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 tephra's 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.