A Quick & Dirty Guide to Wine Soils
Soil is just one component of ‘terroir,’ but it’s an important one. Here’s a look at the key soil types in the world’s greatest vineyards.
At its root, high quality wine depends on great vineyards and top-notch farming. But farming and vineyard quality depend on soils. In recent years, wine lovers have started to gain curiosity about the role soil plays in the wines they love. Different soil types have different levels of water holding capacity, which can influence overall vine health. Soils also have different potential temperatures, and that can change how quickly grapes ripen. And then there is the soil pH, its composition, and its geological origin. The details can start to get overwhelming. When it comes to expanding your understanding of the role dirt can play in your favorite wines, here are a few tips to expand your soil vocabulary.
Soil vs. Dirt
Soil consists of both inorganic and organic particles that together provide the structure that supports a plant’s growth (its housing, so to speak) as well as the nutrients a plant needs to grow (its daily meals). Soil is a complex ecosystem of living organisms, like earthworms and insects, as well as flora and fauna too small for us to see, such as fungi and bacteria. Farming influences the health of the soil because it can impact the balance of the microorganisms living within it.
Dirt, on the other hand, is inert. It’s the stuff that covers your clothes and hands after a day of gardening or follows your favorite pet into the house on its messy paws after a walk through the park. The ground that falls downhill during a landslide, or the truckload you move into your front yard for landscaping are both mounds of dirt, but they’re no longer soil. A pile of dirt can have its own structure—such as sand, silt, clay, or gravel—but because it was displaced, it no longer has the organized ecosystem that makes it soil. Dirt plus nutrients and living organisms become soil over time.
Soil vs. Rock
Soil is formed over time. Geological parent material (such as granite, limestone, schist, or volcanic rock) is one of the starting points of soil. Through weathering and erosion, these rocks are broken apart and transformed into smaller particles that eventually can combine with living organisms to create soil. The characteristics of the parent material influence the properties of the soil it eventually forms.
Different rocks weather and erode at differing rates. Granite, for example, is much harder than basalt and takes far longer to transform into soil. The characteristics of the original geological parent material also directly informs the properties it will deliver to its resulting soil. Generally, the larger the texture of the original parent material, the larger the texture of the resulting soil. The darker the original rock, the darker the resulting soil is too. And, the mineral composition of the original rocks influences the fertility of the resulting soils. Vineyards grow in soils from all sorts of rock types but below are a four of the most common.
Granite is a lighter colored, coarsely textured rock that weathers into grainier soils such as rough sand. Granite includes significant portions of feldspar, mica, and quartz. As a result, it tends to be high in potassium and sodium. Because granite is also very hard, it takes a long time for it to break down into smaller particles. The structure of granite-based soils can mean less water holding capacity and lower soil fertility. The result can be vines that need to either push roots deeper to capture the moisture and nutrients needed, or smaller vines and lower yields.
Common wine regions: Stellenbosch in South Africa; parts of the Dão and Douro Valleys of Portugal; the Northern Rhône; Alsace; Muscadet in the Loire Valley; the coastal mountains of Chile.
Basalt forms as a darkly colored, volcanic rock that is high in iron, magnesium, and calcium but low in potassium and sodium. It forms with very small crystals and weathers more easily than harder materials such as granite. Basalt becomes finer particles of powdery silt and clay, and as a result, basalt-based soils can have very good moisture retention as well as more abundant soil fertility. The result can create higher vigor and yields in vineyards growing in basalt-based soils.
Common wine regions: Canary Islands; Madeira; Mount Etna, Sicily; the Rocks District of Walla Walla Valley; parts of the Willamette Valley; the Yarra Valley in Australia.
Limestone is a sedimentary rock rich in calcium carbonate and can include significant amounts of magnesium carbonate as well. Limestone can include fossilized animal skeletons and seashells, as it is commonly formed underwater or in wet transitional zones where land intersects with a sea. How limestone weathers depends significantly on its surrounding climate. It can be soft and pliable in wetter climates, but very hard in drier ones. As a result, its influence on a vineyard also depends on the region’s weather. The calcium natural to limestone seems to help grapes retain their acidity more readily.
Common wine regions: Barolo and Barbaresco in Piedmont, Italy; Champagne; Burgundy; the Jura of France; Tuscany; Jerez in Spain; the Robertson Valley of South Africa; Ballard Canyon in California.
Though schist and slate are not the same rock from a geologist’s point of view, in winegrowing regions around the world the two names are often used interchangeably. Both are dark and brittle, quick easily fractured. And both also tend to retain warmth when exposed to the sun or heat of the day, which can encourage grape ripening. The hardness and coarse grain of the rock though mean the resulting soil tends to have less water holding capacity, meaning vines push to grow deeper roots.
Common wine regions: Galicia in Spain; the Mosel of Germany; Banyuls in France; the Languedoc; Anjou in the Loire Valley; parts of the Douro Valley in Portugal; Central Otago of New Zealand.