” The role of soil in expressing terroir “
While agronomists have been able to demonstrate the role of climatology and topography in the expression of terroir, they have never provided more than very partial scientific explanations of the role of soil. This is because they have approached the soil from a purely physico-chemical standpoint, ignoring its biological dimension. The fauna is responsible for the porosity of the soil, allowing air and water to enter at depth, while the microflora is responsible for the formation of negative elements that can be assimilated by plants, such as nitrates : NO3-) -, phosphates: PO2-/4 or sulfates: S02-/4.
These elements, which are oxides, can only be formed if the soil is well aerated by fauna, and fauna can only penetrate deeply into the soil if the roots sink into it. So it’s easy to see why we destroy soil fauna with pesticides, and compact soils with increasingly heavy machinery, thereby blocking soil aeration and microbial activity. The vine roots come to the surface to breathe, and we replace the work of the soil microbes with the same fertilizers.
The tendency is to standardize wines, slowly moving away from terroir wines to varietal wines. By neglecting the biology of terroir soils, the wine industry has standardized wines, making them easy to copy and exposing them to foreign competition. For the past four years, our laboratory has been working to define the physical, chemical and biological characteristics of wine-growing soils, particularly in Burgundy. Indeed, defining a role in typicity would make it possible to go beyond climate, topography, grape variety and winemaking. These four criteria are relatively easy for other countries to copy. On the other hand, the diversity of soils and the fundamental relationships between soil, microbes and plants are so complex that they are impossible to copy, and thus ensure the originality of the wine.
– Define the most important physical, chemical and biological criteria for defining a terroir soil.
– Develop cultivation practices that respect these characteristics.
To illustrate our point, we’ve chosen the Burgundy region, because of its monocépage and geological unity (Jurassic limestone). Indeed, in this region, the difference in taste observed between two wines of the same appellation can only be due to the soil, especially when the two wines are vinified by the same winemaker.
Physical characteristics of terroir soils
Texture
Classical granulometric approaches have never been used to characterize a terroir. In Burgundy, for example, two clos can have the same granulometry and produce different wines: Latricière and Chapelle Chambertin, for example. On the other hand, soils have a remarkable characteristic: the specific surface area of their clays. In other words, the total surface area covered by one gram of clay. Our studies of the Côtes de Nuits and Côtes de Beaune suggest three things:
– The soil of each cru is characterized by its own specific surface area.
– Soils used for red wines contain clays with a higher average specific surface area than those used for white wines.
– Within the same white wine appellation, the finer the soils, the higher the specific surface area of the clay.
On the hillside of Puligny-Montrachet, for example:
– AOC Village: les Houlières: 375 m2/g
– Premier Cru: Clavaillon: 233 m2/g
– Grand Cru: Le Montrachet: 176 m2/g
These initial results have been confirmed in other French regions. The lowest specific surface area measured in vineyards is that of the coulée de Serrant, at 57 m2/g, and the highest, that of Richebourg, at 550 m2/g. As there is a linear relationship between the specific surface area of a clay and its cation exchange capacity, we can hypothesize that each type of clay nourishes vines in a specific way, and that there is therefore a first level of typicity, due to the soil, in the terroirs, that of the specific surface area of the clays of their absorbing complex.
The structure
Great terroirs are characterized by high porosity at depth. These porosities can be of physical origin, as is the case for the surface porosity of the gravel or alluvial pebbles of Bordeaux, the Côtes du Rhône or the Loire Valley, or for the deep porosity of the fissured limestones of Burgundy and Alsace. But these porosities are also of biological origin (galleries and faecal pellets of soil fauna), as is the case for the deep porosity of Bordeaux or Loire Valley clays and the biological porosity of Burgundy subsoil limestones or Champagne chalk, as well as for the surface porosity of Burgundy clay-limestone soils.
These high surface and deep porosities enable rapid re-drying of the soil after rainfall, and therefore rapid warming of the surface soil, which is essential for good grape ripening and good oxygenation of the deep roots. The deep drainage of water ensures a regular supply of water to the vines throughout the summer. By stopping tillage and killing off soil fauna with excessive pesticide use, winegrowers prevent water from flowing deep down into the soil, favoring surface circulation and increasing erosion. With oxygen no longer able to penetrate the soil, rooting becomes superficial.
Chemical characteristics of terroirs
No major assimilable element content could be correlated with terroir typicity. On the other hand, high levels of certain assimilable trace elements seem to characterize certain terroirs: high levels of manganese in Coulée de Serrant, Morgon and Montrachet. This trace element can even be measured out in wines. These terroir trace elements can only be assimilated if the soil microflora is active enough to chelate them. Excessive pesticide inputs can block the biological chelation of trace elements. Another interesting feature is the type of limestone in the soil.
Our microscopic observations show that, depending on water circulation, there are limestones in the soil resulting from microbial recarbonation. The reds are planted on the former, and the whites on the latter. In soils rich in bacterial limestone, there is very little free ferric iron, an element we know is necessary for anthocyanin synthesis.
Biological characteristics of terroirs
They operate at three levels: roots, soil fauna and microbes.
Rooting characteristics
All the great terroirs we studied had deep current or ancient roots. When roots are deep, the taste of the terroir is stronger than that of the grape variety. The widespread post-war practice of choosing productive rootstocks with good fertilizer utilization but shallow roots, such as SO4/2, has favored the varietal taste over that of the terroir. The same problem is found in deep valley soils (Hérault, Aude, Rhine, soils below the main road in the Côte d’Or) which, being too nutritious on the surface and poorly aerated at depth, favor superficial rooting and varietal tastes. The deeper the roots, the farther they are from fertilizers, the more they extract nutrients from the terroir and the more they ensure a regular water supply.
Soil fauna characteristics
In great vintages, we always observe, in contact with the parent rock or deep clay, a strong root necromass serving as food for endogenous fauna (mites, springtails and earthworms). This multiplication of fauna at depth is accompanied by a high level of biological porosity (galleries), and an increase in biological activity against the rock. This activity enables the formation of elements that can be assimilated by the vine and are specific to the terroir.
Microbiological characteristics
Our microscopic observations show that the dominant microflora in vineyard soils is not always the same. Some soils are dominated by fungi, others by bacteria. Certain bacterial groups can be very abundant, such as siderobacteria in Bordeaux soils, or cullet-cycling bacteria in Chablis or Champagne soils. Our measurements of microbial activity in vineyard soils show that activity is linked to the specific surface area of clays, the type and content of calcium carbonate, and the quantity and quality of humus in the soil. Generally speaking, current vineyard practices are leading to a collapse in the biological activity of terroir soils. In some vineyard soils, we have observed biological activities lower than those of Saharan soils. Conversely, on estates that have converted to organic viticulture, we have observed a rise in biological activity and an improvement in root descent. Biodynamically farmed terroirs show the best biological activity in deep soil. This translates into higher levels of assimilable elements in the subsoil.
Conclusion
This brief presentation of the soil’s role in terroir typicity shows that the latter is not a myth, contrary to many assertions from the New World. Soil is involved in terroir typicity at the physical level of the soil, through the C.E.C., which supplies cations to the vine roots. This supply is directly dependent on the specific surface area of the terroir’s clays. It also takes place through the deep and superficial structure of the soil, which enables oxygen and water to circulate properly down to the roots. This action of the soil on the terroir is also reflected in the soil’s biology, as it is the microbes of the terroir that produce the oxides assimilated by the plants. Of course, these oxides can only be formed in well-aerated soil, where aeration is provided by fauna. It is by integrating and applying the new knowledge we have acquired about soil biology that we can improve our understanding of terroirs and develop viticultural practices capable of respecting them. It’s no longer the winemaking process that is the limiting factor in improving wine quality, but the soil, and it’s by respecting it that we get the most out of our terroirs. “Less chemistry and more life in the vineyard soil” must be the leitmotiv of tomorrow’s winegrower.