The ICGC, within its activity and competences, carries out activities directly related to knowledge and information about the soils of Catalonia. In order to develop these functions, the ICGC carries out, in collaboration with other organizations if necessary, soil science work, soil cartography, erosion risk studies and assessments and other work related to the soil and its protection. The ICGC allocates resources to the study of soils, carrying out research projects, databases and cartographies.

Soil is a three-dimensional body that has developed as a result of the interactions between the original material, geomorphology, living organisms and climate over time. These five variables make up the soil-forming factors.

When studying soils, it must be taken into account that these are open and dynamic systems, made up of three phases:

• Solid phase, made up of inorganic components (primary and secondary minerals from the weathering of rocks) and organic components in different states of decomposition. These components are the skeleton of the soil; their organization creates a space of voids that is occupied, in a dynamic and complementary way, by the liquid and gaseous phases.
• Liquid phase, also called soil water, is an aqueous solution made up of liquid water (or ice) and different substances, in solution or in suspension, of very variable composition.
• Gaseous phase, also called air or soil atmosphere, is a very diverse and dynamic mixture of gases and vapors, which fill the empty spaces free of the liquid phase and which moves by diffusion thanks to the concentration gradient between the different points.

The proportion of each phase, as well as its characteristics, internal organization and arrangement, give rise to different soil morphologies, defined as everything that can be seen and felt.

The study of soil in the field begins with the systematic description and characterization of its morphology. The survey must take into account the marked anisotropy that the soil presents in the vertical direction, as is evident from the presence of layers or horizons in the profiles (Figure 1).

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Figure 1. Profile of a soil in the municipality of Aitona

The soil description generally includes the following blocks of information (Figure 2):

• General information: profile reference, date of description, people making the description, location...
• Site description: position of the soil in the toposequence, morphology of the site, presence of outcrops, superficial stonyness, original and underlying material, vegetation and land use...
• Soil profile description, horizon by horizon: soil depth, thickness of the different horizons, characteristics of the boundaries between horizons, humidity at the time of description, color, mottled, granulometry or texture, content of coarse elements, structure, consistency, compactness, presence of cracks, secondary accumulations, biological activity, root system, anthropic activities, field trials to be able to contrast working hypotheses and make a more accurate diagnosis....

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Figure 2. Field sheet models for the description and characterization of observations and pits

The macromorphological study of the soil is complemented in the field with the determination of the apparent densities of the different horizons and with their hydrological characterization (water retention capacity, infiltration, hydraulic conductivity) (Figure 3). In addition, in the laboratory, the physicochemical characterization of the samples collected during the description is carried out.

In research work, other types of more specialized techniques can be used (micromorphology, electron microscopy, X-ray diffraction, nuclear magnetic resonance, fluorescence, differential thermal analysis, image analysis, etc.), which allow for a deeper study of the organization of the soil and the nature of its components.

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Figure 3. Complementary characterization of soil profiles

For many years, technicians have strived to develop appropriate and efficient methods to determine the characteristics of the soils that appear in an area and show their spatial distribution to potential users of this type of information. Soil cartography is the term that defines the set of techniques and procedures that these specialists have developed in order to identify and describe the processes of soil formation, predicting and explaining their arrangement in the territory. These methodologies include the analysis and interpretation of aerial photographs from old and recent flights, orthophotomaps and digital terrain elevation models generated by the ICGC, the description of observations and field surveys, the taking of samples from the most representative profiles, their physicochemical analysis, the determination (both in the field and in the laboratory) of other important properties for an adequate knowledge of the soils, the digitization of all the information collected, the interpretation of the results obtained and, finally, the delineation and preparation of the maps. It can therefore be stated that soil maps are the documents that best show the different types of soils that appear in an area, their morphological characteristics, their physical, chemical and biological properties, as well as their spatial distribution.

The suitability of a soil map to achieve specific objectives is determined by the scale, legend and types of taxonomic and cartographic units used.

To establish the different types of soils shown on maps, the approach proposed by Dokuchaev, rigorously described by Jenny in 1941, is traditionally followed. According to this model, recognized as the paradigm of soil cartography, soil is the result of the combined action of formative processes (additions, losses, transformations and translocations) on a set of ecological factors of formation: original material, position in the landscape, living organisms and climate, over a given time.

Soil = f (Original material, geomorphology, living organisms, climate, time)

These established soil types make up the taxonomic units of the soil map; that is, the classification units of this entity.

Taxonomic units are defined for each cartographic project based on the objectives and the scale of work. They comprise a unique range of characteristics; although, over time, as cartographies progress and information is completed, these ranges may change and must be adjusted. For this reason, most soil classification systems are open and can be easily modified.

The spatial distribution of the different soil types is represented on the maps by delineations, also called polygons or tessellations, which have the same representation: color, pattern, code or symbol. The set of all delineations formed by the same soil types and which, therefore, have the same representation, constitute a cartographic unit.

Cartographic units are composed of one or more main or dominant soil types and other soil types that cannot be represented on the map due to the small area they occupy at the working scale. These other soil types are called map inclusions or impurities, and are usually divided into:

• Similar soils. These are soils that, despite belonging to a different soil type than those represented on the map, respond in a similar way to their use and management.
• Dissimilar soils. These are soils that belong to different soil types than those represented on the map and that, in addition, present a very different response in terms of their use and management.

The ICGC soil maps use two types of cartographic units (Figure 4):

• Consortia. These are cartographic units dominated by one soil type in more than 50% of the surface. In addition, the surface area occupied by dissimilar soils is less than 25%.
• Complex. These are cartographic units dominated, in more than 75% of the surface area, by several types of dissimilar soils that usually give the unit its name.

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Figure 4. Types of cartographic units presented in soil maps

The soil map legend is defined as the ordered set of all cartographic units that appear on the map.

Nowadays, all the data that is collected and generated during the execution of soil cartographies is incorporated into a geographic information system (Figure 5); these allow the capture, storage, updating, editing, analysis and presentation of the same.

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Figure 5. Working with a geographic information system

Als mapes, la representació gràfica dels sòls mostra una clara existència de límits entre les diferents delineacions; tot i que, en la pràctica, els sòls acostumen a presentar una variació lateral contínua i, més o menys, gradual. A més, aquesta variació no acostuma a ser la mateixa per a totes les característiques que defineixen els diferents tipus de sòls. Per tant, en utilitzar els mapes de sòls, s’ha de tenir en compte que els límits, al camp, poden no ser tan clars com farien pensar les línies i els polígons que els representen.

La fiabilitat d’una cartografia de sòls fa referència a la confiança amb la que es poden fer prediccions del comportament dels sòls representats sota unes condicions o escenaris determinats. Els principals criteris per avaluar la fiabilitat d’un mapa de sòls, a més de la professionalitat i experiència de l’equip de treball són:

• Metodologia de treball, que al seu torn es pot subdividir en:
  • Ús d’una metodologia normalitzada, amb protocols i plecs de condicions per a l’execució dels diferents treballs encaminats a l’obtenció de la cartografia.
  • Densitat d’observació adient a l’escala.
  • Grau de sistematització i informatització dels processos.
  • Control de la qualitat dels treballs a mesura que es van executant
  • Correlació de la cartografia.
• Forma de presentar la informació:
  • Llegibilitat del mapa, expressió de la informació i precisió de la representació.
  • Redacció i facilitat de comprensió de la informació.

La importància de la informació de sòls es reflecteix en la gran quantitat d’aplicacions específiques que, partint de la informació bàsica georeferenciada dels sòls, es poden generar. Com a exemples d’aquestes aplicacions es poden citar els mapes d’interpretació de diferents característiques del sòl (classes texturals, nivells de pH, nivells de salinitat, capacitat de retenció d’aigua...) i els mapes d’avaluació que permeten analitzar qualsevol escenari on intervingui la informació de sòls (avaluació de la capacitat productiva del sòl, del risc d’erosió, adaptabilitat de sistemes de reg, aptitud de terres...) (Figura 6).

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Figure 6. Map of agrological capacity classes corresponding to the soils mapped in the Amposta sheet