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The Dirt on Dirt

The Dirt on Dirt

We Renoites are very fortunate to have 90% of US wine being produced virtually in our back yard. But what is it about California that makes it ideal for grapevines? Part of the answer is the soil.

There are many types of soil from Albariza to Rendzina to Tuffeau to Schist. California is home to many of these soil types. Napa alone has over 36 different types of soil. Soils differ in their color, in their structure and in their chemical make-up. At first glance one would think that the chemical make-up of a soil would have a great influence on the flavor of the wine. But one would be wrong.

The chemistry of the soil is its pH, its acidic or alkaline level. Grapes are a little particular in that they won't grow in very acidic soils. A pH below 5.5 will inhibit the growth of grapevine roots. The pH of the soil is determined by the organic matter in the soil. An alkaline soil will have more nitrogen, phosphorus, calcium, magnesium and potassium, while an acidic soil will have more iron, boron, zinc and manganese. You would think some of those minerals and elements would effect the flavor of the wine. But most experts agree that it isn't the chemistry of the soil that effects the wine; it is the soil's structure.

The structure of the soil is important to grape growing for three reasons. The structure determines the soil's drainability; it determines air movement, and it determines root penetration. These three things together effect the ultimate wine more than the minerals within the soil. The deeper the root penetration, the more flavorful the grape. The greater the air movement within the soil, the less likely the plant will develop mold or viral problems. The the level of drainability of the soil will effect the temperature and acidity of the soil. If a soil is well-draining, like a sandy soil, it is better able to retain heat, which is a good quality for many red grapes. Also, because it is well-draining, elements such as calcium and potassium leach out of the soil, creating an acidic environment. A soil that has more of a clay texture will not be as well-draining, and will be wetter and therefore, cooler. It will also retain its calcium and potassium, creating a more alkaline environment. The cooler temperature is good for retaining acidity in the grapes, such as one would want in most white grapes. There is a simple rule of thumb for grape planting; dark soil for red grapes, white soil for white grapes. While this isn't always the case, the general idea is that white soils reflect heat, keeping a cooler environment for the acidic white grapes. Dark soils retain heat which is preferable for the phenolics of red grapes.

Washington State is known for its sandy, iron-rich soils which are great for growing Merlot and Syrah. Does the well-draining soil create a warm environment that produces a richer, more flavorful wine? Absolutely. Can one taste the iron in the wine? Experts would say no.

Chablis is known for its Kimmeridgeon soil, a calcium-rich, clay that maintains a cool environment for the Chardonnay grapes for which it is known. Does the texture of the clay effect the flavor of the wine. You bet! Can one taste the calcium in the Chablis? Experts would say no. But it is something to think about. When tasting wine, can one taste the acidity and chemistry of the soil, or only the effects of the soil structure?