EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7 The WRB applied to Argentinian soils: two case studies ROCA PASCUAL Nuria1 and PAZOS Mabel Susana2 1 Facultad de Biología - Universidad de Barcelona – Spain 2 Facultad de Agronomía – Universidad Nacional del Centro de la Provincia de Buenos Aires – Argentina, Email: [email protected] Abstract Since 1970, the USDA Soil Taxonomy has been utilized for making and interpreting soil maps in Argentina but few experiences have been made with the use of the World Reference Base for Soil Resources (WRB). This paper discusses the correlation between Soil Taxonomy and WRB in two contrasting areas in Argentina. The inferences that can be drawn from the soil names for transferring the soil knowledge to non-soil scientists are also discussed. The first area is Azul, in central Buenos Aires province with temperate subhumid climate. Continental plains in the North have flat-concave relief with alkaline-hydromorphic soils. The southern part has both sierras and piedmont landscapes with rock outcrops and shallow to deep well drained soils. Land use is rainfed agriculture in deep well drained soils and cattle on shallow or alkaline-hydromorphic soils. The second area is Fray Mamerto Esquiú Department in Catamarca Province located in the Dry Chaco region with dry warm climate. It is a narrow valley with alluvial soils surrounded by sierras with weakly developed soils. About 30 profiles were described and sampled within a soil survey project. Land use is intensive crop production under irrigation in the low river terraces and natural desertic shrublands in the highlands. A previous study of taxotransference of the soil series in Azul Co. from the soil maps at 1:50,000 scale to WRB was utilized. The results were updated checking presence of diagnostic horizons and properties from the pedon descriptions in the map reports. Some uncertainties in the process of taxotransference to WRB result from incomplete profile descriptions. The most overlooked or partially described characteristics are the type of substratum in well drained soils and features associated with wetness in lowland soils. Nevertheless, a rather good correlation is found between soil names at the subgroup level of Soil Taxonomy and soil units in WRB. Fray Mamerto Esquiú presents two well defined areas according to the presence or absence of salinity/alkalinity. In the normal area (non-saline, non-alkaline), the correlation between the results in both systems is rather good. However, in the saline/alkaline area, Soil Taxonomy fails to show in the resulting names at the subgroup level those characteristics which are important for irrigation, namely salinity-alkalinity. It is concluded that the flexibility of WRB with the utilization of qualifiers is more sensitive in reflecting soil characteristics in the soil name when compared with Soil Taxonomy. The emphasis put on soil morphology compared with abundant laboratory data required by ST makes the WRB system suitable for application in areas with rather modest facilities, of which Catamarca Province is a good example. Concurrently, recommendations can be readily drawn from soil names in WRB when transferring the knowledge about soils to non- specialist users such as agronomists and land use planners. Keywords: Soil Taxonomy, WRB, Soil correlation, Argentina, Pampean soils, alluvial soils Introduction Since 1970, Soil Taxonomy (ST) (Soil Survey Staff, 1999), at that time the 7th Approximation, has been accepted as the soil classification system to be utilised for making and interpreting soil surveys in Argentina. In most cases the system with all of its subsequent revisions until the second edition, has failed to show the uniqueness of the soils in Argentina and particularly those in the Pampean Region (Pazos, 2000). The need for a national soil classification system has been proclaimed (Pazos, 1993) but no further efforts have been made in this sense. The WRB applied to Argentinian soils: two case studies. Roca Pascual & Pazos. 191 EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7 The World Reference Base for Soil Resources (WRB) (ISSS, ISRIC, FAO, 1998) appears as an interesting option for the region. It is a very flexible system with a higher level of reference soil groups and a lower level of adjectives applied as qualifiers, designed as an easy means of communication among scientists. Little experience has been gained in Argentina with the application of WRB (Pazos, 1996; Pazos and Moscatelli, 1998). Two case studies are presented here from different and contrasting areas (Fig. 1). The first area is Azul Co., Buenos Aires province, in the centre of the Humid Pampa where soil maps are available at the scale of 1:50,000 (INTA, 1974-1997). The second area is Fray Mamerto Esquiú Department in Catamarca province, located in the Dry Chaco region, a less studied area, with soil maps at the scale of 1:500,000, and some semidetailed soil maps from neighbouring areas. In both areas soils are classified according to the WRB and the soil names are compared with those obtained applying ST at the subgroup level. The connotation of both resulting nomenclatures is analysed from the standpoint of non-specialist users of soil information. Materials and methods A previous study of taxotransference (Pazos, 1996) of the soil series present in Azul Co. according to the soil maps 1:50,000 (INTA, 1974-1997) to WRB (ISSS, ISRIC, FAO, 1994) was utilized. The results were updated both to the second edition of ST and WRB (ISSS-ISRIC-FAO, 1998) as follows. First, each soil series present in Azul Co. was reclassified to the subgroup level of ST. As far as possible, all uncertain information in the profile descriptions given in the map reports was checked with the soil survey staff at the Soils Institute of INTA (Gustavo Moscatelli, Ricardo Diaz, personal communication). Finally, the soil series were classified with WRB. In Fray Mamerto Esquiú department 30 soil profiles were described and sampled within a soil survey project at the scale of 1:20,000. Both ST and WRB were utilized to classify the soils. On samples obtained from each horizon the following analyses were made: pH (water 1:2.5), electrical conductivity and soluble ions in the saturation extract, clay content by the pipette method, organic carbon by Walkley and Black, and CaCO3 by the gas volumetric method. Results and discussion Azul Azul Co., in central Buenos Aires province, has a surface area of 6615 km2. The northern part is formed by continental plains with flat-concave general relief and alkaline and/or hydromorphic soils, with tosca layers (calcareous layer with variable depth, CaCO3 content and hardness) at shallow depths towards the east. The southern part has both sierras and piedmont landscapes with rock outcrops and shallow and very shallow soils as well as deep well drained soils with tosca at variable depth. The average topsoil organic matter content is 55 g kg-1. The climate is temperate humid-subhumid with mean annual rainfall of 900 mm. Rainfed agriculture is the main use of the deep well drained soils, with wheat, maize and soybean as the main crops. Shallow and alkaline and/or hydromorphic soils are utilized for cattle raising. In Table 1 the soil series described for Azul Co. (INTA, 1974-1997) are listed together with the corresponding subgroup according to ST, the updating to the second edition of ST and the classification with WRB. Many uncertainties were found in the soil profile descriptions. In some cases, dry color was not indicated which hampered the recognition of mollic epipedons and albic horizons. In other cases, the poor description of features associated with wetness made it difficult to assign an aquic soil moisture regime in ST or apply gleyic and stagnic qualifiers in WRB. The main differences between both subgroups, defined by INTA (1974-1997) and updated ST, are derived from: 1. Changes in ST with the incorporation of new great groups and subgroups. For example: Natrudolls (cf. Santa Paula, Sierra Chica and others), Petrocalcic Paleudolls (cf. Azul, Balcarce and others) and Abruptic Argiudolls (cf. Cobo, Saladillo and El Toro). 2. Names coined by INTA and free application of ST criteria in order to suit local soil peculiarities. For example, Thapto argic Hapludoll and Thapto natric Hapludoll were provided for soils with non-albic E horizons, interpreted as lithologic discontinuities. 3. Lithic Hapludoll, besides naming soils with hard rock within 50 cm from the mineral soil surface, was also utilized for soils with tosca within the same depth. 192 The WRB applied to Argentinian soils: two case studies. Roca Pascual & Pazos. EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7 Figure 1. Map of Argentina. 1) Azul Co. in Buenos Aires Province. 2) Fray Mamerto Esquiú Department in Catamarca Province. Provinces shaded in gray. There is a rather good correlation between great groups in ST and reference soil groups in WRB, viz. Argiudolls and Phaeozems; Argialbolls and Planosols, and Natrudolls, Natraquolls and Natraqualfs with Solonetz. When moving into lower levels in both systems, the correlation remains rather good but WRB attains a better segregation of well drained soils with tosca in the substratum. Petrocalcic Paleudolls are segregated into Luvic and Haplic Phaeozems, both with tosca phase. The difficulties in the classification of soils with tosca according to ST are extensively discussed by Pazos and Mestelan (2002). WRB is able to show more accessory properties than ST. For example, exchangeable sodium content above 6 % in Phaeozems (cf. Videla Dorna, Napaleofú and others) and secondary CaCO3 accumulation in Solonetz (cf. Cacharí, Miranda and others). The particular case of tosca in the substratum of Solonetz (cf. Santa Paula, Sierra Chica and others) is not strictly provided in WRB and should carry the petrocalcic qualifier. For this particular case, ST has a petrocalcic subgroup in Natrudolls but not in Natraquolls, giving rise to some inconsistency in the classification of the soils in the lowlands.