Catalonia's unique soils: impressions of the WRB excursion of 2023

12/12/2023

The journey through the wonderful soilscapes of Catalonia where I got to attend animated discussions between experts, was a transformative experience. Guided by Rosa M Poch and Jaume Boixadera of the University of Lleida, our group examined 24 soil profiles scattered throughout the Catalan region: from the high Spanish Pyrenees through the arid Lleida plain to the Ebro Delta and the Mediterranean Sea. I participated to learn more about soils in the country that has been my home for over ten years, but also to renew contacts with people who love soil.

Text and photos: Kathelijne Bonne. 

A Phaeozem soil (WRB) / Mollic Alfisol (Soil Taxonomy) in Vilamacolum, Empordà.
A Phaeozem soil (WRB) / Mollic Alfisol (Soil Taxonomy) in Vilamacolum, Empordà.

Our planet's soils are complex and diverse, hence there is a need to classify them to be able to understand them, just as is done for animals, plants, minerals, molecules, etc. Most countries have their own soil classification system. But to communicate globally about land management and food production, on a planet with finite resources and a growing population, there was a need for an overarching way to name and map soils. Therefore, the International Union of Soil Sciences (IUSS) developed the World Reference Base of Soil Resources (WRB), which organizes soils into 32 Reference Soil Groups (RSGs).

The purpose of the field trip in Catalonia was to test the efficiency of the WRB and see how compatible it is with existing systems, e.g., those from the tropics and the Americas, such as the Soil Taxonomy, which is also used worldwide. "It is important that we can identify, describe and name soils according to one global system developed and supported by the international scientific community," says Stephan Mantel of the World Soil Museum in Wageningen, Netherlands.

Every year a WRB excursion is held, bringing together soil experts from all over the world. The excursion has taken place previously in South Africa, Mongolia and Iceland, and this time in Spain (Sept. 20 to 27, 2023). The bulk of the group is part of the committee authorized to edit the WRB. The others are students, observers and interested parties, like me.

But why does soil fascinate so much?

Cosmic fun

Seen from a planetary point of view, soil is the interface between our rigid rocky planet and the atmosphere that shields us from the frigid universe; a wafer-thin layer full of life that sustains all of the biosphere. That all-encompassing picture was well summarized by Marcin Świtoniak of the Nicolaus Copernicus University in Toruń, Poland: 'I see the whole cosmos here', was his answer when I asked what he saw in a reddish earth clod he was viewing through a hand lens.

This highlights the fun that soil scientists have, they're having a great time when get the chance to examine a soil in the middle of nature.

Soil scientists let loose.
Soil scientists let loose.
Red earth clod with clay illuviation (Girona).
Red earth clod with clay illuviation (Girona).

Red earth

The red soil like the one Marcin looked at are typical of the Mediterranean and we saw several in Catalonia. They are red or rust-coloured because the iron present is oxidized to rust (= the mineral hematite). The red soils we find here also have a horizon (layer) of clay illuviation (argic horizon), that is formed when clay is leached from higher horizons accumulates at a certain depth. Stefaan Dondeyne from the University of Leuven explained this to me, as I had forgotten these processes. The WRB classifies Mediterranean red soils as Luvisols (from Latin: luere, to flush, sol, soil), and more specifically Chromic Luvisols because of the reddish colour. I also encountered them months ago in Puglia, the heel of Italy. But there is more to red soil. Possibly fertility is increased by dust, blown in from the Sahara, suggests John Galbraith of Virginia Tech.

Unnatural soils

Time and climate clearly affect soils, but so does human activity. When a soil is used for years or centuries to grow grain, a thick dark layer full of humus (a mollic horizon) develops at the top, typical of grasslands and steppes. And then these soils fall into another category, that of the Black Soils, including Chernozems as in Ukraine, Kastanozems (chestnut soil) and Phaeozems, which don't occur naturally in Catalonia or on the Mediterranean, but they do in Central Asia and in the Great Plains.

So, due to farming, there are now Kastanozems (with chestnut color, see photo below) and Phaeozems in Catalonia. Actually, the human influence should be echoed in the name, says Lucia Cunha dos Anjos from Brazil.

And now a few practical things. How is the soil type or Reference Soil Group determined?

By growing grain over many years, a Kastanozem soil formed here, which does not happen naturally in Mediterranean climates. It is a Calciustoll (Mollisol) according to the Soil Taxonomy (location: Talarn, Alt Pirineu i Aran).
By growing grain over many years, a Kastanozem soil formed here, which does not happen naturally in Mediterranean climates. It is a Calciustoll (Mollisol) according to the Soil Taxonomy (location: Talarn, Alt Pirineu i Aran).

Barefoot

Soil pits are dug to expose vertical soil profiles. An ordinary person only sees a few colours, muddy ones, at first. The experts see much more, as they zoom in on the microcosm. They skillfully divide the profile into a series of horizons that differ not only in colour but also in texture (amount of clay) and structure (e.g., solid vs. granular). They loosen soil aggregates with a knife, view them with a hand lens, put them in a bath and drip chemicals on them to see what reactions occur ... the brave ones walk barefoot, which adds an extra dimension to their soil experience.

Color says a lot about the minerals and origin of the soil. Therefore, it is determined with the Muschell color scale: The argic horizon of this Luvisol has color code 2.5YR 3/6 (Hue, Chroma and Value).
Color says a lot about the minerals and origin of the soil. Therefore, it is determined with the Muschell color scale: The argic horizon of this Luvisol has color code 2.5YR 3/6 (Hue, Chroma and Value).
Following guidelines to reach the correct description.
Following guidelines to reach the correct description.

All observations are diligently written down and samples are sent to the lab for analysis (thank you Rosa, Jaume and team for doing all that in advance). Finally, with much collected data, the WRB's Key can be applied to name the soil and assign several 'qualifiers' to it. For example, the aforementioned red soil was named: "Chromic Endoprotocalcic LUVISOL (Pantoloamic, Aric, Cutanic, Epic, Ochric, Amphiraptic)." Chromic for the color, Luvisol because there is clay illuviation. And my brain creaks when I have to think of what the rest means.

Ivory tower?

I also heard the other brains creak, because the WRB is complex and only experts that work on it on a regular basis can fully grasp it. Some discrepancies gave rise to long discussions among the silverbacks of our group, especially when it came to the phenomenon of secondary calcium (in short, dissolved calcium precipitating in soils, which was seen in many Catalan soils). Arguments bounced back and forth between different schools of thought. I could barely follow the threads. The debates also raised (unasked) questions, slighly annoying ones even, such as: Why is secondary calcium such a big deal? Should we break out of the ivory tower of the WRB?

I talked with Curtis Monger of New Mexico about my reservations and why secondary calcium seems so important. Firstly, it says much about how the soil was formed, and secondly, it may play a role in capturing and storing carbon from the atmosphere, because calcite (CaCO3) contains carbon. In that way, this process is linked to climate and feeds into our planet's great biogeochemical cycles.

Gypsum layers

Geology plays an important role and bedrock determines in part the kind of soil that forms on it. Catalonia has a great diversity of rocks and therefore of soils.

Gypsum is a salt (made up of calcium and sulphate), that is quite rare to find as a bedrock. Much of Spain's gypsum was deposited when the Mediterranean Sea dried up during the Messinian Crisis (Miocene) and all the salt from the sea precipitated. But in Catalonia, even older gypsum deposits, from Eocene times, are exposed at the surface. Powdery white Gypsisols developed on those ancient gypsum deposits. Some specialized plants only grow on Gyspisols while common Mediterranean trees (holm oak) also seem to thrive on these exceptional soils.  

A piece of gypsum rock from a Gypsisol.
A piece of gypsum rock from a Gypsisol.

Rice and salt

In the Ebro Delta, we examined a water-saturated swamp soil or Gleysol, on which rice is grown. I mentioned this type of soil in an article on Doñana in southern Spain, where the wetlands are under threat by drought and contamination.

Further inland, we come across a barren Vertisol, suffering from salinization, an increasingly widespread problem. The surface cracks are a sign of salinity. Vegetation is adapted to this saline environment and some edible shrubs taste nice and salty. I talk with Maxine Levin of Maine, USA, what one can do with this "useless" soil. It may not be good for farming, then why not let nature take its course here, as nature thrives everywhere, unlike demanding agricultural crops.

Rice field on a Gleysol in the Ebro Delta.
Rice field on a Gleysol in the Ebro Delta.
Surface cracks on a saline soil.
Surface cracks on a saline soil.

In the shadow of Jaume Porta

As our bus drove up across spectacular limestone landscapes and finally reached the granitic heart of the high Pyrenees, Jaume Boixadera shared sad news: the mentor of Rosa M Poch and himself, professor Jaume Porta Casanellas, had died that morning (Sept. 21, 2023). He was president of the Spanish Soil Science Society and former rector of the University of Lleida (UdL). Thanks to Porta, since the 1990s the UdL has become a renowned soil and agronomy hub, attracting many exchange students. "The assignment to hold the excursion in Catalonia is a token of international recognition," Porta still wrote in the foreword of our excursion guide.

The only way to honour him was to delve deeper than ever into 'his' beloved soil profiles, Rosa said, as we stood face to face with an Umbrisol, literally a "shadow soil," at more than 2,100 meters above sea level.

Mountain soils

A Podzol in Cerdanya, Pyrenees. Note the ash-colored layer (zola in Russian = ash).
A Podzol in Cerdanya, Pyrenees. Note the ash-colored layer (zola in Russian = ash).

Umbrisols with their dark topsoil full of organic matter arise in cool humid mountain climates, in temperate rain forests such as in Galicia or in high mountains in the tropics. They form only on acidic rocks such as granite. To cultivate this soil it needs more lime, but steep slopes make it more suitable as grazing or forest soil.

Not far from this Umbrisol there's a Podzol, also typical in rainy, temperate areas and on granite or sand. A Podzol has three distinctive colours, a dark topsoil and below it an ash-white and a rust-coloured horizon. In the ash-coloured horizon, all the iron and humus have been leached, leaving only the pale white colour of the bare rock or sand grains (quartz).

Podzols are probably the only soils a non-expert can recognize.

Wine and soil

Cooperativa l'Olivera in Vallbona de les Monges.
Cooperativa l'Olivera in Vallbona de les Monges.

In the village of Vallbona de les Monges, we saw chalky Regosols and Cambisols on terraces, in a farm where grapes and olives are grown sustainably. This terrace farming is part of Catalonia's cultural heritage and has been practiced for millennia, but it can never compete with large-scale industrial agribusiness.

We tasted wine followed by dinner at the Cooperativa L'Olivera winery. Rosa gave a toast: "Everything is connected to soil, good food, good wine, and good company!"

Meanwhile, there is already talk about where the next soil excursions will take place. Will I join this group again?

A nice group on a red soil in a beautiful landscape.
A nice group on a red soil in a beautiful landscape.

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I mentioned a few people, especially those who ended up next to me on the bus and with whom I could talk a little more. But all the participants contributed in their own way to a unforgettable week. Thanks to all and especially the organizers!

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Read more: The most fertile soils in the world are the black soils or Chernozems found in Ukraine. But now they are there exposed to the violence of war, which impacts soils too. In Spain soil erosion is a huge problem, the land is becoming increasingly barren. Is there a chance the Dust Bowl disaster will come to Spain? The land is being deforested not only for large-scale agriculture and fodder production, but also for speculative housing construction by untouchable developers, as I see with my own eyes in my village in the Sierra de Guadarrama. The Doñana Natural Park perhaps symbolizes best the indifferent attitude of Spanish governments. Another phenomenon, that of soil salinization, not only occurs in naturally saline areas but can also be a result of through over-irrigation. Also important is the effect of livestock production on land degradation and climate, as highlighted in this article on Argentine beef, of which the meat lobby claims is not harmful to the environment.

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Sources

Boixadera, Poch (coord.), Jiménez de Santiago, Simó, 2023, WRB - Catalonia Soil Excursion Guide, Publicacions Universitat de Lleida, Sept 2023.

Mantel S, Dondeyne S, Deckers S, 2023, World reference base for soil resources (WRB), Encyclopedia of Soils in the Environment (Second Edition), Vol 4, 206-217.

ISRIC website for details on Soil Orders, https://www.isric.org/explore/world-soil-distribution/

IUSS Working Group WRB. 2022. World Reference Base for Soil Resources. International soil classification system for naming soils and creating legends for soil maps. 4th edition. International Union of Soil Sciences (IUSS), Vienna, Austria.

Own field notes, observations and conversations with participants (but all views are my own).

A few keywords: WRB excursion 2023, wrb excursion, catalonia soils, soils of catalonia, soils of spain