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Geology: A central aspect of terroir
John Wooldridge, Senior Soil Scientist, ARC Infruitec-Nietvoorbij, Private Bag X5026, Stellenbosch, 7599, firstname.lastname@example.org
Terroir is a Latin concept which, in French, refers to a body of land whose natural criteria: soil, sub-soil, relief and climate, form a unique assemblage of values which confer specific characteristics on the wines produced on that land (Hancock, 1997). Following a study of the French winegrowing regions, Wilson (1998) added geology to the factors which define a specific terroir.
Figure 1: The soils of the Coastal Winegrowing Region, west and north west of Stellenbosch, are mainly underlain by Malmesbury shale (in shades of pale yellow and brown) and granite (pink), although wine grapes are increasingly being grown in valleys in the folded Table Mountain sandstone mountains (blue). The tendency for structures to be orientated in a general worth west to south east direction is a relic of past geological processes. - Map scanned from the 1:1 000 000 scale geological mapsheet of the Republic of South Africa, gravity version (1984), and published by kind permission of the South African Council for Geoscience.
However, Wilson's work, which appears to be predicated on the principle that because wines and their associated geologies differ, geology must be the controlling factor, has been criticised by Hancock (1999) on the grounds that Wilson never explains how this control is imposed. This criticism may be a little harsh in view of the fact that Wilson concludes by defining the 'totality of the elements of the vineyard habitat' - of which geology is clearly a component, as the very 'essence of terroir'. However, it is certainly true that, at least amongst a minority of the delegates to the international colloquium on terroir held in Angers, France, in July 1996, terroir was proclaimed, in terms of a 'magical mystique' (Hancock, 1997). This emotional, verging on spiritual, form of special pleading contrasts sharply with the analytical, factual approach of Morlat (1997) and Mesnier (1997), who apparently were influenced by the work of Saayman in South Africa. Saayman (1977) and Saayman & Kleynhans (1978) sought to link definable wine characteristics with such measurable, or identifiable, environmental parameters as climate and soil.
In South Africa the terroir concept forms the basis of the work which is currently being carried out at Nietvoorbij to define wine producing areas. In accordance with the principles of Saayman, Morlat and Mesnier, the approach is strictly that of defining, measuring and assessing environmental factors, seeking reasons based on quantified parameters, in contrast to the more subjective approach of Wilson (1998), and others.
The role of geology, and geological setting does not fall into the frame of reference of this existing work on terroir and it is reasonable to ask what possible role might geology play in wine production and wine quality? After all, most soils have been transported far from their source rocks of origin, and the bedrock in many locations lies well below effective vine root depth.
That geology is an essential component of terroir can not be doubted. Where doubts have been cast, these have merely tended to reflect the view that geology means nothing more than a categorisation of the rock that underlies the vineyard (Fig 1). This view, nevertheless, has its logic. If, as in the previous example, these rocks are covered by many metres of alluvium or glacial till, then their influence on the vines will indeed be negligible. However, not all soils are derived from transported materials, and geology means a great deal more than mere bedrock.
As defined by Wilson (1998), geology is nothing less than the scientific study of the origin, history and structure of the earth, including the material that composes it. And, one might add, of the life forms that evolved, not to mention the mechanics of the processes involved. A consequence of this definition is that it extends to include such undeniable wine-affecting factors as the shape of the landform, its topographic highs and lows and the orientation of those features relative to the sun and prevailing winds; even the present latitude and longitude, and thus the broad climate experienced by that landform (Fig 2). All are the result of ongoing, though slow, geological processes. If such a definition is allowed, then it becomes apparent that the link between geology, vine and wine is strong indeed.
Figure 2: Vineyard soils at the foot of the Constantiaberg rest on either Malmesbury shale or granite. The mountain is a remnant, dissected by erosion, of younger, principally sandstone rocks which were laid down on a previous landscape of granite and Malmesbury shale. The north-facing vineyards at the foot of the mountain are exposed to cool winds from both the Atlantic and the Indian Ocean, and to the morning sun. - Photograph: Nietvoorbij
The soils in which vines grow today are the products of weathering and soil forming processes. They nevertheless represent only a single stage in the cyclical geological processes of erosion and deposition. Graphic proof of this is provided by the occurrence of sedimentary rocks that contain repeated sequences of recognisable soil profiles, complete with fossilised root channels. Climate, which affects vines directly, and ancient climates which, through their effects on past weathering and transport, and on long term soil development, also affects present day vine performance, may also be linked to geology through the effects on climate of continental drift, mountain building and sea level change. Certain aspects of geology are undeniably abstract. Others, as the following examples illustrate, are of direct, practical significance right now.
1. Of specific importance to winemakers is the supply of potassium (K), mainly because the pH of the juice tends to increase with K availability (Iland, 1988). Research has shown that soils on the flanks of the Paarl granite are capable of releasing K from the mineral grains, sometimes in substantial quantities (Wooldridge, 1988, 1990). However, these granite-derived soils, some of which have developed in situ (Fig 3), have little capacity to buffer or protect this K, or K which is applied as fertiliser, against excessively rapid uptake, or against loss through leaching. Potassium availability therefore rapidly alternates between excess and deficiency in these soils.
Figure 3: Soils which develop in situ may retain features inherited from the parent rock. Here a quartz vein remains as a residual feature in a Hutton soilform on granite parent material. - Photograph: Nietvoorbij
In contrast, certain shale-derived soils and some alluvial, principally sandstone-derived soils may have considerable K buffer capacities, although the natural K content of such soils is invariably low. This buffer capacity for K is, however, highly dependent on pH (Wooldridge, 1990). Such soils may be relied upon to deliver fertiliser K in a controlled and predictable manner, but only where the pH remains stable. Other work (Wooldridge, 1989) showed that variation in terms of cation exchange capacity (CEC) responses to liming, and to applied K, was marked in populations of soils derived from Table Mountain sandstone, whilst CEC responses to liming and K were both marked and consistent in Bokkeveld shale soils. The effects of liming and K on the CEC of granite soils were small. From the data of Wooldridge (1988, 1989, 1990) it was apparent that, in the soil population under test, geology had a predetermining effect on the clay mineral suites found in the mature soils. This is a clear example of a direct link, through conferred mineralogy, between geology and a critical wine-affecting soil chemical characteristic. In view of the fact that recent research by Smuts (2000) has shown that organic material has a dominant effect on cation exchange, the effects of soil mineralogy on vine nutrition may be expected to be greater under Western Cape conditions where the soil organic contents are low, than in most European vineyards, where the soil organic levels tend to be relatively high. Soil organic content has a marked positive effect on the nitrogen content of the must, and hence on wine style (W J Conradie, 2000, personal communication).
2. Work at Nietvoorbij by Van Schoor (2000) shows that soil texture is related, via soil parent material composition, to rock type, and therefore to geology. Geology thus has an indirect effect on such physical soil characteristics as drainage, compaction, water holding capacity and, probably, temperature, organic content and aeration, all of which affect vine performance.
3. Geology is also likely to affect vine growth and wine characteristics through its effects on the form (morphology) of the landscape. Soft, easily erodible rocks tend to erode into valleys whilst harder rocks are preserved as topographic highs. This situation is clearly apparent in the Boland where the Malmesbury shale has eroded away, whilst the more resistant granites remain as rounded hills around which catena sequences of soils have developed in accordance with slope position, texture and slope direction (aspect). The existence of these granites is a reflection of geological processes which resulted in the closure of a sea and the raising of a mountain range on the same site (the Saldanhian Orogeny) about 500 million years ago (Dunlevey, 1988). Geographical orientation may also be dictated by geological processes. That of the wine producing Franschhoek valley, for example, is defined by a fault zone that runs from Saldanha to Franschhoek. Thus geological events in remote antiquity are able to shape modern topography, and thereby the potential for wine production. The Ceres earthquake of 1969, which was caused by movement along a fault that runs parallel to the Worcester Fault, was a recent reminder that certain geological faults dating back to distant events are still active. This earthquake caused marked changes in the drainage patterns of the area, resulting in damage to orchards and vineyards as well as to buildings and irrigation equipment (Truswell, 1977).
4. Past changes in sea level have been considerable. In mid Pliocene times, a little less than about 5 million years ago, the Cape Flats appear to have been submerged to a depth of about 90 metres, probably as a result of a tectonic shift. Table Mountain would then have been an almost sheer sided island and home to millions of sea birds (Olsen, 1983, according to Cooper & Stuart Irwin, 1985). In more recent times, during the Pleistocene, which began about 1,8 million years ago, the sea level around the Western Cape appears to have ranged from more than 100 metres below, to 60 or more metres above the present level. These changes were direct responses to the advance and retreat of ice during the successive phases of the ice age. Climatic effects apart, these changes would have had drastic effects on erosion, deposition and drainage. Since sea levels were low enough to expose most of the Agulhas Bank a mere 20 000 years ago, and since the last ice age ended only about 11 000 years ago (Truswell, 1977), it is clear that great changes occurred over a period of time which was sufficiently recent to have affected the origin and development of many soil parent materials, if not of some of the soils which sustain some vineyards today.
5. The Elgin fruit and wine producing area is situated high in a belt of mountains which overlook the eroded Malmesbury shale and granite lowlands of the coastal plane to the north west. At present the altitude of the Elgin area and its proximity to the coast - the coastline itself is a consequence of the breakup of a pre-existing continent a little over 130 million years ago - confers a relatively cool, maritime climate suitable for the planting of wine grapes. Past apple and pear orchard experience in the Elgin area strongly suggests that future vineyards will preferably be planted in those areas where Bokkeveld shale is preserved in downfolds in the surrounding, older, and more resilient, Table Mountain Sandstone (TMS) highlands (Fig 4). By comparison with the Bokkeveld shale-derived soils, the white, sand soils which have developed on the sandstone have extremely low nutrient buffering and water holding capacities, such that even phosphorus, which is immobile in most soils, may leach below root depth. Although wine production is possible on such TMS soils, open hydroponic methods are likely to be necessary. The effects of open hydroponics on wine quality are, however, uncertain at present. Structural geological considerations therefore affect both local climate and the suitability of soils for vineyards and winemaking. Since Bokkeveld shale soils are usually more suitable for orchards and vineyards than TMS soils, and are also of comparatively limited extent, their price of land on high quality Bokkeveld shale soils is likely to be commensurately high. Geology is therefore also a price-determining factor.
Figure 4: Valleys in the mountains of the Cape Fold Belt are now being planted with wine grapes. Such valleys may have Table Mountain sandstone-derived soils at their margins, and Bokkeveld shale derived soils on the lower and foot slopes. Patches of alluvium, composed mainly of sand from the surrounding Table Mountain sandstone highlands, locally overlie the Bokkeveld shale of the valley floor. - Photograph: Nietvoorbij
From these examples it is apparent that the effects of geology and of geological processes are extraordinarily diverse, and cannot be said to begin, or end, with the rock which underlies a particular vineyard. Geology, which, in the case of the Coastal region at least, is older and more complex than is the case in most French winegrowing areas, effectively, the linkage between topography, soils, climate, vine and wine. It also links current and past earth processes. Since no two areas are truly alike, a sense of uniqueness is also conferred. This uniqueness is potentially capable of being incorporated into marketing strategy. In fact, geological maps, and interpretations of geological features in terms of wine characteristics and qualities, now feature prominently in the marketing strategies of European vineyards and wine making regions. That of the Franciacorta DOCG area of Italy is a particularly impressive example.
Unfortunately, because the geology of South African vineyards has not yet been adequately investigated, no South African wines can yet be marketed, domestically or internationally, with the added value that such a researched geological framework and history can confer.
Our European competitors are currently ahead of the game.
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WOOLDRIDGE, J. 1989. The effect of lime, KCl and parent material on the cation exchange capacity of some acid subsoils of the Western Cape. S. Afr. J. Plant Soil 6, 143-147.
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