Monday, September 29, 2014

The Myth of "Rich Volcanic Soils" Or How I Learned to Love Molisols.

A lot of Washington grape growers tout the "rich volcanic soils" that our grapes are grown on. There is even a writer in the area writing a book about it. Perhaps that isn't the case and a little clarification is necessary.

Yes, the occasional volcano blows up to the west in the Cascade Mountains and some ash blows eastward. Does this make up our soils? Big ole Mt. Mazama gave us about 12 inches of new soil but that was about 8000 years ago. In 1980 Mt. St. Helens deposited about 1/2 inch of dust on the Rattlesnake Hills. Often the wind blows this dust to the west as it did the day Mt. St. Helens covered Portland in ash.

Volcanic soils are called Andisols and occur in the close vicinity of volcanoes. Hawaii, the South Pacific, Japan and even soils within the Cascade Range are known for Andisols. The grapes around Mt. Etna in Sicily are grown on Andisols.

So, if the soils in wine country aren't Andisols, what are they? It turns out the majority of the soils in Eastern Washington are Molisols with some Aridisols. They are less than 1% Andisols.

Molisols are formed by grasslands and you might know that the desert steppe of eastern Washington was at one time a great grassland covered by the native blue bunch wheat grass. The surface horizon was formed by wind-blown (Aeolian) sediments from the retreating glaciers of the ice age with the occasional deposit of volcanic ash for the Cascades.

Aridisols are the desert soils that occupy arid shrublands. They are found in the center of the Columbia Basin (known to many as the Columbia Valley.)

The next time some well-know and often-quoted grape grower spouts off about "rich volcanic soils" you may now correct him. They are Molisols and Aridisols. Sorry, no Andisols.

By the way, Molisols and Aridisols are great for growing wine grapes!


  1. Interesting stuff, Gail. I was just down in the Snake River Valley. They have a lot of cinder cones on the Sunny Slope, which are fascinating to look at and do apparently have some effect on the soils (a fair bit different than those found in Washington). I'll have to ask a geologist friend down there about them.

    Thanks for posting this.

    1. Thanks for the comment. Judging from the map, the chances of grapes being grown on Andisols in Idaho are pretty good.