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.
- 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.
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.
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.
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.
©Wine -- Mise en abyme