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Soil Science Society of America Journal
TerrHum: An iOS Application for Classifying Terrestrial
Humipedons and Some Considerations
about Soil Classication
The name TerrHum is an abbreviation of the words “Terrestrial” (not hydro-
morphic, not submerged) and “Humipedon” (organic and organic-mineral
humus horizons). With this application, it is possible to describe and classi-
fy terrestrial forest and grassland topsoils in a system published as a Special
Issue entitled “Humusica 1– Terrestrial Natural Humipedons” in the jour-
nal Applied Soil Ecology. The iOS application TerrHum allows the storage of
the main content of Humusica 1 on a cellular phone. Images, diagrams and
simplied tables of classication may be recalled with a few touches on the
screen. Humus forms, representing ve humus systems, are classied based
on the vertical arrangement of diagnostic horizons and their attributes.
TerrHum allows accessing specic gures that are stored in a virtual cloud
and can be downloaded the rst time the user recalls them. Once all gures
have been opened in the device, the application is ready to use, without any
further internet connection. The application is in continuous evolution.
In 2003, 26 soil scientists gathered in Trento, Italy to standardize methods and
rules for classifying the biologically most active upper part of the soil. For do-
ing so, it was essential to develop a common terminology and to exchange
data on soil carbon dynamics and its relation to morphological characteristics of
A. Zanella*
Univ. di Padova
Dip. TESAF
Viale Dell Univ.
16, 35020 Legnaro, Italy
K. Katzensteiner*
Univ. of Natural Resources and Life
Sciences Vienna
Dep. of Forest and Soil Sciences
Peter Jordanstr. 82
1190 Vienna, Austria
J.-F. Ponge
Muséum National d’Histoire Naturelle
57 Rue Cuvier
75005 Paris, France
B. Jabiol
AgroParisTech
14 Rue Girardet
54000 Nancy, France
G. Sartori
Museo di Scienze Naturali
Corso del Lavoro e della Scienza, 3
38122 Trento, Italy
E. Kolb
Technische Univ. München
Wissenschaftszentrum Weihenstephan
Hans-Carl-von-Carlowitz-Platz 2
85354 Freising, Germany
R.-C. Le Bayon
Univ. de Neuchâtel
Lab. d’écologie fonctionnelle
Institut de biologie
Rue Emile-Argand 11
2000 Neuchâtel, Switzerland
M. Aubert
Univ. de Rouen
Lab. Ecodiv URA IRSTEA/EA 1293
SFR SCALE 4116, Bâtiment IRESE A
place E. Blondel UFR Sciences et
Techniques
F-76821 Mont Saint Aignan cedex
France
J. Ascher-Jenull
Univ. of Innsbruck
Institute of Microbiology
Technikerstr. 25d
A-6020 Innsbruck, Austria
M. Englisch
Federal Research and Training
Centre for Forests
Natural Hazards and Landscape
Dep. of Forest Ecology and Soils
Seckendorff-Gudent-Weg 8
1131 Vienna, Austria
H. Hager
Univ. of Natural Resources and Life
Sciences Vienna
Dep. of Forest and Soil Sciences
Peter Jordanstr. 82
1190 Vienna, Austria
Core Ideas
•A common humus classication
system improves communication
among soil scientists.
•A cellular phone application can
be used for global soil mapping and
monitoring purposes.
•The humus classication can
be combined with different soil
classication systems.
Soil Sci. Soc. Am. J. 83:S42–S48
doi:10.2136/sssaj2018.07.0279
Received 30 July 2018.
Accepted 18 Mar. 2019.
*Corresponding authors (augusto.zanella@unipd.it; klaus.katzensteiner@boku.ac.at).
© 2019 The Author(s). Re-use requires permission from the publisher.
North American Forest Soils Conference – International Symposium on Forest Soils
Published June 13, 2019
www.soils.org/publications/sssaj S43
North American Forest Soils Conference – International Symposium on Forest Soils
organic and organic-mineral topsoil. The definitions of organic
and organic-mineral horizons of the soil, for example, diverged a
lot. These scholars came from nine different European countries
and immediately realized that the task was not an easy one because
each country had its own historical classification systems, often in-
compatible in-between. It took more than 5 yr of meetings and
discussions to publish, on behalf of a part of the members of the
“European Humus Group”, then simply “Humus Group”, a first
synthesis (Zanella et al., 2009).
The main problem was not the existing classifications, but the
fact that for bringing together the existing pieces of knowledge to
form a frame compatible with all points of view, it was necessary
to enlarge the picture, and also to understand what the different
classifications had not considered individually. As an example, there
were some humus forms called “humus gemellare” (twin humus)
by an Italian classification. In France, these forms were considered
as “double” forms, but rather classified as Mulls (Amphimull)
because of their thick and impressive crumby organic-mineral
horizon (Jabiol et al., 1994). In other countries, these same forms
were classified as Moder, because of their very thick OH horizon
(Broll et al., 2006). Because of all these disparities, it was then
proposed to bring all these “double” forms together into a new
category called “Amphi”. This decision caused a rift between (i)
those who had seen and supported the existence of such forms of
humus, and (ii) those who instead preferred to stick to the current
classification systems without considering such forms, because they
were not very common in their respective countries. The issue was
solved only after a workshop in San Vito (2005, Italy), with field
trips to see “Amphi” and comment on it. Elsewhere, a German
member of the Humus Group published an article explaining
to his compatriots the existence of such forms in Germany, also
highlighting that they could become more frequent and therefore
be used as indicators of ongoing global warming (Graefe, 2007).
However, the story went on. When members of the Humus
Group came together to see these Amphi forms in Vienna (2004,
Austria), other humus forms already described many years before
did not match this category, and then fell more or less in oblivion
(Hartmann, 1952, 1965; Kubiëna, 1953). So are Tangel forms
(sometimes called “Alpenmoder” by local forest managers), found
at high altitude on calcareous substrates, while corresponding
Mor humus forms, with different morphological and chemical
properties, are related to siliceous bedrock and acidic soils. The
fact opened a diatribe concerning differences between Mor
and Tangel that lasted for years and was solved only recently
(Kolb and Kohlpaintner, 2018; Meynier and Brun, 2018). To
clarify the situation, we decided to set up a classification linked
to the soil forming substrate (only if it influences the topsoil:
Amphi and Tangel on calcareous substrates, Moder and Mor on
“acidic” ones, with Mull in-between in neutral or nearly neutral
situations). All members of the Humus Group were then in
agreement with this solution, except supporters of “Mor rather
without pedofauna” (French point of view) and others who
wanted instead “Mor with rather relevant pedofauna” (German
supporters who described Mor as extreme Moder forms).
One day, inevitably, the discussion on classification became
bogged down on the definition of soil (Zanella et al., 2018a,
2018g; Zanella and Ascher-Jenull, 2018) a point of contention
for more than 2 yr. For some of us, abiotic factors predominate
pedogenesis, while for others, biotic factors outshine the abiotic.
When the soil was proposed as a “digestion system”, a “belly”, with
living organisms having it inside (in their belly), the group started to
crumble. It spread to individual members when the idea came out
that soil binds life and death (for many soil scientists, philosophy
should remain separated from what they call “real science”,
“numbers”, “mathematical evidence”, even if all of us—as living
beings—are destined to die an incalculable day coming). Against
all odds, however, the group remained united, but encountered
very often many other difficulties, for example when the various
humus forms were arranged on a global scale (Zanella et al., 2018f,
2018h). It was then necessary to introduce the concept of “humus
system”, comprising ecologically and functionally similar humus
forms as well as creating new systems for collecting strange, unusual
or never surveyed humus forms (Zanella et al., 2018f ) or for those
in water (Zanella et al., 2018b, 2018c, 2018d) and in strongly
anthropogenics-shaped environments (Zanella et al., 2018l).
Nevertheless, there are still many questions that may be
resolved in the near future. New methods at the frontier of science
in soil biology can answer issues linked to co-evolution of soil
and organisms, biogeography of soil organisms, soil biodiversity
etc. Anyway, all these questions give a huge importance to the
soil and may be reflected in its morphological characteristics.
Humans need to better know the soil, but what about “humus”?
Recently it has been suggested to abandon the use of this term,
because “humus” cannot be clearly characterized from a chemical
point of view (Lehmann and Kleber, 2015). We strongly suggest
to keep the term and to speak of “humus” when organic particles
and accompanying biota are present.
HOW DOES TERRHUM WORK?
Let us consider a user facing a soil profile to be classified. A
cubic hole (50 × 50 × 50 cm) dug into the ground is generally
sufficient for studying humus systems and forms in a forest
environment, while a larger hole or many well-distributed small
holes are necessary for a representative survey of a heterogeneous
area (Zanella et al., 2018k). A detailed description of the main
actors of soil biodegradation and their relationships with
the horizontal and vertical distribution of humus horizons is
reported in (Zanella et al., 2018e).
As a humus form is made of superposed humus horizons,
the application asks the user to indicate one by one which types
of humus horizons are present in the observed profile. Organic
horizons (OL, OF and OH, corresponding to USDA Oi, Oe,
Oa) were clearly distinguished from organic-mineral ones
(different types of A horizons) (Fig. 1).
In general, when in the field a horizon appears totally
consisting of organic remains, it is classified as an organic horizon.
It is well accepted that the organic matter (OM) composing
such horizons amounts to more than 1/3 of the total weight,
S44 Soil Science Society of America Journal
which corresponds to 20% or more organic carbon (OC). That
allows the separation of organic and organic-mineral horizons
in the field and the adjustment of the classification, if necessary,
after laboratory analyses of the OC concentration in a horizon
sample. The organic horizons are divided into OL, OF and OH
following their decreasing content of “recognizable remains”
(e.g., a leaf, a needle, a piece of bark…): more than 90% in OL,
from 90 to 30% in OF, and less than 30% in OH.
Another diagnostic criterion is, if the process of litter
transformation (decomposition) is accomplished mainly by animals
or by microbes. Respectively, the process generates zoogenic zoOF
or non-zoogenic nozOF horizons described in the application and
recognizable in the field. According to its structure and genesis,
the organic-mineral A horizon is classified in zoogenic biomacro,
biomeso and biomicro A horizons (maA, meA, miA), and in non-
zoogenic massive and single-grain A horizons (msA and sgA). All
these horizons are described and illustrated in the application.
The user is asked to answer a series of Yes/No questions in a
dichotomous key; an example: “is OH horizon present”? (Fig. 2;
Fig. 3a-d). Slightly differing from the application, the simplified
key (Fig. 2) requires some field experience, but allows a faster,
equivalent, correct classification. For detailed definitions of all
diagnostic horizons and criteria of classification, refer to Zanella
et al. (2018i, 2018j).
Fig. 1. Basic concept of humus classication and the designation of organic layers on forest oors (O-horizons) and organic-mineral soil horizons
in the European classication of humus systems (TerrHum).
Fig. 2. Dichotomous key for identifying Terrestrial Humus systems and forms. The rst bifurcation divides “3. Terrestrial” (described in TerrHum)
from “1. Histic and Aqueous” or “2. Para” systems (1 and 2 are described in Zanella et al. 2018b and Zanella et al. 2018f, respectively).
www.soils.org/publications/sssaj S45
A touch-button located at the bottom of the screen allows
the user to recall at any time definition and photographs of each
diagnostic horizon, allowing users to more accurately define the
real horizon (Fig. 4a-d).
Other keys at the bottom of the screen allow retrieving
examples of humus systems and forms (Fig. 5), as well as tables
of composition and/or classification of humus horizons, or
groups of animals and their droppings (Fig. 6). At the end of the
classification process, a photograph of the target humus form
appears, along with a list of the chosen horizons.
By touching the screen, each photograph may be magnified
(Fig. 3d; Fig. 4b; Fig, 5b; Fig. 6b). A caption at the bottom of
each picture provides access to the morpho-functional features
of the soil profile and leads finally to the classification of the
humus system.
Many examples of soil horizon features are documented in
the application. Future updates of the application will include
useful links to enriched external data banks. We are currently
collecting information and photographs to further improve the
classification of charcoal, charred mass and other dark-colored
sublayers that can interfere with the definitions of standard
Fig. 3. TerrHum screens. (a) Initial screen. The main key of classication is accessible by touching the red button (“Yes/No” key). In addition, four
options allow accessing to illustrations of humus horizons, forms, systems and types of transition between organic and organic-mineral horizons. The
third option (“Systems and Forms”) allows opening specic tables containing helpful specic information for the classication (e.g., percentage of
recognizable remains in different diagnostic horizons). The last option (“About TerrHum”) corresponds to a link to an external site, where the user
may nd a complete manual of the application; (b) A single click on the “Yes/No” key (red button) opens a new window where a Yes/No question is
proposed to the user; touching the screen on either “Yes” or “No” keys and subsequently on the “Next” red button activates a series of further Yes/
No questions; (c) At the end of the series of answers, a humus form is proposed as solution, along with the list of horizons chosen during the run/
classication process; (d) By clicking on the proposed image it is possible to magnify the photograph and, in addition, to see other examples of the
same humus form, in different environments.
Fig. 4. TerrHum screens. (a) Clicking on the rst touch-button “Horizons” (Fig. 3a) allows to see examples of diagnostic horizons; (b) Example of
the result of clicking on “zoOH horizon”; (c) By clicking on the touch-button named “O/A transitions”, the user may display examples of passages
between O and A horizons; (d) An example of a very sharp O/A transition.
S46 Soil Science Society of America Journal
diagnostic horizons (e.g., Ponomarenko et al., 2018). We also are
collecting new photographs and drawings of soil animals, related
to the assessment of soil biological quality.
IS IT POSSIBLE TO INTEGRATE TERRHUM
INTO DIFFERENT SOIL CLASSIFICATION
SYSTEMS?
The purpose of Te r rH u m is to share a morpho-functional
classification of topsoils at global level. All over the world,
students, researchers, forest managers may recognize humus forms,
take a georeferenced photograph and send it to one of the authors
of this article. Collected and checked data may help to improve
published maps (Zanella et al., 2018h) or to prepare new soil and
humus maps. The photographs being georeferenced, it is relatively
easy to create new maps if the number of points is sufficient.
Fire is an integral process of carbon transformation and
pyrogenic features of humus horizons are currently overlooked,
and still have to be incorporated into the Te r r Hu m system.
Adding these features to the Terrhum application may enable
collecting an important layer of information on past fire events
at both local and global levels.
An Android version of Te r rH u m for smartphones will be
available in the near future.
Te r r Hu m (iOS) and/or the up-coming Android version or
in general the concept of Te r rH u m can be an example also for
soil classification. For this purpose, it would be important to
adopt a soil model subdivided into Humipedon, Copedon and
Lithopedon published earlier (Zanella et al., 2018i, 2018j) We
Fig. 5. TerrHum screens. (a) Clicking on the third touch-button “Systems and Forms” (Fig. 1a) opens a list of the latter categories; (b) By selecting
one of the denitions listed in the rubric “Humus Forms”, e.g., Leptoamphi, a photograph of this humus form appears on the screen; which can be
magnied by clicking on it; two other photographs are accessible by sweeping the screen with a nger; (c) By clicking on the fourth touch-button
called “Help” (Fig. 3a), the user can display information (“Help”) concerning groups of animals (c) and produced soil aggregates (d).
Fig. 6. TerrHum screens. (a) Selecting the fourth touch-button “Help” (Fig. 3a) opens the related help information; (b) By selecting “Pedofauna and
droppings” and then, e.g., Arthropods, two photographs of these animals collected in Petri dishes may be recalled and magnied. In addition, schemes
with info about the composition of humus horizons (c), or tables of humus systems and forms (d) are available.
www.soils.org/publications/sssaj S47
programmed to work on applications like this for Copedons
and Lithopedons. Histosols are also considered as humus forms
and will be later integrated in Te r rH u m . People interested in
the project are kindly asked to contact the authors of this paper.
Given Humipedons, Copedons and Lithopedons could then be
associated to provide a full profile and name for each type of soil.
This is the reason we would like to widen the concept of
soil. We would like to divide the soil profile into three parts, each
of them being studied either separately or brought together to
better understand the overall soil functioning (Fig. 7; Fig. 8). We
know that this concept may hurt some among the pedologists,
however, it is crucial to understand that each humus form
functions relative to soil and vegetation but has also its own
spatiotemporal scale of formation, functioning and dynamics
(Bernier and Ponge, 1994).
The soil may be divided into three layers which may be
described and classified relatively independent each from each
other: Humipedon, Copedon and Lithopedon. The first
depends mainly on the source of organic matter and animals
and microorganisms that live in the soil: they generate the
Humipedon composed of organic (OL, OF and OH) and
organic-mineral (A) horizons (Hole, 1981).
Beyond these, in the most evolved soils, different mineral
horizons like E and B can be distinguished. Their development,
though linked to root dynamics and turnover of organic matter
is strongly dependent on physical and chemical soil processes. As
for those belonging to the Humipedon, there are different types
of Copedon horizons, according to climate and parent material
(Muhs et al., 2001).
The bottom of the profile completely depends on the
original rock. These are the fragmented C horizons and the
hard R rock layer. There are different types of Lithopedon
(e.g., carbonatic, silicatic; all possible morpho-functional types
or more or less transformed initial rocky basements). The
Lithopedon is not always at the bottom of a well-differentiated
soil profile. A very young soil is just a thin “biofilm” laying on a
Lithopedon, like in deglaciated environments (Wynn-Williams,
1996). Then Humipedon and Lithopedon are built-up, the soil
profile evolving toward a complete sequence of Humipedon,
Copedon and Lithopedon (Fig. 5). Occasionally the soil can
either lose some of its more superficial parts by erosion (truncated
soils; Desmet and Govers, 1995) or one part grows till taking
all the place (e.g., Humipedon in submerged soils- Histosols, or
Copedon in tropical Vertisols).
Once mastered this classification, things become simpler
and easier to understand; and the definitive/detailed name of the
soil could arise from combining the names given separately to the
humipedon and the classification of the soil profile in any system.
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