Terroir - Soil

Soil. Huh, what is it good for?

As winegrowers we try to produce the best wine possible. We want vines that don’t grow excessive foliage. We want vines that produce limited amounts of flavorful fruit. We want vines where the crop and the foliage are in balance. Soil has much to do with getting us there, though some soils are better suited to do this than others.

Soil does several things for the grape vine.
1) Soil supplies water and oxygen to the vine
2) Soil provides mineral nutrition to the vine
3) Soil provides a home for the vine to anchor itself

Each of these things can influence how the vine grows and the fruit it bears. As a general rule, the poorer the soil for agriculture, the better it is for growing fine wine. (But there are exceptions.) A vine planted in deep, moist, fertile soil will want to make more vine, not flavorful grapes. Soils that are poor at holding water and short on nutrients are less fertile, and more desirable for producing fine wine.

Soil moisture has a great effect on vine physiology and its structure relating to drainage is important. Vines need to be under mild water stress to improve red wine color and flavor. This can come from structural and textural factors that influence the depth to which vine roots grow and the amount of water held in the root zone. In texture we are talking about proportions of stones, sand, clay, silt and organic matter. In all great vineyard soils you see water moisture that recedes in August and stops vine growth around the time when grapes begin to turn color. The best soils in wet regions with summer rainfall drain freely, preventing excessive soil moisture after heavy rainfall. While a drainage problem in the wet regions obstructions to the roots in dry regions can keep the roots out of the water table. Water stress is where climate and soil interact.
Many who write about terroir see irrigation as
practiced as reasons for their belief terroir differences are smaller and not appreciated in the New World. Sometimes irrigation is presented as a bad thing. It is true, that soil differences affect vine water supply and vine physiology. But unlike California, in Europe there is an abundance, often an oversupply, of rainfall on un-irrigated soils, yet terroir shows itself each year. If the Old World growing regions received the perfect amount of rainfall each growing season would terroir go away? Can we argue that the concept of "terroir" can only be seen in excessively watered soils? If this were true, then a New World vineyard manager would merely over water his vineyard to get an expression of terroir.

Fortunately this is not necessary. The vine doesn’t know what is causing its water stress. Anyone who has made dozens of wines from the same variety, clone and rootstock from different vineyard blocks on Spring Mountain knows there is a wide variation block to block in wine aroma and flavor. We drip irrigate in many vineyard blocks. We have ways of managing water stress in vines that are relatively precise. The pressure bomb and the use of its readings in water stress management is a good example. Yet despite careful control, we still see striking differences in wines from block to block that are unrelated to water management. This is terroir.

One of the most commonly heard ideas is that wine flavor compounds are present in the soil, and are taken up by the vine roots. (Usually the proponent of this idea is not a student of vine physiology and wine chemistry.) This misconception is understandable and probably comes from a century or two of anecdotal observations that saw a direct connection between the soil under the vine and wine quality. However, looking at this experimentally, we see little evidence of any direct transfer of flavor components from the soil through the vine and into the grape. To the contrary, we see a great body of evidence that the differences in grape and wine composition that result terroir are compounds synthesized in the leaves and fruit of the vine. This is confirmed by studies in Bordeaux, the Loire, Languedoc, Australia, and here in California

Grapevine roots do, in fact, take up mineral nutrients from the soil selectively. There can be differences in the soil - excesses and deficiencies - which can impact the way the vine grows and matures fruit. The best crus in Europe are often grown in soils with deficiencies that limit the vigor of the vine. The classic example is the limestone soils in many of the famous European wine regions. These soils are high in calcium carbonates which makes them high in pH which, in turn, limits the availability of several micronutrients. These deficiencies, in turn, limit the size of the vine, the density of its foliage, the size of its crop, and the way flavor compounds develop. This is how soil composition can contribute to the wine’s terroir. Terroir is a real taste, it just isn’t the taste of the dirt.

A grape vine needs a place to put down an anchor. It will grow almost anywhere it can hold on and put down enough roots to bring it enough water and minerals to sustain itself. On some of the rockiest sites the vine may not produce, or be allowed to produce, usable fruit for nearly a decade until it establishes itself. On Spring Mountain with shallow soils and steep slopes soil erosion is a constant concern. Maintaining our soils is part of the process of maintaining our terroir. Just like soil is the foundation for the vine, soil is the foundation of terroir.

Soil is one of the reasons I take the winegrower out of the discussion of terroir. Though the grower may make adjustments to the way they fertilize, their root stocks and clones, their vine spacing, their canopy, and their crop size…any variable that they can control; terroir will likely be there, exhibiting what couldn’t be changed.

Soil and its effects on terroir are many and complicated. I would like to hear what other winemakers have to add or say about soil.

Terroir is the product of the interaction between soil and climate. In the next part, we will talk a bit about climate and weather.

(Photos from the top down.... a vine in the Northern Rhone, Hermitage on the Rhone, a Cabernet Sauvignon cluster on Spring Mountain, and a Cabernet vine at Spring Mountain Vineyard.)