The soil on which a vineyard is grown is undoubtedly reflected in the wine it produces, and for this reason an understanding of the Douro soils will help to explain something of the nature of Port. Douro soils are generally shallow and very rocky, consisting of a layer of light clay-schist spread thinly over the underlying schistous rock. Although relatively soft, and gradually eroded into a silt-like dust, the rock is found throughout the soil profile. It is therefore necessary to clear away an immense amount of stone before planting a new vineyard – and often explosives are required to break up the larger boulders. Traditionally this stone was used to construct the walls of the old terraces, although these days it is buried by bulldozers.
Despite the viticulturists’ best efforts, there will undoubtedly be a considerable amount of stone left in the soil which, although making vineyard operations more difficult, has two main beneficial effects. During the winter it increases water penetration and stabilises the soil against erosion which is especially important where the terraces are cut by bulldozer and therefore have no supporting wall. In the summer the heat-retention of the stones keeps the root-zone warm during the night and extends the useful hours of ripening each day. This ensures complete fruit ripeness virtually every year.
Moving further to the eastern end of the Douro, or upriver, the underlying schist is occasionally interrupted up by large bands of granite which sometimes break the surface. Where this occurs it creates the only truly unplantable terrain in the Douro, however any vineyards planted where granite is the principal underlying rock type are penalised points in their Casa do Douro classification. Even further to the east, around Foz Côa, the classic blue schist used for vineyard trellising posts is found. As a general rule, the darker the schist, the harder it is.
Douro soils are typically quite acidic, characterised by low pHs which make some of the important macronutrients (such as potassium and phosphorus) very unavailable. These nutrient deficiencies can have a major effect on reducing yield and retarding fruit ripening and have to be addressed by the application of fertilisers. Use of lime to correct pHs is also common, although less is required in the Douro Superior where soils are less acidic. Organic matter levels are very low across the region, but again many producers are attempting to increase the percentage because of the beneficial effect sustaining the vines during times of drought. Boron deficiency is the other major Douro problem, and unless corrected with foliar sprays can cause poor flowering and reduced fruit set.