Elsevier

Applied Soil Ecology

Volume 122, Part 1, January 2018, Pages 10-21
Applied Soil Ecology

Review
Humusica 1, article 1: Essential bases – Vocabulary☆

https://doi.org/10.1016/j.apsoil.2017.07.004Get rights and content

Highlights

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    What is soil?

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    One soil, three functional units.

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    Basic concepts necessary for investigating the topsoil functionning.

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    Humus systems of planet Earth – general overview.

Abstract

The Special Issue Humusica 1 corresponds to a field guide for the classification of terrestrial humus systems and forms. The present first article of the issue defines vocabulary, objects and concepts necessary for: (a) field investigation, (b) understanding the process of classification, (c) assigning ecological significance to the defined morpho-functional units, (d) discussing and exchanging scientific data about humus systems. The article starts with general considerations, as the necessity humans have to classify natural objects for sharing ideas and information on them. Then the article focuses on soil as functional element of every ecosystem. Historical and recently published international definitions of soil are reported and compared to the more biological definition of the authors of the paper. Once the concept “soil” clarified, the soil profile is parted in three new sub-units, for specialised further investigations. The superficial and richer in organic matter sub-unit is labelled Humipedon. In the rest of the article, authors explain the different constituents of a general humipedon, introducing even novice soil scientists to field practice and topsoil observation. A general overview of the variety of humipedons that one may expect to find all over planet Earth, ranged in humus systems and humus forms, concludes the article.

Section snippets

Why do we classify things?

To classify objects is no more in fashion, even if the modern natural science is based on a genial Darwinian observation and classification of living organisms. We cannot understand the soil functioning without imitating Darwin and observing/classifying living soil profiles as they were living systems. We always need to compare at least two objects or concepts for understanding the world in which we stay and act. We are richer or poorer than our next-door neighbour; we cannot be rich or poor in

What’s soil?

Let us consider official definitions first:

a) Soil is organic or lithic material at the surface of planets and similar bodies altered by biological, chemical, and/or physical agents (Johnson, 1998).

b) Soil in “Soil Science Glossary 2008” (GSST, 2008): (i) The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the earth that has been

What’s humus in the soil?

“Humus” is a Latin name of Greek origin, meaning “earth, ground, soil”, sharing the same Indio-European root than “Homo” (Humans), which was introduced in soil science at the end of the 18th century for designating any kind of soil organic matter without any recognizable plant structure. This is a strictly chemical sense, which was used mainly by soil analysts when trying to desperately separate it from what they called “raw” organic matter. Thorough microscopic investigations made by Bernier

The pedon, a “sample of soil”

Pedon. If we want to study the soil and carry “pieces” of soil in a laboratory, we need to delimitate in the field one of these units of soil (Fig. 1(b)). According to Van Wambeke (1966), an ideal basic unit for soil classification should: (a) be an object which is observable and measurable in three dimensions and includes the whole vertical thickness of the soil; (b) be independent of all taxonomic systems; (c) have clear boundaries, although arbitrarily fixed; and (d) be of a size convenient

One soil, many functional units

Since many years, organisms are recognized as important factors of soil formation (Darwin, 1881, Jenny, 1941). Soil is increasingly recognized as a living entity (Sugden et al., 2004, Gobat et al., 2010, Ritz and Young, 2011). However, soil scientists have mainly studied soil for its physic-chemical properties. Even soil ecologists did not look at soil from the view point of the main biological theory: Evolution (Barot et al., 2007). In recent years, ecology and evolution became each other

Soil and humus profiles and horizons

Horizons. The natural soil of our planet is made of visible layers, called “horizons” because they gradually merge one into the other. Horizons are the result of an intense biological activity (by micro-, meso-, and macro-organisms) which interacts with physical and chemical alterations of the mineral substrate (Fig. 5).

Soil profile = side (commonly 100 × 100 cm) of a trench considered as representative of a soil (Fig. 5). Strictly speaking, the OL horizon and the R layer are not part of newly

In short: real objects and concepts

To separate real objects and concepts is a crucial phase. The first are observed and described in the field; the second are tided in mind as imaginary references, collecting many similar virtual objects.

Humus systems and forms classifications, historical overview

The humus form (a specific “form = aspect” of a humus profile within a given humus system) corresponds to the part of the topsoil that is strongly influenced by organic matter and coincides with the sequence of organic (OL, OF, OH, HF, HM, HS) and underlying organic-mineral horizons (A, AE). Plant remains like leaves, needles, pieces of wood and bark, roots, root exudates, etc., form a prominent part of the primary production of forest ecosystems.

During the 19th century, scientists noticed that

Present version of the key contents, changes and innovations

The present version of the key contents the following innovations (Fig. 7).

Concepts and real objects necessary for a clearer classification have been newly defined or better refined: humus system, humus form, humus profile, pedon, humipedon, copedon, lithopedon, humus horizon and diagnostic horizon;

Each humus system (type of process of biodegradation and consequent implementation of the litter transformation in a soil profile) is composed of 3–4 variations, called humus forms, ensuring the

General overview of humus systems and forms of planet earth

All humus systems have been set in a draft that looks like a spherical system (Fig. 7a): a nucleus made of 5 Terrestrial, 5 Histic plus 2 Aqueous “white circles” and 2 Anthropogenic “black circles”; “grey circles” representing the Para humus systems and “gravitating” around the others, as they do on the surface of our planet, occupying the place still not taken by non-Para main humus systems. Hydro and Epihisto semi-terrestrial intergrades are set at the centre of the figure. Terrestrial humus

Authors’ contribution

Zanella A., Ponge J.F.: coordination of authors’ contribution, conception and redaction of the article, definition of basic vocabulary.

Blouin M., Juilleret J., Aubert M.: contribution to the development of the concept of pedon parted in three sub-units.

Gobat J. M., Chetov O., Rubio J.L.: participation to discussions and improvement of the text.

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