April Petrocalcic Red Dermosol - Emeritus Professor Robert E White.
During the International Year of Soils, each month we ask a soil researcher to select his or her favourite soil. In April Emeritus Professor Robert E White, University of Melbourne has selected his favourite soil type.
Professor White has been an active member of the soil science community both in Australia and overseas. He has been a federal and branch president, chair of the Certified Professional Soil Scientist Accreditation Board and recognised for his contributions to soil science with honorary life membeship of the International Union of Soil Science and the Australian Society of Soil Science Inc.
If you had to choose a special Australian soil type what would it be?
The famous Terra Rossa, formally known as a Petrocalcic Red Dermosol, occurring in the Coonawarra wine region of the Limestone Coast in South Australia.
Plate 1 - Profile of a Coonawarra Terra Rossa showing the red soil profile over calcrete capping of the porous limestone parent material (from White R. E. (2015) Understanding Vineyard Soils, 2nd ed., Oxford University Press, New York).
What are the main properties of this soil type?
This soil has formed on a sandy porous limestone laid down during the Miocene epoch (about 22 million years ago). Subsequently, during the Pleistocene epoch when sea level fluctuated markedly during successive glacial and interglacial periods, a series of sandy beaches formed on limestone ridges. These have been exposed to soil formation during uplift of the land as the sea retreated to the west.
Insoluble impurities like quartz and clay in the limestone have accumulated as the limestone dissolved. The resulting depth of the soil is highly variable and has been increased during the Pleistocene interglacial periods by deposition of wind-blown silt and fine sand (loess).
Ferric iron oxides give the soil its bright rusty-red colour and its excellent structure throughout the profile, which allows good drainage, a very important property for grapevine growth (see Plate 2).
The soil pH is neutral to slightly alkaline, maintained by the dissolving calcium carbonate. This pH tends to restrict the availability of some nutrients such as phosphorus and the micronutrients iron, copper, zinc and manganese.
Why do you find this soil type particularly interesting?
These Red Dermosols have a limited extent of about 5,000 ha in the Limestone Coast zone. Typically they occur on slightly elevated ridges where there is often a calcrete capping on top of the underlying limestone (see Plate 1). The capping is of secondary calcium carbonate (petrocalcic means cemented with secondary carbonates) which forms on top of the more porous limestone below. The calcrete curbs deep root growth and reduces access to water, although the roots of older vines appear to penetrate the underlying limestone in the same way that old vines do in St Emilion and the Cote d’Or, France. The slow rate of water supply favours the build up of colour and flavour compounds as the fruit ripens, which contributes to the distinctive character of the wines, especially Cabernet Sauvignon and Shiraz.
Do the properties of this soil type have consequences for its management, e.g. in terms of land use, soil quality, conservation?
In places where the depth of soil over limestone is shallow, deep ripping may be required during vineyard establishment to fracture the calcite capping and encourage deeper rooting of the vines. However, given an annual rainfall of about 650 mm, the combination of clay content (up to 40 percent) and good structure means that many vineyard blocks, once established, can survive with no, or minimal, irrigation. Producers of premium wines try to moderate the use of irrigation in their vineyards. The iron oxides and abundant content of calcium ions keep the soil structure very stable so there is little erosion or transport of colloidal sediment through the profile.
Plate 2 Photograph of the excellent prismatic structure of a Terra Rossa subsoil. The scale is 10 cm (from White R. E. (2015) Understanding Vineyard Soils, 2nd ed., Oxford University Press, New York).
Can you tell us your most memorable story concerning this soil type?
More than a decade ago, when the Coonawarra wine Geographic Indication (GI) was being established, there was much dispute among wineries as to which should be included in the much-acclaimed GI. The region is approximately 27 km long but only 2 km wide, extending in a north-south direction from the town of Coonawarra in the north to just south of Penola. Eminent soil scientists were called to testify before an Administrative Appeals Tribunal (AAT) as to the properties of the soils that would qualify particular vineyards to be included in the GI. After much argument, the AAT accepted the proposition that the Terra Rossa soil was a defining feature of the Coonawarra wine region. Some appellants challenged the consequent ruling of the AAT, as implemented by the GI Committee (GIC), in the Federal Court on matters of law. Eventually, however, the revised GIC delineation of the Coonawarra GI was confirmed and came into being in January 2003, after nearly 20 years of debate. Although many examples have occurred in Europe, this case is one of the few examples in Australia where a particular soil type has played such an important part in underpinning the reputation of a region for the distinctiveness of its produce (there may be others looming in the confrontations between farmers on fertile agricultural soils and mining exploration companies seeking to exploit oil and gas reserves). In 2014, Dr Rob Fitzpatrick reported in detail on the history of the Coonawarra GI and its implications for identifying a particular wine terroir in an article entitled ‘Coonawarra – the trauma of defining the terroir’ in the Australian and New Zealand Grapegrower & Winemaker, 601, 33-40.