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17/3/2006

Definition

Soil type

Soil type classification is determined in the test pit.

The soil class is the upper part of the ground or regolith (the Earth's crust) that has been influenced by soil formation factors e.g. climate and organisms. This influence has often resulted in the formation of visible soil horizons. Soil class includes the humus layer but not the detritus/S-layer.

The following soil types can be distinguished:

Code

Name

 

With visible B horizon

(1)

Cultivated soil

(2)

Brown forest soil

(3)

Transition type

(4)

Iron podsol

(5)

Iron humus podsol

(6)

Humus podsol

 

Without visible B horizon

(7)

Poorly drained soil

(8)

No B horizon due to dense soil texture

(9)

No B horizon due to coarse soil texture

(10)

Boulder soil

(11)

Lithosol (has occasionally, but not always, visible B horizon)

(12)

Bedrock outcrop

 

Special case:

(13)

Disturbed soil

There follows a description of the characteristics of each soil class and the rules for soil sampling. The diagrams illustrating the soil classes should only be regarded as typical examples, as within each soil class there is wide variation in appearance.

In certain cases, the conditions in the test pit can agree with the description of more than one soil class. In this case, the soil class that comes first in the following order should be recorded:

    Bedrock outcrop
    Boulder soil
    Disturbed soil
    Lithosol
    Humus podsol
    Iron humus podsol
    Iron podsol
    Poorly drained soil
    Cultivated soil
    Transition type
    Brown forest soil
    ’Coarse soil‘
    ’Dense soil‘

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The soil classes are characterised in the following way:

Code and name

Description


(1) Cultivated soil

Cultivated soil -photo

The soil is a previously ploughed arable soil with a distinct boundary between the topsoil (Ap horizon) and the subsoil. The topsoil (Ap horizon) is generally approx. 20 cm thick. Under the Ap horizon, the B horizon is generally present. However, there may be relics of a former bleached horizon that was present at the time the soil was first ploughed (the plough did not reach so deep that the entire bleached horizon layer was ploughed in). This ’old‘ bleached horizon is not considered in the soil classification.

Ploughing has resulted in a topsoil (humus classified as either a mull or a mull-like moder), but if the soil has been colonised by mor-forming vegetation, an O horizon can form above the topsoil and the humus form can change to a mor or a moder.

Even if the podsolisation process has given rise to ’staining‘ under the O horizon, the soil must still be classified, as a cultivated soil if the boundary between the Ap horizon and subsoil caused by ploughing is clearly visible. If the podsolisation process has proceeded so far that an E horizon has formed, the soil must be classified as a podsol (most often iron podsol).

There can be a Bs horizon in the B horizon.

Humus sampling depends on the humus form. Note that the A horizon is neither measured nor sampled if the humus form is of the mor type.

B-samples are taken from the uppermost 5 centimetres of the B horizon.

BC-samples are taken from the 10 cm layer located 45-55 cm below the soil surface. This parameter should be omitted if the variable J-depth has a code less than or equal to 50.

C-samples are taken from the 10 cm layer located 55-65 cm below the upper surface of the mineral soil (most often the Ap horizon). This parameter should be omitted if the soil is not sufficiently deep.


(2) Brown forest soil

Brown forest soil -photo

The humus form is mull or mull-like moder. In patches it may even be moder but not mor, peaty mor or peat. There is one exception to this rule. In cases where the soil profile does not have a uniform humus form due to a new humus form having developed on top of an older (if the variable H-uniformity is given the code 3), the soil can be classified as a brown forest soil even if the humus form is mor, peaty mor or peat.

The podsolisation process must not have given rise to an E horizon or even a ’staining‘ of the A horizon, see below under ’Transition type‘. There should not be a clear boundary between the topsoil and the subsoil (Ap horizon), see above under ’Cultivated soil‘.

The A horizon has a well-developed aggregate structure.

The B horizon is commonly brownish, sometimes grey-greyish brown. There may be a Bs horizon in the B horizon. The transition layer between the A and B horizons (the AB horizon) is diffuse and often extended in depth. If the texture of the soil is very fine (clay or silt), the difference in colour between the A and B horizons can be indistinguishable and class distinction can then be made with the help of the structure of the soil material (aggregation).

The factor that distinguishes a brown forest soil from the class ’no B horizon formed due to dense soil texture‘ with humus form mull or mull-like moder is that in the latter soil type there is no gradual transition between an A horizon and a B horizon (AB horizon) and the A horizon ends in a sharp boundary to the C horizon.

Humus sampling depends on the form of the humus, which is nearly always mull or mull-like moder. Note that the A horizon is neither measured nor sampled in the very rare cases where the humus form is moder.

B-samples are normally taken from the upper 5 centimetres of the B horizon, although not deeper than in the layer 40-45 cm below the soil surface. If the mull layer is thick and/or the AB horizon extended in depth, the B-sample can thus end up above the B horizon (in the AB horizon, or in very rare cases in the A horizon).

BC-samples samples are taken from the 10 cm layer located 45-55 cm below the soil surface. This parameter should be omitted if the variable J-depth has a code less than or equal to 50.

C-samples samples are taken from the 10 cm layer located 55-65 cm below the upper surface of the mineral soil (= A horizon). This parameter should be omitted if the soil is not sufficiently deep.


(3) Transition type

Transition type -photo

The name transition type refers to soils that possess more or less both brown forest soil characteristics and podsol characteristics. Transition type is thus used for the transition phase brown forest soil >>> (towards) podsol (usually iron podsol) and podsol >>> brown forest soil. However the term transition soil should not be used for the transition phase cultivated soil >>> podsol.

There can be an O horizon above an A horizon, but the O horizon can also be absent. The humus form can thus be anything from mor to mull, but the most common is moder or mull-like moder.

The A horizon seldom has a well-developed aggregate structure. It is typical to find 'staining' in the upper part of the A horizon, which indicates the start of podsolisation. However, the process should not have gone so far that an E horizon has been formed. If there is an E horizon, the soil must be classified as a podsol (usually iron podsol).

Sites where the soil is of transition type are characterised by unstable conditions in the soil, i.e. some soil formation factor has been altered so that the thickness and structure, particularly of the upper soil profile horizons, has been affected.

The most common is that a brown forest soil is transformed into a podsol, but of course the opposite can also occur. The latter process is often initiated by the establishment of dense field layer (mainly grass). In such a situation, the mor ’disappears‘ in the root blanket and the surface of the topsoil is loosened by the interweaving roots, so that the characteristics of the bleached horizon is erased. Both the mor layer and the bleached horizon can also be loosened and develop a granular structure (the podsol has changed into a transition soil). If a ’brown forest soil-forming‘ tree species becomes dominant thereafter, the transformation may continue towards a brown forest soil. In this succession, the boundary between the transition soil and the brown forest soil can be very difficult to distinguish. If two horizons with distinct characteristics (the root-filled 'old' mor layer and the friable bleached horizon) can be distinguished within the layer loosened mainly by grass roots, the classification transition type is retained. However, when the loosened layer is completely homogeneous, the soil is classified as a brown forest soil.

The Bs horizon may be present and is more common than in brown forest soils.

Sampling is carried out as specified for brown forest soil above.

  • Note that humus of mor type is more common in transition type soils than in brown forest soils.


(4) Iron podsol

Iron podsol -photo

The humus form is generally mor type 1 or mor type 2, but can also be moder or peaty mor. Other humus forms are very rare.

There must be an E horizon present (with one exception, see below). In patches, this horizon may only appear as a very thin strip. In such cases, one must be guided by the humus form in distinguishing between iron podsol and transition type. In other places the E horizon can be very thick and in extreme cases it can be thicker than 50 cm.

The boundary between the E and B horizons is often distinct but can sometimes have a transition zone. The thickness of the E horizon must be specified in the variable E-thick. If there is a transition zone between the E and B horizons, the E horizon measurement should include half this transition zone. If the soil depth is shallow, the E horizon can rest directly on the bedrock. In such cases, the thickness of the E horizon is measured down to the bedrock surface. Note that if the thickness of the mineral soil is less than or equal to 10 cm, the soil is classified as a lithosol.

Under the E horizon there is often a Bs horizon. There must not be a Bsh horizon in the upper part of the B horizon. In such cases the soil must be classified as an iron humus podsol, see below.

Iron podsols are mainly formed on dry to mesic sites, more seldom if the soil water content is mesic-moist or wetter, see iron humus podsol and humus podsol.

Soils rich in fine particles (with a high content of clay and/or silt) can appear very light in colour in the upper part of the mineral soil when they dry out - note that this is not an E horizon. The soil class on such sites is often ’No visible B horizon due to dense soil texture‘.

There is one exception from the rule that there must be a distinct E horizon: dune sand areas with a still unformed soil profile are classified as iron podsols with bleached horizon 0 cm. A note about dune sand must be recorded in the observations menu.

Humus sampling depends on the humus form.

EB-samples are taken only if the bleached horizon thickness is less than 2.5 cm, i.e. if the variable E-thick has code 0, 1 or 2. The sample is taken from a 5 cm layer starting from the upper edge of the E horizon.

B-samples are taken from the upper 5 centimetres of the B horizon. This sample should be omitted if the humus form is of mor or peat type and the variable H-thick has a code greater than or equal to 51 (rare in iron podsols).

BC-samples are taken from the 10 cm layer located 45-55 cm below the soil surface:

  • If the variable J-depth has code 51-55, the sample should be taken from 45 cm depth down to the bedrock.

  • If the variable J-depth has a code less than or equal to 50, a BC-sample should not be taken.

If the humus form is of mor or peat type, the following apply:

  • If the variable H-thick has code 45-50, the sample should be taken immediately below the humus layer down to 55 cm depth.

  • If the variable H-thick has a code greater than or equal to 51, a BC-sample should not be taken. Such thick humus layers are rare in iron podsols.

C-samples are taken from the 10 cm layer located 55-65 cm below the upper surface of the mineral soil (E horizon - or A horizon if one exists above the E horizon). This sample should be omitted if the humus form is of mor or peat type and the variable H-depth has a code greater than or equal to 31. This sample should also be omitted if the soil is not sufficiently deep.


(5) Iron humus podsol


This type of soil profile develops mainly on somewhat wetter sites than iron podsols. As a rule, the humus form is peaty mor, but can also be either a mor type or a peat.

As in iron podsols, there must be a distinct E horizon. It is often very similar to the E horizon in an iron podsol.

The boundary between the E and B horizons is often distinct but can have a transition zone. The thickness of the E horizon must be specified in the variable E-thick. If there is a transition zone between the E and B horizons, the E horizon measurement must include half this transition zone.

The main characteristic of an iron humus podsol is that the upper part of the B horizon contains a dark-brown layer with strong humus enrichment (Bsh horizon). The Bsh horizon is often well-developed and continuous, but sometimes there is instead a quantity of isolated dark-brown patches or stripes, often with granular structure. Sometimes there is a hardpan (ortstein) layer within the Bsh horizon. Under this layer there is a Bs layer, which is often similar to the B horizon in an iron podsol.

In the C horizon there is often gley formation that sometimes extends up into the B horizon.

Iron humus podsols can also develop on considerably drier sites, preferentially on sites with a mineralogically poor parent material and high humidity. This soil class is particularly common on till within the NW Svealand sandstone and porphyry bedrock region.

Sampling should be carried out as specified for the iron podsol


(6) Humus podsol

Humus podsol -photo

Humus podsols are formed on wetter sites than iron podsols and iron humus podsols. The humus form is often peat or peaty mor.

The E horizon is often thick (sometimes over 50 cm) and humus-impregnated (Eh), and has therefore often a dirty grey to dirty brown colour. Sometimes it can be so dark that it no longer merits the name bleached horizon, but its minerals are weathered in the same way as in an ’ordinary‘ bleached horizon.

A large part of the B horizon is often humus-impregnated too (Bh or Bsh) and can then have a colour indistinguishable from that of the Eh horizon. In some cases there is a transition zone between the E and B horizons.

Hardpan can occur in the upper part of the B horizon.

The thickness of the E horizon must be specified in the variable E-thick. If there is a transition zone between the E and B horizons, the E horizon measurement should include half this transition zone.

  • If a bleached horizon cannot be distinguished, the soil should be classified as a fen soil.

Gley formation often occurs in the C horizon, but also in the B and E horizons.

Sampling should be carried out as specified for iron podsols.


(7) Poorly drained soil

Poorly drained soil -photo

Poorly drained soils are formed on even wetter sites than humus podsols, and are characterised by reducing conditions more or less up to the soil surface. The humus form is often peat.

Under the H horizon there is often an A horizon that changes to a C horizon with a grey-blue to grey colour. The soil often smells like LP-gas (marsh gas). In the C horizon there is often gley formation (Cg). There are no distinct E and B horizons. (Variable Bs thus has code 0.)

If the humus form is of mor or peat type and the variable H-thick is greater than or equal to 51, all mineral soil sampling should be omitted for practical reasons. In such cases, it is not necessary to dig down to the mineral soil, since the soil can be classified as a fen soil if a bleached horizon is not visible in soil core samples.

Humus sampling depends on the humus form (most often peat).

B-samples should be taken from the uppermost 5 centimetres of the mineral soil (C horizon, or A horizon if one exists above the C horizon). This sample should be omitted if the humus form is of mor or peat type and the variable H-thick has a code greater than or equal to 51.

BC-samples should be taken from the 10 cm layer located 45-55 cm below the soil surface:

  • If the variable J-depth has code 51-55, the sample should be taken from 45 cm depth down to the bedrock.

  • If the variable J-depth has a code less than or equal to 50, a BC-sample should not be taken.

If the humus form is of mor or peat type, the following apply:

  • If the variable H-thick has code 45-50, the sample should be taken immediately below the humus layer down to 55 cm depth.

  • If the variable H-thick has a code greater than or equal to 51, a BC-sample should not be taken.

C-samples should be taken from the 10 cm layer located 55-65 cm below the upper surface of the mineral soil (C horizon - or A horizon if one exists above the C horizon). This sample should be omitted if the humus form is of mor or peat type and the variable H-depth has a code greater than or equal to 31. This sample should also be omitted if the soil is not sufficiently deep.


(8) No B horizon due to dense soil texture

No B horizon due to dense soil texture -photo

If the soil consists solely of clay and/or silt, and possibly also of coarse silt, it is often very difficult to distinguish a B horizon.

The humus form can be of mor, peat or mull type. The humus layer has a sharp boundary with the C horizon.

If the choice of soil class is between this and fen soil, fen soil should be chosen.

For distinguishing from a brown forest soil:

    The B horizon should not be present and the Bs variable thus has code 0.

Humus sampling depends on the humus form.

B-samples should be taken from the uppermost 5 centimetres of the C horizon.

    This sample should be omitted if the humus form is of mor or peat type and the variable H-thick has a code less than or equal to 51.

BC-samples should be taken from the 10 cm layer located 45-55 cm below the soil surface:

  • If the variable J-depth has code 51-55, the sample should be taken from 45 cm depth down to the bedrock.

  • If the variable J-depth has a code less than or equal to 50, a BC-sample should not be taken.

If the humus form is of mor or peat type, the following apply:

  • If the variable H-thick has code 45-50, the sample should be taken immediately below the humus layer down to 55 cm depth.

  • If the variable H-thick has a code greater than or equal to 51, a BC-sample should not be taken.

C-samples should be taken from the 10 cm layer located 55-65 cm below the upper surface of the mineral soil (C horizon - or A horizon if one exists above the C horizon). This sample should be omitted if the humus form is of mor or peat type and the variable H-depth has a code greater than or equal to 31. This sample should also be omitted if the soil is not sufficiently deep.


(9) No B horizon due to coarse soil texture


If the soil texture is coarse (gravel and/or coarse sand, possibly with boulder and stone inclusions), it is often very difficult to distinguish a B horizon.

The humus form can be mor, peat or (very seldom) mull type.

If the choice of soil class is between this and fen soil, fen soil should be chosen.

The B horizon should not be present and the Bs variable thus has code 0.

Humus sampling depends on the humus form.

B-samples should be taken from the uppermost 5 centimetres of the C horizon.

    This sample should be omitted if the humus form is of mor or peat type and the variable H-thick has a code less than or equal to 51.

BC-samples should be taken from the 10 cm layer located 45-55 cm below the soil surface:

  • If the variable J-depth has code 51-55, the sample should be taken from 45 cm depth down to the bedrock.

  • If the variable J-depth has a code less than or equal to 50, a BC-sample should not be taken.

If the humus form is of mor or peat type, the following apply:

  • If the variable H-thick has code 45-50, the sample should be taken immediately below the humus layer down to 55 cm depth.

  • If the variable H-thick has a code greater than or equal to 51, a BC-sample should not be taken.

C-samples should be taken from the 10 cm layer located 55-65 cm below the upper surface of the mineral soil (C horizon - or A horizon if one exists above the C horizon). This sample should be omitted if the humus form is of mor or peat type and the variable H-depth has a code greater than or equal to 31. This sample should also be omitted if the soil is not sufficiently deep.


(10) Boulder soil


Sites where boulders or dense collections of stones occur in piles, boulder sinkholes, talus or along former shorelines exposed to wave action (raised beaches). Fine material is lacking or is present to a very limited extent between stones and boulders. The soil mineral type is generally till.

These are distinguished from soils with distinct mineral soil horizons because due to the accumulation of boulders it is not possible to dig out profile walls for a reliable soil classification.

The B horizon should not be present and the Bs variable thus has code 0.

If the stones and boulders shows signs of strong abrasion (well-rounded) and the position in the terrain indicates that the site is a former shoreline, the soil is classed as a sediment to denote the fact that this is a shingle field.

Humus sampling depends on the humus form. Sampling can be difficult to perform if the sampling points end up above gaps between the boulders. Compensate by taking up humus by hand from these gaps under the actual sampling area (= soil corer area).

No mineral soil samples are taken


(11) Lithosol (skeletal soil)

Lithosol (skeletal soil) -photo

A soil is classified as a lithosol in two cases:

  1. If the bedrock is covered by an organic soil horizon and/or by a thin layer of mineral soil, which is at most 10 cm thick.

  2. When a well-developed mull lies directly on limestone bedrock or the mull lies in the spaces between limestone fragments, (rendzina soil). This soil type is only encountered in areas with limestone bedrock and is abundant on alvar ground on the islands Íland and Gotland (Cambro-Silurian bedrock).

If there is an E horizon, its thickness must be recorded in the variable E-thick. Sometimes the E horizon lies directly on top of the bedrock (B horizon lacking). The thickness of the bleached horizon should then be measured down to the bedrock surface.

The Bs horizon can be present.

Humus sampling depends on the humus form.

No mineral soil samples are taken.


(12) Bedrock outcrop

Bedrock outcrop -photo

Bedrock outcrop is not actually a soil class but indicates bare bedrock at the surface, i.e. rock surfaces more or less clad in mosses or lichens, without a humus layer and without mineral soil.

The humus form is always ’humus layer lacking‘.

Both humus sampling and mineral soil sampling are omitted.


(13) Disturbed soil

Disturbed soil -photo

A soil is classified as disturbed if the normal sequence of the soil horizons has been clearly mixed/reversed due to a disturbance so that e.g. remains of a bleached horizon or humus layer are lying some way down in the B horizon. The mixing/reversal can either be due to human activities or have 'natural' causes such as trees uprooting or soil freezing.

Former charcoal pits with a clear layer of charcoal and former arable soils from which the topsoil has been scraped off are also classified as disturbed soils.

    Note that the disturbance is only noticed in conjunction with digging of the test pit. Otherwise the sampling site lies on AVM (aberrant ground) or MBA (scarified ground), where soil classification/soil sampling should never be performed.

    Note when measuring the thickness of the humus layer (H-thick) that if several humus layers are found, their thicknesses should be combined. However, this only applies to humus layers found within 30 cm depth from the soil surface.

Both humus sampling and mineral soil sampling are omitted.

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