If You Like Volcanic Wine, Thank Fungi

“Each teaspoon of dirt is teeming with millions and millions of bacteria, fungi and other microbes,” says Sam Kaplan, winemaker of Arkenstone Vineyards in Napa Valley, which has been organically farmed on Howell Mountain’s volcanic soils since the winery’s inception in 1988.

Indeed, no matter where you are, if you reach down and scoop up a handful of earth, you’re not just holding dirt. You have in the palm of your hand a vast collection of microscopic organisms.

These microbes, particularly fungi, made it possible for plants to evolve to inhabit a once inhospitable Earth some 500 million years ago. Fast forward to today, and they still play a vital role in plant life. One “we’ve only just begun to understand,” says David Montgomery, professor of geomorphology at the University of Washington and author of several books like What Your Food Ate and Dirt.

How many times have you heard a wine professional wax poetic about a vineyard’s soil? But how do the vines, and subsequently the grapes, actually reap those supposed benefits? Especially when vines are grown in volcanic soils, which typically have very little organic matter and nutrients that are still locked away in rocks.

Yet, volcanic soils manage to produce some of the most coveted wines in the world. Emerging science would suggest that the microbes toiling away under the surface make it possible.

Spores (shown in blue) and hyphae (shown in green) of the arbuscular mycorrhizal fungus Rhizophagus irregularis. / Image Courtesy of Vasileios Kokkoris

On a long enough timeline, all soil is volcanic. Sort of.

“Volcanic soil is a vague term because all of the rocks on the earth’s surface, whether they be sedimentary(1) or metamorphic,(2) have been derived from volcanic rocks,” says Robert White, professor emeritus of agriculture and food and ecosystem sciences at the University of Melbourne. He’s also coauthor of Healthy Soils for Healthy Vines and author of several other books on wine and soil. In other words, when we talk about volcanic soils, we are really talking about (relatively) young soils.

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But you can classify something as volcanic if “you drill down into the soil and find volcanic rock below, then one can assume that’s the parent material,” says White. Some common types you’ll find in vineyards include basalt, andosol and dolerite. However, we aren’t journeying down quite that far. Instead, we’re going to hang out in the rhizosphere,(3) with the roots.

“Microbes and plants have a symbiotic relationship,” says Montgomery. “The microbes serve as miners and truckers that are going and getting things out of the soil to deliver them to the plant. That’s where it becomes interesting ecologically because the plants are feeding the microbes with carbohydrates, proteins and fats they drip into the soil.”

Naturally, depending on where you are, the composition of volcanic soil can and will vary greatly. But in general, you will find similar minerals in volcanics across the board (the ratios will differ between andesitic soils and basaltic soils though), according to Scott Burns, professor emeritus of geology at Portland State University in Oregon.

Dust seeds so small and light that they can be dispersed by the wind—about 60 micrometers around; they will not germinate until they have met fungi that will provide energy and nutrient / Image Courtesy of Vasileios Kokkoris

Because volcanic soils are quite young, there’s a wider range of nutrients and minerals “because the microbiota,(4) including the fungi, haven’t been working on them as long,” says Greg Pennyroyal, head of vineyard management at Wilson Creek Winery in Temecula, California, and professor of viticulture at Mt. San Jacinto College. However, while there is an abundance of minerals and nutrients, they are often locked away in rocks that haven’t been broken down yet. And, as mentioned, there is often very little organic matter. This means that the microbiota, and subsequently the grapevines, have to work harder to reap the benefits to survive.

Luckily, “Fungi actually do quite well in young soils,” says Pennyroyal, “because they can produce very, very strong acids that biochemically break down nutrients in the soil.”

Take arbuscular mycorrhizal fungi,(5) which, according to a study by the journal Microbes and Environment, “plays a significant role in the establishment and resilience of vegetation in harsh environments, such as volcanic slopes.” Not to mention, this fungi is particularly savvy at acquiring, and thereby supplying, plants with phosphorus and can even reduce nitrogen loss. The latter is especially helpful for vintners, as volcanic soils are often nitrogen deficient (which can result in myriad issues including stuck fermentations).

The microbes really have to struggle to get minerals like iron and magnesium out of the rocks. “But that being said, there’s a greater microbial diversity [in volcanic soils],” says Pennyroyal. “This diversity allows plants and microbes to establish a system that allows for a consistent nutrient flow to the fruit.”

Speaking of microbial diversity, volcanic vineyards are often full of porous rocks due to the nature in which they were originally formed. But “what’s really fascinating about those vesicles(6) is they create the perfect habitat for microorganisms,” says Jessica Cortell, owner of Vitis Terra Vineyard Management, in Willamette, Oregon. “And to my knowledge, no one has studied those potential colonies.” In other words, we’ve barely scratched the surface on what’s going on just below it.

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Pictured is the moment the plant seed and fungi meet. The lower seed has connected with a mycorrhizal fungus and begun to sprout—a very rare sight. The fungal hyphae connect it to the as-yet-unsprouted seed above it. This image was produced via confocal laser scanning microscopy by biologist Merlin Sheldrake / Image Courtesy of Vasileios Kokkoris

Coming Up For Air

There’s no denying that, despite all the underground fungal assistance, it’s tough for vines in volcanic soils. “Our soils are void of readily available nutrients and water—volcanic soil drains exceptionally well,” says Jake Krausz, estate director of Arkenstone in Napa Valley. This inevitably leads to a smaller crop load, but one that the vines have put almost all their resources and energy into.

The resulting wines tend to have concentrated acidity and a minerality sensation on the palate. The latter can be a controversial tasting term. But Claire Jarreau, associate winemaker at Brooks, a biodynamic winery in Willamette Valley that sources from about 30 different volcanic vineyards, argues, “We have to trust our palates. I don’t shy away from the word minerality. I love mineral-driven wines in general. I don’t know how much of that is attributed to the soil directly; that’s a great unknown and I hope more research is done on that.”

If we want to have a better understanding of what’s in our glass, we need to get down with the microbes. And volcanic soil is a good place to dig in.

(1) Deposits left behind by wind, water or ice

(2) Substantially changed from their original form

(3) The narrow zone of soil nearest the soil directly influenced by root systems

(4) Microorganisms of a particular location

(5) AMF develop symbiosis with root systems for an exchange of essential nutrients and minerals

(6) Holes, or tiny cavities, formed by bubbling gas in cooling magma

This article originally appeared in the Winter 2024 issue of Wine Enthusiast magazine. Click here to subscribe today!

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