Claude Bourguignon is an agricultural engineer with a Ph.D. in soil microbiology. Lydia Bourguignon is an agri–food scientist and enologist. They met while working as researchers for France’s Institut National de la Recherche Agronomique (INRA). Together, they have consulted for numerous wineries in France and abroad, including Selosse, Huet, Dujac, Château le Puy, Keller, Elio Altare and Harlan Estate, helping growers understand the complex biological makeup of their vineyards. Their book, Le sol, la terre et les champs, has been referenced by several contributors to this issue. Recently, I reached them in their offices in the Côte d’Or.
Lydia: It’s a laboratory. We go to the site to put together a profile of the soils.We analyze the soil health and determine what the vignerons need to continue to do—and what they need to stop doing—to get the vine to really express its place. The objective is to allow the farmer to better know the terrain and its composition to better manage the soil and have a higher quality product.
Claude: We do an analysis that is physical, chemical and also biological. To measure a soil’s health, you have to measure its biological activity. There are a lot of effectively dead soils out there—soils that don’t have any detectablemicrobial activity. The more the soil microbes are active, the more there will be enzymatic activity. So, we measure the quantity of enzymes.
By biological activity, we’re really talking about bacteria from actinomycetes [filament or rod-shaped bacteria] and fungi. We first measure their activity on the surface and then deeper, close to themother rock. A living soil, especially organic or biodynamic, has more microbial activity on the surface than a soil treated with chemical fertilizer. These soils are alsomuch richer in fauna: earthworms, acaroids [mite-like insects] collembolans [small, wingless hexapods]… which in turn help the vine roots to grow deeper.
Is there, in fact, a means by which the soil influences the scent, flavor or texture of a wine? How would you describe minerality in a wine?
Lydia: Minerality is the perception of the rocks in the soil, by the palate.
Claude: Minerals themselves don’t have an odor…with the exception of silex. That one gives off a scent of pierre à fusil [gunflint]. All other minerals are odor-free—sand, limestone. However, each soil gives tactile perceptions in the mouth.
We understand this mineral link to the soil by the complexity of the aromas in the wine. The dissenters will say that aromas are carbon-based molecules; they don’t contain oligo elements like manganese, zinc, boron, selenium. But all aromas are synthesized by enzymes that have metallic cofactors.* And these metals all originate in the soil. It’s the microbes that make the metals available to the plant. This is why hydroponic plants—plants grown without soil—don’t have any flavor.
Lydia: In fact it’s not an olfactory analysis of a wine but the mouthfeel that’s a function of the place the vine was planted. We hold terroir tastings called “geo-sensorial” tastings. They were first practiced in 1312 by Philippe le Bel. It’s a sort of taste-training, in which you actually touch, even taste, different rocks to then be able to find the same sensations on the palate with the wine. For example, touching granite gives a cold impression, while limestone seems warm.
If you have two syrahs, one from a granite soil and the other from limestone, tasted in the same cellar and at the same temperature, you will have two distinct sensations. The one grown in granite will seem colder on the palate, and the limestone one will seem warmer. These impressions involve extremely weak levels of concentrations, immeasurable by our instruments, but we do feel them. Similarly, we can’t physically measure the sapidity that we experience when we taste a wine grown on chalk or explain the metallic taste of flint that we perceive in a Pouilly-Fumé grown on silex.
With a properly managed soil, the roots will dig deep into the subsoil, and it’s the subsoil that determines the goût de terroir. Without deep roots, all you taste is the grape variety and the cellar technology. A “vin de terroir” is characterized by its length on the palate, which is due to the quality of the polyphenols, which in turn depend greatly on place and thus on terroir. So we’re talking about mouthfeel.
Claude: To have minerality and complexity in a wine, of course the roots have to dig deep. You’ll have limestone, or clay, and the variety of elements found in each of them will be different—maybe more zinc, or less potassium. When you have these different elements, the mouthfeel of the wine will be different. We can taste it and say “this vine grows on clay,” when we’ve never even seen the terrain.
Lydia: When you physically touch chalk or granite—that’s really the test—you’ll have different sensations. And you’ll find these differences also on the palate when you taste the wines. It’s a curiosity.
*A cofactor is a helper molecule bound to an enzyme that allows its biochemical transformations. Cofactors are frequently metal ions.
Excerpted from Le sol, la terre et les champs, by Claude and Lydia Bourguignon (Éditions Sang de la terre, ne wedition, 2008; translation below by Carson Demmond).
Until recently, studies on the soil/terroir relationship had revealed that permeability, the presence of heat-retaining stones on the surface or deep layers of clay that release water slowly over time are all factors that allow the vine to avoid water stress in the summer.We’ve known that overly deep soils don’t allow the crop stunting necessary for the vine to mature its grapes. We’ve known that surplus potassium in soils creates a drop in wine’s acidity and that soils too rich in nutrients promote the spread of disease and unbalance in wine. But we didn’t know why, with the same grape variety and at a distance of 20meters,we had a Romanée Conti or a Richebourg, a Côte Rotie Brune or Blonde…Knowing there is no truth with a capital T,we have studied soil in our laboratory not on one single level but in its entirety…and in this obscure and unknown world that is the soil, hints of light have appeared that allow us to confirm that soil has a role in terroir expression, alongside climate, geology and topography…
Each rock, according to its composition, will liberate elements in a particular concentration to be assimilated by the roots. Thus, the clay soils of Burgundy or the granite in Morgon are rich in manganese, an element that we also encounter in the wines…The andesite in Brouilly is rich in zinc and copper. So each rock will liberate its goût de terroir. Onemay object to this statement, as a wine’s aromatic molecules are carbon-based and don’t contain oligo elements like manganese, barium or zinc. But in fact, these aromatic molecules are synthesized by enzymes, and these enzymes are proteins with metallic cofactors. These oligo elements that the vine extracts from the rocks can be incorporated in the enzymatic cofactors and thus participate in the synthesis of perfumes. It is plant physiology that can deliver the definitive response to this question. But physiologists do not yet know all of the enzymes and their cofactors implicated in the synthesis of aromatic molecules…
Microbes nourish plants by making the nutritive elements in a soil soluble. In a dead soil, the vigneron must bring in chemical fertilizers—and these are the same throughout the world, creating uniformity in the taste of wines. The vine not only feeds itself on nitrogen, phosphorus and potassium but on 28 other elements that the microbes bring it according to their proportion in the mother rock. We have seen that these elements need oxygen to be formed by the microbes, and oxygen can only enter into a soil if the fauna are abundant.
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