How does anything possibly grow enveloped by rocks? Although the sight of slate in vineyards might seem ominous, the soil type is perfect for growing wine grapes. From the Clare Valley vineyards in Australia to the Cederberg mountains in South Africa, slate soils are utilized all over the world.
But what is slate, exactly?
“Slate is a low-grade metamorphic rock that is formed when muds that were deposited on ancient ocean floors are buried and heated up,” says Amanda Albright Olsen, associate professor of geochemistry at the University of Maine.
The word “slate” is believed to come from the Old French word esclate meaning “to split or break into pieces.” Used in roofing, tiling and flooring, slate’s durability comes from its mineral composition. This fine-grained foliated rock is composed mainly of clay minerals from pre-existing rocks like quartz and mica.
The slate rocks most often found in soils are gray, but slate also comes in myriad striking hues. Some winemaking regions with slate soil are known for their distinct colors, like Priorat’s deep black slate composites and the iconic blue or red slate of the Mosel.
“The colors generally have to do with chemical impurities,” says Olsen. “Green is rich in magnesium, red contains iron that has been oxidized, black contains organic matter.”
Dark hues also make slate a great conductor of heat. This is especially important for one of the coolest wine regions in the world, Germany’s Mosel. The vines capture the intense sunlight during the day and transfer that stored energy to the soil whenever vines need sustenance.
“The slate does conduct heat for the soil, but the greenery on the slate also prevents it from getting too hot,” says Thorsten Melsheimer, head winemaker and owner of Weingut Melsheimer. “They don’t tell you about that.”
Slate’s slick exterior also promotes drainage, which is very helpful for climates that undergo heavy rainfall and flooding.
“We have Mosel floods very often,” says Melsheimer. “Especially at the bottom of my steep vineyards, it gets overloaded often with water, and the soil helps.”
Slate also decreases harmful soil erosion caused by wind, water and sunlight.
“The interlocking, jagged slate fragments give the soil a cohesion that helps reduce erosion on these remarkably steep slopes,” Alex Maltman writes in Vineyards, Rocks, and Soils.
On the other hand, the heavy, cumbersome structure of slate soils makes the vine work harder for its water. Wines’ pH levels can be influenced by the slate content in the soil, too.
Despite these potential challenges, there are many thriving, well-regarded, successful slate vineyards that hold their rocky terroir with great pride.
“I’ve been working in the winery in Mosel for 54 years,” says Melsheimer. “Slate soils are great and all I know, really. This is my home.”