M. Tejedor

Volcanic materials as mulches for water conservation

Abstract Arid regions are characterised by a limited rainfall, a circumstance that acts as a barrier to dryland farming. Lanzarote in the Canary Islands (Spain) is one of the most arid regions in Europe, with less than 150 mm annual rainfall and potential evapotranspiration in excess of 2000 mm. A traditional farming system developed on the island has led to a diversified and productive form of agriculture that uses no irrigation. The system utilizes a layer of 10–20 cm of tephra, covering the natural soil. The tephra acts as mulch and is highly effective for soil water conservation. In this work, we present the results of a 3-year monitoring of soil moisture content in three plots covered with two types of basaltic tephra with different grain sizes. The results are compared to those obtained on adjacent plots that were not covered with the mulch. Sampling was conducted once a month every 10 cm, up to a depth of 1 m. Compared to the noncovered soils, the tephra-covered soils managed to retain eight times more water in the surface layer during the driest months. At a depth of 1 m, twice the amount of water was retained in the tephra-covered plots. This positive effect was explained by the tephras physical properties, particularly its low water retention capacity and high porosity, enhancing water infiltration and reducing evaporation rates. Differences observed in the water conservation depending on the mulch type was explained largely by grain size. The pyroclastic materials proved to be very effective for soil water conservation under arid conditions.

Soil Moisture Regime Changes in Tephra-Mulched Soils

Large areas of the Canarian archipelago are catalogued as aridic according to U.S. soil taxonomy. A traditional system of cultivation, based on the application of a mulch layer of tephra, is widely practiced. The objective of this work is to compare the classification of soils with and without mulch using U. S. soil taxonomy, and illustrate the problems in estimating the soil moisture regime on the basis of meteorological data. In this study the moisture regime of a natural soil and an adjacent soil mulched with a 12-cm layer of basaltic tephra was characterized on the basis of gravimetric moisture measurements, taken over a period of >1 yr, at 10-cm intervals to a depth of 1 m. The moisture regime of the soil was also estimated using meteorological data. Results obtained for the natural, nonmulched soil using measured values and those estimated from atmospheric data are very similar, and both indicate an aridic soil moisture regime. The difference with the soil covered with basaltic tephra is noteworthy. In the latter case, the soil remained moist throughout the year, representing a udic moisture regime. This noteworthy difference is reflected in the classification of the soils: Aridisols in the first case and Inceptisols in the second. A proposal is made to include mulching with tephra as a cultural practice along with irrigation and fallowing in the definition of the classes of soil moisture regimes in U.S. soil taxonomy. This would prevent the practice of mulching with tephra from changing the soil order placement in U.S. soil taxonomy.

Soils of volcanic systems in Spain

The Canary Islands are the most important volcanic zone anywhere in Spain, considering both extension and age of the eruptive events. On mainland Spain, the most important volcanic areas are found in Campo de Calatrava (Ciudad Real, Central Spain), Cabo de Gata (Almería, Southeast) and Olot (Girona, Northeast). In former two, which are older, soils do not present andic characteristics (Vizcayno Muñoz et al. 1979ab, Simón 2005). In Olot, Andisols developed on basaltic scoria and lapilli under perudic climatic conditions have been described (Bech 1976ab). Andisols on non-volcanic materials have also been described in Galicia, on easily weatherable parent rocks such as gabbros, amphibolites and fine grained schist rich in biotite (García Rodeja et al. 1987). The soils are well drained and formed under a udic moisture and mesic temperature regime. They are non-allophanic Andisols in which the colloidal fraction is dominated by Al-humus complexes. Of a similar nature are the Andisols described in Navarre on phyllites under perudic conditions (Iñiguez and Barragán 1974). Given the already mentioned greater significance of the soils of the Canary Islands, the variety of soil forming factors, the importance and peculiarity of land use in recent volcanic landscapes and the extensive research performed in the soils of the archipelago from different Soil Science disciplines, this chapter will focus mainly in canarian soils, with brief description of the others-limited by the scarcity of the research. We will describe the environmental conditions in the Canaries, the soil types and their distribution, with special attention to Andisols, and will situate in this context the Canarian reference profiles used in the COST622 European project.