Wednesday, May 8, 2013

Beaujolais and Douro (Portugal): Contrasts in the utilization of granite and schist soils

The Beaujolais and Douro wine regions are both "blessed" with granite and schist rock types in their wine growing regions but they take contrasting approaches in dealing with this reality. In this post I will define the two rock types, address their distribution in the wine regions, and describe how they are incorporated (or not) into the vine-growing strategy.

The Rocks

According to geology.com, "granite is a light-colored, igneous rock with grains large enough to be visible with the unaided eye." Its primary constituents are quartz, feldspar, small amounts of mica and what Wilson (Terroir: The Role of Geology,Climate, and Culture in the Making of French Wines) calls "dark" minerals.

Source: granite4less.co.uk
Granite formation (Source: tovisitportugal.blogspot.com)

The feldspar, mica, and other minerals in granite are susceptible to chemical weathering which, if it occurs, alters the feldspar and mica to "easily removed" clays and causes the remaining rock structure to disintegrate to a sandy granite wash (Wilson).

Schist, again according to geology.com, "is a metamorphic rock with well-developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces."

Schist (Source: earth.ox.ac.uk)

Granite and Schist in Beaujolais and Douro

The Beaujolais wine region is effectively divided into an upper and lower Beaujolais based on the soil composition north and south of the Nizerand River. The landscape south of the river is comprised of flat limestone-clay plains interspersed with sandy patches. North of the river, light granite- and schist-based soils predominate in the hills and stone and clay soils on the lower slopes.

The granite in Beaujolais was formed at great depth over 300 million years ago and was brought to the surface in the mid-Tertiary period by the uplift of the Massif Central and the Morvan. On its way to the surface the molten mass intruded into a crust of ancient lava and ash and metamorphosed that material into schist which accompanied the granite on its upward journey (Wilson). The principal vineyards of upper Beaujolais reside on a large massif known as the Odenas granite, a rock which weathers readily to clay minerals.

Beaujolais (Source: vineyardsandvignobles.com)

The north of Portugal consists almost exclusively of granite, the uniformity of which is interrupted by a massif of shale that extends from Barca d'Alva almost to Régua. This schist is a slate-like metamorphic rock which frequently splits into vertical layers below the surface, allowing moisture to seep in as well as providing growth pathways for the vine roots.

Lithology of the Douro region (Source: ivdp.pt)

The soils

Beaujolais is divided into Haut- and Bas-Beaujolais, based on distinctions in the soil and the intervening Nizerand river valley, with Haut-Beaujolais to the north and Bas to the south.  The north is characterized by rolling granite hills pock-marked with patches of clay and limestone and with granite and schist in the upper slopes and higher stone and clay content in the lower slopes.  The south has a flatter topographical profile than the north and has primarily clay and sandstone soils. For the purposes of our analysis, we will concentrate on the soils of Haut-Beaujolais.

The hills in the north of Beaujolais are convex and have no bellies in which to collect slope wash. As a result, the soils here are thin and primarily crumbled granite bound together by sparse clays resulting from the weathering of feldspars and micas. The sandy soils are known locally as "arene," "gore," or "gorrhe." These soils are thin, acidic, and lacking in nitrogen but contain high concentrations of mineral nutrients (potassium, phosphorous, magnesium).

The Douro soil is schistose with granite at the borders and, in some cases, penetrating horizontally into the schist layers.  Prior to its current state, the Douro land under vine was characterized by "the presence of bedrock at less than 15 cm (5 inches) below the surface" (Instituto dos Vinhos do Douro e Porto (IVDP)).  This was untenable for vine growth given the rooting requirements of the vine.

According to IVDP, the Douro soil has been created by man "digging down deeply and forcing the vertically layered rock to break up, thus totally altering its original disposition and creating changes to its original morphology, added to which he has applied fertilizers."  This scarification has resulted in soils with depths of between 1 and 1.3 meters.  In both cases the roots take advantage of the pliability/makeup of the underlying bedrock to dive to great depths.

The Douro soil profile consists of a 12.5-cm layer -- "the result of the digging that is done around the roots of the vines every year" -- followed by a layer of between 87.5 centimeters and 1.17 meters thickness, and then bedrock.  The composition of this topsoil is a clay-rock mix with (IVDP): little organic matter; low calcium, magnesium, and phosphorous levels; and medium-high levels of potassium. The soil pH is predominantly acid (4.6-6.5).

Vineyard characteristics and practices

There are approximately 3600 vineyards in the 55,000 acres that constitute Beaujolais. The vineyards tend to be small to mid-sized plots owned by hundreds of farmers and carrying between 9000 and 13,000 vines per hectare.  For the most part, the fruit is sold to negociants who produce and market the wines.  In the north the vineyards sit on gently sloping hills at elevations that range between 500 and 2000 feet. The resultant exposure to the sun allows quicker ripening and harvesting in the north when compared to the south.  Vine training is primarily Goebelet but, recently, some Guyot training has been utilized in the south.

The low nutrient levels in the Douro will act to reduce yield and retard fruit ripening if unaddressed.  The solution is the application of fertilizers for the macronutrients and foliar sprays for the boron deficiency. Lime is added to the soil as necessary to counteract the effects of low pH. The vines reside on hillside terraces and are Guyot- or cordon-trained. Planting density is on the order of 6000 vines/hectare.  Drip irrigation is shunned.

Comparative utilization of available soils

Gamay has found a home in the granitic soils of Haut-Beaujolais. This vine was ripped out of the primary areas of Burgundy by Philip the Bold in 1395 due to its fecundity but the thin, sandy soils of Beaujolais are the "right challenge" for its voracious growth. The principal vineyards of Haut-Beaujolais are situated on a large massif known as the Odenas Granite which contains black mica, a material which weathers easily to clay minerals. The thinness of the Beaujolais soil is, therefore, compensated for by high levels of nutrients such as potassium, phosphorous, and magnesium. It should be noted that the wines produced from this combination of soils and vine are not prone to extended longevity.

The opposite is the case in the Douro region where schist reigns supreme. Its water acquisition and retention properties, as well as its provision of growth pathways for the vine roots, make it the soil of choice in the region. Its low nutrient levels are compensated for by the addition of fertilizers for macronutrients and foliar sprays for boron deficiency. When granite breaks through to the surface, the terrain is considered unplantable and vineyards planted on granite are penalized in their Casa do Douro classification (rating of Douro vineyards on a quality scale ranging from A to F) as assigned and maintained by the regulatory authorities. The grapes produced from this regime yield intensely flavored wines which can stand up to the intensity of 20% alcohol fortification and long aging while retaining their fruit characteristics.


©Wine -- Mise en abyme

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