What is special about WA wine regions?
The ability of Western Australia to produce quality wines is widely acknowledged across the world. The recently published 8th Edition of the World Atlas of Wine (Johnson and Robertson 2019) states that Margaret River’s “free draining ironstone gravels are most prized for the region’s exceptionally fine reds”, grouping this region with other west coast regions of Bordeaux, Bolgheri, Napa/Soma and the Limestone Coast in their “propensity to turn the rays of the setting of the sun into some of the most satisfying, and age worthy, red wine in the world”. The Atlas also classes Margaret River’s chardonnay as outstanding.
In the past, many authors, including Gladstones (1965, 1992), tended to use comparisons of the latitude and climate data of WA regions with older, established wine regions, particularly those in France and California. Gladstones even raised comparisons between the soils in Margaret River and Bordeaux based on the presence of gravel. While regions such as Margaret River and the Great Southern undoubtedly share some widely recognised characteristics seen as essential to the production of high quality wines, WA Wine Regions are far from being carbon copies of their Northern Hemisphere counterparts.
We think it is time to start showcasing the characteristic, or combination of characteristics, unique to WA that contributes to its reputation for producing such fine wines. While we are still a long way from a full understanding of the interactions between environmental conditions in WA and wine production, three features of the south-west corner stand out: the ubiquitous ironstone gravelly soils; the ancient granitic geology; and the influences of afternoon sea breezes such as the Fremantle and Albany Doctor. In the remainder of this article we discuss what we suspect is the most significant of these features.
The predominance of ironstone gravels
Unlike so much of the wine world, limestone or calcareous soils are of limited importance for Western Australian viticulture. Here, the prized soils are something completely different. As far as we have been able to ascertain, there are no other wine growing areas around the globe that have soils so broadly dominated by lateritic ironstone gravels. These soils are widespread throughout the south-west of WA, covering approximately a third of the combined area of all the wine regions (Figure 1). The gravels have definitely been sought out by vignerons, as by our estimates, they account for over 40% of all plantings in the state.
These ironstone gravels are of pedogenic origin, that is, they have formed within the soil. This is a strong point of difference with other regions, particularly those in Europe, where gravelly soils comprise fragments of quartz, quartzite, limestone, basalt, flint and other rocks (Figure 2).
Viticulture researchers have long identified the potential of ironstone gravels. Despeissis (1902) observed that the loamy ironstone gravels associated with the growth of marri trees (Corymbia calophylla) were “par excellence the best suited for making high class wine; clean to the taste, rich in colour, and pleasant of bouquet”.
Gravels in general are globally appreciated for their association with good drainage and reflectance of heat which impacts on ripening. But for now, just why these lateritic gravels should produce such good quality grapes remains a mystery. The argument that they improve profile drainage doesn’t necessarily apply to WA where the gravel rarely extends far into the clayey subsoils. Here, it is old tree root channels that provide the main pathway for water movement down the profile.
Farmers in WA were quick to recognise the ability of their gravelly soils to produce good pastures and broadacre crops. To a soil scientist, this appears counter intuitive. In an environment where soil water storage is extremely important, the high proportion of topsoil gravel in these soils (often in excess of 50%) would seem to significantly reduce the plant available water in the main root zone.
Clearly, understanding the value of these soils requires us to view ironstone gravels as something more than inert “ball bearings” that only reduce the soil volume and make them more droughty. For a start, these gravels do absorb and store water. Of course, the question remains of how much of this water the vines’ roots are capable of utilising.
More recent investigations by Pate et al (2001) highlighted surprising relationships between the lateritic materials and the native vegetation in WA. They suggest that the vegetation has a role in the creation of the gravel. They argue that symbiotic bacteria associated with proteaceous plants (e.g. banksias, dryandras) may play a part in the formation of ironstone gravels as a way of manipulating the availability of water and nutrients in the soil to their advantage. We do not know exactly how grape vine roots interact with the gravels. However, if the native plants have indeed engineered soil conditions to suit themselves, the results also seem to suit the production of high quality wine grapes.
Despeissis A (1902). In: The handbook of horticulture and viticulture of Western Australia. Government Printer, Perth.
Gladstones JS (1965). The climate and soils of south-western Australia in relation to vine growing. Journal of the Australian Institute of Agricultural Science 31: 275-88.
Gladstones JS (1992). Viticulture and Environment (Winetitles: Adelaide).
Johnson H and Robinson J (2019). The World Atlas of Wine (8th edition). Mitchell Beazley, London.
Pate JS, Verboom WH and Galloway PD (2001). Co-occurrence of Proteaceae, laterite and related oligotrophic soil: Coincidental associations or causative inter-relationship? Australian Journal of Botany 49: 529-560