The O-layer can be added to any soil profile as long as there is in fact organic material on top of the mineral soil layers underneath. It does not matter what type of soil it is on top of. So you can have regolith with or without an organic layer - depending on the situation on-site.

So, yes, I would say the O horizon is part of the regolith profile description as you have to include all horizons present.

Should be part of the "general definition" though? Probably not. Organic material is almost always present so it would need to be included in most soil type definitions, which does not make much sense.

Dear Jerôme,

The definition of "regolith" is not completely fixed, as some authors include soil in the regolith, and some authors exclude it. This question is a subject of debate. In my sense, the organic horizon should be grouped with the other horizons of soil (in the pedologic definition) and distinguished from weathering horizons.

Thanks Michael and Robert.

Michael, as soil is part of the Regolith according to Merrill (1906) definition and as topsoil (Organic horizon) are part of Soil, I agree that we can include O horizon in the Regolith profile.

However, I agree with Robert that this special upper part of the Regolith should be distinguished from pure  mineral material.

I agree with Robert, too. More generally, I am reluctant of a common definition of organic and mineral horizons as "soil type", because they do not evolve at the same rate and are not heterogeneous at the same scale. Organic horizons are, at least locally, more directly influenced by vegetation dynamics than underlying mineral horizons. Hence the concept of humus form, and its classification independent of "soil type".

I agree with Jean-Francois. Together with the mineralic A horizon is the O horizon part of the topsoil which is biologically the most active part of the soil profile and should be classified as humus form separately from the soil type. This becomes quite evident by the fact that the same humus form can occur on different soil types and the same soil type can develop different humus forms depending on changing environmental conditions.

There are different opinions. You know that many soil classifications do not include purely organic horizons such as 0, F, H on the mineral one. I think that we should study and classify the whole soil-regolith as a single entity. However the presence, absence or degradation of organic horizons can be fast, and thus the inclusion of organic horizons in soil taxonomies could be not a recommendable (very transient horizons in many environments) . But in any caser these horizons are essential parts of the soil system.

Thanks by follow me in Researchgate.

All the best

Juanjo

Yes, that's the point ! Can we exclude such an important compartment from the soil profile, given what we know of the influence of  organic matter (and associated organisms: plant roots, animals, microbes) on soil developemnt ? My answer is no, but we must consider scale effects (both in time and space). Different depth levels do not evolve at the same rate: in a first approximation the deeper the horizon (or layer) the slower the rate of change and the broader the scale of heterogeneity.

Dear Juan,

In 2002, you noted that “The question is not to know whether the soil continuum can or cannot be classified, but to know how it should be made and the purpose”.

I completly agree with you, the whole soil-regolith should be classify and as member of WRB working group and Humus Group we are working on the soil and topsoil.

I also proposed a classification system of the subsolum in WRB style to complete the taxonomy of the first 2 m of the Earth surface as it should be, the publication is accepted in Geoderma and will be available soon.

If you want to join in a specifc working group, you are of course welcome.

I should precise to  Robert, whom is also specialist in Regolith caracterisation but in a geological perspective that our proposal in more soil scientist oriented. Hence the classification is morpho-fonctional and not only based on weathering and type of material.

Best regards

Ibáñez, J.J., Boixadera, J., 2002. The search for a new paradigm in pedology: a driving force for new approaches to soil classification. In: Soil Classification 2001. E. Micheli, F. Nachtergaele R.J.A. Jones, and L. Montanarella, (eds). European Research Report No.7, p.93-110. Office for Official Publications of the European Communities, Luxembourg.

Thanks by you interest. If I could help to your WG I will be happy. Please notice thatas far as I know the WRB Soil Classification will fall down very soon being replaced by the product that must be delivering by the Universal classification WG (news from the FAO staff in Rome). In any case I agree with you.

All the best

Juanjo

Dear Juan

we should become aware of the things we are doing. If we describe a soil profile we should describe the whole soil regolith as a single entity. The organic horizons, like you said, are essential parts of the soil system. But if we classify a soil profile, we should make a difference. Is the formation of the topsoil still conform to the formation of the subsoil? Then we have an harmonic profile and no problem. Imagine, on the other hand, a drained blanket bog. The soil type will remain Histosol whether the land is used as pature or as forest. Only the topsoil changed into another system, driven by other soil organisms.

For classification purposes we should clearly define the object of classification. Topsoils or humus forms can be an object of classification apart from soil classification. The main criterion in classification of complex systems is the similarity relationship between the classified objects. I try to illustrate this in the attached graph.

All the best

Ulfert

Der Ulfert,

Completely agree with you. If you do not understand me sorry. My English is not good as you can see.

I think it should be classified forms of humus, but regardless of a soil taxonomy such as understand them. My experience in Mediterranean forests can be illustrative. In my PhD time ago (1986) I sampling in different forest types of a given mountainous range. I take a morphological classification of the humus layers above of the A horizon. In addition I carry out a biochemical characterizations of the humus of the A horizon. With 150 samples I expected to find differences between the different types of forests in biochemistry of A horizons A. However the samples close to tree trunks that were oriented toward at the top mountain (in the French terminology used in Spain) and three organic horizons L, F , H., while samples near the trunk opposite one horizon over L on the A  In addition there was also conspicuous differences in the runoff of the rainfall in the tress, with greater abundance in the trunk and the edge of the canopy. After that I carry out statistical analysis and result that such as differences affect to the humus biochemistry of the A horizon. In other words multivariate analysis show that the highest part of the variance was obtained by under a single tree (accordion to their location respect to the trunk) that between very different forest types.  Summarising the spatial variation of the Organic layers on the A horizons was terrific. A single sample do not say nothing.

Sorry my English is not good. I hope that you can understand me.    

All the best

Juanjo

You may be interested in the following publication:

Cremeens, D.L., Brown, R.B. and Huddleston, J.H. (Eds) 1994. Whole Regolith Pedology. Soil Science Society of America Special Publication 34, 136pp.

There is a chapter on regolith classification - follows Soil Taxonomy. 

Also see: Pain, C.F. 2008. Field Guide for Describing Regolith and Landforms CRC LEME, c/o CSIRO Exploration and Mining, PO Box 1130, Bentley WA 6102, Australia, 94pp. (Download from http://crcleme.org.au/Pubs/guides/regdescmeth.html)

Classification here is about materials. Years ago I considered trying to classify regolith profiles in the same way that soil profiles are classified. However, it turned out that regolith is extremely variable - which is why we went for materials classification.

I would certainly include the soil, including organic horizons, in the regolith.

Eggleton's glossary can be downloaded from: http://crcleme.org.au/Pubs/Monographs/BookRegGloss.html

http://crcleme.org.au/Pubs/guides/regdescmeth.html

Dear Colin,

thanks for your answer, happy that you answer that O horizon are also for you part of regolith (even if they are not of "lithic" origin)

I already know the Whole Regolith Pedology and especially classification of Buol.

I also used your definition as you are for me a big reference on this topic.

Regolith classification is mainly geological oriented and made by geologist, my intention is to propose a more ecofunctionning approach, based not on intensive regognition of the real weathering processes but more a field base approach for mapping purpose during soil survey, to complete classification of the first 2 m of the earth surface  (as Soil taxonomy and WRB should do)

As you I attempt to classifiy "regolith profiles" in a soil scientist perspective and with my coauthors we propose a first classification system in a WRB style way. This first attempt will probably evolved with time.

My intention was not only to classify based on lithology but to take into account the morphological and funtionalities of Regolith. Indeed, as I am working in hydropedology, hydrologist highlighted the influence of cracks network and density on subsoil waterpathway, see the work of Wrede et al (2014).

Best regards

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Definitely, We may include organic horizon or organic layers in the general definition of Regolith.Whether it is exposed to air for activating soil forming processes (as a part of solum) or permanently remains under water (as a part of parent material of overlying organic soil horizons. Because, parent material (inorganic or organic) is included in the definition of regolith.

I agree that O-horizon be taken as part of Regolith. In soil classification, the organic layer has a lot of impact on the soil type. This is better appreciated in the tropics where the organic layer of the soil is so thin. When comparing Tropical and Temperate soils, O-horizon is a key player. If not included as part of Regolith what then remains?