making it a very diverse soil unit and different from the Regosols of the FAO system. Mineral soils which contain less than 8 clay All three classification systems have textural requirements for the cambic horizon. In addition to a texture of sandy loam or finer, the FAO system requires that there must be at least 8 of clay in the horizon in question. Many soils have a texture of sandy loam or finer but do not have at least 8 clay (Mikkeli 01, Table 1 and Jokioinen 02, Table 3). They are Regosols according to the FAO system but may qualify as Cambisols in the WRB system and Inceptisols in Soil Taxonomy if the B horizons are sufficiently altered, i.e. have adequate formation of structure and indication of weathering. The WRB system has no clay requirement, therefore, Cambisols of WRB system are more extensive in Finland than those of the FAO system. Table 1. Selected physical and chemical properties of pedons from Finland. Horizon depth cm Color matrix (H 2 O) Org. C Clay Silt Sand CEC cmol kg -1 Base saturation Jokioinen 09, cultivated Eutric Cambisol (FAO), Haplic Phaeozem? (WRB), Aquic Haplocryoll? (Soil Taxonomy) Ap, YR 3/2, YR 5/2 dry Bw1, YR 5/ Bw2, YR 5/ Bw3, YR 4/ BC YR 4/ Cg YR 4/ Mikkeli 01, cultivated Dystric Regosol (FAO), Dystric Cambisol (WRB), Typic Dystrocryept (Soil Taxonomy) P 2 O 5 mg kg -1 Ap, YR 3/ YR 6/2 dry Bs, YR 4/ n.d. BC1, YR 6/ n.d. BC2, YR 6/ n.d. C, Y 6/ n.d. P was extracted with 1 citric acid. Clay stands for particles <0.002 mm, silt for mm and sand for mm. CEC was determined at Not determined = n.d. Many clayey soils of Finland have clay contents between 30 and 70 in the plough layer. They have micaceous/illitic mineralogy, with some vermiculite and smectite (Sippola, 1974). According to the FAO criteria, they have vertic properties because cracks and wedge-shaped structural aggregates are present, therefore, they are classified as Vertic Cambisols. In the Soil Geographical Database of Europe these clayey soils dominate the polygons in the southern coastal region of Finland. Almost all clay soils of Finland are cultivated and drained with subsurface pipes. Problems encountered when classifying the soils of Finland. Yli Halla & Mokma. 185

4 Initially in the WRB system (FAO, 1998), the vertic horizon required a soil have, among other characteristics, tilted wedge-shaped aggregates and intersecting slickensides. The clayey soils of Finland have vertical, not tilted, wedge-shaped aggregates, and lack slickensides, even though they have numerous pressure faces. Therefore, these soils fail to meet the criteria of vertic horizon of the initial WRB system. They were classified as Eutric Cambisols, together with loamy and silty soils. This name neglects essential characteristics of the clayey soils where water flow is dominated by cracks. A revision of the criteria for the vertic soil unit of the WRB system (Driessen et al., 2001) allows the clayey soils of Finland to be classified as Vertic Cambisols, which is a welcomed change. Acid sulphate soils Acid sulphate soils are common on the coasts of Finland (Yli-Halla et al., 1999). The sulphidic materials are covered with non-sulphidic deposits. These soils are mostly cultivated and, like other agricultural soils of the country, drained with subsurface drainage pipes placed at a depth of about 1 m. Drainage causes the sulphidic materials to be oxidized quite deep, producing acidic drainage waters and fish kills in the recipient waters (Hildén and Rapport, 1993). The sulphuric horizon and sulphidic materials are used as diagnostic criteria when they occur within 100 cm, 125 cm and 150 cm of soil surface in the WRB and FAO systems and Soil Taxonomy, respectively. In the artificially drained acid sulphate soils of Finland, the horizons producing hazardous acidity are commonly below 100 cm (Helsinki 07, Table 2). The thionic characteristics of these soils are often not reflected in their classification according to WRB, but are frequently in FAO and usually in Soil Taxonomy. The failure of the WRB system to recognize drained acid sulphate soils was also pointed out by Greve et al. (2000). They requested a revision of the criteria in the WRB system in order to express the thionic characteristics of these soils when these characteristics occur within 200 cm of soil surface. Podzolised soils Originally, Finland was covered by forests that were dominated by coniferous trees (pine and spruce). Acidic solutes from the litter from these trees causes podzolization of the coarse mineral soils developed from felsic magmatic and metamorphic rock. Podzolisation is the principal soil forming process in the forested soils of Finland (Aaltonen, 1952; Petäjä-Ronkainen et al., 1992). In the current Soil Geographical Database of Europe, Podzols dominate the country. Soils formed on sandy deposits are often well developed Podzols with albic horizons and cemented spodic horizons. However, podzolisation processes have been quite weak in many pedons. Table 2. Selected physical and chemical properties of a drained and cultivated acid sulphate soil in Helsinki, Finland (Pedon Helsinki 07). Horizon and depth, cm Color matrix Org. C Clay Silt Sand Fresh Inc. SO 4 -S mg kg -1 Total S g kg -1 Ap, YR 3/ n.d. n.d YR 6/2 dry Bg1, YR 5/ n.d. n.d. 1.2 Bg2, Y 5/ Bj, Y 5/ Cg, Y5/ C, GY 3/ Clay stands for particles <0.002 mm, silt for mm and sand for mm. Soil was determined in water (1:1) upon sampling (Fresh) and after six weeks of aerobic incubation (Inc.). SO 4 -S was determined in the incubated samples. The soil is a Thionic Gleysol (FAO), Dystric Gleysol (WRB) and a Sulfic Cryaquept (Soil Taxonomy). Not determined = n.d. Soils formed in glacial till, containing more weatherable minerals, often only partly meet the criteria of Podzols or Spodosols (pedon Jokioinen 01, Table 3; more examples are in Mount et al., 1998, Mokma et al., 2000). Some loamy glacial tills may not have developed podzolic morphology (pedon Jokioinen 02, Table 3). 186 Problems encountered when classifying the soils of Finland. Yli Halla & Mokma.

5 Sometimes Bhs horizons are not dark enough to meet the color criteria of the WRB system (7.5YR or redder: value of 5 or less and chroma of 4 or less; 10YR: value of 3 or less and chroma of 2 or less). Such soils have been presented by Petäjä-Ronkainen et al (1992). Greve et al. (2000) also presented such a podzolised soil from Sodankylä, Northern Finland (Table 3). That particular soil fails to meet the color criteria in terms of chroma. However, it meets the chemical criteria. It is a Spodosol according to Soil Taxonomy, a Podzol according to the FAO, but following the WRB criteria, this soil becomes a Dystric Cambisol. This name neglects the most important pedogenic process of the soil. Because of the low chroma requirement (4 or less) in the WRB system, many soils with a podzolic morphology fail the criteria of Podzols of the WRB system, even though they may be Podzols according to the FAO system and Spodosols of Soil Taxonomy. Quaternary deposits and soils of Finland Glacial till is the most widespread deposit in Finland and is primarily covered by forest vegetation (Table 4). Peatlands, formed in depressions in glacial till, are also widespread. Most of the thick peat deposits are not used intensively, however, some are used for peat mining. Thin peatlands are primarily covered by forest, but some are cultivated. Agriculture is practiced mostly on clayey, silty and loamy deposits and on fine sands. Table 3. Selected physical and chemical properties of pedons in Finland. Horizon and depth, cm Color matrix (H 2 O) Org. C Clay Silt Sand CEC cmol kg -1 Base saturation Jokioinen 01: forested Haplic Podzol (FAO, WRB), Entic Haplocyod (Soil Taxonomy) Oa, YR 3/ n.d. n.d. n.d. n.d. n.d. n.d. E, YR 4/ n.d. n.d Bs1, YR 4/ n.d. n.d Bs2, YR 4/ n.d. n.d BC, YR 4/ n.d. n.d C, YR 4/ n.d. n.d Jokioinen 02: forested Dystric Cambisol (WRB), Dystric Regosol (FAO), Typic Dystrocryept (Soil Taxonomy) Ah, YR 3/ Bw1, YR 4/ Bw2, YR 4/ Bw3, YR 4/ C, Y 5/ Sodankylä 01: forested Dystric Cambisol (WRB), Haplic Podzol (FAO), Oxyaquic Haplocryod (Soil Taxonomy) O, 0-5 n.d n.d. n.d. n.d n.d. Ah, YR 2.5/ n.d. n.d. n.d n.d. E, YR 4/ n.d. n.d Bs, YR 4/ BC, Y 4/ C, Y 4/ Clay stands for particles <0.002 mm, silt for mm and sand for mm. Index stands for 0.5 x Fe + Al, extracted with acid oxalate, 3.0, and expressed as percentage of soil mass. Not determined = n.d. Index Problems encountered when classifying the soils of Finland. Yli Halla & Mokma. 187

6 Table 4. The distribution of Quaternary deposits (adapted from Kujansuu and Niemelä, 1992) and their dominant land use Type of deposit land area () Land use Glacial till 58 Forest (and cultivated) Sandy glacio-fluvial deposits 5 Forest and cultivated Clayey and silty deposits 8 Cultivated Peatlands (>30 cm) 15 Forest, peat mining, no use (and cultivated) Exposed bedrock 14 Within forested areas According to the Soil Geographical Database of Europe, Podzols are by far the most common soils of Finland (Table 5). On the basis of podzolic morphology (E horizon and accumulation of Fe and Al in the B horizon), all soils formed on glacial till have traditionally been included in Podzols. As mentioned earlier, some weakly podzolised soils, particularly the loamy-textured ones (Yli-Halla and Mokma, 2001), may not meet the criteria of Podzols but instead should be classified as Dystric Regosols/Cambisols, and the most coarse-textured soils of esker areas are likely to be Cambic/Haplic Arenosols. Table 5. Soils recognized in Finland in the Soil Geographical Database of Europe at Scale 1:1,000,000, with respective percentages of land surface. Soil class Land area () Haplic Podzols 56 Vertic and Dystric 8 Cambisols Dystric and Vertic Gleysols 2 Dystric and Eutric Histosols 29 Leptosols 5 It has commonly been claimed that one third of the area of Finland is peatland. However, when applying the thickness requirement of 40 cm, the area of Histosols is smaller than reported in the Soil Geographic Database of Europe, where obviously shallow peatlands have also been included as Histosols instead of being mineral soils with histic surface horizons. This is obvious when the area of peatlands in Table 4 and the area of Histosols in Table 5 are compared. In addition, Leptosols occur only in the polygons along the coasts of Finland and in extreme northern Lapland where they are indeed abundant. According to topographic maps, exposed bedrock, and associated shallow soils, occur throughout the country. Therefore we assume that Leptosols should occur in most mapping units of Finland. Recently, soil profiles representing Arenosols, Regosols and man-made Phaeozems have been found in Finland (Yli-Halla and Mokma, 1999; Mokma et al., 2000;Yli-Halla and Mokma, 2001), and Umbrisols will likely be found, but, it is not known how widespread these soils are. Conclusions Comparison of the Soil Geographical Database of Europe at scale 1:1,000,000 with the information of land cover of Finland and the soil profiles recently reported suggests that the soil map of Finland needs revision. Artificial drainage has promoted structure formation and it has increased the depth of oxygen penetration into the subsoil. Cambic horizons have formed in these soils, but may not occur in the native state. Structure formation is thought to make the cambic horizon a permanent soil characteristic. Difference in textural requirements of a cambic horizon in the FAO and WRB systems makes the distribution of Cambisols of the two systems potentially different. It is not clear whether the sandy loams of Finland with less than 8 of clay meet the other requirements of a Cambisol of the WRB system. 188 Problems encountered when classifying the soils of Finland. Yli Halla & Mokma.

7 Long cultivation (up to 400 years of cereals and grasses), tillage, manuring and liming have created dark Ap horizons particularly in sandy soils or loamy sands. According to the present criteria, some of these soils are Mollisols of Soil Taxonomy and Phaeozems of the WRB system. It is subject to discussion whether the man-made mollic epipedon should be taken into account in soil classification. Soils developed in light-textured glacial deposits are often weakly podzolised and only marginally meet the requirements of Podzols of the WRB system. Since podzolization is the most important pedogenic process in these soils, there should be a way to express it in the name of the soil. Sulphidic materials and sulphuric horizons should be recognized in the WRB system when they occur within 200 cm of soil surface in artificially drained soils. References Aaltonen, V Soil formation and soil types. Fennia 72: Driessen, P., Deckers, J., Spaargaren, O. and Nachtergaele, F. (Ed.) Lecture notes on the major soils of the World. World Soil Resources Reports pp. FAO, Rome. European Soil Bureau Georeferenced Soil Database for Europe. Manual of Procedures Version 1.0. European Soil Bureau, Scientific Committee. 170 pp. EUR EN. FAO FAO/Unesco Soil Map of the Word. Revised Legend, with corrections. Word Soil Resources Report 60. FAO, Rome. FAO World reference base for soil resources. World Soil Resources Reports pp. FAO, Rome. Greve, M.H., Yli-Halla, M, Nyborg, Å.A. and Öborn, I Appraisal of World Reference Base for Soil Resources from a Nordic point of view. Danish Journal of Geography 100: Hildén, M. and Rapport, D Four centuries of cumulative impacts on a Finnish river and its estuary: an ecosystem health approach. Journal of Aquatic Ecosystem Health 2, Kujansuu, R. and Niemelä, O Surficial deposits. P In Alalammi (ed.) Atlas of Finland 124. National Board of Survey and Geographic Society of Finland. Mount, H.R., Newton, D.L., Räisänen, M.-L. and Lee, S.E Morphology of the soils in Central Finland. Soil Survey Horizons 36: Mokma, D.L., M. Yli-Halla, and H. Hartikainen Soils in a young landscape on the coast of southern Finland. Agricultural and Food Science in Finland 9: Öborn, I Properties and classification of some acid sulphate soils in Sweden. Geoderma 45: Petäjä-Ronkainen, A., Peuraniemi, V. and Aario, R On podzolization in glaciofluvial material in northern Finland. Annales Academiae Scientiarum Fennicae. Series A. III. Geologica-Geographica 156: 19. Sippola, J Mineral composition and its relation to texture and some chemical properties in Finnish subsoils. Annales Agriculturae. Fenniae 13: Soil Survey Staff Keys to Soil Taxonomy. U.S.D.A. -N.R.C.S. 8th Ed. 326 pp. Yli-Halla, M. and Mokma, D.L Soil temperature regimes in Finland. Agricultural and Food Science in Finland 7: Yli-Halla, M., and Mokma, D.L Classification of soils of Finland according to Soil Taxonomy. Soil Survey Horizons 40: Yli-Halla, M. and D.L. Mokma Soils in an agricultural landscape of Jokioinen, southwestern Finland, Agricultural and Food Science in Finland 10: Yli-Halla, M., Mokma, D.L. and M. Starr Criteria for frigid and cryic temperature regimes. Soil Survey Horizons 42: Yli-Halla, M., Puustinen, M. and Koskiaho, J Area of cultivated acid sulfate soils in Finland. Soil Use and Management 15: Problems encountered when classifying the soils of Finland. Yli Halla & Mokma. 189