A Key for the Classification of Forest Humus Types
Prepared by M. D. HOOVER AND H. A. LuNT1
A report of the Committee on Forest Humus Classification, Forest Soils Subdivision, Soil Science
Society of America
HE presence of organic layers at the surface is a
T
characteristic feature of forest soils and serves to
distinguish them from agricultural soils in which cultivation destroys the natural arrangement of organic
horizons. The forester uses the term forest humus to
cover the organic portions of the soil profile. From experience and observation, foresters have learned that
there are great differences in the humus beneath various kinds of forest stands and that these differences influence the physical, chemical, and biological properties
of forest soils. Also the character of the mineral soil
may have some influence on the type of humus. It has
been learned, further, that the type of forest humus
can be altered by management with such practices as
thinning, changing stand composition, controlled burning, and final cuttings.
Because of the importance of forest humus to forest
and watershed management, many attempts,have been
made to classify the different types which can be recognized. The first such generally acceptable classification
is accredited to P. E. Miiller (2). Revisions, new names,
and new systems have appeared at intervals since then.
This previous work has been ably reported by Romell
and Heiberg (3), Wilde (5) and Heiberg and Chandler
(1). None of the previous classification schemes was
applicable for use over wide ranges in climate, soil and
forest conditions, nor were they meant to be. Thus
classification had not been standardized, and workers
in different regions did not talk the same language
humus-wise. There is still much to learn about the
origin of the various humus types and the practical
significance of their differences as they may relate to
timber and watershed management.
The scheme proposed by Heiberg and Chandler (1)
was the product of committee work and represented
the best thinking up to that time for the Northeastern
United States. It was a marked improvement over
earlier classifications and has found wide usage. However, questions have arisen in the field because some of
Research Center Leader, Southeastern Forest Experiment Station; and Soil Scientist, Connecticut Agricultural Experiment Station, respectively. The writers wish to acknowledge the outstanding contributions of Professor S. O. Heiberg to the problems of
forest humus classification and the significance of humus types in
forest
management.
2
Members of the Forest Soils Subdivision Committee were: W.
N. Broadfoot, T. S. Coile, E. A. Colman, P. W. Fletcher, S. O.
Heiberg, H. A. Lunt, H. F. Morey, J. Retzer, R. Earl Stone, S. P.
Gessel, S. A. Wilde and M. D. Hoover. This committee was discharged at the annual meeting of the SSSA, August 1951.
the descriptions were subject to several interpretations,
and some rather common humus types were not clearly
identified. An effort to remedy these faults and develop
a more usable classification was made by the Northeastern Forest Soils group in 1946 when a committee
was appointed to consider the subject. The committee's
approach was to introduce a greater degree of consistency in the type descriptions and to set up the classification in the form of a key to facilitate identification
of the various humus types in the field.
On the recommendation of the Northeastern group,
the Forest Soils Subdivision of the Soil Science Society
of America appointed a committee2 consisting of representatives from all forest regions of the United States
to work out a forest humus classification which would
be applicable, to a greater or lesser degree, to the entire
country. Members of this committee tested the Northeastern key in their localities and suggested the changes
needed to make it more useful. The present key is the
result of this coordinated committee effort. With such
a classification to serve as a frame of reference, it will
be possible to determine the practical significance and
genesis of the humus types as now identified. Further,
with the greater understanding of forest humus types
thus obtained, it will be possible to devise classifications
which will be more realistic as regards both development of humus types and their application to forest
management.
No pretense is made that this key covers all humus
types. It is believed, however, that the major categories
of Mull, Duff Mull, and Mor are universally applicable
and that further subdivisions of these major types can
A comparison of humus types of new key with those
proposed by Heiberg and Chandler.
Heiberg and Chandler (1)
Firm mull
Coarse mull
Medium mull
Fine mull
Twin mull
Granular mor
Laminated mor
Matted mor
Fibrous mor
Greasy mor
New key
Firm mull
Sand mull
Coarse mull
Medium mull
Fine mull
Twin mull*
Thick duff mull (or simply duff mull)
Thin duff mull
Granular mor
Felty mor
Greasy mor
Thin mor
Imperfect mor
*More restricted definition than previously allowed for twin mull.
HOOVER AND LUNT.' KEY FOR CLASSIFICATION OF FOREST HUMUS TYPES
be made as needed for any region. The classification
applies to forest humus occurring on well-drained and
moderately well-drained soils. It is not meant to apply
to peats nor to mineral soils where the water table is
close to the surface most of the time. Nor does it apply
to grassland or tundra vegetation types.
The basic criteria in the classification are: (1) the
presence or absence of an H layer; (2) the degree of incorporation of organic matter into the upper mineral
soil layer, and (3) the structure, thickness and organic
matter content of the H layer and/or the AI horizon.
These characteristics are recognizable in the field with
a minimum of training. Further refinements should be
avoided until shown to be significant.
Definitions of the terms used in describing forest
organic layers are:
L layer —
(Litter) the surface layer of the forest
floor consisting of freshly fallen
leaves, needles, twigs, stems, bark
and fruits. Where decomposition
and incorporation are rapid, this
layer may be very thin or absent
during the growing season. Jn standardized horizon nomenclature this
is the A00 horizon.
F layer —
A layer of partially decomposed litter still recognizable as to origin.
The A01 horizon.
H layer —
A layer consisting of well decomposed organic matter unrecognizable
as to origin. The A02 horizon.
Aj horizon — The surface mineral-soil horizon
containing incorporated or infiltrated organic matter. It is frequently very thin or absent in Mor humus
types. When present in Mor types,
there is an abrupt transition between the A! and the next underlying horizon.
Key for the Classification of Forest Humus Types (1)*
Prepared by the Committee on Humus Classification
Forest Soils Section, Soil Science Society of America.
A. No H layer; A,, horizon an intimate mixture of organic matter
and mineral soil, with gradual transition between the A! and
the horizon beneath. F layer may or may not be present.
MULL (2j,4)
1. A! essentially single-grain or massive, without aggregates.
Organic matter appears to be more or less uniformly distributed throughout.
(a) Massive and firm with generally less than 5% organic
matter by weight
Firm Mull
(b) Loose, with low to medium organic matter content
(usually less than 10%) and consisting of a mixture'of
mineral soil and organic matter as single grains. Typically on sandy soils
Sand Mull
2. A! horizon, granular or crumb structure. Concentration of
organic matter and the granular structure most pronounced
in the upper At and decrease gradually with depth.
(a) Coarse granular or crumb structure; many granules y$"
(2-3 mm) or larger. Usually 5-20% organic matter.
Coarse Mull
(b) Medium granular or crumb structure; larger granules
about 1/16" (2 mm) or slightly smaller. Wide range of
organic matter content, usually 5-30%.. .Medium Mull
•"Italicized numbers refer to explanatory notes at the end of
the key.
369
(c) Fine granuar structure; frequently has the appearance
fine black sawdust; organic matter content high, usually
over 30%
Fine Mull (6)
3. Complex mull types. Distinct structural differences between
layers within the zone of organic matter incorporation.
(a) Fine mull underlain by coarse or medium mull.
Twin Mull
B. H and F layers present with an underlying At horizon essentially similar to that of a true mull. Gradual transition from
the H to A! and mineral soil beneath. (This type possesses
some of the characteristics of both mulls and mors).
DUFF MULL (4j)
1. Combined F and H layers more than 1 inch thick.
Thick Duff Mull
2. Combined F and H layers less than 1 inch thick.
Thin Duff Mull
C. H layer present (except in 3 below). Practically no mixing of
organic matter with mineral soil. Abrupt transition from
surface organic matter to underlying horizon. .. MOR (4)
1. The H layer more than i/fc inch thick
THICK MOR
(a) The H layer has a fine granular structure.
Granular Mor
(b) H layer structureless, feels greasy when wet, but hard
and brittle when dry
Greasy Mor
(c) H layer feels and looks felty, due to presence of fungal
hyphae and/or plant residues but not living roots.
Felty Mor
2. H layer less than yz inch in thickness
THIN MOR
3. H layer lacking or present only as a thin film in depressions
IMPERFECT MOR
EXPLANATORY MATERIAL
(1) This key does not apply on areas where the upper A horizon
shows evidence of prolonged water saturation, such as mottling, peat layers, or bog conditions.
(2) Following disturbance of the forest cover, a mull may develop
on an old podsol. As a result, a remnant of a leached layer
may be present in the profile even though the layer above it
resembles the Aj of a mull. In such a case, the humus type is
typed as a mull on the basis of the characteristics of this Aj
horizon.
(3) A complete description of a mull or duff-mull type should furnish the depth of organic matter incorporation in inches.
For grouping data and reconnaissance use the following depth
classes are suggested: very shallow, less than 1"; shallow, 1-2";
deep, 2-4"; and very deep, more than 4". For example, a sand
mull with organic matter incorporated to a depth of li/fc"
would be a "Shallow Sand Mull."
(4) When it is apparent that plowing or grazing have modified or
eliminated the natural humus type, this should be indicated
by adding the letter "P" or "G" to the name of the humus
type. For example, Firm Mull-P or Firm Mull-G or Firm
Mull-PG if both plowing and grazing have caused present
conditions. On previously cultivated land, there is frequently
an old plow layer which is comparatively homogeneous
throughout but may usually be recognized by the sharp line
of demarcation at the base of the plow layer. The humus
type should be based on the characteristics of the H and/or
A! horizon, and not on the properties of the entire plowed
horizon. Grazing causes compaction of the organic horizons
and may reduce a mull with granular structure to firm mull,
or may mix the H-layer of a mor with mineral soil creating a
mull-like condition. Again, humus type should be based on
the H and/or Aj horizon, adding the letter "G" to indicate
that grazing was responsible.
(!) As stated in explanatory note No. 3 the depth of organic
matter incorporation should be given in description. The
adjectives for the depth classes should be used as prefixes in
describing the At portion of the duff-mull. For example,
"Thick Duff Mull with shallow Aj" would be used to describe
a duff-mull with F and H layers more than 1" thick and
the A! horizon 1-2" deep.
(6) Because of the high organic matter content in the Aj horizon
of fine-mull it may occasionally be confused with the H layer
of granular mor. This is particularly true when the horizon
or layer is shallow or thin. In this case, if transition to the
mineral soil horizon below is rather abrupt and the organic
content so high it cannot be determined in the field, whether
it is actually fine mull or a granular mor, the layer should be
classed as an H layer and typed as mor.
SOIL SCIENCE SOCIETY PROCEEDINGS 1952
370
MOR
DUFF MULL
MULL
L
L
M,
L
F
F
F
H
1
1
A,
.'•/•":'' V.
.. •••'_..••.
H
A .1
A
.
A,,
- '.•:'•:
12
B
A
•'
12
B
B
FIG. 1. — Diagrammatic representation of the three basic humus types. The Mull type may or may not have an F layer. Duff Mull and
Mor are distinguished from Mull by the presence of a well-defined H layer. Mor differs from Duff Mull in that there is an abrupt
transition from the H layer to the underlying mineral soil, which is essentially devoid of incorporated or infiltrated organic matter.
As shown in the sketch, a thin, rather ill-defined At horizon and/or a leached A2 horizon may be present in the Mor type.
The organic debris above the mineral soil is frequently referred to collectively as the A0 horizon.
The principal distinguishing characteristics of the
three basic humus types are illustrated in figure 1. No
attempt is made to depict any of the subdivisions, for
they cannot be satisfactorily shown in drawings, nor
can they be photographed to advantage. Rather, one
must depend upon written descriptions, mounted
samples, and field observations. Mounted samples, prepared by the cellulose acetate (or cellulose nitrate)
technic (4) with minor variations, have proven to be
very effective for student teaching and training courses
forfieldmen.
Literature Cited
1. Heiberg, S. O., and Chandler, R. F. Jr. Soil Sci. 52: 87 (1941).
2. Miiller, P. E. Berlin: Studien iiber die natiiralichen Humusformen (1887).
3. Romell, L. G., and Heiberg, S. O. Ecology 10:567 (1931).
4. Smith, Henry W., and Moodie, C. D. Soil Sci. 64:61 (1947).
5. Wilde, S. A. Forest Soils and Forest Growth. Waltham, Mass.:
Chronica Botanica Co. (1946).