INSIGHTS N.5:

Soils of Etna Terroir

A contribute from Marco PerciaboscoRegione Siciliana, Dept. of Agriculture, Soil Science Operative Unit

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It is not very simple to describe complexity and variability of Etna soils, which strongly contributes to characterize the Terroir of this area.

In the Etna vine environments is it possible to find specific factors which characterize the different Terroirs and also influence soil formation and evolution; these factors are climate, anthropic landscape and pedogenic substrate or parent material.

Climate of Etna area is defined, according to Köppen Macroclimatic Classification, as “mesotermic humid sub-tropical with dry summer (Csa type)”, that is a typical Mediterranean climate characterized by an average temperature, in the coldest month, less than 18°C, and an average temperature, in the warmest month, more than 22°C, and a rainy period mostly concentrated in autumn-winter seasons. Rainfalls register values comprised between 1.000 and 1.200 mm/year on the northern, eastern and south-eastern mountainsides, while in the south-

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western mountainside are much lower, with values of about 500 mm/year. Thermic excursions are mitigated by the sea influence, except in the south-western mountainside where they are more marked.

With regarding to anthropic landscape factor, it is important to remember, first of all, that the volcanic activity of Etna is both effusive and explosive. In the first case lava flows are generated, in the other case, airborne ejecta, called pyroclastics like lapillus and ashes, are produced. Human activity, in Etna area, was dedicated to model a characteristic landscape which adapt to the complexity of the volcanic cone slopes, produced by the several lava flows which followed one another through the centuries. So anthropic terraces, which are a peculiarity of Etna landscape, were created with the aim of decreasing steepness, increasing rain water infiltration in the soil, protecting soil by erosion, achieving a suitable soil depth for cultivation and facilitate soil tilth management.

Several stone clearance were also carried out to obtain suitable surfaces for agriculture and the stony material was used to build the walls for terraces. It was calculated that approximately 70% of vineyard Etna area is interested by

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terraces, some of them quite ancient, but there are also recent terraces built according to the requirements of modern viticulture.

Parent material finally, that is the mineral fraction on which evolve Etna soils, is mainly constituted by the pyroclastic material (like lapillus and ashes) produced by the explosive activity of the volcano. These materials generate a coarse texture (particle size distribution) producing soils with an aerated hypogeal environment, very suitable for the development and growth of vine roots; for the same reason in these soils water stagnations are very rare and, in the meanwhile, they warm very easily, generating suitable conditions for the occurrence of the chemical reactions for the weathering of the finest materials. But the main characteristic of the volcanic soils is the occurrence of Allophanes, which are amorphous (non crystalline) clay minerals, consequent of the chemical weathering of volcanic ash. These minerals are chemically very reactive, and produce some agronomic important properties, like high exchange cation capacity, which means a high capacity to retain fertility elements (like nitrogen, potassium and phosphorus) in the exchange complex of the soil particles, which will be gradually available for plant nutrition. These materials also produce soils with high water retention, despite of the their coarse texture, due to their high capacity to retain water on the suface of the soil particles. So, volcanic soils with good percentage of allophanes are characterized by both chemical and physical fertility, due to their high exchange cation capacity and high water retention; at the same time they are also characterized by a good water drainage and aeration, producing a suitable hypogeal environment for root growth.

In the pedogenesis of the volcanic Etna soils, pedogenic factors thus described, have great importance and are useful to understand the distribution model of the soils in this area. As formerly described, the parent material is constituted, almost exclusively, by pyroclastics and the chemical weathering of these materials in allophanes is strictly connected to soil water content and temperature; at the same time, terracing facilitate the maintenance of wetness in the soil and accumulation of fresh material coming from eruptive explosive activity of the volcano. Finally, the lava flows, which occurred through the centuries, produce “natural holder” of the airborne parent material, implemented by terracing, and created a complex pedological distribution model.

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To understand the distribution model of Etna soils, a physiografic photointerpretation of the entire area was carried out, using satellite images and digital elevation model, with the aim of identifying the convexity or concave areas of the slopes and characterizing the hydrological fluxes along the slopes and the rates of accumulation of airborne volcanic materials.

Exposition of the slope plays also an essential role in soils’ pedogenesis, because it influences intensity and duration of solar radiation and so having effects on evapotranspiration, water content in the soil, weathering of primary minerals and accumulation of organic matter.

According to soil survey dataset and their elaboration, it was possible to identify five different landscape systems, defined as follows:

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• Northern landscape

• North-eastern landscape

• Eastern landscape

• South-eastern landscape

• Southern landscape.

Soils of these environments differ between them for the grade of weathering of primary clay minerals: coming from north towards south, the wetness of the soils decreases and, consequently, the weathering of the volcanic constituents of parent material. So the soils of Northern landscapes, compared with soil of Southern landscapes, are characterized by a finer texture, a higher content of

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organic matter and a higher values of cation exchange capacity, which are agronomic properties of great importance.

A soil map of Etna area, at a scale of 1:25.000, was produced and it is visible trough an interactive WebGis at this website http://www.sias.regione.sicilia.it/SIT/, clicking on “SUOLI”, after “Aree DOC” and finally “DOC Etna”. In this work, environmental peculiarities of the area, factors influencing soil formation, methodology followed for the soil survey and the production of cartography, physical and chemical soil properties are described.

Soil map of Etna area is structured in 60 map units, which are related to the landscape systems described before; in the report associated to the soil map, are

described several properties and qualities of the soils, useful for their agronomic management.

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Try to access this map at this direct link:http://151.99.146.37/SIAS/APD/Viewer/viewer_etna_suoli.aspx?id=etna_suoli&n=t

For a more specific use for viticulture, some “Vineyard management papers” were produced for every single map unit significantly interested by viticulture, and they are visible in the WebGis formerly described. These Papers report technical agronomic advice, to apply in the specific soil type, for the vineyard management. In every paper are emphasized advised practices and practices to avoid for general agronomic themes like soil management, tilth, use of grassing in the rows and fertilization, and for specific viticulture themes, like the choose of rootstock, training system, plant vineyard density and foliage management.

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The organizers wish to thank Marco Perciabosco for his kind availability to share with us this document.

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