Retisols

[FreddyNachtergaele]

FREDDY NACHTERGAELE wrote

I trust you have read the historical reviews on the use of criteria for classification of WRB RSGs from 1974 to 2022 (on the KU Leuven WRB website). In the 6 editions of related classification systems (FAO and WRB), the differences in criteria and limits used for diagnostic horizons and definitions of RSGs accepted by “consensus” by these different groups of high-level experts is extremely high. Consensus seems to be a very flexible and changing matter. Decisons concerning the Retisols are a case in point.

PETER SCHAD replied

I can only contribute with my personal memory. And I remember a broad consensus in the WRB Board. But the two decisions were different. The Fluvisols was just one decision. The Retisols was an evolutionary process with suggestions, decisions, new suggestions, re-decisons etc. (meetings in Freising, Ulm, Rome and Hannover).

Albeluvic tonguing and retic properties required and require a marked textural difference between the lighter-coloured and the intensively-coloured parts. This has often been overlooked. People saw the different colours and concluded Albeluvisol and Retisol without checking if the texture is different. Many of the soils called Albeluvisol or Retisol in fact always have been Stagnic Alisols, Stagnic Luvisols or Stagnosols, because the lighter-coloured and the intensively-coloured parts have the same texture. They are not Retisols, and this is not because of the position in the key, but because they fail criterion 4 of the retic properties (criterion 4 of the albeluvic tongues). ***

FREDDY NACHTERGAELE replied and PETER SCHAD commented in his text (in bold)

I am not sure why you state that the textural difference between the glossae and the argic horizon is often not fulfilled.
If there is a clear albic horizon over an argic with glossae, one would safely presume that the texture in the glossae would be the same as in the albic. The problem occurs when the albic horizon has been ploughed and one is only left with claric material in the argic. I doubt that routine sampling would involve separating the glossae from the argic for textural determination. The difference in field texture and the colour would suffice in that case.

I disagree. One can only claim retic properties, when the texture difference has been proven and not presumed. The parts with light colour in the argic horizon may have two reasons: (1) interfingering of an E horizon that has lost iron oxides together with clay minerals due to clay migration and/or (2) redox processes. Only if the first process has happened, we have retic properties. The first to do is hand texturing. The WRB Field Guide requires that you describe coarser-textured parts and finer-textured parts in the same layer, separately, in order to decide on retic properties. The second step is the lab analysis. Of course, you have to sample and analyze the coarser-textured and finer-textured parts, separately.

Three decisions made since 2014 have made the classification of Retisols indeed problematic: (1) the raised clay increase required to have an argic horizon (2) allowing albeluvic glossae in Stagnosols (3) the weakened requirements to qualify for a Stagnosol.

a) Yes, but this is relevant for all RSGs with argic horizon.
b) This was one of the wisest decisions ever made in WRB history. The splitting of soils with strong stagnic properties at shallow depth over two RSGs was not justified, neither by soil genesis, nor by soil functions. But again, I don't think that at global level this decision changed many proven Albeluvisols into Stagnosols. In the Soil Atlas of Europe (JRC, 2005), the large area of Albeluvisols in eastern Europe was never real. This was always a mixture of Luvisols, Alisols, Retisols, Stagnosols and others. It is rather rare that soils with proven retic properties develop strong stagnic properties at shallow depth, because the water runs well through the E parts of the Bt/E (or E/Bt) horizon.
c) In 2014, the requirements were made weaker, and in 2022, a bit stronger again. Now, they are at the same strength level as Gleysols which makes sense.

FREDDY NACHTERGAELE replied:
The requirements for retic properties concerning the difference in clay content between the glossae and the argic horizon are unrealistic and not practical during soil surveys. At least a field texture determination of coarser versus finer and claric colours should be acceptable as an alternative.
The Retisol process you describe is not in line with the generally major accepted one given by Langohr and Van Vliet (1982) and Dondeyne et al (2019): Glacial desiccation cracking and infilling of the tongues with overlying material – redox processes may enhance the textural difference due to ferrous soluble iron, dislodging clay bonds and clay may start moving out of the albeluvic tongues.
In WRB2022, the phrasing of the Stagnosols criteria is over-complex . I understand that a layer of 20 cm of reducing and oximorphic features is needed in a layer of 60 cm, while in WRB 2014 you needed more than half of the layer: hence for a 60 cm layer 30 cm would have reducing and oximorphic features. To me this implies a weakening of the stagnic properties required in WRB2022 in Stagnosols, not a strengthening as you write.
Most Albeluvisols occur in Russia and Canada not in Central Europe. Soil maps made then should be judged against the criteria used at the time, not against the ones created now
The bottom line remains that when the Retisols were introduced they were presented as an expansion of the Albeluvisol concept (WRB2014 p 2: ‘The only change at the Reference Soil Group (RSG) level is to replace Albeluvisols by Retisols. Retisols have a broader definition and include the former Albeluvisols’. In fact what has happened in practice was a reduction of the Albeluvisol concept to the point that they are now considered redundant.
At the same time WRB expanded the Stagnosols that are very localized (nearly never dominant at smaller scales), temporary or relict and bound to disappear in large areas with climate change.