Soil impact on wine debunked

PORTLAND — Geologists say wine critics may be stretching things a bit when they talk about the soil giving a vintage a distinct flavor.

Wine critics often use the French phrase, "gout de terroir", which means "taste of the soil."

But scientists who met in Portland this week for the annual Geological Society of America conference say much of that taste may be imagined.

The geologists say wines may vary in levels of dissolved minerals, but those variations aren't related to the levels in vineyard soil. And they say the concentration of minerals in wine is below the threshold of human taste and smell.

"I am not saying that chemistry and geology have no effect on the wine. It may have effects that we don't understand," said geologist Alex Maltman. "But whatever 'minerality' in wine is, it is not the taste of vineyard minerals."

Debunking mistaken notions about terroir — how landforms, soils, climate and other local conditions define the character of wine — was one goal of the scientists during the meeting.

"When people talk about terroir, it all sounds very fancy and all very marvelous and it makes it sound like we really know something. But I guarantee you, we know very little," said Jonathan Swinchatt, a geologist and consultant to several high-end California vineyards. Swinchatt, however, said he's found subtle differences in soil texture that make a huge difference in grape quality in California's Napa Valley.

One grower told Swinchatt about a "sweet spot" in his vineyard that consistently produced his best grapes. By digging pits across the vineyard to study the soil, Swinchatt found that the sweet spot coincided with a clear geological feature: ancient volcanic debris lay much shallower there than in the rest of the vineyard.

At another Napa site, vines growing on adjacent, similar-looking gravel-and-sand soils produced grapes consistently different in character.

Swinchatt found that the two soil areas had different geologic histories: Floods laid down one, while volcanic debris flows deposited the other.

"Some factors in the geology are reflected in the winemakers' tastebuds," Swinchatt said.

For example, the soil's water-holding capacity can make a difference. Different soils create better or worse conditions for roots and the fungi that help roots extract nutrients. But scientists have barely started to explore these factors in vineyards.

Climate also is shaped by geology. In eastern Washington, the landscape is wrinkled into giant folds by north-south compression of the bedrock. As a result, "there are spectacular differences in climate over small distances," said Kevin Pogue, a geologist at Whitman College in Walla Walla.

Pogue found one site that enjoys on average 65 more days without frost than another just a mile and half away.

In Oregon's Rogue Valley near the California border, climatologist Greg Jones of Southern Oregon University combined information from soil surveys, climate records, zoning and other sources to map areas most likely to produce excellent wine grapes.

Jones found that more than half of existing vineyards are planted on land that is only marginally suitable for growing grapes. Nearly a third of the planted acreage is mismatched to climate: Cool-climate grapes such as pinot noir are growing where it's too warm, and varieties requiring more heat are growing where it's too cold.

Scott Burns, a geology professor at Portland State University who helped organize the session, conducted a test that did little to settle the question. He offered fellow scientists wines made in the same style from the same pinot noir clones on two different soils: volcanic jory versus marine-sediment Willakenzie soil.

In a show of hands for a taste test, geologists picked the Willakenzie 40 to 15. But Burns, who has repeated the test with other audiences, said the running score is close to 50-50.

"Nobody is right," Burns said. "It goes back and forth."

Share This Story