Saturday, 15 July 2006

Soil Toposequence on Pumice Tuff in the Semiarid Canary Islands (Spain). Properties and Agronomic and Ecological Significance.

J. Asterio Guerra Sr., Antonio Rodríguez-Rodríguez, Carmen D. Arbelo, and Jesús S. Notario. University of La Laguna, Avda. Astrofísico Fco. Sánchez, La Laguna (Canary Is.), Spain

Pumice deposits locally known as jables cover a great surface of the semi-arid, southeastern side of the Canary Islands, and were generated during highly explosive volcanic eruptions, at a recent geological past (300.000-200.000 years ago). This paper examines a set of relevant ecological and agronomical properties of these soils including detailed analyses of ten soil profiles developed over pumiceous, trachyphonolitic tuffs, so as to represent their main features and typology, on a climatic toposequence comprised between 80 and 2350 m above the sea level and subjected to an aridic to xeric soil moisture regime. All the soils are clay-poor (41-186, sandy- to sandy-loam-textured, often with a high content in coarse (> 2 mm) pumice fragments that gives them a very high moisture retention capacity at 1500 kPa (60-210, and available moisture retention capacity (60-340 The bulk densities are always lower than 0.9 Mg.m-3, whereas the proportion of volcanic glass invariably exceeds 30% of the coarse sand. This leads to a more or less marked vitrandic character, based upon the oxalate-extractable Al (Alo) content (0.01-1.38%), P retention capacity (9-32%), pHNaF (> 9.5), and a calculated allophane content between 2.0-22.7%. Near the coastline, soils are typically alkaline (pHH2O = 7.5-8.9), to progressively turn into slightly acidic with the elevation (5.7-6.2 above 1000 m). CEC values (neutral 1N ammonium acetate) range from 16 to 78, and relate to the Alo content, rather than the organic C content, which suggests that their chemical surface reactivity relates basically to amorphous constituents (CEC:clay > 0.9). Unlike other volcanic soils developed over alkali-poor, felsic parent materials described elsewhere, soils along this sequence are highly base-saturated (100%) near the coastline, to moderately to high saturated (35-88%) at higher elevations. Exchangeable K+ occurs at high concentrations (7-33, together with exchangeable Na+ at lower elevations (19-42 and exchangeable Ca+2 at middle areas (10-21 At the highlands, under a cool and dry climate, pumices are less weathered, as shown by the low clay content and CEC, so the base saturation decreases, potassium is the most representative exchangeable cation and soils are quite closer to the parent material. Organic C contents range between 11-29, and are related to the vegetation type and coverage degree. A certain trend for the organic C to increase with the environmental moisture can be observed. Provided the steepness of the slopes (20-50%) water erosion predominates over soil formation processes. In addition, weathering of pumices is hindered by the soil moisture regime (aridic or xeric), whereby soil are mostly Entisols: Vitrandic Torripsamments near the shoreline, Aridic, Lithic or Typic Ustipsamments at middle elevations, and Lithic or Vitrandic Xerorthents near the summit (Leptosols, Arenosols, Regosols). Soils with a higher degree of development can only be found in flatter areas (slopes < 20%), with a more marked andic character and showing a cambic endopedion with texture finer than very fine sand: Andisols (Typic Vitritorrands), Aridisols (Vitrandic Haplocambids), and Inceptisols (Vitrandic Haplustepts and Vitrandic Distrustepts) (Andosols, Cambisols). These soils occur at the lowest and middle elevations, and show the greatest potential to be cropped, because of their high capacity to keep the environmental moisture, high fertility and feasibility for tillage. As a matter of fact, they have been cropped during the last centuries and constitute singular agrosystems over a unique terraced landscape, although agricultural practices have been given up in the last three decades in favour of the touristic development or the cash cropping along the coast, which was coincident with the shift of the Canarian economy towards the tertiary sector, giving rise to more or less intense degradation processes in these semi-arid lands. The man-made terraced hillslopes often observable along this soil sequence have made it possible to increase the agricultural surface, but also have been useful for water harvesting and soil conservation, constituting valuable agroscapes, both environmentally and aesthetically. They are also a cultural legacy from our ancestors that must be conserved as examples of sustainable soil use and management in adverse conditions.

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