Diagnostic horizons, properties and materials

Diagnostic horizons

Diagnostic horizons, in contrast to the more general horizon designations O, A, E, B and C, have defined and more precise criteria that must be fulfilled for a horizon to be designated as diagnostic. In both cases, they refer to identifiable soil horizons that can be traced horizontally around the profile wall of a soil pit. (Note also that a given type of diagnostic horizon may occur in more than one soil reference group. A cambic horizon, for example, may also occur in an Umbrisol, but in that case it is the surface horizon (umbric) that is the decisive diagnostic horizon.)

Cambic

General description

A cambic horizon (from Latin cambiare – to change) is a fine-textured B horizon that shows signs of weak to moderate alteration due to soil-forming processes. Compared with underlying horizons, it has a higher content of oxides and/or shows evidence of carbonate removal. Overlying mineral horizons have, by comparison, a higher content of organic matter and a darker and/or less intense colour. Alteration due to pedogenesis should be demonstrable through the presence of a developed structure.

Diagnostic criteria

A cambic horizon

1. has a texture of sandy–silty till or loamy sand, or finer;
   and
2. lacks any original stratification or varving of the parent material in at least half of its volume;
   and
3. shows evidence of alteration in one or more of the following respects:
   - redder hue, higher chroma, or higher clay content than the underlying layer;
   or
   - rredder hue, higher chroma, or higher value than the overlying mineral horizon, and a developed aggregate structure in at least half of the volume;
   or
   - evidence of carbonate loss;
   and
4. is not part of a former plough layer and is not part of a spodic or umbric horizon;
   and
5. has a thickness of at least 15 cm.

Folic

General description

A folic horizon (from Latin folium – leaf) is a surface horizon or a horizon close to the soil surface. It consists mainly of well-aerated organic material (usually a humus form of mor type).

Diagnostic criteria

A folic horizon consists of organic material that

1. is water-saturated for less than 30 days during a normal year;
   and
2. has a thickness of at least 10 cm.

Histic

General description

A histic horizon (from Greek histos – tissue) is a surface horizon or a horizon close to the soil surface. It consists of organic material whose decomposition is inhibited by limited oxygen supply (usually a peat-type humus form).

Diagnostic criteria

A histic horizon consists of organic material that

1. is water-saturated for at least 30 consecutive days during a normal year or is artificially drained (by ditching, lake lowering, etc.);
   and
2. has a thickness of at least 10 cm.

Mollic

General description

A mollic horizon (from Latin mollis – soft) is a thick, dark-coloured surface horizon with good structure, a high base saturation, and a moderate to high content of organic matter. A mollic horizon is an A horizon.

Diagnostic criteria

The key below is exactly the same as that for an umbric horizon (see below), except for point 4. A mollic horizon has, on average, the following properties in the upper 20 cm (or down to bedrock if it occurs within 20 cm):

     4. a base saturation (in 1 M NH₄OAc) of 50% or higher, calculated as a weighted mean for the entire horizon (i.e. a fertile and easily weathered material).

Spodic

General description

A spodic horizon (from Greek spodos – wood ash) is a B horizon in a Podzol. It contains translocated and precipitated substances consisting of organic matter, aluminium and iron. These precipitated substances are characterised by a high proportion of pH-dependent charges, a large specific surface area, and good water-holding capacity. A spodic horizon is usually overlain by albic material.

Diagnostic criteria

A spodic horizon

1. has, in the upper 2.5 cm, one of the following Munsell colours:
   - a hue of 5YR or redder;
   or
   - a hue of 7.5YR with a value of 5 or lower and a chroma of 4 or lower (6 or lower where cementation – ortstein – occurs in at least half of the volume);
   or
   - a hue of 10YR with both value and chroma of 2 or lower, or the colour 10YR 3/1;
   and
2. has a thickness of at least 2.5 cm.

Umbric

General description

An umbric horizon (from Latin umbra – shade) is a thick, dark-coloured surface horizon with a low base saturation and a moderate to high content of organic matter. An umbric horizon is an A horizon.

Diagnostic criteria

(The key below is exactly the same as that for a mollic horizon, except for point 4.)

An umbric horizon has, on average, the following properties in the upper 20 cm (or down to bedrock if it occurs within 20 cm):

1. a structure sufficiently strong that the horizon does not become massive and hard or very hard when dry (aggregates larger than 30 cm in diameter are considered massive unless a secondary structure occurs within the aggregates);
   and
2.  at least 0.6 wt-% organic carbon (1 wt-% organic matter);
   and
3. one or both of the following:
   a). a crushed sample must have a Munsell value of 3 or lower and a chroma of 3 or lower;
   or
   b). a texture of sandy–silty till or loamy sand, or coarser, and a Munsell value of 5 or lower and a chroma of 3 or lower, and at least 2.5 wt-% organic carbon (4.3 wt-% organic matter);
   and
4. a base saturation (in 1 M NH₄OAc) of 50% or lower, calculated as a weighted mean for the entire horizon;
   and
5. the following thickness requirements:
   a). at least 10 cm if it lies directly on bedrock;
   or
   b). at least 20 cm.

Diagnostic properties and materials

Gley colours

General description

Soil material develops gley colours (from the Russian term gley – sticky soil mass) if it is saturated with groundwater for a period long enough for oxygen in the soil water to be consumed through oxidation of organic matter, leading to reducing (anaerobic) conditions. Under reducing conditions, divalent iron forms and dissolves in the soil water. When the groundwater table falls, the iron is oxidised to the trivalent form and precipitates along root channels and in cracks between aggregates where oxygen first penetrates.

Gley colours are often characterised by a mottled pattern that develops when alternating reducing and oxidising conditions occur in the transition zone between groundwater and soil water, resulting in an uneven distribution of red iron (hydr)oxides. Under permanently reducing conditions, colours dominated by grey/white (sandy soils), blue/green (clayey soils), or black (sulphidic soils) prevail.

Diagnostic criteria

Gley colours fulfil one or both of the following:

1. at least 95% of the exposed surface has colours indicating permanently reducing conditions (grey/white, blue/green, or black);
   or
2. at least 5% of the exposed surface consists of mottles (often red-brown or yellow-brown) indicating oxidising conditions through accumulation of iron (hydr)oxides.

Reducing conditions

Definition

Reducing conditions imply oxygen deficiency in the soil and that iron and other substances occur in reduced form. This can be demonstrated by applying a 1% K₃Fe(CN)₆ solution to a fresh break of an aggregate in a moist soil sample. If the reagent turns dark blue, reducing conditions prevail.

Albic material

General description

Albic material (from Latin albus – white) is usually light-coloured fine earth that has been leached of iron (hydr)oxides and organic matter, so that the colour of the horizon is determined by the colour of the sand and silt particles rather than by coatings on these particles. The material has a single-grain structure or a weakly aggregated structure. Layers of albic material often occur beneath a humus layer, but may reach the soil surface through erosion or disturbance. The material is associated with an eluvial horizon (E horizon) and then usually overlies an illuvial horizon (B horizon). Albic material in an eluvial horizon often has a coarser texture than the underlying (spodic) B horizon, although the difference may be small (sometimes one texture class, but often less than this). Thick layers of albic material are often linked to sandy deposits and humid climatic conditions. Albic material also occurs under reducing conditions.

Diagnostic criteria

Albic material is fine earth that, in at least 90% of its volume, has:

Munsell colours:  

• a value of 6, 7 or 8 and a chroma of 4 or lower;
  or
• a value of 5 and a chroma of 3 or lower;
  or
• a value of 4 and a chroma of 2 or lower. A chroma of 3 is permitted if the parent material has a hue of 5YR or redder and the chroma is due to the colour of the clean silt and sand particles.

Organic material

General description

Organic material consists of a large amount of dead organic residues accumulated on the soil surface under either moist or dry conditions, in which any mineral content does not significantly affect the properties.

Diagnostic criteria

Organic material contains at least 20 wt-% organic carbon (35 wt-% organic matter) in the fine earth (< 2 mm).