Why is soil so important in vineyards?

Grapevines are deep-rooted and long-lived plants. Unlike some crops like corn and soybeans, you can't move your vineyard a year after it was planted, so it's of utmost importance to choose a suitable site.

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Terroir: wine's sense of place

Terroir is both a constant source of headache for me and one of the reasons I got into this field in the first place. First, let's try to define it.

Terroir is a French term that Americans are probably pronouncing incorrectly. I usually hear "tare-wah" or "turr-wah" but I'm lazy and pronounce it more like "turr-warr."

Terroir derives from the Latin word terratorium meaning “land.” There is not a direct translation into English, so many definitions exist. In 2006, the international conference “Terroir 2006” was held at the University of California, Davis. Even after three days of discussion they couldn't agree on a definition! Some definitions include:

A 19th-century French dictionary defines terroir as “the earth considered from the point of view of agriculture” (Trubek, 2008).
“The French term ‘terroir’ refers to the relationship between a particular wine and the specific place where it is produced” (Foroni et al., 2017).
“The terroir concept describes the link between the quality of wine and the physical environment in which the vines are grown. Terroir can be defined as a trait of land where natural characteristics of the environment, namely soil, subsoil, relief and climate, form a unique assemblage of factors that provide the wine, through the interaction with vine’s genotype, with specific and high quality characteristics” (Costantini et al., 2012).
Terroir “relates the sensory attributes of wine to the environmental conditions in which the grapes are grown” (Van Leeuwen and Seguin, 2006).
“The ‘terroir concept’ is a complex approach to vine growing and wine making including environmental factors as well as socioeconomic and historical aspects of a given area” (Zsófi et al., 2011).
“The concept of terroir […] includes specific soil, topography, climate, landscape characteristics and biodiversity features of each winemaking region” (Fraga et al., 2014).

I define terroir as the impact of the combination of culture and vineyard ecology -- including climate, soils, landscape, microbial communities -- on the wine. It's what makes wine so special compared to other types of drinks, because it matters where it comes from. A Sauvignon Blanc from France will be different from a Sauv Blanc from New Zealand and a Sauv Blanc from California. Some places have great cultural histories of making certain styles of wines, such as Champagne, France.

Terroir is neither positive nor negative. Those who say a wine has “good terroir” usually mean that the wine seems to particularly express the place it was grown. Way back in the middle ages, monks first acknowledged the relationship between soil and wine and started keeping track of which soils resulted in the best wine.

How does soil contribute to terroir?

Soil is tremendously important for all crops. But isn't it even cooler that soil may impact the taste of wine?? There's an old phrase: goût de terroir, which means “the taste of the soil.” But the soil aspect of terroir is particularly difficult to determine. Many instances of soil influencing wine sensory attributes are anecdotal or from wine journalism and are often the result of years of winegrowing and drinking experience:

WineGeeks has a list of the “World’s Top 10 Wine Soils.” Topping the list is Burgundy, France. According to the author, these soils contain limestone clay which is “heavy with nutrients from the fossils of shellfish from eons ago.” The best soils, those with a mix of limestone, flint, and sand, produce wines with a “haunting mix of earth, fruit, structure and nuance that is impossible to duplicate elsewhere.” Mosel’s slate soils come in second, and Bordeaux’s gravelly soils third.
LetItWine.com describes the wines produced by different soils: clay soils produce wines that are rich, flavorful, and complex while sandy soils produce smooth, silky wines. Calcareous soils, according to them, produce wines with excellent flavor, complexity, and minerality.
Wine Folly includes four primary soil types: sandy, clay based, silt, and loam. Sandy soils produce “elegant wines with high aromatics, pale color and low tannin.” Clay based soils are “muscular” with “high extract and color” because they “stay cooler and also retain water.” Silt soils, such as loess soils in Washington State, are “smooth and round” with “slightly less acidity.” Finally, they claim that loam soils are too fertile so wines “have very little flavor and color.”
VinePair acknowledges that it is difficult to break soils into categories, especially when geology is considered a soil factor. They categorize soil types by geology and texture (alluvium is inexplicably included as a texture). Igneous volcanic soils impart “ashy, rusty taste” while granite (which, I might point out, is an igneous rock) soils produce acidic, subtle, and layered wines. Schist-derived soils produce “big, powerful wines with rich minerality” and flint-derived a “rich, flinty minerality.” Clayey soils make wine “thick, round, and generous” while loamy soils produce “powerful, voluptuous” wines.

What does science say about these claims?

Some of the description terms used in wine journalism aren't exactly scientific (what is a "muscular" or “generous” wine?). Scientists haven't been able to find a like between soil and generous wines yet.

All kidding aside, it's really difficult to extract the specific impact of soil on wine. How do you set up an experiment that only compares different soils? You might think that you could fill pots with different soils and grow grapevines in a greenhouse. But soil is a living natural body that has structure, water, organic matter, hydrology, horizons, macro- and microorganisms, etc. When you dig it up and put it in a pot, it loses many of those features. It's no longer really soil.

How about comparing the same variety of grape at different vineyards with different soil? This is proper soil at least. But you will have different weather/climate between vineyards, and maybe other differences like the age of the vines.

Let's say that you managed to set up an experiment that had differences in soil and nothing else. Now, how do you figure out which part of the soil is most important? Is it the nutrients the soil provides? Or how much water the soil holds?

Here's what we do know:

Soil provides nutrients to the grapevines. Some of these such as nitrogen and potassium, can influence wine quality.
Soil also provides water to the vines. Different soils will hold onto more water than others.
In general (and I mean very general), the highest quality wines are from vines growing on soils with a limiting factor such as low fertility and/or low plant available water.
Soils with low silt content (which holds a bunch of water), rocky soils (which don't hold much water), soils on backslopes (which usually drain really well), and soils with thin topsoil (which are less fertile) are more likely to produce with positive sensory attributes including increased color intensity, sugar content, and phenolics.
This is especially important for red wine production where tannins and color are central quality characteristics.
Climate may still be more important than soil during certain years. If there's a ton of rain, the driest, most well-drained soils may be superior. But in a dry year, you want soils that hold onto a lot of water.

In conclusion, we may not know what specifically creates a wine's terroir, but it's more than just a romantic notion. It is a complex association of environmental factors that come together to create the complex wines we get to drink. Soil is an important (and really cool!) part of a wine's terroir, though it's difficult to scientifically test. It definitely can influence the wine, which is one of the reasons wine is so unique and interesting to drink!

Van Leeuwen, in his chapter Soils and Terroir Expression in Wines, says: “Wine can, to some extent, be compared to art. Wine stimulates the senses, as does art. Great wines can be a speculative investment, as can be pieces of art. If a great wine is a piece of art, then this is a form of art that is shaped by the soil” (van Leeuwen, 2009).

For more reading on minerality, I recommend

Costantini EAC, Bucelli P, Priori S. 2012. Quaternary landscape history determines the soil functional characters of terroir. Quaternary International 265:63–73.
Deloire A, Vaudour E, Carey VA, Bonnardot V, Van Leeuwen C. 2005. Grapevine responses to terroir: a global approach. OENO One 39:149–162.
Foroni F, Vignando M, Aiello M, Parma V, Paoletti MG, Squartini A, Rumiati RI. 2017. The smell of terroir! Olfactory discrimination between wines of different grape variety and different terroir. Food Quality and Preference 58:18–23.
Fraga H, Malheiro AC, Moutinho-Pereira J, Cardoso RM, Soares PMM, Cancela JJ, Pinto JG, Santos JA. 2014. Integrated Analysis of Climate, Soil, Topography and Vegetative Growth in Iberian Viticultural Regions. PLOS ONE 9:e108078.
Karakis S, Gulbranson E, Cameron B. 2018. Insight into the source of grapevine water acquisition during key phenological stages using stable isotope analysis: Vine water source assessment using stable isotopes. Australian Journal of Grape and Wine Research 24:252–259.
Priori S, Martini E, Andrenelli MC, Magini S, Agnelli AE, Bucelli P, Biagi M, Pellegrini S, Costantini EAC. 2013. Improving Wine Quality through Harvest Zoning and Combined Use of Remote and Soil Proximal Sensing. Soil Science Society of America Journal; Madison 77:1338–1348.
terroir - definition of terroir in English | Oxford Dictionaries. Oxford Dictionaries | English. as found on the website (https://en.oxforddictionaries.com/definition/terroir).
Trubek AB. 2008. The Taste of Place: A Cultural Journey Into Terroir. University of California Press.
van Leeuwen C. 2009. Soils and Terroir Expression in Wines. In Soil and Culture. pp. 453–465.
van Leeuwen C, Trégoat O, Choné X, Bois B, Pernet D, Gaudillère J-P. 2009. Vine water status is a key factor in grape ripening and vintage quality for red Bordeaux wine. How can it be assessed for vineyard management purposes? 1 43:121–134.
van Leeuwen C, Seguin G. 2006. The concept of terroir in viticulture. Journal of Wine Research 17:1–10.
Zsófi Zs, Tóth E, Rusjan D, Bálo B. 2011. Terroir aspects of grape quality in a cool climate wine region: Relationship between water deficit, vegetative growth and berry sugar concentration. Scientia Horticulturae 127:494–499.