Wednesday, July 6, 2016

Volcanic soils

During my recent visit to Sicily, I was able, thanks to the connectedness of my good friend Brandon Tokash, to spend three days in the Mt Etna region visiting producers and tasting a broad selection of wines. A key determinant of the character of those wines is the volcanic soil on which the vines are grown. I would, therefore, like to spend some time reviewing volcanic soils, in general, and Mt. Etna soils, specifically, prior to sharing my actual experiences in the region. I begin with the following mini review of volcanic soils.

According to V. E. Neall, volcanic soils cover only 1% of the earth's surface but supports approximately 10% of the world's population, the latter fact attributable to its high natural fertility. Volcanic soils are wholly derived from volcanic parent material and are distributed globally as shown in the map below.

Parent material escapes the volcano either as a result of lava flows or volcaniclastic events. According to Neall, only strong tropical weathering will reduce lavas to finer-grained volcanic soils and soils formed from this source are usually of low fertility. Further, of soils formed from lava, basaltic lavas (low viscosity, ability to flow large distances on low gradients) are the most significant source of parent material.

Soils from volcaniclastic parent material are, on the other hand, very productive. Volcaniclastics are usually classified as of pyroclastic (explosive) or epiclastic (erosional) origin. The figure directly below shows the origin and components of these two classes while the figure following shows the grain-size classification of volcanic fragments and rocks.

Formation of volcanic soils from parent material is effected through a combination of two processes: (i) the formation of non-crystalline materials (active Al and Fe compounds) and (ii) the accumulation of organic matter (Neall). This combination of processes is termed "andosolization" and its extent is affected by time, climate, the proportion of volcanic glass in the parent material, and grain size and vesicularity of the parent material (Neall; Ugolini and Dahlgren):
  • Andisols form rapidly in humid climates and alter to other soil orders as soil age and degree of weathering increases
  • Volcanic soils contain differing levels of volcanic glass
    • The lower-silica, higher mafic (high magnesium and iron) volcanics weather more rapidly than higher silica, lower mafic (high sodium and potassium) volcanics
  • A dense, high-silica, igneous volcanic rock will weather more slowly than a less dense and highly vesicular pumice (rocks containing holes made by gas escaping from cooling lava) of identical composition.
The first component of volcanic rocks to undergo weathering is volcanic glass and, as shown in the figure below, it weathers to a clay type called a short range order clay (SROC) which is characterized by a weak degree of crystallinity. The allophane and imogolite clays differ primarily in their shapes, with the allophane presenting as hollow spherules and the imogolite as a "tubular, thread-like material."

Selected characteristics of Andisols are presented in the figure below while the development of volcanic soils in varying climatic environments are presented in the figure following.

According to Neall, volcanic soils are usually the dominant soil in young volcanic landscapes. "Surrounding the volcano, and for large distances downwind, there may be a wide variety of landscapes upon which fine ash has accumulated over thousands of years by tephra accretion."On lowlands, that tephra could have accumulated to great thickness, providing deep, fertile volcanic soils. In hilly landscapes, erosional activity may have resulted in relatively thin soil profiles.

Shoji and Takahashi view the significance of volcanic ash soils thusly:
  • The periodic additions of volcanic ash generally improve the soil physical and chemical properties and renew the soil productivity
  • They accumulate a large amount of organic carbon and nitrogen as important components of soil organic matter that are the main source of nitrogen for plants and various nutrients and energy for soil organisms
  • The accumulation of carbon is important in the sequestration of atmospherreic carbon dioxode, one of the most important greenhouse gases
  • Soils with well-developed structure can hold a large amount of plant-available water.
Giovanni Ponchia, lead oenologist for the Soave Consorzio, identifies volcanic rocks as having a high level of macro-porosity in soils and "these pores allow the rocks to store water up to 100% of their weight and then release it very slowly thanks to their high water retention coefficient." This quality renders the rocks a very important source of water for vine root systems, "especially in years with little rainfall or even drought."

Fiorenzo C. Ugolini and Randy A. Dahlgren, Soil Development in Volcanic Ash.
Daniel Scheffer, Volcanic Soils produce unique wines, Style, South China Morning Post, March 2015.
McDaniel, P.A., Lowe, D.J., Arnalds, O., Ping, C.-L., 2012. Andisols. In: Huang, P.M., Li, Y,  Sumner, M.E. (editors) “Handbook of Soil Sciences. 2nd edition. Vol. 1: Properties and Processes”. CRC Press (Taylor & Francis), Boca Raton, FL, pp.33.29-33.48. 
Murcia et al., Volcaniclastic deposits: terminology and concepts for a classification in Spanish, Rev. Geol. Amér. Central n.48 San Pedro de Montes de Oca Jun. 2013.
Sadao Shoji and Tadashi Takahashi, Agricultural and Agricultural Significance of Volcanic Ash Soils
V. E. Neall,Volcanic Soils, Land Use, Land Cover, and Soil Sciences -- Vol III, Encyclopedia of Life Support Systems., Andisols.

©Wine -- Mise en abyme

No comments:

Post a Comment