<figure><img src="image_1.png" /></figure>

HAL
open science

# Humusica 1, article 5: Terrestrial humus systems and forms - Keys of classification of humus systems and forms

Augusto Zanella, Jean-François Ponge, Bernard Jabiol, Giacomo Sartori, Ekart Kolb, Renée-Claire Le Bayon, Jean-Michel Gobat, Michaël Aubert, Rein de Waal, Bas van Delft, et al.

<figure><img src="image_2.png" /></figure>

## To cite this version:

Augusto Zanella, Jean-François Ponge, Bernard Jabiol, Giacomo Sartori, Ekart Kolb, et al. Humusica 1, article 5: Terrestrial humus systems and forms - Keys of classification of humus systems and forms. Applied Soil Ecology, 2018, 122 (Part 1), pp.75-86. 10.1016/j.apsoil.2017.06.012. hal-01658808

HAL Id: hal-01658808

https://hal.science/hal-01658808

Submitted on 7 Dec 2017

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Public Domain
## Humusica 1, article 5: Terrestrial humus systems and forms – Keys of classification of humus systems and forms

Augusto Zanella<sup>a,*</sup>, Jean-François Ponge<sup>b</sup>, Bernard Jabiol<sup>c</sup>, Giacomo Sartori<sup>d</sup>, Ekart Kolb<sup>e</sup>, Renée-Claire Le Bayon<sup>f</sup>, Jean-Michel Gobat<sup>f</sup>, Michaël Aubert<sup>g</sup>, Rein De Waal<sup>h</sup>, Bas Van Delft<sup>h</sup>, Andrea Vacca<sup>i</sup>, Gianluca Serra<sup>j</sup>, Silvia Chersich<sup>k</sup>, Anna Andreetta<sup>l</sup>, Raimo Kölli<sup>m</sup>, Jean-Jacques Brun<sup>n</sup>, Nathalie Cools<sup>o</sup>, Michael Englisch<sup>p</sup>, Herbert Hager<sup>q</sup>, Klaus Katzensteiner<sup>q</sup>, Alain Brêthes<sup>r</sup>, Cristina De Nicola<sup>s</sup>, Anna Testi<sup>t</sup>, Nicolas Bernier<sup>b</sup>, Ulfert Graefe<sup>l</sup>, Ugo Wolf<sup>l</sup>, Jérôme Juilleret<sup>u</sup>, Andrea Garlato<sup>v</sup>, Silvia Obber<sup>v</sup>, Paola Galvan<sup>w</sup>, Roberto Zampedri<sup>x</sup>, Lorenzo Frizzera<sup>x</sup>, Mauro Tomasi<sup>y</sup>, Damien Banas<sup>z</sup>, Fabrice Bureau<sup>g</sup>, Dylan Tatti<sup>h</sup>, Sandrine Salmon<sup>b</sup>, Roberto Menardi<sup>a</sup>, Fausto Fontanella<sup>a</sup>, Vinicio Carraro<sup>a</sup>, Diego Pizzeghello<sup>a</sup>, Giuseppe Concheri<sup>a</sup>, Andrea Squartini<sup>a</sup>, Dina Cattaneo<sup>a</sup>, Linda Scattolin<sup>a,†</sup>, Serenella Nardi<sup>a</sup>, Gianni Nicolini<sup>A</sup>, Franco Viola<sup>a</sup>

<footnote><sup>a</sup> University of Padua, Italy</footnote>

<footnote><sup>b</sup> Muséum National d'Histoire Naturelle, Brunoy, France</footnote>

<footnote><sup>c</sup> AgroParisTech, Nancy, France</footnote>

<footnote><sup>d</sup> Museo delle Scienze, Trento, Italy</footnote>

<footnote><sup>e</sup> Technical University of Munich, Germany</footnote>

<footnote><sup>f</sup> University of Neuchâtel, Switzerland</footnote>

<footnote><sup>g</sup> Normandie Université, Rouen, France</footnote>

<footnote><sup>h</sup> Bern University of Applied Sciences, Zollikofen, Switzerland</footnote>

<footnote><sup>i</sup> University of Cagliari, Italy</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: augusto.zanella@unipd.it (A. Zanella), ponge@mnhn.fr (J.-F. Ponge), bernard.jabiol@agroparistech.fr (B. Jabiol), giacomo.sartori@sfr.fr (G. Sartori), kolb@wzw.tum.de (E. Kolb), claire.lebayon@unine.ch (R.-C. Le Bayon), jean-michel.gobat@unine.ch (J.-M. Gobat), michael.aubert@univ-rouen.fr (M. Aubert), rein.dewaal@wur.nl (R. De Waal), bas.vandelft@wur.nl (B. Van Delft), avacca@unica.it (A. Vacca), lserra@tiscali.it (G. Serra), silvia.chersich@gmail.com (S. Chersich), anna.andreetta@unifi.it (A. Andreetta), raimo.kolli@emu.ee (R. Kölli), jean-jacques.brun@irstea.fr (J.J. Brun), nathalie.cools@inbo.be (N. Cools), michael.englisch@bfw.gv.at (M. Englisch), herbert.hager@boku.ac.at (H. Hager), klaus.katzensteiner@boku.ac.at (K. Katzensteiner), alain.brethes@orange.fr (A. Brêthes), kridn@libero.it (C. De Nicola), anna.testi@uniroma1.it (A. Testi), bernier@mnhn.fr (N. Bernier), ulfert.graefe@ifab-hamburg.de (U. Graefe), ugo.wolf@unifi.it (U. Wolf), jerome.juilleret@list.lu (J. Juilleret), agarlato@arpa.veneto.it (A. Garlato), obbber@arpav.it (S. Obber), paola.galvan@gmail.com (P. Galvan), roberto.zampedri@fmach.it (R. Zampedri), lorenzo.frizzera@fmach.it (L. Frizzera), tomasi@panstudioassociato.eu (M. Tomasi), damien.banas@univ-lorraine.fr (D. Banas), fabrice.bureau@univ-rouen.fr (F. Bureau), dylan.tatti@bfh.ch (D. Tatti), sandrine.salmon@mnhn.fr (S. Salmon), roberto.menardi@unipd.it (R. Menardi), fausto fontanella@unipd.it (F. Fontanella), vinicio.carraro@unipd.it (V. Carraro), diego.pizzeghello@unipd.it (D. Pizzeghello), giuseppe.concheri@unipd.it (G. Concheri), squart@unipd.it (A. Squartini), dina.cattaneo@unipd.it (D. Cattaneo), serenella.nardi@unipd.it (S. Nardi), gianni.nicolini@alice.it (G. Nicolini), franco.viola@unipd.it (F. Viola).</footnote>

<footnote>† Deceased.</footnote>
j Freelance Researcher, Cagliari, Italy

k Freelance Researcher, Milano, Italy

l University of Florence, Italy

m Estonian University of Life Sciences, Tartu, Estonia

n IRSTEA, Grenoble, France

o Research Institute for Nature and Forest, Geraardsbergen, Belgium

p Bundesamt für Wald, Vienna, Austria

q Universität für Bodenkultur, Vienna, Austria

r Office National des Forêts, Boigny-sur-Bionne, France

s Università La Sapienza, Roma, Italy

t Institut für Angewandte Bodenbiologie GmbH, Hamburg, Germany

u Luxembourg Institute of Science and Technology, Belvaux, Luxembourg

v Agenzia Regionale per la Protezione e Prevenzione dell'Ambiente del Veneto, Treviso, Italy

w Freelance Researcher, Trento, Italy

x Research and Innovation Centre, Fondazione Edmund Mach, Trento, Italy

y Freelance Researcher, Bolzano, Italy

z Université de Lorraine, Nancy, France

A Servizio Parchi, Provincia Autonoma di Trento, Italy

Keywords: Humus; Humus systems; Humus forms; Humus classification; Terrestrial humus forms; Humusica

ABSTRACT

This article is an as simple as possible key of classification of terrestrial (aerobic, not submerged) topsoils (organic and organic-mineral series of soil horizons). Based on the introduction exposed in Humusica 1, article 1, and using vocabulary and definitions listed in article 4, a classification is proposed for better understanding the biological functioning of the soil, partially disclosing the process of litter digestion. Five types of terrestrial topsoils, called terrestrial humus systems, are described and illustrated with the help of photographs. Within each humus system, 3-4 humus forms are also revealed, corresponding to similar series of soil horizons generated in a relatively homogeneous environment whose range of ecological factors is not so large to overstep and cause
the genesis of another different humus system. The article ends with a figure that shows the relationship between Tangel and Amphi humus systems, and a dichotomous key of classification that one can easily print and bring in the field for practicing humus classification.
## Foreword

Even if published as an independent article, if you are not accustomed to soil or humus field classification, this paper lacks of basic information you can find in:

*   **Humusica 1, article 1:** Essential bases – Vocabulary (Soil and humus profiles and horizons, Humus systems and forms classifications, historical overview...);

*   **Humusica 1, article 3:** Essential bases – Quick look at the classification (for beginners);

*   **Humusica 1, article 4:** Terrestrial humus systems and forms – Specific terms and diagnostic horizons.

Humusica recovers keys of classification published in preceding works (Zanella et al., 2011a, b; Jabiol et al., 2013), which are still valid but incomplete. Here an enlarged group of authors updated the old units, created few new references and better illustrated the whole.

## 1. Key of classification of humus SYSTEMS

On a morpho-functional basis, Terrestrial humipedons are subdivided in five systems (Mull, Moder, Amphi, Mor and Tangel), hereafter identified and described based on diagnostic features.

Essential legend (complete definition in Humusica 1, article 4): biomacro A = biomacrostructured A horizon; biomeso A = biomesostructured A horizon; biomicro A = biomicrostructured A; zoOF or OF = zoogenic OF horizon; nozOF = non zoogenic OF horizon. OH= implied zoOH (zoogenic OH) and/or possible szoOH (slightly zoogenic OH) horizons.

Caution: "and" written at the end of a phrase means that the exposed preceding diagnostic criteria are not sufficient and need to be completed with others; "or" reported between criteria allows to select among them. The sign ";" is used between two sentences and indicates that the process of classification is not finished.

### 1.1 Mull

To be identified as Mull, a topsoil must display the following properties:

1. absence of any OH horizon; and

2. presence of biomacro A;

or

2. Presence of biomeso A and at least two of the following:
* presence in the A horizon of living earthworms or their casts, except in frozen or desiccated soil;

* presence of a very sharp transition (< 3 mm) between organic and organic-mineral horizons;

* pH<sub>water</sub> of the A horizon ≥ 5.

Correct lecture/interpretation for Mull:

1) must be without OH horizon; and

2) must show biomacro

or

3) biomeso A horizon and two of the listed three criteria.

## 1.2 Moder

To be identified as Moder, the topsoil must display the following properties:

1) presence of an OH horizon (even if sometimes discontinuous); and

2) absence of nozOF; and

3) absence of biomacro A; and one of the following:

* no sharp transition OH/A horizon (transition ≥ 5 mm);

* pH<sub>water</sub> of the A horizon < 5;

or

3) presence of biomeso A or biomicro A, or A single-grain or (rare, in case of intergrades to Mor) A massive, and one of the following:

* no sharp transition OH/A horizon (transition ≥ 5 mm);

* pH<sub>water</sub> of the A horizon < 5.

## 1.3 Amphi

To be identified as Amphi, the topsoil must display the following properties:

1) simultaneous presence of OH and biomacro or biomeso A horizons; and

2) absence of nozOF; and

3) thickness of A horizon ≥ thickness of ½ OH horizon; and
4) absence of massive or single-grain A; and

5) presence of biomacro A and one of the following:

* living earthworms in the A horizon;

* sharp transition between A and OH;

* pH<sub>water</sub> of the A horizon ≥ 5,
or

5) presence of biomeso A and one of the following:

* living earthworms in the A horizon;

* no sharp transition between OH and A;

* pH<sub>water</sub> of the A horizon ≥ 5.

## 1.4 Mor

To be identified as Mor, the topsoil must display the following properties:

1) never biomeso or biomacro or biomicro A horizon; and

2) presence of nozOF and one of the following properties:

* pH<sub>water</sub> of E or AE or A horizon < 4.5;

* A absent, or massive A, or single-grain A,
or

2) presence of OH horizon in very sharp (< 3 mm) transition to A, AE or E horizon and one of the following properties:

* pH<sub>water</sub> of E or AE or A horizon<4.5;

* A absent, or massive A, or single-grain A.

## 1.5 Tangel

To be identified as Tangel, the topsoil must display the following properties:

1) Organic zoogenic horizons present and thick (zoOF + OH)>10 cm; and

2) nozOF absent; and

3) Hard limestone and/or dolomite rock fragments in or at the bottom of the humus profile; and
4) A horizon absent or present. If present:

4) Biomeso A; and A < 1/2 OH

or

4) Massive A horizon and both the following:

* A<1/2 OH;

* pH<sub>water</sub> of A ≥ 5

The name of a humus system is always written with capital letters, or with a beginning capital letter.

Example: TANGEL or Tangel, never tangel.

## 2. General character and distribution of the humus SYSTEMS

It is very useful to associate an ecological frame of genesis and development to each humus system. It allows beginners to avoid serious errors of classification. We reported main ecological conditions, dominant actors of biodegradation, actors' actions, pH<sub>water</sub> of the A horizon, key diagnostic horizons and, sometimes, concise dynamic considerations. An entire paper (Humusica 1, article 8) has been written for describing/illustrating the biological activities of humus systems.

### 2.1. General characters and distribution of Mull

* ecological conditions: temperate or tropical climate and/or nutrient-rich siliceous or calcareous parent material and/or easily biodegradable litter (C/N < 30) and/or no major environmental constraint;

* dominant actors of biodegradation: anecic and large endogeic earthworms, bacteria; actors' action: fast biodegradation and rapid disappearance of litter from the topsoil (≤ 3 years), carbon mainly allocated in the A horizon;

* pH<sub>water</sub> of the A horizon: generally ≥ 4.5;

* key diagnostic characters (morpho-functional result of specific biological activities): OH never present, biomacro or biomeso A, very sharp transition (< 3 mm) between organic and organic-mineral horizons.

**Nota Bene:** Even if a very low soil pH is observed (≤ 4.5) in the equatorial zone, temperature and moisture compensate for unfavourable soil conditions (Sanchez et al., 2003) and a very active Mull humus system occurs in all this area (Lavelle et al., 1993), except in white sand or inselberg sites (with very low base content), where Mor and Moder dominate, respectively (Hartmann, 1970; Klinka et al., 1981; Coomes and Grubb, 1996; Kounda-Kiki et al., 2008). The equatorial Mull shows a large
number of roots at its surface (it is often a Rhizo Mull), which can absorb the nutrients thanks to mycorrhizal symbiotic partners (Nasto et al., 2014). Nitrogen fixing bacteria ensure a good amount of nitrogen in the soil and compensate for the leaching effect due to intense rainfall. On the contrary of temperate and boreal soils which often lack nitrogen, tropical soils are frequently poor in phosphorus. Despite their acidity, equatorial soils may be very fertile. Their fertility depends on a closed nutrient cycle between living biomass and topsoil. This biological phenomenon explains the relative fragility of the equatorial Mull systems when the growing biomass is exported by deforestation, letting a humus system that rapidly lacks essential nutriments and collapses...

## 2.2. General characters and distribution of Moder

* ecological conditions: mild to moderately cold climate, frequently on acidic substrate;

* dominant actors of biodegradation: arthropods, epigeic earthworms and enchytraeids; fungi;

* actors' action: slow biodegradation (2-7 years), carbon stocked in both organic and organic-mineral horizons;

* pH<sub>water</sub> of the A horizon: generally < 4.5;

* key diagnostic characters: OH always present (presence includes discontinuous presence too), nozOF never present, biomicro A, massive or single grain A, gradual transition (≥ 5 mm) between organic and organic-mineral horizons.

Nota Bene: When erosion bring away organic horizons, or in case of evolution from Moder toward Mull and absence of OH horizon, it is necessary to focus on the structure of the A horizon and/or to observe equivalent humipedons in areas not altered by erosion.

## 2.3. General characters and distribution of Amphi

* ecological conditions: strongly seasonal climate conditions (dry summer or winter frost), generally on calcareous and/or dolomitic or nutrient-rich substrate; an artificial substitution of vegetation, with a consequent shift from rich and palatable broad-leaf litter (C/N < 20) to recalcitrant coniferous litter (C/N>40), leads generally to a transformation of the original Mull into Amphi (this dynamic process can also generate a Moder on acidic substrates or in cold climate conditions);

* dominant actors of biodegradation: endogeic and/or anecic earthworms in the organic-mineral horizon; arthropods, enchytraeids and epigeic earthworms in the organic horizons; fungi;

* actors' action: slow biodegradation (2-7 years), high carbon content in both organic and organic-mineral horizons;

* pH<sub>water</sub> of the A horizon: generally ≥ 5;

* key diagnostic characters (morpho-functional result of specific biological activities): OH always present, nozOF never present, thickness of A horizon ≥ ½ OH; biomicro A and sharp
transition (< 5 mm) between organic and organic-mineral horizons, or biomeso A (biomicro A
possible in addition to biomeso A) and no sharp transition (≥5 mm) between organic and
organic-mineral horizons.

2.4. *General characters and distribution of Mor*

*   ecological conditions: cold climate, and/or very nutrient-poor siliceous substrate (mostly sand or sandstone), poorly degradable litter (rich in resins and/or phenols, thick cuticle, C/N > 40);

*   dominant actors of biodegradation: fungi (mostly mycorrhizal) and other non-faunal processes;

*   actors' action: very slow biodegradation (> 7 years), highest carbon content in organic horizons;

- pH<sub>water</sub> of E or AE or A horizon < 4.5;

*   key diagnostic characters (morpho-functional result of specific biological activities): nozOF (always present but sometimes difficult to recognize especially in wet conditions), E horizon or massive A or single-grain A, very sharp transition (< 3 mm) between organic and organic-mineral (or mineral) horizons.

2.5. *General characters and distribution of Tangel*

*   ecological conditions: mountain humid climate (subalpine or upper montane belts) on hard limestone and/or dolomite rock/rock fragments;

*   dominant actors of biodegradation: epigeic earthworms, enchytraeids and arthropods within organic horizons; fungi;

*   actors' action: very slow biodegradation (> 7 years), carbon stocked mainly in organic horizons;

- if presence of A horizon: pH<sub>water</sub> of the A horizon ≥5;

*   key diagnostic characters (morpho-functional result of specific biological activities): nozOF never present but thick organic horizons [(zoOF +OH) > 10 cm], if presence of A horizon: thickness of A horizon < ½ OH; A biomeso or A massive.

In Table 1, the main diagnostic horizons and their specific features are synthetically
associated to the main Terrestrial humus systems.

3. Key of classification of humus FORMS
In this new version of the key of classification of humus forms, we added a Tangel form and the names of the three Tangel forms were changed in order to fit with the corresponding forms of an Amphi system. The prefix “Dys” (reminiscent of poor nutrient availability) was abandoned because not suited for a humus form that can be even calcareous.

Terrestrial humus forms correspond to the topsoil never submerged and/or water saturated, or only for a few days per year, having:

Step 1

1) Organic zoogenic horizons present and thick (zoOF + OH)> 10 cm; and

2) nozOF absent; and

3) Hard limestone and/or dolomite rock fragments in or at the bottom of the humus profile; and

4) A horizon absent or present. If present:

4) Biomeso A; and A < 1/2 OH

or

4) Massive A horizon and both the following:

* A < 1/2 OH;

* pH<sub>water</sub> of A ≥ 5

TANGEL (Fig. 1), and either:

a) thickness of organic horizons (zoOF + OH)>50 cm: **Pachytangel** (Fig. 2);

b) thickness of organic horizons (zoOF + OH) comprised between 15 and 50 cm: **Eutangel** (Figs. 3a and b)

c) thickness of organic horizons (zoOF + OH) < 15 cm: **Leptotangel** (Fig. 4).

OR

Step 2

1) never A biomeso or biomacro or biomicro; and

2) presence of nozOF and one of the following:

* pH<sub>water</sub> of E or AE or A horizon < 4.5;

* A absent, or A massive, or A single grain,

or

2) presence of OH horizon in very sharp (< 3 mm) transition to A, AE or E horizon and one of the following:
* pH<sub>water</sub> of E or AE or A horizon < 4.5;

* A absent, or A massive, or A single grain.

**MOR** (Fig. 5) and either:

a) nozOF continuous, OH absent: **Eumor** (Fig. 6),

b) nozOF continuous, OH present and continuous: **Humimor** (Fig. 7),

c) nozOF discontinuous and OH present and continuous: **Hemimor** (Fig. 8),

**OR**

Step 3

Other topsoils, never submerged and/or water saturated, or only for a few days per year, having:

1) OH horizon present (even if sometimes discontinuous); and

2) nozOF absent; and

3) Biomacro A horizons absent; and

4) Biomeso or biomicrostructured, or massive, or single grain A horizon present, and one of the following:

* Gradual transition OH/A horizon (transition ≥ 5 mm); or

* pH<sub>water</sub> of the A horizon < 5

**MODER** (Fig. 9) and either:

a) Biomeso A absent, OH horizon continuous and ≥ 1 cm, **Dysmoder** (Fig. 10),

b) Biomeso A absent, OH horizon continuous and < 1 cm, **Eumoder** (Fig. 11),

c) Massive or single grain A absent, OH horizon discontinuous or in pockets, **Hemimoder** (Fig. 12),

**OR**

Step 4

Other topsoils, never submerged and/or water saturated, or only a few days per year, having:

1) nozOF horizon absent; and

2) Thickness of A horizon> ½ that of OH horizon;

and either:

3) OH and biomeso A horizons present; and one of the following:
* Living earthworms (or freshly deposited earthworm faeces) in the A horizon; or

* Gradual transition (≥5 mm) between A and OH horizons; or

* pH<sub>water</sub> of the A horizon ≥ 5;

**AMPHI** (Fig. 13) and either:

a) OH horizon ≥ 3 cm, **Pachyamphi** (Fig. 14),

b) OH horizon < 3 cm, **Eumesoamphi** (Fig. 15),

or

3) OH and biomacro A horizons present; and one of the following:

* Living earthworms (or freshly deposited earthworm faeces) in the A horizon; or

* Sharp (< 5 mm) transition between OH and A horizons; or

* pH<sub>water</sub> of the A horizon ≥ 5

**AMPHI** and either:

c) OH horizon ≥ 1 cm, **Eumacroamphi** (Figs. 16a and b),

d) OH horizon < 1 cm, **Leptoamphi** (Fig. 17),

OR

Step 5

Other topsoils, never submerged and/or water saturated, or only a few days per year,
having:

1) OH horizon absent; and

2) Biomacro A horizon present; or

2) Biomeso A horizon present and at least two of the following:

* Presence in the A horizon of living earthworms or their casts, except in frozen or desiccated soil;

* Presence of a very sharp transition (< 3 mm) between organic and organic-mineral horizons;

* pH<sub>water</sub> of the A horizon > 5

**MULL** (Fig. 18) and either:

a) OF horizon present and continuous, **Dysmull** (Fig. 19),

b) OF horizon missing or discontinuous and vOL horizon continuous and thick, **Oligomull** (Figs. 20a and b),

c) OF horizon missing and vOL horizon present but discontinuous, **Mesomull** (Fig. 21),

d) OF and vOL horizons missing, **Eumull** (Figs. 22a and b)
The name of a humus forms is written in a single word, beginning with a capital letter.
Example: Eumull, not Eu Mull, not Eu-Mull, not Eu-mull, not eumull.

We strongly suggest adding survey date and geographic coordinates to the name as
minimum information in a dataset.

Example July 2016 – Eumull – long +44.28.59; lat +09.41.25.

3.1. Tangel status and comparison with thickness of Amphi diagnostic horizons

It is sometimes difficult to distinguish Amphi and Tangel. They gradually pass the one into the other. Subjectively, it was decided to consider the relative thickness of A and OH horizons (Fig. 23) for distinguishing a system (Amphi) with strong biological activity in both organic-mineral (A) and organic (OH, OF, OL) horizons, from another system (Tangel) with strong activity only in the organic horizons. Amphi is generated even at low altitude, in Mediterranean climates; Tangel develops only at high altitude, in alpine or subalpine climates. We think that the low temperatures of these mountain climates (and the consequent low rate of rock weathering) does not allow the formation of mineral soil, resulting in a lack of habitat for large anecic earthworms (which live in depth during the bad season) and the evolution of the Tangel toward an Amphi humus form. Considering that temperature could be of minor importance in soil development with respect to rainfall, an alternative explanation could be that Tangels develop on carbonates and therefore on parent material that are easily dissolved, but because of their chemical composition (theoretically no Si and Al), soil minerals can only form from the impurities contained in calcite or dolomite.

3.2. Field dichotomic key of classification

This field key (Fig. 24) is elaborated starting from a French classification (Jabiol et al., 2007),
completed with Amphi and Tangel forms (Zanella et al., 2011a, 2011b), updated with new codes
horizons and slight modifications (R.-C. Le Bayon, unpublished), completed in September 2016 by A.
Zanella, J.F. Ponge, B. Jabiol and M. Auber considering Histo and Para systems, pedofauna features
and presence/absence of A diagnostic horizons.

In general, the criteria for humus system classification are assimilated by heart after few
utilisations of the indications reported in Section 1. It is a good habitude to control whether the
detected systems fit the main criteria reported in Section 2. If incoherence between systems
classified with Section 1 and described in Section 2, a second attempt of classification may be
necessary. Each humus system is shared in a few humus forms which range in intergrades and create
bridges between systems. A doubtful situation can be solved using two names of humus forms and
evaluating the surface occupied by each of them. The faster way for recognizing a humus form is to
pass through the key of Section 3, at the level of the right humus system, or go straight to the tables
with annexed photographs (Section 3).
The dichotomic field key (Fig. 24) is built considering even essential biological data. It is cautious to use biological criteria after acquiring some experience in the field, following the instructions of an expert. In *Humusica* 1, article 8, curious autodidacts may find supplementary information about pedofauna, droppings and other biological features related to each terrestrial humus system. The dichotomic field key is a very efficient mean for a rapid and sure field classification of humus systems and forms.

## Authors' contributions

A. Zanella, J.-F. Ponge, B. Jabiol, G. Sartori, E. Kolb, J.-M. Gobat, R.-C. Le Bayon, M. Aubert, R. De Waal: redaction of the text and elaboration of the key of classification.

Other authors: re-lecture and correction of the text, participation to researches and meetings, field investigations, discussions for improvements of the content of key and article.

Not cited author of photographs: A. Zanella.

## References

Coomes, D.A., Grubb, P.J., 1996. Amazonian caatinga and related communities at La Esmeralda, Venezuela: forest structure, physiognomy and floristics, and control by soil factors. Vegetatio 122, 167–191.

Jabiol, B., Zanella, A., Ponge, J.F., Sartori, G., Englisch, M., Van Delft, B., De Waal, R., Le Bayon, R.C., 2013. A proposal for including humus forms in the World Reference Base for Soil Resources (WRB-FAO). Geoderma 192, 286–294.

Hartmann, F., 1970. Gli Humus Forestali. Cedam, Padova.

Klinka, K., Green, R.N., Trownbridge, R.L., Lowe, L.E., 1981. Taxonomic Classification of Humus Forms in Ecosystems of British Columbia: First Approximation. Province of British Columbia, Ministry of Forests, Victoria.

Kounda-Kiki, C., Ponge, J.F., Mora, P., Sarthou, C., 2008. Humus profiles and successional development in a rock savanna (Nouragues inselberg, French Guiana): a micromorphological approach infers fire as a disturbance event. Pedobiologia 52, 85–95.

Lavelle, P., Blanchart, É., Martin, A., Martin, S., Spain, A., Toutain, F., Barois, I., Schaefer, R., 1993. A hierarchical model for decomposition in terrestrial ecosystems: application to the humid tropics. Biotropica 25, 130–150.
Nasto, M.K., Alvarez-Clare, S., Lekberg, Y., Sullivan, B.W., Townsend, A.R., Cleveland, C.C., 2014. Interactions among nitrogen fixation and soil phosphorus acquisition strategies in lowland tropical rain forests. Ecol. Lett. 17, 1282–1289.

Sanchez, P.A., Palm, C.A., Buol, S.W., 2003. Fertility capability soil classification: a tool to help assess soil quality in the tropics. Geoderma 114, 157–185.

Zanella, A., Jabiol, B., Ponge, J.F., Sartori, G., De Waal, R., Van Delft, B., Graefe, U., Cools, N., Katzensteiner, K., Hager, H., Englisch, M., Brêthes, A., Broll, G., Gobat, J.M., Brun, J.J., Milbert, G., Kolb, E., Wolf, U., Frizzera, L., Galvan, P., Kôlli, R., Baritz, R., Kemmers, R., Vacca, A., Serra, G., Banas, D., Garlato, A., Chersich, S., Klimo, E., Langohr, R., 2011a. European Humus Forms Reference Base.
http://hal.archivesouvertes.fr/docs/00/56/17/95/PDF/Humus Forms ERB 31 01 2011.pdf.

Zanella, A., Jabiol, B., Ponge, J.F., Sartori, G., De Waal, R., Van Delft, B., Graefe, U., Cools, N., Katzensteiner, K., Hager, H., Englisch, M., 2011b. A European morphofunctional classification of humus forms. Geoderma 164, 138–145.
## Figure captions

**Fig. 1.** Tangel system and forms. Table: diagnostic horizons in line, sequence as in real profile; humus forms in columns: Pachytangel, Eutangel, Leptotangel. Profile: Typic Tangel diagnostic horizons with very thick OF and OH horizons. Earthworms generating an A horizon may be present at the bottom in contact with the bedrock or between rock blocks (photograph of the humus profile: E. Kolb).

**Fig. 2.** Pachytangel or Bryo Pachytangel considering the moss carpet (refer to Humusica 2, article 13 for a detailed description of Bryo humus systems and intergrades to Terrestrial humus systems). Thickness of OF + OH horizons > 50 cm (photograph, E. Kolb).

**Fig. 3.** Eutangel. Thickness of OF + OH horizons about 30 cm, less than 50 cm. a) OF and OH horizons directly on hard calcareous bedrock; b) OF and OH horizons within a loose accumulation of dolomitic rock.

**Fig. 4.** Leptotangel or Bryo Leptotangel considering the moss carpet (refer to Humusica 2, article 13 for a detailed description of Bryo humus systems).

**Fig. 5.** Mor system and forms. Table: diagnostic horizons in line, sequence as in real profile; humus forms in columns: Hemimor, Humimor, Eumor. Profile: Typic Mor diagnostic horizons with sharp transition between organic OH and mineral E horizons. Common on Podzols, as in this picture.

**Fig. 6.** Eumor or Bryo Eumor, considering the overlying moss carpet (see chapter 2.2 in Humusica 2, article 13). nozOF (with yellow fungal hyphae) is dominant in the organic layer; sharp transition with an E mineral horizon at the bottom.

**Fig. 7.** Hemimor. Presence of a thick organic layer with a thick black nozOH horizon; sharp transition with the mineral light grey horizon of a Podzol.

**Fig. 8.** Hemimor. Sharp transition between a thin organic nozOH and a mineral clear E horizon.

**Fig. 9.** Moder system and forms. Table: diagnostic horizons in line, sequence as in real profile; humus forms in columns: Hemimoder, Eumoder, Dysmoder. Profile: Typic Moder diagnostic horizons with gradual transition between organic OH and organic-mineral A horizons. Common on Luvisols, as in this picture.

**Fig. 10.** a) Dysmoder. Alpine, between the dark brown OH and clear E horizons it is possible to notice the presence of a black organic-mineral A horizon, in gradual transition with the above OH horizon. b) Dysmoder. Brown organic OH horizon in gradual transition with a clearer organic-mineral A horizon in a Mediterranean forest ecosystem.

**Fig. 11.** Eumoder. Thin continuous OH horizon over a thin organic-mineral biomicrostructured A horizon, in a Mediterranean forest ecosystem.

**Fig. 12.** Hemimoder. Discontinuous OH horizon laying over an organic-mineral biomicrostructured A horizon. a) Earthworms can consume all the OH horizon which becomes discontinuous; b)
two types of A horizons are often possible: dark and thin at the soil surface, clearer and thick in contact with the mineral part of the soil profile.

**Fig. 13.** Amphi system and forms. Table: diagnostic horizons in line, sequence as in real profile; humus forms in columns: Leptoamphi, Eumacroamphi, Eumesoamphi, Pachyamphi. Profile: Typic Amphi diagnostic horizons with biological organic OH and organic-mineral A horizons.

**Fig. 14.** Pachyamphi. Thick brown zoOH horizon in gradual transition to an organic-mineral biomesostructured A horizon, (unfortunately the structure is not visible on the picture) in a Mediterranean forest ecosystem.

**Fig. 15.** Eumesoamphi. Thick but < 3 cm black organic zoOH horizon in gradual transition to a brown-grey thick organic-mineral biomesostructured A horizon; In an Alpine pure spruce forest, on calcareous lithopedon.

**Fig. 16.** Eumacroamphi. Large aggregates in a grey organic-mineral biomacrostructured A horizon, overlaid by a black OH horizon. a) In a broadleaf and coniferous forest, b) in a beech forest, both in the Alps on calcareous lithopedon.

**Fig. 17.** Leptoamphi. Like a Mull, but with a thin OH horizon covering the biomacrostructured A horizon. In an Alpine beech forest.

**Fig. 18.** Mull system and forms. Table: diagnostic horizons in line, sequence as in real profile; Humus forms in columns: Names of humus forms in Mull system: Eumull, Mesomull, Oligomull, Dysmull. Profile: Typic Mull diagnostic horizons, absence of OH horizon, gradual change in the colour of the A horizon, darker at the top.

**Fig. 19.** Dysmull. Presence of a continuous OF horizon overlying a biomacrostructured A horizon.

**Fig. 20.** Oligomull. a) Presence of a discontinuous OF horizon overlying a biomesostructured A horizon. b) Oligomull. Presence of pockets of OF horizon.

**Fig. 21.** Mesomull. Absence of any OH and OF horizons. Presence of a continuous OL horizon (grass leaves in this case) and a discontinuous vOL horizon.

**Fig. 22.** Eumull. a) absence of OH, OF and vOL horizons, presence of a discontinuous nOL and a crumby mA horizon visible even at the surface. b) Presence of a biomacrostructured mA horizon. The horizon is generally darker at the surface because the numerous anecic earthworms living in this humipedon progressively integrate the litter in the underlying soil by moving vertically through the soil profile. c) typical biomacrostructure of a Mull A horizon.

**Fig. 23.** Amphi and Tangel. Amphi and Tangel can be distinguished considering the relative thickness of A and OH horizons. Amphi =thickness A ≥ OH/2; Tangel = thickness OH > 2 x A. Tangel can also be without an A horizon.

**Fig. 24.** Dichotomic key of classification of Terrestrial Humus systems and Forms. The first bifurcation shares (or separates) Terrestrial from Histic, Aqueous and Para systems. Specific articles have been prompted for these hydromorphic or specialized complex systems, which are collected in Humusica 2, articles 9, 12 and 13, respectively. Slightly different from the keys reported in
the text of this article, this simplified Terrestrial field key requires some field experience but
allows a faster, equivalent, correct classification. Legend: zo= zoogenic; noz = non zoogenic;
szo =slightly zoogenic. Example: nozA corresponds to a non-zoogenic A horizon, which groups
msA (massive A) and sgA (single grain A). For rigorous definitions of all diagnostic horizons
and criteria of classification, please refer to Humusica 1, article 4, in which pictures and
tables solve a large number of doubts raised during field activities of humipedon
classification.
<table><caption>**Table 1**
Diagnostic horizons and features of the five Terrestrial humus systems (five references = biological activity types). In the table, the adjectives "active" or "inactive" refer to the presence or absence of living organisms in the diagnostic horizons.</caption><thead><tr><th>Diagnostic horizons</th><th>MULL</th><th>MODER</th><th>AMPFI</th><th>MOR</th><th>TANGEL</th></tr></thead><tbody><tr><td>OL<br>OF</td><td>possible<br>possible, zoogenically<br>transformed</td><td>present<br>present, zoogenically<br>transformed, active, with living<br>organisms</td><td>present<br>present, zoogenically<br>transformed, active, with living<br>organisms</td><td>present<br>present, zoogenically<br>sometimes discontinuous; zoogenically transformed possible<br>(accompanied), inactive or partially active<br>present or absent, if present: inactive or partially active</td><td>present<br>present, zoogenically<br>transformed, active, with living<br>organisms<br>present, active, thick (but ≤ 2<br>times thickness of A)</td></tr><tr><td>OH</td><td>absent</td><td>present, active, sometimes<br>discontinuous</td><td>present<br>present, zoogenically<br>transformed, active, with living<br>organisms</td><td>present<br>present, zoogenically<br>sometimes discontinuous; zoogenically transformed possible<br>(accompanied), inactive or partially active<br>present or absent, if present: inactive or partially active</td><td>present<br>present, zoogenically<br>transformed, active, with living<br>organisms<br>present, active, thick (but ≤ 2<br>times thickness of A)</td></tr><tr><td>Transition O/A or O/<br>AE or O/E</td><td>very sharp<br>(< 3 mm)</td><td>not sharp<br>(≥ 5 mm)</td><td>if A biomacro: sharp (< 5 mm)</td><td>very sharp (< 3 mm)</td><td>present<br>present, zoogenically<br>transformed, active, with living<br>organisms<br>present, active, thick (but ≤ 2<br>times thickness of A)</td></tr><tr><td>A</td><td>biomacro or biomeso<br>A (aneatic and endogeic<br>earthworms)</td><td>biomeso or biomicro or single<br>grain or massive<br>OF (feeding)</td><td>if A biomeso: not sharp (≥ 5<br>mm)<br>biomacro or biomeso, biomeso<br>accompanied by biomicro<br>possible<br>OF (feeding)</td><td>absent (= E) or present, if present: not zoogenic or<br>discontinuously biomicro.<br>OH (weak or traces of old activity)</td><td>absent or: present: If present:<br>massive or biomeso<br>OF (feeding)</td></tr><tr><td>Horizon of dominant<br>faunal activity</td><td></td><td>OH (accumul. droppings)</td><td>OH (accumul. droppings)</td><td></td><td>OH (feeding and accumulated<br>droppings)</td></tr><tr><td>Earthworms</td><td>Organic<br>horizons</td><td>Epigeic and<br>Anecic<br>Endogeic and Anecic</td><td>A (earthworms)<br>Epigeic<br>Endogeic and/or Anecic</td><td>Epigeic absent or rarely present<br>absent</td><td>Epigeic possible<br>Endogeic possible</td></tr><tr><td></td><td>Organic<br>mineral<br>horizon</td><td></td><td></td><td></td><td></td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>Fig. 1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2</figcaption></figure>
a

<figure><img src="image_5.png" /></figure>

b

<figure><img src="image_6.png" /><figcaption>Fig. 3</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4</figcaption></figure>
<table><thead><tr><th colspan="2">DIAGNOSTIC HORIZONS<br>Trans: O-A</th><th colspan="3">MOR</th></tr></thead><tbody><tr><td rowspan="2">HYDRO</td><td>(typical)</td><td>Hemimor</td><td>Humimor</td><td>Eumor</td></tr><tr><td>nOL</td><td colspan="3">possible and not discriminant</td></tr><tr><td rowspan="5">gOL, gOF<br>posible not<br>sufficient, gOH<br>sufficient for<br>Hydro prefix</td><td>vOL</td><td colspan="3"></td></tr><tr><td>nozOF</td><td>disc pock</td><td colspan="2"></td></tr><tr><td>zoOF</td><td>possible</td><td colspan="2"></td></tr><tr><td>szoOH</td><td colspan="3">possible</td></tr><tr><td>zoOH</td><td colspan="3">possible</td></tr><tr><td colspan="2">Transition O-A (mm)</td><td colspan="3">&lt; 3 mm (*)</td></tr><tr><td rowspan="6">gAE, gnozA,<br>gzoA (gmaA,<br>gmeA, gmiA)<br>sufficient for<br>Hydro prefix</td><td>AE, EA</td><td colspan="3">OR</td></tr><tr><td>nozA</td><td colspan="3">A absent OR sgA, OR msA</td></tr><tr><td>miA</td><td colspan="3"></td></tr><tr><td>meA</td><td colspan="3"></td></tr><tr><td>maA</td><td colspan="3"></td></tr><tr><td colspan="2">(*) mandatory &lt; 3 mm</td><td colspan="3"></td></tr><tr><td colspan="2">disc pock = discontinuous or in pockets</td><td colspan="3"></td></tr><tr><td colspan="2">Possible hydromorphic (g) terrestrial diagnostic horizons</td><td colspan="3"></td></tr><tr><td colspan="2">Terrestrial diagnostic horizons</td><td colspan="3"></td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>Fig. 5</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 6</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 9</figcaption></figure>
a

<figure><img src="image_13.png" /></figure>

b

<figure><img src="image_14.png" /><figcaption>Fig. 10</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 11</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 12</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Fig. 13</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Fig. 14</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Fig. 15</figcaption></figure>
a

<figure><img src="image_20.png" /></figure>

b

<figure><img src="image_21.png" /><figcaption>Fig. 16</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>Fig. 17</figcaption></figure>
<table><thead><tr><th colspan="2">DIAGNOSTIC HORIZONS<br>Trans: O-A</th><th colspan="4">MULL</th></tr><tr><th>HYDRO</th><th>(typical)</th><th>Eumull</th><th>Mesomull</th><th>Oligomull</th><th>Dysmull</th></tr></thead><tbody><tr><td rowspan="6">gOL, gOF<br>posible not<br>sufficient,<br>gOH<br>sufficient for<br>Hydro prefix</td><td>nOL</td><td>disc pock</td><td></td><td></td><td></td></tr><tr><td>vOL</td><td></td><td>disc pock</td><td></td><td></td></tr><tr><td>nozOF</td><td></td><td></td><td></td><td></td></tr><tr><td>zoOF</td><td></td><td></td><td>disc pock</td><td></td></tr><tr><td>szoOH</td><td></td><td></td><td></td><td></td></tr><tr><td>zoOH</td><td></td><td></td><td></td><td></td></tr><tr><td colspan="2">Transition O-A (mm)</td><td colspan="4">&lt; 3 (*)</td></tr><tr><td rowspan="6">gAE, gnozA,<br>gzoA (gmaA,<br>gmeA, gmiA)<br>sufficient for<br>Hydro prefix</td><td>AE, EA</td><td></td><td></td><td></td><td></td></tr><tr><td>nozA</td><td></td><td></td><td></td><td></td></tr><tr><td>miA</td><td></td><td></td><td></td><td></td></tr><tr><td>meA</td><td></td><td></td><td></td><td>OR</td></tr><tr><td>maA</td><td></td><td></td><td></td><td></td></tr></tbody></table>

(*) at least two of the following:

1) Presence of a very sharp transition (< 3 mm) between organic and organo-mineral horizons;

2) Presence in the A horizon of living earthworms or their casts, except in frozen or desiccated soil;

3) pH water of the A horizon > 5

disc pock = discontinuous or in pockets

Possible hydromorphic (g) terrestrial diagnostic horizons

Terrestrial diagnostic horizons

<figure><img src="image_23.png" /><figcaption>Fig. 18</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>Fig. 19</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>Fig. 20</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>Fig. 21</figcaption></figure>
a

<figure><img src="image_27.png" /></figure>

b

<figure><img src="image_28.png" /></figure>

c

<figure><img src="image_29.png" /><figcaption>Fig. 22</figcaption></figure>
<figure><img src="image_30.png" /><figcaption>Fig. 23</figcaption></figure>
<figure><img src="image_31.png" /><figcaption>Fig. 24</figcaption></figure>