Organic Horizons and Layers

The soil horizon and layer definitions and methods for field description that follow are taken or modified from Soil Classification Working Group (1998), Green et al. (1993), and Luttmerding et al. (1990).

16. Horizon/Layer

Record the organic horizon or layer designation. Two groups of master organic horizons are recognized: L, F, H ("upland") horizons, and O ("wetland") horizons. All contain > 17% organic C by mass. These two groups are differentiated primarily by the features outlined in Table 2.19.

TABLE 2.19. Guidelines for differentiating between upland and wetland organic horizons

Property	L, F, and H horizons	O horizons
Physiography	Sloping to level	Depression to gently sloping
Soil drainage	Very rapid to imperfect	Poor to very poor
Water table	Absent in organic horizons (may fluctuate in response to water input)	At or near ground surface for significant duration during the frost-free period
Origin of materials	Organic residues from plant communities typically associated with soil moisture regimes 0-6	Organic residues from plant communities typically associated with soil moisture regimes 7-8

Codes for master organic horizons:

L	An upland horizon consisting of relatively fresh organic residues that are readily identifiable as to origin.
F	An upland horizon comprised of partly decomposed plant residues in which fragmented plant structures are generally recognizable as to origin.
H	An upland horizon comprised of well-decomposed plant residues in which plant structures are generally not recognizable.
O	A wetland organic horizon comprised of materials in varying degrees of decomposition.

Codes for Subordinate oganic horizons:

Ln	An L horizon composed of newly accreted and essentially unfragmented plant residues.
Lv	An L horizon exhibiting initial decay and strong discoloration.
Fm	An F horizon in which plant residues are aggregated in a matted structure, with a tenacious consistence. Fungal mycelia are clearly a predominant biotic component; some faunal droppings may be present.
Fz	An F horizon in which plant residues are weakly aggregated with a loose or friable consistence. Faunal droppings are typically numerous and easily observed under magnification with a hand lens or binocular microscope; fungal mycelia may be present.
Fa	An F horizon in which plant residues are aggregated into a weak to moderate, non-compact matted structure. This is an intergrade between the Fm and Fz horizons, and as such, reflects properties of both, but neither fungal mycelia or faunal droppings predominates.
Hh	An H horizon dominated by fine substances with very few, if any, recognizable plant residues.
Hz	An H horizon dominated by fine substances with very few, if any, recognizable plant residues; faunal droppings constitute most of the fabric.
Hr	An H horizon dominated by fine substances, but that also contains recognizable plant residues, usually from fine roots, wood, or bark; typically dark reddish-brown hues, around 2.5YR.
Of	An O horizon comprised largely of poorly decomposed plant residues that are readily identifiable as to origin. It has 40% or more rubbed fibre (i.e., fibre that remains after rubbing a sample about 10 times between thumb and forefinger). These materials are classified in the von Post scale of decomposition (defined below, in Item 18, "Fabric") as class 1 to class 4.
Om	An O horizon comprised of partly decomposed plant residues which are at a stage of decomposition intermediate between Of and Oh horizons. Rubbed fibre usually ranges between 10 and 40% by volume. These materials are classified in the von Post scale of decomposition as class 5 or 6.
Oh	An O horizon of well-decomposed plant residues that for the most part have been transformed into humic materials. The rubbed fibre content is less than 10% by volume. These materials are usually classified in the von Post scale of decomposition as class 7 or higher, and very rarely as class 6.
Oco	Coprogenous earth, deposited or modified by aquatic organisms.

Lowercase modifiers:
The following lowercase modifiers may be applied to any organic horizon without restriction.

i	An organic horizon that contains intermixed mineral particles finer than 2 mm, with 17-35% organic C by mass. This intermixing of mineral particles with organic materials may result from several different processes (e.g., colluvial, eolian, alluvial, cryoturbation, silvoturbation, and zooturbation).
p, u, y	May also be used with organic horizons, and are defined under "Mineral lowercase modifiers" in Item 24.
w	An organic horizon that contains significant amounts (> 35% of the volume of solids) of coarse woody debris in various stages of decomposition.

Codes for organic layers:

S	A distinct ground surface layer of living materials such as bryophytes or "soil crusts."
Limno	A layer or layers 5 cm or more thick of sedimentary peat, diatomaceous earth, or marl.
Cumulo	A 5-30 cm thick layer or layers of mineral material in Organic soils.
Terric	An unconsolidated mineral substratum not underlain by organic matter, or one continuous unconsolidated mineral layer more than 30 cm thick in the middle or bottom tiers underlain by organic matter within a depth of 160 cm.
Lithic	Bedrock occurring within 10-160 cm in Organic soils
Hydric	A layer of water that extends from a depth of not less than 40 cm from the organic surface to a depth of more than 160 cm.

Tiers:
Tiers are arbitrary depth intervals used in classifying wetland Organic soils, and consist of the surface (0-40 cm), middle (40-120 cm) and bottom tiers (120-160 cm). They are not recorded.

17. Depth

Record the average depths (in centimetres) of the upper and lower boundaries of the horizon being described. The depth of organic horizons in mineral soils are measured upward from zero depth (e.g., L 12-9, Fm 9-2, and Ah 2-0), and in organic soils they are measured downward from the ground surface, or uppermost soil horizon (e.g., S 4-0, Of 0-35, and Om 35-110).

18. Fabric

Describe the structure and consistence of the upland organic horizons and record the von Post classes for wetland horizons. Structure is important in distinguishing between Fm, Fz, and Fa horizons, and the von Post scale of decomposition helps to distinguish the Of, Om, and Oh horizons.

Structure:

Describe structure according to the degree and kind of the macromorphological aggregation of the material within a horizon. Record the structure "degree" code (Table 2.20) in the first column and the "kind" code (Table 2.21) in the second column.

TABLE 2.20. Degree of aggregation codes

Code	Class	Description
W	Weak	Disaggregated materials are dominant; < 20% distinctly aggregated
M	Moderate	Some disaggregated materials are found; 20-60% distinctly aggregated
S	Strong	Aggregated materials are dominant; most material conforms to the same arrangement; > 60% distinctly aggregated

TABLE 2.21. Kind of aggregation codes

Code	Class	Description
SP	Single particle	An incoherent mass of individual particles with no aggregation
BK	Blocky	Faces rectangular and flattened; vertices angular
GR	Granular	Spheroidal and characterized by rounded or subrounded vertices
NM	Non-compact	Materials arranged along horizontal planes matted with no compaction
CM	Compact	Materials arranged along horizontal planes matted with evident compaction
ER	Erect	Materials arranged vertically
RC	Recumbent	Materials arranged in recumbent (reclining) position
MA	Massive	A coherent mass showing no evidence of aggregation

von Post scale of decomposition:

Squeeze a sample of the O horizon and observe the colour of the solution that is squeezed out between the fingers, the nature of the fibre, and the proportion of the original sample that remains in the hand. Record the class (Table 2.22).

TABLE 2.22. von Post scale of decomposition classes

Code/Class	Description
1	Undecomposed; plant structure unaltered; yields only clear water coloured light yellow brown.
2	Almost undecomposed; plant structure distinct; yields only clear water coloured light yellow brown.
3	Very weakly decomposed; plant structure distinct; yields distinctly turbid brown water, no peat substance passes between the fingers, residue not mushy.
4	Weakly decomposed; plant structure distinct; yields strongly turbid water, no peat substance escapes between the fingers, residue rather mushy.
5	Moderately decomposed; plant structure evident, but becoming indistinct; yields much turbid brown water, some peat escapes between the fingers, residue very mushy.
6	Strongly decomposed; plant structure somewhat indistinct, but more evident in the squeezed residue than in the undisturbed peat; about one-third of the peat escapes between the fingers, residue strongly mushy.
7	Strongly decomposed; plant structure indistinct, but recognizable; about one-half of the peat escapes between the fingers.
8	Very strongly decomposed; plant structure very indistinct; about two-thirds of the peat escapes between the fingers, residue almost entirely resistant remnants such as root fibres and wood.
9	Almost completely decomposed; plant structure almost unrecognizable; nearly all the peat escapes between the fingers.
10	Completely decomposed; plant structure unrecognizable; all the peat escapes between the fingers.

19. Mycelial Abundance

In most cases, fungal presence is indicated by masses of hyphae called mycelia. While individual hyphae are generally too small to be seen, the mycelial mass is usually visible. Determining mycelial abundance helps to distinguish the Fm, Fz, and Fa horizons, and therefore the humus form classification. Describe fungal mycelia by noting their abundance class as indicated in Table 2.23.

TABLE 2.23. Mycelial abundance classes and codes

Code	Class	Description
X	None	Fungal mycelia are not visible
F	Few	Fungal mycelia are occasionally present, but are scattered and not easily observed
C	Common	Fungal mycelia are commonly observed
A	Abundant	Fungal mycelia are observed continuously through- out the horizon, often "matting" materials together and creating a "felty" tactility

20. Fecal Abundance

The presence of soil fauna may be observed directly, or indirectly by the presence of fecal droppings or casts. Determining fecal abundance helps to distinguish the Fm, Fz, and Fa horizons, and therefore the humus form classification. Describe the presence of soil fauna by noting their abundance class as indicated in Table 2.24.

TABLE 2.24. Fecal abundance classes and codes

Code	Class	Description
X	None	No feces or fauna observed
F	Few	Fecal droppings or fauna occasionally observed, but scattered
C	Common	Droppings or fauna commonly observed
A	Abundant	Droppings or fauna frequently observed (droppings in relatively large numbers throughout the horizon)

21. Roots

Since root distribution in organic horizons differs substantially from that in mineral soils, the abundance and size classes and the reference unit areas are somewhat different from those used for mineral horizons (Table 2.25). Record the most abundant size first; secondary roots can be recorded by using a slash (/) in the columns as shown below:

Example: Abundant fine and plentiful medium roots.

ROOTS
AB.	SIZE
A/P	F/M

TABLE 2.25. Root abundance and size classes and codes

Size class	v. fine	fine	medium	coarse	very coarse
Code	V	F	M	C	K
Size (mm)	< 1	1-2	3-5	6-15	> 15

Abundance code and class	Reference area
	----------------25 cm2----------------	----------------------100 cm2--------------------

X None	0	0	0	0	0
F Few	< 10^a	< 10	1	1	1
P Plentiful	10-50	10-50	2-10	2-5	2-5
A Abundant	> 50	> 50	> 10	> 5	> 5
^a Values observed in reference area represent number of roots of size class

22. pH

Record pH, noting the method of measurement in the column header (e.g., pH/3 for Hellige-Truog) (Table 2.26), and the determined values for each horizon to one decimal place.

TABLE 2.26. Codes for methods of pH measurement

Code	Method	Code	Method
1	Bromothymol blue	6	pH meter (0.1 M CaCl2)
2	Cresol red	7	Phenol red
3	Hellige-Truog	8	Soiltex
4	Lamotte-Morgan	9	Thymol blue
5	pH meter (H2O)	10	pHydrion
5a	pH meter for ground water sample	11	Litmus paper

23. Comments Section

Record any observations or measurements that are unique, unconforming, or could be of particular significance to the study, classification, or management interpretations. Examples include: consistence, character, faunal species, colour of mycelium, percentage of decaying wood, presence of charcoal, and disturbance history. When coding a property, be sure to note the property being described.

Consistence:
This describes the nature and strength of forces holding materials together. It is determined by the kind of deformation or rupture that occurs when pressure is applied and then released. Use the codes in Table 2.27 to describe consistency.

TABLE 2.27. Consistency classes and codes

Code	Class	Description
LO	Loose	Material has no consistence
FR	Friable	Material crumbles easily under gentle pressure
FM	Firm	Material can be crushed under moderate pressure; resistance is noticeable
PL	Pliable	Material is soft and plastic
RE	Resilient	Material is springy or elastic; assumes its original shape after pressure is released
TE	Tenacious	Material is cohesive and not easily pulled apart

Character:

This describes tactile qualities, particulate shapes, and other noteworthy qualities of materials in organic horizons. Determining the character requires a qualitative examination of the fabric. Use the codes in Table 2.28 to describe character.

TABLE 2.28. Character classes and codes

Code	Class	Description
MS	Mushy	Soft and spongy tactility; materials wet or saturated
MK	Mucky	Smooth and sticky tactility; materials usually wet; silt- and clay-sized mineral particles usually present
GR	Greasy	Smooth and greasy tactility; materials easily workable when moist; fine mineral particles are usually absent
GT	Gritty	Rough tactility produced by mineral granules or coarse fragments
LF	Leafy	Tactility of materials produced by deciduous foliage showing a shingle-like layering (banded structure)
GA	Grassy	Tactility of materials produced by graminoid remains
MO	Mossy	Tactility produced by bryophytes with more or less preserved vegetative structures
AC	Acerose	Tactility produced by particles having a tip, such as the needles of conifers
FE	Felty	Tactility produced by abundant fungal mycelia
FI	Fibrous	Tactility produced by an abundance of fibrous plant residues which do not break down when rubbed between fingers (i.e., fine roots)
LG	Ligneous	Tactility produced by coniferous or deciduous wood fibres
CR	Crusty	Hard and brittle tactility of dry or desiccated materials

Fauna:

When describing soil fauna, use the name (Figure 2.3), e.g., few earthworms, several nematodes.

FIGURE 2.3. Major kinds of soil fauna.

soil34.jpg (42546 bytes)

Label Fauna Label Fauna

A Mites (Acarina) G Woodlice (Isopoda)

B Springtails (Collembola) H Centipedes and millipedes (Myriapoda)

C Spiders (Araneida) I Termites (Isoptera)

D Fly larvae (Diptera) J Earthworms (Lumbricida)

E Beetles and larvae (Coleoptera) K Potworms (Enchytraeida)

F Ants (Hymenoptera) L Nematodes (Nematoda)

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Label	Fauna	Label	Fauna
A	Mites (Acarina)	G	Woodlice (Isopoda)
B	Springtails (Collembola)	H	Centipedes and millipedes (Myriapoda)
C	Spiders (Araneida)	I	Termites (Isoptera)
D	Fly larvae (Diptera)	J	Earthworms (Lumbricida)
E	Beetles and larvae (Coleoptera)	K	Potworms (Enchytraeida)
F	Ants (Hymenoptera)	L	Nematodes (Nematoda)