At present, there is a clear tendency to put forward the specificity of a terroir to describe all the uniqueness of a wine from one vintage to another as well as from one parcel or region to another.
But what is terroir?
The terroir describes the natural environment of the vineyard and its many interactions. It therefore encompasses not only the soil of the vineyard of origin, but also the complex interrelationship between soil, topography, climate and human influence (e.g. the type and style of winemaking, the choice of grape variety, the winemaker)
It is a set of elements that complement and add up throughout the process from the vine to the wine making.
We will focus on the soil and how its composition can lead to specific characteristics during the harvest.
The soil and its mysteries
The formation of a soil comes from the alteration by fragmentation of the bedrock and its mixing with the organic matter deposited on the surface which will be transformed into humus by homogeneous layers called “horizons”. The soil is therefore the part located between the atmosphere and the subsoil made up of the mother rock.
The soil is necessary for the vine because it draws from it the nutrients and water it needs. The pH of the soil is also important. It should be between 4.5 and 8.5 to provide the ideal soil for nutrients. Without this, deficiencies can occur and its share of diseases such as chlorosis.
What are nutrients anyway?
Well, nutrients are a whole set of organic or inorganic elements necessary for the life of the plant, such as :
- Organic matter
- Nitrogen, resulting from the degradation of organic matter and responsible for the green matter of the vine
- Minerals such as phosphate for root development and grape ripening, potassium for sap production etc. For the record, earthworms are responsible for the formation of clay-humus complexes (the mixture of clay and humus). These are negatively charged and will attract the minerals (positively charged). Winegrowers have understood this with more environmentally friendly farming practices.
- Trace elements such as copper, zinc, manganese, iron etc. Iron is very important for photosynthesis
Types of rocks
Above the bedrock, the soil thickness varies from a few centimeters to several meters. Several types of rocks can be found whose degradation will produce sand, clay or pebbles from the parent rock or from later deposits.
There are mainly 3 kinds of rocks:
- Magma which comes from the magma. Granite is a volcanic rock
- Sedimentary which comes from the sedimentation of minerals, organic matter or other rocks. This can take place in the open air, under water or ice. For example, clay, marl, sand are from this category
- Metamorphic which comes from the pressure exerted between the magmatic and sedimentary rocks. For example, the shales
The topography also has an influence on the final quality of the grapes if the vineyard is located at the top, middle or bottom of the slope. The transformed and decomposed constituents of the bedrock will evolve and be distributed differently. On the other hand, the subsoil will remain the same for several hectares. The wine appellations and their classification take these factors into account.
Soils are not simply homogeneous. Their actual soil landscape is a mosaic of soils that gradually change from one to another. They are on reliefs that will nuance the exposures as well as the water circulation at the bottom of the slope and modify the internal character of each soil. Each parcel is thus unique by its natural assembly of products.
Types of soil
We can find different types of soils more or less fertile:
- Clay-limestone which is suitable for Merlot in Pomerol or for Rioja’s trempanillo for round and fruity wines
- Clay and gravel, suitable for Merlot or Cabernet Franc for powerful and elegant wines
- Marly limestone for pinot noir or chardonnay. These are wines with body (marl) and finesse (limestone)
- Chalky soils (almost white soil with brown clay, porous and filtering. Water is retained in the deeper layers of the soil) for chardonnay or chenin. We obtain wines of great finesse
- Granite for Syrah in the Rhone Valley, Viognier or Pinotage in South Africa for expressive and mineral but also fine wines
- Graves: rich in pebbles, like the land on the left bank of the Garonne, which is the origin of the best Médoc and Graves wines. These are warm, well-drained soils that give the wine a sumptuous finesse.
- The schist soils as in the Rhone Valley with Grenache, Syrah for wines of character, Mosel with its great Rieslings or Douro Ports.
In the great vineyards of the world, limestone, clay-limestone, granite and gravel soils are the most common.
The vine and its soil
The healthiest vines grow on deep and fertile soils that allow their roots to dive 3 to 5 meters deep or even more to draw water and the necessary mineral elements. Their root mesh is therefore important as well as the accessibility to water which influences the depth of their roots. For example, on gravelly-sandy soils with little clay, water retention is low and the roots dip up to 7 m. This allows them to better resist drought. However, the nature of the soil and its granulometry (pebbles, stones of different sizes and humus) are the essential points to define the nature of the grapes produced. However it will be necessary to take care of the fertility of the soil. It will promote the growth of the vine with a high yield but its fruit will be poor in character.
Thus, the quality of the grapes requires the opposite. Poor and shallow soils for a lesser growth of the vine and more focused on the grape. These stressful conditions result in firmer, more balanced berries. The wine produced is more complex with strong aromas.
In general, it is best to avoid very clayey, very stony, poorly drained and very alkaline soils. The water supply to the vine also plays an essential role in the terroir effect. It is not the only element, the nitrogen supply and the soil temperature are also involved. But what differentiates the terroirs is the specificity of the factors that govern its water supply.
Ideally, the vine needs water at the beginning of its vegetative cycle and then a slight hydric stress during the veraison to encourage the maturation of the grapes. If the soil is too clayey, it will retain too much water at the risk of rotting the plant’s roots. If the soil is too sandy or stony, it will be necessary to irrigate abundantly even in a region with high rainfall.
The soil has an influence on the visual aspect (color of the grapes) and the character of the wines (aroma, power, finesse etc.) but not on the quality of the wine. It should be noted that depending on the type of soil, the same grape variety will reveal different gustatory notes. The soil will influence the composition of the harvest (sugar content, acidity, polyphenols, anthocyanins, etc.) and will modify the character of the wine in a complex way.
Thus, as in all things, it is the balance between the different components of the soil that will allow the grapes to mature with a maximum of qualitative potential. It is up to the cellar master to express it through a wine that is surprisingly elegant and delicate, but unique because of its vintage and its terroir.