Growing grapes in soil-challenged environments: Carso (Friuli-Venezia Giulia, Italy) and Douro (Portugal) wine regions

I have previously noted the importance of soil as a key influencer on the growth of the vine plant through the provision of: (i) a supply of water; (ii) anchorage in the ground; and (iii) a source of nutrition.  The Douro (Portugal) and Carso (Friula-Venezia Giulia, Italy) wine regions are separated by the land mass that is Portugal and Spain, the full width of the Mediterranean Sea, and the Italian land mass but they have in common a lack of a classic soil profile as represented in the graphic below.  Even with this challenge, both

Source: westone.wa.gov.au

regions still manage to produce high-quality wines.  In this post I will detail the challenges presented to both regions and compare and contrast the solutions they have implemented in order to mitigate said challenges.

Carso DOC lies on a Karst landscape and is subject to all of its vicissitudes: lack of a surface-water web; no water retention; hard, rocky surface with no surface soil or vegetation.  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.

In both Carso and Douro, viticulturists have found ways to get around nature's lack of gifts and to create environments wherein grape vines can thrive.  In the case of Carso, vineyards are prepared by cutting away any vegetation existing on the rocky surface and then digging down into the limestone to remove the roots.  Yellow soil trucked in from Trieste is laid to a depth of 3 meters and is then overlaid with 0.5 meter of Carso red (terra rossa) soil.  This red soil -- found in collapsed caves called Dolina -- is rich in iron and lime but poor in organic components.  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.

In both regions the manufactured soil profile consists of three layers.  Carso soil has a 0.5-meter layer of terra rossa, a 3-meter layer of yellow soil, and then bedrock.  The pH of this soil is essentially neutral.  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).  The low nutrient levels in the region 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.

Carso terra rossa soil

Douro surface soil at Quinta do Vesuvio

The resulting vineyard architectures are similar in that both Carso and the Douro employ terracing.   In Carso, the vineyards reside on stone terraces and vines are Albaretto-trained.  In the Cante vineyard, the density is 8500 vines/hectare, a situation necessitated by the expense associated with creating each acre of vineyard.  In the Douro 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 utilized in Carso but is shunned in the Douro.

Albaretto-trained vines in Carso

Terracing in Carso

Terraces in Douro (Quinta do Vesuvio)

Production levels differ greatly between the two regions.  Given the ready availability of raw materials, and the size of the region, the Douro has by far the greater number of hectares under vine.  But due to the relatively harsh conditions in which the grapes are grown, the berries from both regions are small and thick-skinned with high skin-to-pulp ratios.

These regions were both handed a similar challenge -- lack of a readily apparent wine growing region -- but pioneers living therein saw it not as a problem but as a challenge. And both regions met the challenge in their own inimitable way.