Physical Properties of the Canary Islands’ Volcanic Pyroclastic Materials as Horticultural Substrates

In the Canary Islands and in other parts of the world where it can be found in its natural state, basaltic tephra, or “Picón” as it is known locally, is commonly used as a soilless substrate for crops. The aim of this study is to learn more about the physical properties of the Canary Islands’ tuff, and to find a simple method to predict the hydraulic behaviour of these substrates due to their heterogeneity. To accomplish this, 32 tuff samples were collected from all the quarries on the island of Tenerife (Canary Islands) that were authorised for the study. The tuffs had hydraulic properties that were highly influenced by the particle size. Coarse tuffs had an aeration capacity greater than 35% v/v and easily available water of less than 5% (v/v), while fine tuffs had aeration capacities below 20% v/v and elevated water retention (20 to 26% v/v). The intermediate tuffs had characteristics that varied between those of the two previous groups. Particle size fractions of less than 1 mm demonstrated the best correlation to common air:water ratios and present the best predictive capacity to relations involving air. By focusing on air:water ratios and the previous assumption, an attempt was made to predict the suitability of new pyroclastic material samples emitted by the La Palma Volcano as growing substrates for vegetables.