Geology and Wine

"Like man, the vine — and vitis vinifera in particular — has a remarkable ability to adapt itself to the conditions in which it finds itself."¹⁾ Wine is produced in every state of the USA except Hawaii and Alaska, and wine grapes thrive in a wide range of sites — chalks and limestones, heavy marls, granites, schists, gravels, sands, and volcanic soils²⁾. Claims are made by wine-writers of the special qualities imparted although few demonstrable scientific connections have been found. In the Loire, for instance, the same grapes, Chenin Blanc and Cabernet Franc, grow equally happily on the pre-Cambrian Armorican shield to the north as on the chalky Cretaceous to the south, but taste quite different.³⁾

The geology of underlying rocks influences wine quality through the two aspects of their weathering — physical and chemical. Physical weathering is largely responsible for the structure and texture of the soil. The prime need of vines is for good drainage; they hate being waterlogged. Roots can go as deep as 6 metres; the deeper they are the better. If the bedrock is shallow and the soil thin the vines cannot stand up to drought. On the other hand, fertile alluvial soil is only good for table grapes which lack the concentration necessary to produce flavour in wine, and this is why wine vineyards tend to be on well-drained slopes. Limestone areas, with good, natural drainage, appear to predominate in wine production. Where the parent rock is limestone, the soil will be clays and silts which favour moisture retention, whereas granites and sandstones weather to gritty shales, often sandy and drying out easily. Stony surface soils enable heat to be absorbed to greater soil depth and shield against surface evaporation.

Limestone-derived soils also have an advantage in chemical weathering as carbonates help the decomposition of organic matter. Acidic soils can produce fine wines — the granites of Haut-Beaujolais, the Côte Rôtie, and Alsace are examples — but active calcium carbonate is the one chemical soil constituent most commonly associated with wine quality. Clay minerals are essential to the function of vine plants enabling the steady take-up of mineral nutrients — montmorillonite (remember S260?) and vermiculite being particularly important for their cation-exchange capability.

Vines need a blend of nutrients, and it seems probable that the fine-wine producing areas which have been discovered by trial and error in the past are those which contain the right balance. The macro-nutrients, needed in quantity, are oxygen, nitrogen, phosphorus and potassium. The last three are sometime deficient because they leach out easily, and are sometimes supplied in chemical fertilisers. Micro-nutrients needed are iron, calcium, magnesium, sulfur, zinc, manganese, copper, boron, molybdenum and chlorine. The pH of a soil is vital to the take-up of nutrients. Iron tends to become a poorly-soluble oxide in alkaline soils, so the yellow leaves of chlorosis are often seen in limestone areas, and manganese, copper and zinc are also better absorbed in acid soils, but all the remaining nutrients prefer a high pH of around 8.5.

In general, the wider the range of rock-types from which the soil of vineyards derives, the more likely it is that the necessary minerals and nutrients can be derived, and the more varied the soil structure. Climate also pays a vital role. Calcareous soils seem to be less advantageous in Mediterranean regions, with greater summer heat and dryness, where schists, sandstones and ancient alluviums are reckoned to give finer wines.⁴⁾ The Kimmeridge Chain running from southern Champagne through Chablis and Sancerre to the Loire produces distinctive wines, but Kimmeridge clay in the Midi does not.

On our domaine viticole near Montpellier, chemical soil tests have shown higher magnesium levels than our neighbours, giving an impression of acidity and freshness in white wines which Midi wines often lack. We are in an area of extensional normal faulting to the east of the volcanic belt running into the sea at Cap d'Agde described by Brian Dawson in the November 2000 Newsletter. There are traces of small lava flows locally, and I have found basalt stones in the vineyards among the limestone, possibly giving a slightly more mafic soil mix.

Geology is only one of the elements involved, as the French concept of terroir recognises. Climate, the choice of the best grape varieties for the terroir, the skills of the vineyard manager in selecting rootstocks which can mitigate an unfavourable balance of minerals or physical structure, in trellising and canopy management, and the skills of the winemaker are all vital contributory factors.

Ann Cripps

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¹⁾ Jancis Robinson, Vines, Grapes and Wines: The Wine Drinker's Guide to Grape Varieties, 1986, Mitchell Beazley, London
²⁾ Charles Pomerol (Ed.), The Wines and Winelands of France, Geological Journeys, 1989, English edition Robinson McCarta, London
³⁾ James E. Wilson, Terroir: The Role of Geology, Climate and Culture in the Making of French Wines, 1998, Mitchell Beazley, London
⁴⁾ John Gladstones, Viticulture and Environment, 1992, Winetitles, Adelaide