How do different soils affect grapes?

There are no two ways around the fact that great wine is made on the vine. While many factors affect grapes in the vineyard— such as local weather, microclimate, and topography—perhaps the most important is soil.

What is soil?

In viticulture, soil is composed of weathered rock, air, water, minerals and humus (decomposed plant and animal matter). Together these elements create an ecosystem in which a vine can thrive.

The ground under a vineyard isn’t one homogeneous block of soil. It’s divided into layers: topsoil, subsoil, and bedrock. Topsoil is the soil that you can see/that sits on the surface. Below the topsoil is the subsoil, which is harder and more compact, and contains less organic material but generally more minerals. Often, when we refer to soil, we are referring to both of these layers. Below the subsoil is the bedrock, which is rock that hasn’t been broken down. Vine roots often reach and in some cases penetrate the bedrock, therefore it is important to understand not just the soil a vine is planted in, but what’s underneath.

What is high-quality soil?

There is no single set of characteristics that constitute high-quality soil. Certain characteristics that are beneficial in one region, may be detrimental in another and certain characteristics that work well with one varietal may not work well with another. It all depends on region, varietal, and winemaking intent and style.

In general though, high-quality soils have two things in common: they lack nutrients, and retain just enough water to support a vine but not enough to make the vine overly vigorous. In other words, high-quality soil is well-draining and infertile. Well-draining soil forces vines to spend most of their energy on root growth in search of water, resulting in smaller more concentrated grapes that are ideal for winemaking. Nutrient scarcity further inhibits vine vigor, resulting in smaller grapes.

While we know for certain that soil affects grapes, we cannot scientifically say an aroma or flavor in a wine was caused by a certain aspect of the soil. However, we often talk about a wine’s minerality or a wine having certain stone aromas or flavors. Minerality and stoniness are real characteristics but not directly linked to minerals or stones in the soil.

Soil Types

There are tens of thousands of soils in the United States. Napa Valley AVA, one of the smallest wine regions by area in the world, contains 33 different soil series and more than 100 different soil variations. A cross section of one vineyard in Napa, or anywhere else for that matter, can reveal a mosaic of different topsoils, subsoils and bedrocks.

Like the primary colors of , there are basic types of soil from which the rest are derived: clay, silt, sand and loam.

Burgundy expert Jasper Morris eloquently dispelled our common confusion with these categories: “We tend to think of clay, sand, and gravel as the entities which we come across in real life: sand as small particles on the beach, clay as a thick glutinous substance which sticks to the boots in wet weather. To a scientist though, these words denote different sizes of particles.” Clay is composed of the smallest particles, silt is made up of slightly larger particles, and sand is made of even larger particles. Loam is a combination of equal parts clay, silt, and sand. Loam is too fertile for high-quality viticulture but when mixed with one or multiple other soils can produce some of the most sought-after soil in the world.

Certain soil and varietal pairings are considered classic, like Limestone with Chardonnay and Pinot Noir, made famous by Burgundy, or Cabernet Sauvignon in gravelly soil, see Oakville and Rutherford. However it’s not as simple as saying, “X varietal goes with Y soil.” Climate, geography, and viticultural history are significant factors that contribute to those varieties success in those soils.

Some Soils of Note

Russian River’s Sandy Loam

The Goldridge sandy loam of the Russian River Valley is light, fluffy, and lacks nutrients. The high percentage of sand improves drainage, forcing roots deep for moisture, which limits canopy vigor, resulting in small bunches of highly concentrated grapes.

Napa’s Alluvial Benchlands

The western edge of Napa Valley is dominated by alluvial fans composed of materials eroded down the Mayacamas Mountains over millennia. These gently sloped fans are composed of an array of coarse gravels, clay loams, and bale loams. Their coarse texture and gentle slopes make these soils very well-draining, while a small percentage of clay retains water and remains cool during hot summers—a perfect combination for cultivating robust world-class Cabernet Sauvignon. 

Carneros’s Clay

10,000-15,000 years ago the mouths of Napa and Sonoma Valleys was submerged under the San Pablo Bay. As the bay receded, it left behind a shallow dense clay topsoil sitting atop an impenetrable clay hardpan. Today, these lowlands form Carneros AVA which receives the lowest average annual rainfall in the region, so the dense clay soils help retain any precious water that does fall and the hardpan forces roots to fan out, keeping them near the surface, allowing them to soak up that scarce rain.

Burgundy’s Limestone

Burgundy contains a variety of different limestone but one of the most common is marl, a mixture of clay and limestone. In marl, clay provides nutrients that limestone lacks, while limestone improves drainage. Limestone’s unique chemical properties allow it drain well in cold weather and retain water in warm weather. Its natural alkalinity complements high-acid varieties, like Pinot Noir and Chardonnay.