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A GENERAL COMPARISON BETWEEN SOIL AND CLIMATIC FACTORS IN SOUTH AUSTRALIA
AND THE WESTERN CAPE IN SOUTH AFRICA
Lesley Toombs & Eben Archer, Department of Viticulture and Oenology,
University of Stellenbosch
INTRODUCTION
A comparison of vineyard cultivation, and the qualitative and quantitative
results thereof, in the Western Cape of South Africa and South Australia
requires investigation into a wide variety of soil and climatic factors.
Both of these Southern hemisphere areas are renowned for world class
vineyards and world class wines. Situated oceans apart, these two vine
growing regions provide an interesting comparison with regards to both
climatic and soil factors. Varied and diverse climatic and soil factors
co-exist in each of these regions yet in both regions the vine develops
well, providing good examples of vine adaptation to climate and soil
conditions. It is almost impossible to separate soil factors from climate
factors as the soil develops under the influence of climate and serves to
modify the effects of climate thereby playing a very important role in
determining the quality of the crop. The correct combination of climatic and
soil factors is therefore of the utmost importance in vineyard cultivation.
This balance between soil and climate determines the ultimate performance of
the vines under cultivation.
As a basis for climatic comparison, data obtained from the weather stations
at the Nietvoorbij Institute of Viticulture and Oenology in Stellenbosch
(Western Cape) and at Coonawarra (South Australia) have been used. The
choice of specific weather stations allows for an accurate interpretation of
the prevailing conditions within the two regions. The Western Cape and South
Australia, with unique and classic features of their own, boast ideal yet
differing viticultural conditions. In comparing these two regions the
balance between soils and climate can be demonstrated. The most important
climatic and soil features are taken into account in this comparison and
allow a general overview of the factors specific to each region.
COMPARISON OF
CLIMATIC FACTORS
Temperature
Temperature is a basic factor used to distinguish one region from another.
Temperature is related to the situation of a specific region, in other words
the longitudinal and latitudinal position of the area in question and the
proximity to the moderating influence of the sea. The Western Cape, situated
at approximately 32¡ to 35¡ S and 18¡ to 22¡ E (Fullard & Darby, 1964) is
characterised by a typical mediterranean climate which is influenced by
westerly winds originating from the Atlantic Ocean (Encarta ¨ Encyclopedia,
1997). South Australia's most important vine growing region, situated at
approximately 33¡ to 36¡ S and 136¡ to 140¡ E (Fullard & Darby, 1964) is
characterised by a mild, temperate climate. Both of these regions have 4
distinct seasons with cool winters and warm summers (Encarta ¨ Encyclopedia,1997).
Figure 1: The mean monthly temperatures of Coonawarra, South Australia
(Austr. Bur. of Meteorology, 1998) and Stellenbosch, South Africa (Agromet,Nietvoorbij, 1998).
Figure 2: The mean monthly rainfall of Coonawarra, South Australia (Austr.Bur. of Meteorology, 1998) and Stellenbosch, South Africa (Agromet,Nietvoorbij, 1998).
The hottest month of a typical year in the Western Cape is the month of
February with a mean monthly temperature reaching 21.60¡C (Agromet,
Nietvoorbij, 1998). Similarily in South Australia February is the hottest
month with a lower mean monthly temperature of 20.25¡C (Australian Bureau of
Meteorology, 1998). When taking the coldest month into account, July is seen
to represent both regions. The mean monthly temperature in South Australia
reaching 9.25¡C (Australian Bureau of Meteorology, 1998) while the Western
Cape demonstrates a warmer winter with an average low of 12.35¡C (Agromet,
Nietvoorbij, 1998).
The mean monthly temperatures experienced in South Australia are visibly
several degrees lower than those experienced in the Western Cape (Fig. 1).
The minimum temperature at which a vine is still physiologically active
varies from cultivar to cultivar and is also related to the specific area in
which a vine is grown. It can therefore been seen why cultivar selection is
so important and why the choice of cultivars might differ between South
Australia and the Western Cape. A factor influencing the selection of
cultivars includes fluctuations in temperatures within these regions. It is
however important to note that not only climatic factors will influence the
choice of a specific cultivar in a certain region. Cultivars of the same
type can be chosen in both areas but will be subject to variations in
temperatures, therefore producing wine with varying properties and nuances.
Cultivars such as Riesling, Sauvignon blanc, Bukettraube, Cabernet Sauvignon
and Pinot noir are grown in both these regions, but will exhibit varied
characteristics which will partly be due to the influence of the climate in
which they are grown. Delayed budburst occurs to a lesser extend in South Australia than in the Western Cape.
Rainfall
The Western Cape experiences higher levels of rainfall than South Australia
(Fig. 2). The rainfall in the Western Cape is, however, much higher than
South Australia over the entire rainfall season. The Western Cape
experiences its first rainfall at an earlier stage than South Australia,
with effective precipitation beginning as early as April when the total mean
monthly figure reaches approximately 135 mm. At the same time the mean
monthly figure for South Australia reaches approximately 55 mm (Fig. 2). On
the other hand, limited rainfall is received in both regions during the
summer months. The storage water in the soil which is utili-zed during the
warmer summer months of the year is critical in both regions. The lower
rainfall in South Australia is the reason why irrigation is used more freely
than in the Western Cape. This, together with lower summer temperature,
results in less stressed vines in South Australia compared to the local situation.
Radiation and Sunshine Hours
The accumulation of cold units by the vine will result in the initiation of
budding after a period of dormancy. After budding, heat and sunshine hours
affect active shoot growth. The amount of radiation received by the vine in
combination with the number of sunshine hours experienced in a specific
region influence the rate of growth of the vine (Jackson & Spurling, 1992).
The data obtained from the Nietvoorbij Institute was representative of the
average number of sunshine hours per day in the Western Cape (Agromet,
Nietvoorbij, 1998). It is evident from Fig. 3 that the average number of
sunshine hours per day are higher during the spring and summer months.
Data obtained from the Coonawarra weather station was in the form of the
number of mean clear and mean cloudy days in South Australia (Australian
Bureau of Meteorology, 1998). From Fig. 4, it can be seen that during the
winter months the mean number of cloudy days exceeds the mean number of
clear days and during the summer months the mean clear days exceed the mean
cloudy days.
An interesting comparison between the Western Cape and South Australia with
regards to radiation and sunshine hours can be made when taking the periods
for harvesting into account. January, February and March are the months for
harvesting in a typical year in the Western Cape compared to the months of
March, April and May in South Australia. When related to the sunshine hours,
it can be seen that as the period of ripening occurs in the Western Cape so
the average number of sunshine hours increases (Fig. 3). In South Australia
it can be seen that during the period of vŽraison, the mean number of cloudy
days exceeds the mean number of clear days (Fig. 4). From this it can be
concluded that during vŽraison, the vineyards in the Western Cape receive
more radiation and sunshine than the vineyards in South Australia. This
without doubt, exerts an important influence on the rate of ripening of the different constituents in the berry.
Figure 3: Average daily sunshine hours at Stellenbosch, South Africa(Agromet, Nietvoorbij, 1998).
Evapotranspiration
The relationship between rainfall and evapotranspiration contributes to the
aridity of a region (Jackson & Spurling, 1992) and is a significant factor
in viticulture in both the Western Cape and South Australia where the warmer
growing season is also the time of lowest rainfall (Fig. 2). In areas where
the effective evaporation exceeds rainfall for much of the year there will
be a marked contrast in stress levels when compared to areas where the
effective rainfall exceeds evaporation (Jackson & Spurling, 1992).
Unfortunately, no evapotranspiration figures could be obtained from
Coonawarra. Taking the data concerning temperature, rainfall, radiation and
sunshine hours in consideration, it can be surmised that evapotranspiration
in the Western Cape exceeds that in South Australia. This may have important
effects on the stress level occuring in vines during ripening in these two regions.
COMPARISON OF
SOIL FACTORS
Soil is the most important factor that must be taken into account when
considering vineyard cultivation. This applies to any viticultural region in
the world and will ultimately result in determining the growth and
productivity of a vine. Grapes are adapted to a wide range of soil types and
it is evident that there is a preference for a certain soil type in nearly
every grape growing district (Saayman, 1981). Soils are difficult to deal
with in a broad sense due to the fact that the type of soil can range from
gravelly, sandy to heavy clays, from shallow to very deep and from low to
high fertility (Saayman, 1981). This applies to both South Australia and the
Western Cape where soil classification is a very involved and complicated task.
Texture
Texture is a fixed soil characteristic which cannot be influenced by soil
manipulation, but does, however, have a marked influence on manipulation
techniques (Van Huyssteen, 1981). The classification of soils in South
Australia was mainly developed by Dr Keith Northcote in 1979. According to
Dr Northcote the most common soil types to be found in South Australia
include the uniform texture profiles (the U soils) and the duplex texture
soils (the D soils). These U soils include sands, loams, non-cracking clays
and cracking clays. The D soils consist of red duplex soils, brown duplex
soils, yellow and yellow-brown duplex soils and black duplex soils. The most
prominent soils found in areas under intense vineyard cultivation consist of
a variety of soil textures. These include solonetz and solodized solonetz,
solodized brown soils, desert loams, calcareous desert soils and desert.
The most common soil types to be found in the Western Cape include duplex
soils with sandy or loamy topsoils on clay, structureless red- and
yellow-brown apedal soils, structured soils, wet and darkly coloured soils
and shallow soils on rock (Saayman & Van Huyssteen, 1981). It is
difficult to evaluate soil texture as a comparative feature between these
two regions due to the fact that the soils vary so widely within each
region. It is however recognised that vines will thrive best on well aerated
soils - medium sands to loams are ideal, but vines will tolerate well
drained clays (Gordon Cook, 1965). It is further evident that the soils in
the Western Cape occur in a more complicated pattern than in South Australia
and that within the same area the local soils are much more varied in
physical properties. It can therefore be stated that, based on texture, the
soils in the Western Cape are of a lower potential for vine growth than those in South Australia.
PH / Acidity
The pH or acidity relates to the presence of hydrogen ions in the soil. The
amount or level of acidity in the soil is measured by the pH. The ideal pH
for viticultural purposes is a pH (KCl) which ranges between 5.0 to 7.5
(Saayman, 1981). Acidity varies widely from region to region and is
influenced by both climatic and soil factors. The soils of the Western Cape
are mostly characterised by high le-vels of acidity and therefore low pH
values, a feature mainly found in areas of high rainfall (Fig. 2). South
Australia which has lower levels of annual rainfall than the Western Cape is
considered to be a more arid area (Jackson & Spurling, 1992) and it is in
such areas that soil alkalinity occurs (Encarta ¨ Encyclopedia, 1997).
Differences in levels of pH or acidity will necessitate differing methods in
soil preparation or management, for example the addition of pH stabilising m
aterials in order to obtain optimum vine growth conditions. The higher
levels of lime addition to soils in the Western Cape necessitate more
intensive soil preparation procedures than are generally found in South Australia.
Structure
Both the Western Cape and South Australian soils demonstrate the full
spectrum of soil structures. The principle forms of soil structure have been
classified as being blocky, spheriodal or polyhedral, prismlike and platey
(Macvicar, et al., 1991). In the Western Cape the soils are known to be
hampered by depth limitations and weak physical structures (Saayman & Van
Huyssteen, 1981). These structures will consist of natural aggregation
within the soil which can be to such a great degree that vine root
penetration is prevented. Compound units or peds form within the soil and
are separated from one another by planes or surfaces of weakness (Macvicar,
et al., 1991).
Most of the soils in South Australia exhibit characteristics which suggest
that root penetration will be medium to deep. Solodized brown soils and
calcareous desert soils (Northcote, 1981) are very prominent as vineyard
soil. The solodized brown soils exhibit higher levels of restriction to root
penetration due to the presence of hardened subsoil layers (Northcote, 1981)
and are more accurately compared to the soil structures of the Western Cape.
Structure is therefore a difficult comparative factor due to the
considerably wide range of structures which occur within these two regions
and from vineyard to vineyard. In general, however, it is evident that soils
in the Western Cape are shallower than those found in South Australia.
Soil Management
In the Western Cape soil management is intensively applied (Saayman & Van
Huyssteen, 1981). This intensive soil management is necessary because of the
weak physical structure and limited depth characteristics of Western Cape
soils (Saayman & Van Huyssteen, 1981). It is important to increase the
potential zone for root penetration and to improve on the soils moisture
holding capacity. The soils of South Australia require different forms of
soil management (Barnes, 1957). With the sandy soil types that predominate
it is the constant aim to protect the soil surface from wind erosion and to
improve their ability to store the rainfall water and plant nutrients. Where
vines are planted in loamy soils farming methods used must be those that
will protect the soils from water erosion and increase water storage (Wells,
1959). The emphasis on storing rainfall water in both the soils of the
Western Cape and South Australia is essential because a shortage of water
(Fig. 2) in the summer can be an important contributory cause of low yields.
Figure 4: Mean cloudy and clear days in Coonawarra, South Australia
(Australian Bureau of Meteorology, 1998)
Conclusion
The comparison between the Western Cape and South Australia has shown how
two successful Southern hemisphere viticultural regions demon-strate both
similarities and differences with regards to climate and soils. The
successful growing of the vine is not solely dependent on one factor, but is
as a result of the combined influences of both the climate and the soils.
The fluctuations in climate and variations in soil types within these two
regions will contribute to the development of unique grape and wine
characteristics. In general it is evident that South Australia has a cooler
climate during ripening than the Western Cape. Therefore the ripening
process is slower and the harvesting date later than in the Western Cape.
This enables the production of different wine styles.
Soil is a distinguishing characteristic in any region and varies widely
according to its origin. It is the more intricate of the two factors to
compare as such a wide spectrum of differences can occur from vineyard to
vineyard. Both South Australia and the Western Cape provide soils which
support this statement. Good soil management is essential to establish the
most favourable soil conditions in a specific region. Soils can vary greatly
within a given region, and even within a vineyard. The basic factors of
soils are demonstrated clearly within classification systems and enable a
comparison to be carried out.
A general comparison between soil and climatic factors within the Western
Cape and South Australia has demonstrated that vineyard cultivation takes
place under a wide variety of conditions. When analyzing these two regions
it can be seen that both soil and climatic differences do occur which
provide conditions under which different wine styles can be produced.
REFERENCES
Barnes CP, 1957. Soil management in regions. In: The yearbook of
agriculture, US Dept. Agric., Washington DC
Encarta ¨ Encyclopedia, 1997. Microsoft ¨ Corporation
Fullard H & Darby HC (Eds.), 1964. The University Atlas, 11th ed. George
Philip & Son Ltd., London
Gordon Cook J, 1965. Your guide to the soil, 2nd Ed., Nerrow Publ. Co. Ltd.,
England
Jackson DI & Spurling MB, 1992. Climate and Viticulture in Australia. In:
Viticulture Vol. I, Resources in Australia (Eds. BG Coombe & PR Dry). Austr.
Ind. Publ., Adelaide
MacVicar CN, et al, 1991. Soil classification. A taxonomic system for South
Africa, NIGB, Dept. ATS, Pretoria
Saayman D & Van Huyssteen L, 1981. Grondvoorbereiding. In: JD Burger & J
Deist, Reds. Wingerdbou in Suid-Afrika, 120-140. Stellenbosch, NIWW
Saayman D, 1981. Klimaat, grond en wingerdbougebiede. In: JD Burger & J
Deist, Reds. Wingerdbou in Suid-Afrika, 48-66. Stellenbosch, NIWW
Van Huyssteen, 1981. Bewerking. In: JD Burger & J Deist, Reds. Wingerdbou in
Suid-Afrika, 283-307. Stellenbosch, NIWW
Wells CB, 1959. Soils and land use in the Barosa district, South Australia.
Series no. 30, CSIRO, Australia
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