Suelos volcánicos de la provincia del Chubut

Abstract

Los suelos volcánicos de la provincia del Chubut evidencian cambios oeste-este en el régimen de humedad edáfico (údico-xérico) y en la génesis de arcillas (alofano-imogolita-haloisita), generando un gradiente Andisoles-Molisoles ándicos-Molisoles. Existe una gran variabilidad edáfica a pequeña escala, relacionada con la posición en el paisaje, la profundidad y granulometría de los depósitos volcánicos y las características del material subyacente (i.e., rocas, depósitos glaciarios, coluviales o aluviales). Si bien predominan suelos profundos y bien drenados, son frecuentes las limitantes en profundidad y/o drenaje. Las cenizas de los suelos de la provincia son de naturaleza mesosilícica. Los perfiles volcánicos, típicamente sueltos, muestran secuencias A/Bw/C o A/AC/C. Predominan las texturas medias y gruesas (franco limosas a areno francas); los suelos son porosos (densidad aparente < 0,9 g cm-3) y con alta capacidad de retención de agua. Los contenidos de materia orgánica (MO) son altos, y varían de acuerdo al régimen de humedad (údico: MO>15%; xérico: MO = 5-12%) y al uso de la tierra. Los suelos alofánicos presentan pH ligeramente ácido, altos valores de CIC (> 100 cmolc kg-1) y nutrientes (N, Mg, Ca), y bajos valores de P disponible, debido a la alta retención de aniones. Hacia el este, donde la arcilla predominante es haloisita, disminuyen la MO y la CIC y los pH se incrementan. Los suelos volcánicos de Chubut, que sostienen la actividad forestal de la provincia y gran parte de la actividad agrícola ganadera, son extremadamente erosionables cuando pierden la cobertura vegetal.

The edaphic-bio-climatic gradient, typical from Patagonian Andean Region, can also be noticed in Chubut province, where mean annual precipitation varies from 3,000 to 300 mm along ca. 90 km. Volcanic soils, from holocenic ashes and pumicites, show west-east changes in soil moisture regime (udic - xeric - aridic) and in clay genesis (allophane - imogolite - halloysite), developing a soil gradient: Andisols - Andic Mollisols - Mollisols. At a detailed scale, a great edaphic variability exists, associated with the position in the landscape, the depth and granulometry of the volcanic ashes and the characteristics of the subjacent material. Since ashes are distributed by western winds, topography (i.e., aspect, slope degree, convexity) controls the depth of the deposits, and thus, the effective depth for downward roots to develop. Although moderately deep and well-drained soils predominate, limitations in depth and or drainage are common. These limitations are given by the material on which the ash has been deposited: rocks carved by the glaciers, glacifluvial, glacilacustrine, colluvial or alluvial deposits. For example, in profiles where ash is intermingled with alluvial materials, or where ash is on a clayey glacial deposit, it is common to find redoximorphic features near to the surface. On the other hand, profiles on steep slopes where ash was deposited on andesite rock, usually are shallow soils. Volcanic ashes in Chubut province are of mesosilicic nature, with dominance of slightly altered volcanic glass. Mineralogical studies show slightly weathered ashes and periodic additions of fresh ashes in the soils. Volcanic profiles, light and easily excavated, show a scarce horizons development and A/Bw/C or A/AC/C are the most common sequences. Soil colours correspond to 10YR hue; A horizons are dark toward the west, with value and chroma lower than 3 and 2, respectively, becoming lighter towards the east. A horizons are usually granular structured and subsurface horizons are non-structured (i.e., massive or loose). Although soil textures can widely vary in short distances, medium and coarse textures dominate (silty loam to loamy sand). As usual in Andisols, given their large surface area and the stable aggregation between organic matter and non-crystalline minerals, soils result in high porosity and low bulk densities (< 0.9 g cm-3). Soils also show high water retention capacity, which varies according to texture and dominant clays, showing silty loam allophanic soils the highest values. Soils under udic moisture regime show high soil chemical fertility, with high organic matter contents (>15%), moderately to slightly acid H2O pH, KCl pH values near to H2O pH, high cation exchange capacity (> 100 cmolc kg-1) and appropriate levels of nutrients (N, Mg, Ca). Only P levels are low, due to the high anions retention, typical of these soils. Towards the east, organic matter contents and cation exchange capacity diminish, while pH and exchangeable cations increase. Under the xeric moisture regime, organic matter contents vary conditioned by land use, reaching values ca. 12% in preserved native forests, and ca. 6% in rangelands and exotic conifers plantations. In the semiarid sector, with steppe vegetation and where soil moisture regime corresponds to a xeric-aridic transition, volcanic deposits are restricted to certain positions in the landscape, highly protected from the winds. In this eastern limit of volcanic soils distribution, the dominant clays are crystalline (i.e, halloysite), organic matter contents diminish (ca. 2%) and pH is slightly alkaline. The volcanic soils from Chubut are extremely erodible when they lose the vegetation cover. In the subhumid sector (i.e., xeric moisture regime), based on isotopic techniques, erosion rates as high as 30 m3 ha−1year−1 are found. Since organic matter and mineral fraction are stably combined in volcanic soils, erosion does not involve individual particles, but rather highly stable microaggregates, enriched in fine particles. These volcanic, fertile and erodible soils support the forest activity in Chubut Province, and much of crops and cattle, thereby, land management should have special emphasis on soil conservation.