Soil System Sciences

Soil System Sciences

Working for the recovery of burned soils

Fire is a natural agent that occurs in most terrestrial ecosystems. In Mediterranean areas, for example, fire is a natural agent that has contributed to shape the history of vegetation, soils, and ultimately, the landscape we know today. Also, since ancient times, men have also used fire as a tool for the management of ecosystems. As a result, the Mediterranean vegetation has developed mechanisms of adaptation to fire, but Man has contributed to the intense transformation of the original forest systems in crops, pastures and meadows and dehesas from the fifteenth century to encourage farming, sylvopastoral use of forests, and human supply.

Prescribed burn in Portugal. Photo by Vicky Arcenegui, University Miguel Hernández, Spain.

Prescribed burn in Portugal. Photo by Vicky Arcenegui, University Miguel Hernández, Spain.

In countries like Spain, the concentration of population in urban areas that began in the second half of the 20th century has led to a shift away from rural areas and declining crop and livestock pressure. The abandonment of traditional rural labors contributed to people forgetting how to efficiently manage agricultural and forest resources, such as maintaining terraces on the slopes, taking care of roads or forest clearing. All this, coupled with the pressures of tourism, the expansion of urban areas and other social reasons, has led to a large increase in the number and damage caused by forest fires during the 1960s, 1970s and 1980s. Since 1990, the number of forest fires has increased progressively, although affecting a lower annual total. In recent years, though the number of forest fires has declined high intensity fires are still occurring. High intensity fires occur under certain environmental conditions (high temperatures, wind and low humidity of vegetation and soil), but also due to a management of the forest environment that favors the spread of fire.

In the context of global change, scientists expect the number of high intensity wildfires increase, as well as the severity of its impact on the environment, the productive capacity and natural resources. For these reasons, scientists who study the impact of fire on soils have participated in the development of practice guidelines for the management planning burned soils that facilitate managers to decide when, where and how to act.

Soil scientists and firefighters during a prescribed burn in Sevilla. Photo by Antonio Jordán, University of Seville.

Soil scientists and firefighters during a prescribed burn in Sevilla. Photo by Antonio Jordán, University of Seville.

According to the Spanish Network Forest Fire Effects on Soils, a concern that is necessary to transfer the decision-makers that it is not always necessary to act in the post -fire, and that in many cases, both the vegetation and the soil can recover themselves in relatively short periods of time. Also, that actions cannot be the same on all systems, and the planning and management of burned areas should be based on local characteristics of the environment.

Good forest management practices should be based on scientific research. Lots of money have been used for fight against forest fires, but just to prevention activities, and scarcely for the study of their impact on soil, water and vegetation. The knowledge of the effects of fire on soil properties and the proposing and use of impact factors becomes essential when performing management decisions, restoration or prevention of the areas affected by fire.

To this end, scientists from Galicia (Forest Research Centre Lourizán, Institute of Agrobiological Research from Galicia, University of Santiago de Compostela and University of Vigo) have developed the first guide for urgent action planning against soil erosion in fire-affected forest areas. This initiative has been supported by the Spanish Government and FEDER funds of the European Union.

Guide for urgent actions against soil erosion in burned forest areas

Guide for urgent actions against soil erosion in burned forest areas

The first part of the text studies the risk of erosion and soil hydrological response after fire in Galicia, where soils, vegetation and climate are very different from neighbor regions. The second part details the urgent treatments to combat erosion risk in the post -fire, proposing methodologies for assessing the severity of fire impacts on soil and vegetation, and recommends a guide for urgent decision-making.

We hope this work, a collaboration of scientists deep and managers, help the recovery of degraded areas in a region hard hit by the effects of fire.


This post was also published simultaneously in G-Soil.

Soils at Imaggeo: when a soil is born

Artemi Cerdà, University of Valencia, Valencia – Spain

Mosses on granite rock initiate the soil development, by Artemi Cerdà. Click to see the original picture at Imaggeo.

Soil development is based on the weathering of rocks and the deposition and decomposition of litter and roots, which are the main source or soil organic matter. Mosses are one of the key actors on those processes, as they are present at the initial stages of pedogenesis.

This post was also published simultaneously in G-Soil.

Ladies and gentlemen: the Rolling Stones

Racetrack Playa valley. Photo by Jon Sullivan. Click on the image to see the original image at Wikimedia Commons.

Racetrack Playa is a plain without vegetation of a dry located above the northwestern side of Death Valley, in Death Valley National Park, Inyo County, CA, USA (click here to see in Google Maps). Although “playa” is the Spanish word for beach, it is also used in English to refer to a dry lake. Racetrack Playa occupies an area of 4.5 km (north-south) by 2 km (east-west) which is 1,130 m above sea level between the Cottonwood Mountains and Last Chance Range. The surface is extremely flat and dry for most of the year, when the surface is covered with small hexagonal mud curls, although floods partially during the rainy season forming a shallow lake that evaporates quickly. In winter it forms a relatively thick layer of ice.

The sailing stones

Despite its geomorphological and environmental interest, Racetrack Playa is known around the world for the phenomenon of sliding rocks or sailing stones, because on the surface of the basin appear scattered stones leaving a trail behind him, so it seems that something or someone had dragged over the surface of the ground without anyone’s seen them move ever. The phenomenon is so striking that it has been “investigated” by pseudoscientists who have attributed the movement of the stones to energy phenomena, gravity field anomalies, extraterrestrial activity and other funny hypotheses.

Sailing stone in Racetrack Playa. Photo by Laurence G. Charlot. Click on the image to see the original picture at Wikimedia Commons.

Looking for answers

The first scientific approaches to the study of this geomorphological process suggested the hypothesis that wind was the main cause of stone movements. Louis G. Kirk, a National Park Service Ranger speculated that local strong winds caused the movement of stones over the muddy surface after heavy rainfall. An experiment was conducted by Jim McAllister and Allen Agnew (USGS) in 1948, who had the idea that the movement of the stones was due to strong winds blowing over the flooded surface. The two researchers flooded a small part of the plain and a used an aeroengine to create a strong air flow to move the stones, but failed to replicate the natural result. Moreover, local winds can reach 150 kilometers per hour, but not enough to move some of the stones, which may weigh hundreds of kilograms in some cases.

Stones with divergent trajectories. Photo by Daniel Mayer. Click on the image to see the original picture at Wikimedia Commons.

During the following decades, the researchers could not explain the nature of this phenomenon, although it was suggested a possible link with the ice layer formed on the lake at certain times of the year. John Reid (Hampshire College) and his team also reported that wind alone is not enough to move stones, and hypothesized that the ice layer was pushed by the wind during the winter, dragging the stones. But Paula Messina (San José State University) analyzed the trajectory of different stones using GPS. She noted that some trails were linear, suggesting the influence of wind, but other are curve or irregular. She estimated that wind velocity needed for stones to move this way was of several hundred kilometers per hour. You can visit Paula’s website for more complete information.

Undecided sailing stone. Photo by Jon Sullivan. Click on the image to see the original image at Wikimedia Commons.

Ice and wind

More recently, Ralph Lorenz (Johns Hopkins University) and his team replicated the phenomenon in a very simple way. He realized that in some cases, rocks contrails direction abruptly changed when crossed with each other, as if the rocks had hit and taken different directions. The only way this happens is that there is a mass of ice around each rock upon impact with another, without stopping deviate due to the low coefficient of friction of ice. He tested his theory in his own house with stones, the freezer and a couple of tupperwares… and stones moved!

Details of Ralph Lorenz’s home experiment (Lorenz et al., 2008). Click to see larger image.

Lorenz’s team suggested that the movement of stones in Racetrack Playa is due to the effect of weak winds on buoyant stones that are included in “ice cakes”, as also occurs in arctic tidal beaches. Ice cakes allow the stones to move over the flooded bed.
Stones arrive at the playa from the slopes around or by other processes. During the rain, water has no outlet possible, so that it accumulates and the area is flooded. If the temperature is low enough, a layer of ice is formed on the surface of liquid water. The stones partially embedded in the floating ice rise slightly above the bottom with the increasing level of water. Both the friction between the ice and water and between the stones and the bed are very small, so that blowing wind with some intensity pushes the ice (and the rocks embedded). If the stones and mud at the bottom have a light touch, the dragged stones leave a trail that remains once the ice has melted and the water has evaporated.
Why moving stones have not been observed? According to Lorenz, “movement happens for only tens of seconds, at intervals spaced typically by several years”, and “this would demand exceptional patience as well as luck” (see comments here). So, the rocks are probably traveling on the coldest and windiest days that occur over a period of several years. The most likely time would be in the very early dawn. Do you dare?

Know more

Bacon, D., Cahill, T. and Tombrello, T.A. 1996. Sailing Stones on Racetrack Playa. Journal of Geology 104: pp.121-125.

Lorenz RD, Jackson BK, Barnes JW, Spitale J, Keller, JM. 2011. Ice rafts not sails: Floating the rocks at Racetrack Playa. American Journal of Physics 79: 37.

Kirk LG. 1952. Trails and rocks observed on a playa in Death Valley National Monument, California. Journal of Sedimentary Petrology 22: 173-181.

Messina P, Stoffer P. 1999. Differential GPS/GIS analysis of the sliding rock phenomenon of Racetrack Playa, Death Valley National Park. In: Slate JL (Ed.), Proceedings of Conference on Status of Geologic Research and Mapping, Death Valley National Park. US Geological Survey Open File Report 99-153: 107-109.

Reid JB, Bucklin EP, Copenagle L, Kidder J, PackSM, Polissar PJ, Williams ML. 1995. Sliding rocks at the Racetrack, Death Valley: What makes them move? Geology 23: 819-822.

Sharp WE. 1960. The movement of playa scrapers by wind. Journal of Geology 68: 567-572.

Sharp RP, Carey DL. 1976. Sliding Stones, Racetrack Playa, California. Geological Society of America Bulletin 87: 1704-1717.

Sharp RP, Carey DL, Reid JB Jr, Polissar PJ, Williams ML. 1996. Sliding rocks at the Racetrack, Death Valley: What makes them move: comment and reply. Geology 24: 766-767.

This post was also published simultaneously in G-Soil.

What is soil structure?

Soil aggregates from a dark clayey soil. Photo courtesy of Pepe Álvarez (Technical University of Cartagena, Spain).

Soil aggregates from a dark clayey soil. Photo courtesy of Pepe Álvarez (Technical University of Cartagena, Spain).

Soil structure is the result of the spatial arrangement of the solid soil particles and their associated pore space. Aggregation mainly depends on the soil composition and texture, but is also strongly influenced by other factors such as biological activity, climate, geomorphic processes or the action of fire. Structure is a typical morphological soil property, which allows differentiating soil of geological material. Because of its importance, structure is a property commonly described in soil studies

Organic and mineral soil particles are not isolated from each other, but form structural aggregates (also called “peds”). In 1961, Blackmore and Miller observed how the Ca-montmorillonite may be arranged in groups of four or five particles, depending on various soil characteristics.

Thin section of a surface sandy soil under cross polarized light showing sand grains and cellular plant material. Photo by Laura Gargiulo. Click to see the original picture at Imaggeo.

The fact that soil particles do not form a continuous and compact mass, but are associated, involves an interconnected pore space, makes possible the development of life in the soil. The volume formed by pores, channels, chambers and cracks allows the movement of fluids (air and water) in the soil, providing a favorable environment for microbial activity and facilitating root growth of plants.

Some authors consider that more than one property, structure is a state of soil, because when dry, it becomes clear, but if it is wet, the soil becomes massive, no cracks are distinguishable, and structure disappears.

Texture, biological activity and a number of physicochemical conditions allow the aggregation of soil particles. The predominance of one or other process creates various types of structure. Aggregation is strongly conditioned by colloids (clay and organic matter) and soil cementing substances (carbonates, sesquioxides, etc.), which coat solid particles, including them in groups (aggregates). If the proportion of colloids or cementing substances is too low, solid particles remain dispersed. Flocculation of colloids gives rise to the co-precipitation of colloidal particles (clay and organic matter), forming microaggregates (<250 μm), which then evolve resulting in macroaggregates (>250 μm). In the formation of small fabric units (cluster and domains), the inorganic bonds are the most important, while in the aggregate stabilization the organic ones play a more relevant role (humic cements).

Flocculation induced by cations in the soil solution plays an important role in the development of aggregates. Calcium and magnesium (in calcareous soils) or iron and aluminium (in acid soils) favour the formation of stable aggregates. In contrast, monovalent cations as sodium contributes to dispersion of aggregates. Also, cementing agents as calcium carbonate (in calcareous soils) or iron oxides (acid soils) may enhance soil aggregation. In the formation of macroaggregates, biological agents are also involved, as plants (roots), animals (earthworms, arthropods, etc.), microorganisms (bacteria and, especially, fungi) are also important.

Fragment of calcium carbonate from an unearthed petrocalcic horizon. Photo by A. Jordán. Click to see the original picture at Imaggeo.

The degree of development of the structure and aggregate stability depends on the type of particles present and the forces of attraction/repulsion taking place. This can lead to particle packing or aggregate formation. Packing is important when the forces of attraction / repulsion are negligible in the absence of electric charge (such us between sand particles). In sandy soils, the surface tension of the film of water adsorbed on the surface of the grains may cause a certain binding capacity.

Aggregate stability

Soil structure is not a stable parameter; it may vary depending on weather conditions, management, soil processes, etc. In general, the most important causes of the degradation of soil structure are:

  • Expansion of swelling clays (montmorillonite type) during wet periods.
  • Rain, especially if it results in a violent dilution of cations, which promotes flocculation of the colloids.
  • Loss of organic matter (common in cropped or eroded soils).
  • Acidification, resulting in destabilization of microaggregates.

Classification of structure

Soil structure can be classified according to the presence of colloidal soil particles and their interaction with coarser particles. According to this, soil structure can be classified in three broad categories:

Poorly developed soils on sandy plains under Scots pines in the Dzukija National Park (Lithuania). Photo by A. Jordán. Click to see the original picture at Imaggeo.

  • Single grained. Colloids are scarce and soil textural composition is dominated by coarse particles without aggregation capacity, the grain structure is particularly loose.
Sandy soil from Moguer (Huelva, SW Spain)

Sandy soil from Moguer (Huelva, SW Spain).

  • Aggregated. The presence of colloids is moderate and coarse particles are arranged in small clods or aggregates. Aggregates are relatively porous, promoting aeration and soil permeability.
  • Massive. If the presence of colloidal particles is dominant, the soil appears cemented in one great mass due to the decrease of the pore volume, thereby decreasing aeration and drainage. In the dry season, clayey soils become massive. If swelling clays are present, shrinkage cracks appear

In the case of aggregated soils, aggregates can be classified by morphology in different groups. According to FAO, the types of soil structure are:

  • Blocky. Blocks or polyhedrons, nearly equidimensional, having flat or slightly rounded surfaces that are casts of the faces of the surrounding aggregates. Subdivision is recommended into angular, with faces intersecting at relatively sharp angles, and subangular blocky faces intersecting at rounded angles.

Soil students describing a blocky aggregate from a forest soil developed on a weathered schist near Almadén de la Plata (Sevilla, SW Spain). Photo by A: Jordán. Click to see the original picture at Imaggeo.

Angular blocky aggregate from a red clayey soil (Chromic Luvisol) showing cutans and sharp edges. The faces of the aggregate intersect mostly at relatively sharp angles as a result of swelling and shrinking of clay. Photo by A. Jordán. Click to see the original picture at Imaggeo.

  • Granular. Spheroids or polyhedrons, having curved or irregular surfaces that are not casts of the faces of surrounding aggregates.
Granular structure at the soil surface. Photo courtesy of Juan Gil (University of Córdoba, Spain).

Granular structure at the soil surface. Photo courtesy of Juan Gil (University of Córdoba, Spain).

  • Platy. Flat with vertical dimensions limited; generally oriented on a horizontal plane and usually overlapping.

Soil aggregates showing platy structure in the foot of a marly hill near Rota (Cádiz, SW Spain). Photo by A. Jordán. Click to see original picture at Imaggeo.

  • Prismatic. The dimensions are limited in the horizontal and extended along the vertical plane; vertical faces well defined; having flat or slightly rounded surfaces that are casts of the faces of the surrounding aggregates. Faces normally intersect at relatively sharp angles.
Prismatic soil aggregates from Posadas (Córdoba, SW Spain). Photo courtesy of José M. Recio (University of Córdoba, Spain)

Prismatic soil aggregates from Posadas (Córdoba, SW Spain). Photo courtesy of José M. Recio (University of Córdoba, Spain)

  • Columnar. Prismatic structures with rounded caps are distinguished as columnar.
  • Rock structure Rock structure includes fine stratification in unconsolidated sediment, and pseudomorphs of weathered minerals retaining their positions relative to each other and to unweathered minerals in saprolite from consolidated rocks.
  • Wedge-shaped. Elliptical, interlocking lenses that terminate in sharp angles, bounded by slickensides; not limited to vertic materials.
  • Crumbs, lumps and clods Mainly created by artificial disturbance, e.g. tillage.


Agronomically, well-structured soils easily crumble spontaneously when dry. So, when relatively dry it can be easily tilled, and when wet it does not adhere to the tools.

The surface of aggregated soils facilitates seedling emergence and water infiltration. A good aggregation prevents soil sealing and further formation of surface crusts.

Sealed soil surface due to rain-splash processes in a cropped area near Sevilla (SW Spain). Photo by A. ordán. Click to see the original picture at Imaggeo.

In aggregated soils, enhanced water infiltration reduces runoff and water erosion, increasing the water available for plants. Also, aggregated soils allow a smooth flow of water, air and nutrients, promotes the development and activity of aerobic microorganisms, soil fauna and the penetration of roots. A well-structured soil is more resistant to erosion than the loose particles of sand, silt and clay and organic matter.

Know more

Barthès B, Roose E. 2002. Aggregate stability as an indicator of soil susceptibility to runoff and erosion; validation at several levels. Catena 47, 133-149.

Barto EK, Alt F, Oelmann Y, Wilcke W, Rillig MC. 2010. Contributions of biotic and abiotic factors to soil aggregation across a land use gradient. Soil Biology and Biochemistry 42, 2316-2324.

Blackmore AV, Miller RD. 1961, Tactoid size and osmotic swelling in calcium montmorillonite. Soil Science Society of America Journal 25, 169-173.

Boix-Fayos C, Calvo-Cases A, Imeson AC, Soriano-Soto MD. 2001. Influence of soil properties of some Mediterranean soils and the use of aggregate size and stability as land degradation indicators. Catena 44, 47-67.

Cerdà A. 1998. Soil aggregate stability under different Mediterranean vegetation types. Catena 32, 73-86.

Colombo C, Torrent J. 1991. Relationships between aggregation and iron oxides in Terra Rossa soils from southern Italy. Catena 18, 51-59.

FAO. 2006. Guidelines for soil description. Food and Agriculture Organization of the United Nations. Rome.

Harris RF, Chesters G, Allen ON, Attoe OJ. 1964. Mechanisms involved in soil aggregate stabilization by fungi and bacteria. Soil Science Society of America Proceedings 28, 529-532.

Hoogmoed WB, Stroosnijder L. 1984. Crust formation on sandy soils in the Sahel. Soil & Tillage Research 4, 5-24.

Kleijn WB, Oster JD. 1982. A model of clay swelling and tactoid formation. Clays and Clay Minerals 30, 383-390.

Le Bissonnais Y. 1996. Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology. European Journal of Soil Science 47, 425-437.

Oades JM. 1984. Soil organic matter and structural stability: mechanisms and implications for management. Plant and Soil 76, 319-337.

Oades JM. 1993. The role of biology in the formation, stabilization and degradation of soil structure. Geoderma 56, 377-400.

Mataix-Solera J, Cerdà A, Arcenegui V, Jordán A, Zavala LM. 2011. Fire effects on soil aggregation: A review. Earth-Science Reviews 109, 44-60.

Rengasamy P, Greene RSB, Ford GW. 1984. The role of clay fraction in the particles arrangement and stability of soil aggregates-a review. Clay Research 32, 53-67.

Soil Survey Division Staff. 1993. Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18. Washington DC.

Spohn M, Giani L. 2011. Impacts of land use change on soil aggregation and aggregate stabilizing compounds as dependent on time. Soil Biology and Biochemistry 43, 1081-1088.

Tisdall JM, Oades JM. 1982. Organic matter and water-stable aggregates in soils. Journal of Soil Science 33, 141-163.

This post was published simultaneously in G-Soil.