EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7
Problems encountered when
classifying the soils of Finland
YLI-HALLA1 Markku and MOKMA2 Delbert L.
1
2
MTT Agrifood Research Finland, FIN-31600 JOKIOINEN, Finland
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.
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EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7
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 1:250000 (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 1997-2001. 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 500 – 600 mm) climate, drainage is required. Most
agricultural soils of Finland have subsurface drainage systems installed at the depth of 100-120 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 pH 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 pH range is commonly 5.5 – 6.5.
Many soils (Jokioinen 09, Table 1) have sandy Ap horizons over clayey subsoils. The Ap horizons are
commonly dark, 25-30 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 P2O5
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.
EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7
However, they have less than 1500 ppm P2O5 and after 1998 these soils are classified as Haplocryolls in
Soil Taxonomy, instead of Eutrocryepts until 1998. If they have less than 100 ppm bicarbonate-extractable
P2O5, 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
pH
(H2O)
Org. C
%
Clay
%
Silt
%
Sand
%
CEC
cmol kg-1
Base
saturation
%
P2O5
mg kg-1
Jokioinen 09, cultivated
Eutric Cambisol (FAO), Haplic Phaeozem ? (WRB), Aquic Haplocryoll ? (Soil Taxonomy)
Ap, 0-30
Bw1, 30-40
Bw2, 40-45
2Bw3, 45-70
2BC 70-100
2Cg 100-150
10YR 3/2,
10YR 5/2 dry
10YR 5/3
10YR 5/3
10YR 4/2
10YR 4/2
10YR 4/2
6.5
2.4
13
14
73
14.9
86
796
6.8
6.9
7.0
7.2
7.3
0.5
0.1
0.2
0.2
0.3
8
9
45
57
69
19
15
42
37
27
73
76
13
6
4
5.11
4.54
24.8
27.1
29.2
90
89
92
92
92
172
94
195
459
489
Mikkeli 01, cultivated
Dystric Regosol (FAO), Dystric Cambisol (WRB), Typic Dystrocryept (Soil Taxonomy)
Ap, 0-27
Bs, 27-39
BC1, 39-65
BC2, 65-97
C, 97-150
10YR 3/1
10YR 6/2 dry
10YR 4/6
10YR 6/6
10YR 6/3
2.5Y 6/2
6.4
3.4
3
29
68
13.5
81
509
6.4
6.4
6.4
6.3
0.9
0.3
0.2
0.1
0
3
4
2
30
33
31
34
70
64
65
64
4.9
2.2
1.8
1.0
56
56
45
42
n.d.
n.d.
n.d.
n.d.
P was extracted with 1% citric acid. Clay stands for particles <0.002 mm, silt for 0.002-0.06 mm and sand
for 0.06-2 mm. CEC was determined at pH 7.00. 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
EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7
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
Ap, 0-31
Bg1, 31-50
Bg2, 50-68
Bj, 68-84
Cg, 84-117
C, 117-170
Color matrix
10YR 3/3
10YR 6/2 dry
10YR 5/2
5Y 5/1
5Y 5/1
5Y5/1
5GY 3/1
Org. C
%
4.8
Clay
%
47
Silt
%
46
Sand
%
7
pH
Fresh
6.3
pH
Inc.
n.d.
SO4-S
mg kg-1
n.d.
Total S
g kg-1
1.0
2.4
1.9
1.9
3.0
3.2
58
65
67
67
65
39
32
29
28
32
3
3
4
5
3
4.8
4.3
3.9
3.6
6.5
n.d.
3.8
3.9
3.4
2.9
n.d.
95
113
497
4824
1.2
1.4
2.2
4.4
17.9
Clay stands for particles <0.002 mm, silt for 0.002-0.06 mm and sand for 0.06-2 mm. Soil pH was
determined in water (1:1) upon sampling (Fresh) and after six weeks of aerobic incubation (Inc.). SO4-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).
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Problems encountered when classifying the soils of Finland. Yli Halla & Mokma.
EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7
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
pH
(H2O)
Org. C
%
Clay
%
Silt
%
Sand
%
CEC
cmol kg-1
Jokioinen 01: forested
Haplic Podzol (FAO, WRB), Entic Haplocyod (Soil Taxonomy)
Oa, 0-7
7.5YR 3/2
4.0
29
n.d.
n.d.
n.d.
n.d.
E, 7-10
7.5YR 4/1
4.0
2.0
2
9
89
n.d.
Bs1, 10-35
7.5YR 4/4
5.0
0.8
2
8
90
n.d.
2Bs2, 35-64
7.5YR 4/6
5.3
0.3
0
6
94
n.d.
2BC, 64-75
10YR 4/4
5.4
0.3
1
8
91
n.d.
2C, 75-90
10YR 4/4
5.7
0.1
1
3
96
n.d.
Jokioinen 02: forested
Dystric Cambisol (WRB), Dystric Regosol (FAO), Typic Dystrocryept (Soil Taxonomy)
Ah, 0-15
7.5YR 3/2
4.7
5.4
8
40
52
15.8
Bw1, 15-25
7.5YR 4/3
4.9
3.1
8
38
54
11.8
Bw2, 25-45
10YR 4/4
5.1
1.9
6
33
61
8.1
Bw3, 45-55
10YR 4/6
5.3
0.8
4
32
64
5.4
C, 55-70
2.5Y 5/4
5.3
0.4
4
33
63
3.2
Sodankylä 01: forested
Dystric Cambisol (WRB), Haplic Podzol (FAO), Oxyaquic Haplocryod (Soil Taxonomy)
O, 0-5
n.d.
3.7
43
n.d.
n.d.
n.d.
91
Ah, 5-7
7.5YR 2.5/2
3.9
29
n.d.
n.d.
n.d.
55
E, 7-10
7.5YR 4/6
4.2
3.7
6
47
47
n.d.
Bs, 10-17
7.5YR 4/6
4.6
2.1
6
48
46
14.6
BC, 17-65
2.5Y 4/4
4.7
0.6
6
42
52
6.4
C, 65-100
2.5Y 4/3
5.3
0.3
13
40
48
7.6
Base
saturation
%
Index
%
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.07
0.55
0.28
0.33
0.21
14
10
8
10
11
0.55
0.69
1.10
0.80
0.44
15
6
n.d.
1
3
36
n.d.
n.d.
0.22
0.83
0.48
0.37
Clay stands for particles <0.002 mm, silt for 0.002-0.06 mm and sand for 0.06-2 mm. Index stands for 0.5 x Fe + Al,
extracted with acid oxalate, pH 3.0, and expressed as percentage of soil mass. Not determined = n.d.
Problems encountered when classifying the soils of Finland. Yli Halla & Mokma.
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EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7
Table 4. The distribution of Quaternary deposits (adapted from Kujansuu and Niemelä,
1992) and their dominant land use
Type of deposit
Glacial till
Sandy glacio-fluvial deposits
Clayey and silty deposits
Peatlands (>30 cm)
Exposed bedrock
land area (%)
58
5
8
15
14
Land use
Forest (and cultivated)
Forest and cultivated
Cultivated
Forest, peat mining, no use (and cultivated)
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
Haplic Podzols
Vertic
and
Dystric
Cambisols
Dystric and Vertic Gleysols
Dystric and Eutric Histosols
Leptosols
Land area (%)
56
8
2
29
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.
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Problems encountered when classifying the soils of Finland. Yli Halla & Mokma.
EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 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.
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