Like sand or silt, clay is a composite of igneous, sedimentary and metamorphic rocks.
Sand is the coarsest, with particles that measure approximately two millimeters across, according to Alex Maltman, author of Vineyards, Rocks, and Soils: The Wine Lover’s Guide to Geology. “Finer ones are called silt, and the very finest clay,” Maltman writes.
Due to this fine-grained texture, clay retains more water than sand or silt. This is especially useful for winemakers in hot, dry climates.
|VINEYARDS IN POMEROL, BORDEAUX, FRANCE
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“On the winemaking side, I can’t think of any negativity to clay,” says Sam Parra,
Clay soils tend to be rich in nitrogen, he says, which benefits wines in several ways.”
“Grapevines use nitrogen to build essential compounds, including proteins, enzymes, amino acids, nucleoid acids and pigments,” says Parra. “This is great for winemakers. The vineyard is healthy, and the fruit comes in near perfect where minimal additions in winemaking are needed.”
Clay soils also provide the potassium that vines need to form sugars and starches, says Parra, and phosphorus to encourage bud initiation. “This is really good for minimal-intervention winemaking,” he says.
Plus, because clay retains water, it maintains cool, consistent temperatures below the vine. That’s helpful in sunny vineyards, where fruit might otherwise ripen quickly and lose acidity.
|SAM PARRA / PHOTO BY SEAN CARR PHOTOGRAPHY
On the other hand, clay soils have poor drainage. In maritime microclimates and areas with ample rainfall, grape vines grown in clay can become waterlogged.
Clay is heavy, too, and requires more labor.
“On the viticulture side, clay makes me think of the long hours my uncle and grandfather would put in,” says Parra, whose relatives worked in vineyards in Napa Valley. Clay soil can strain tractor discs, he says.
Some wine professionals believe clay best serves grape vines in conjunction with other soil types.
“An excess of clay can stifle the vine’s root system, but a proportion of small clay particles mixed with other soils can be advantageous,” writes Tom Stevenson, author of The Sotheby’s Wine Encyclopedia, in a post on TimAtkin.com.
|CLAY SOIL IN RIOJA / GETTY
Napa and Barossa Valley also have notable clay vineyards.
“Some of the most significant domestic Syrahs and Merlots I’ve ever had were grown in Carneros in a type of clay called Diablo,” says Parra. “Clay produces powerful wines, but still with finesse.”