South African Vineyard Soils and Climates

Geology

The traditional South African vineyard areas are located mainly along the coastal zone and toward certain inland areas of the Western Cape Province. This landscape is characterised by very old geological formations that are today still clearly visible due to sustained tectonic uplift (probably due to a hot plume of mantle rock below the southern and central part of the African continent) and subsequent erosion, resulting in steep, folded mountains that roughly parallel the coast, with younger deposits found in the high laying inland areas. The oldest rocks are the Malmesbury group (pre-Cambrian Namibian Epoch, 980-830 Ma) of shale, phyllite, schist and greywacke, that occur in pockets as foothills and lower laying undulating hills around Stellenbosch and Somerset West and also form a prominent range of high hills (Tygerberg) around and north of Durbanville. Here it was baked (hardened) by intruding hot magma, rendering it more resistant to erosion and levelling. The Malmesbury deposits also occurs extensively further north in the Swartland area around Malmesbury, Riebeeck mountain, Moorreesburg and Piketberg.

Cambrian granite magma intruded about 630-500 Ma ago into the Malmesbury deposits and coagulated as plutons (domes) about 10-15 km below the then land surface. Through land uplift and subsequent erosion these plutons were exposed or levelled and then covered again by the deposition of deep sandy material, the present day Cape Supergroup (600-425 Ma) of sandstones and shales. Permian to Triassic (300-200 Ma) orogeny (mountain building through tectonic movement pressure) and subsequent erosion, resulted in the present day Cape Fold Belt Mountains, roughly paralleling the southern- and western coastlines of the Western Cape. Erosion also exposed the granite intrusions, at present clearly discernible in several areas, such as the basements of sandstone mountains like Table Mountain, the mountains around Somerset West, Stellenbosch and Franschhoek and the mountains east of Paarl and Wellington. In other places these granite plutons are prominently visible as rounded hills and low, dome-like mountains like Bottelary Hills, Paarl mountain, Paardeberg and a range of low hills north of Malmesbury, as well as the Darling range of high hills close to the west coast. Isolated exposed intrusions also occur along the south-western coastal belt.

Soils

In the coastal regions, the very old geological formations and the long periods of exposure to weathering, without extensive covering by younger material of glacial or alluvial epochs, resulted in viticulture being practiced on some of the oldest materials and soils in the world. The advanced state of weathering resulted in soils with kaolin and sesquioxides (iron and aluminium oxides) as dominant clay minerals, with a low exchange capacity (3-5 cmol kg-1, compared to 10-15 cmol kg-1 of younger soils) and with increasing acidity with depth (<4.0-5.0, measured in 1N KCl), as most basic cations have been lost through leaching and subsequently replaced by hydrogen ions. This is quite unique, but the effect of this on wine character compared to soils with higher pH or soils containing free lime, is still not yet unravelled. However, the inhibiting effect of too low a pH on root development and functioning, is well documented and measures to increase the soil pH to at least 5.5 by incorporating lime to depths of 1.0-1.2 m by trench ploughing before planting, are standard practice. Such intensive deep soil preparation before planting is a hallmark of South African viticulture and virtually unparalleled world-wide.
The soils of the inland areas beyond the coastal mountain ranges, like the Breede River valley, and further north along the Olifants- and Orange Rivers, are not acid and often contain free lime.

South African vineyard soils are also notoriously low in phosphorus, except for soils along the lower reaches of the Olifantsrivier. This limitation is also readily rectified by incorporating adequate quantities of phosphorus during the deep ploughing soil preparation operations.

Due to the pronounced topographical diversity over short distances, soils also vary similarly, ranging from shallow, rocky soils on plateau- and steep slope positions, to deep, highly weathered yellow- to reddish-brown soils (oxisols)) along mountain foothills in the Constantia, Stellenbosch, Paarl and Wellington areas, as well as on ranges of hills in the Durbanville, Malmesbury and Darling areas. These soils are remnants of a former land surface and soils formed during a tropical epoch, estimated at c. 50 Ma ago. On the Malmesbury shale and schist in lower landscape positions, more strongly structured and heavier textured soil are usually found, often with a duplex character (double soil; sandy/gravelly material top layers with an abrupt transition to heavy textured, structured subsoil material), often prone to periodical wetness. However, when properly ameliorated, both physically and chemically, high wine quality wines can be obtained on these medium vigour soils.

In low laying- and valley positions and along river courses, Quartenary sediments occur on which sandy to organic rich, often hydromorphic soils (prone to waterlogging) developed. These soils are generally not highly regarded for quality wine production in the coastal zone, but are extensively utilised in the warmer inland areas like the Breede River valley and along the banks of the Olifant- and Orange Rivers for high production vineyards, the grapes mainly destined for bulk- and distilling wine. Also in the inland areas, reddish-brown soils, often with calcrete (hardened limestone) and/or duripan (silica cemented material) layers present in the profile, occur on higher laying old terraces and are extensively used for viticulture and fruit production. Again, deep soil preparation is necessary on these soils to break up the hard layers. Once properly done, these soils can induce high vigour, but have a reputation for inducing better wine quality than the alluvial soils.

Climate

The traditional coastal and bordering inland vineyard areas benefit from a Mediterranean climate, but are cooler than similar areas at the same latitudes in the Northern Hemisphere. In the coastal zone this is predominantly due to the influence of the nearby oceans. The cold Benguela current from the South Pole flows northwards along the western coast (Atlantic ocean) and the warmer Mosambique current from the equator (Indian ocean) follows the south coast in a westerly direction, the two currents or oceans meeting each other between Cape Agulhas and Cape Point. The cooling effect of prevailing winds and breezes from these oceans on nearby land, have been scientifically studied and verified.

An old Cape saying goes that “a vineyard that can see the sea, is a good vineyard”. This seems to be largely true in view of the outstanding varietal character that are obtained in such vineyards, especially in the case of terroir sensitive cultivars like Sauvignon blanc, Sémillon, Shiraz and Pinot noir, Concerning temperature, the coastal regions, which usually are not more than 30-40 km away from the sea, generally fall in the Winkler GDD (Growing Degree Days) Region III, with vineyards closer to the coast and at higher altitudes falling in Region II. Further inland and along the river courses, GDD temperature ranges fall into Regions IV and even V. But even in the higher temperature ranges, due to prominent topographical features such as altitude and southern to eastern aspects (slopes) that induce cooler conditions, pockets of terroir are to be found that, with the right cultivars and with competent management, can yield wines of outstanding quality.

Rainfall generally decreases from the coast to the inland areas and from south to north. The varied topography of the Western Cape further causes rainfall to be highly variable, with mountains and associated valleys generally receiving higher rainfall than surrounding areas. Rainfall usually ranges from 400-800 mm per year, with about 30% falling during early summer and virtually nothing during late summer and the ripening period. This forces producers to hoard winter rain in dams where possible and then use the water as irrigation during dry spells as necessary.

Viticulture Development

South African viticulture originated in the vicinity of the Cape in 1652 with the advent of VOC (Vereenigde Oost-Indische Compagnie) Dutch settlers. The first wine was produced I 1658 from gardens around the fort, with the first wine terroir selections probably the Constantia, Vergelegen (Somerset West) and Stellenbosch areas. The industry slowly expanded during the eighteenth century, but exports increased dramatically due to preference tariffs introduce in 1813 by the then British occupants of the Cape. It reached a low point in the early 1850’s due to unscrupulous exports of inferior quality wines and crashed completely with the cancellation of preference tariffs. However, the industry picked up again after overcoming the phylloxera disaster which struck in 1886, but suffered from subsequent overproduction towards the end of the nineteenth century, initiating the establishment in 1918 of the KWV (Co-operative Wine Farmers Association). From then, the KWV controlled production by i.a. issuing grape production quotas, mainly to established producers, prohibiting any unauthorised new plantings. This system guided the wine industry through the troubled early twentieth century but prevented the industry from expanding to new, mainly cooler localities and areas that were at that stage already considered to have potential for high quality wines.

When the regulatory function of the KWV was abolished in 1992, the industry experienced great improvement in vitiviniculture during the nineties and beginning of the twenty-first century as new regions were developed, mainly along the coasts. Among the most prominent is the Darling range of hill on the west coast, where viticulture is practiced mainly on deep, reddish-brown soil derived from granite, at elevations of 200-350 m and exposed to cooling winds from the Atlantic ocean, about 10 km away. Rainfall is relatively low (c. 500 mm/year), with very little possibilities for storing winter rain. However, dry land production is viable because of the extreme depth of the soils and its high water holding capacity.

South of Stellenbosch and Somerset West is another significant development, viz. the mountain basin of Elgin, formally almost exclusively an apple- and pear producing area. This is a cool area, situated 12-20 km from the sea, with mean February (ripening stage) temperatures of 19-20oC. Soils are predominantly ferruginous (iron rich) gravel on highly weathered, soft Bokkeveld Group shale, situated at 200-300 m altitude and surrounded by Table Mountain sandstone mountains at heights of about 500-1 000m, sheltering the area from the often extremely strong south-easterly winds in summer. Rainfall is ample (c. 1 000 mm/year), with a well developed infrastructure for storing water and irrigation, the latter being only rarely necessary in the case of vineyards.

Other localised plantings took place further to the south-east around Walker Bay and in the Hemel-en-Aarde valley in the vicinity of Hermanus. Still further south-eastward are today reputable vineyards at Stanford, Uilenkraal and in the vicinity of Elim. Soils vary considerably, being derived from shale, granite, sandstone or even calcrete material, the main consideration here being the cool climate as most of these plantings are not further than 15 km away from the sea. These areas and plantings recently received their own encompassing region of origin, named Cape South Coast.

Further inland, in the quest for cool locations, sporadic plantings can be found at high altitudes in areas like Ceres, the Cederberge, Sutherland, the Lange- and Swartberge., soils being very diverse and of lesser importance.

Protection of Origin

The diversity of South African vineyard- and wine landscapes are regarded as a great asset, yielding great wines with pronounced but divergent character and styles. The effects of natural factors (climate, topography and soil/geology) on wine character have been scientifically verified since 1977, with follow-up studies receiving high priority and yielding confirming results. Even before this, the importance of the demarcation and legal protection of areas of origin was realised and the South African ‘Wine of Origin’ scheme was instituted in 1973, with a Wine and Spirit Board for the upkeep, administration and running of the scheme. Due to a lack of long term tradition and experience, especially in new areas, it is the policy of the Board to demarcate potential areas or units of origin according to a distinctive pattern of natural factors. This is usually done by request of producers or the industry, who are then allowed, without prescriptions, to develop their own unique wines by means of cultivar choice, production methods and winemaking techniques. Today, the demarcation of areas of origin is generally accepted by the wine industry and is well developed and administered.

Summary

The distinctive and diverse topography and geology of the South African vineyard and wine landscapes, characterised by magnificent sandstone mountains, granite foothill bases, low mountains and prominent ranges of hills, which merge into undulating shale hills, are outstanding natural components and regarded as a great asset. As a result of this diverse topography and geology, even more diverse soil types occur, as well very diverse meso-climates due to variation in altitude, sunlight interception and exposure to prevailing winds and their effects on temperature and water availability. The importance of these natural factors in determining wine character and -style have been realised since the establishment of vitiviniculture at the Cape, have since been scientifically verified and today form the basis on which South Africa’s wines of origin areas are identified, demarcated and protected.