My thoughts on this topic have come about following a question posed during a Zoom wine tasting late in 2020. The winemaker was commenting on wine characters and alluding to different soil types in the various vineyards from which the grapes had been sourced. This led to the immediate question of ‘just what is the link between soil and wine?’
There are some obvious examples of direct influence of soil on wine composition. In Australia, we have an increasing problem with soil salinity as well as higher salt concentration in irrigation water. The uptake of chloride and sodium from the salt can affect the vine’s physiology as well as giving a ‘salty’ character to the wine’s taste. A long-term study on Shiraz led by Dr Rob Walker of the CSIRO has found that the choice of rootstock to which the Shiraz has been grafted can provide a benefit in terms of reduced salt effects on wine sensory attributes (Aust. J. Grape Wine Res. 2019, vol. 25, pp. 414–29). Some years ago, I visited the Hungarian wine region of Somló. This region is situated on the side of an extinct volcano and the high magnesium content of the soil is reflected in the taste of the wine – think Epsom salts! The centuries’ old claims of the health benefits of this wine are still to be justified.
The relationship between vineyard soil and wine composition is more subtle than the direct effects described above. It is clearly a multi-faceted issue, best described as terroir, that involves the soil, the climate, the terrain or topography and the planting material (grape cultivar and rootstock). Human intervention in vineyard management and wine production can reduce or even eliminate the terroir effect if extensive manipulation occurs. If the commercial priority is to produce a wine that is the same or similar in character year after year, akin to beer perhaps, then control of the grape to wine process is important and, when coupled with blending across regions, the terroir influence on the finished product is not relevant.
Terroir has always been seen as a major marketing tool in European wine production, especially in France, Italy and Spain, the regions I know best. It is becoming increasingly important in Australia, especially for smaller wineries that are trying to secure a well-defined market position. Soil plays a critical role in defining terroir. It is the substrate on which the vine grows. Water availability, supply of nitrogen and other nutrients and the capacity to build a stable microflora population (see my July/August 2020 column) are significant aspects of the soil’s contribution to terroir.
The soil, nitrogen, water availability question is something that we studied over a three-year period when I was at the National Wine and Grape Industry Centre. Using a 1.2-hectare experimental vineyard in the Riverina region, we could manipulate the timing of irrigation and nitrogen addition at three critical periods from bud-burst to harvest. Two important outcomes were the variation in the total amino acid amount as well as changes in the individual amino acid profile. Arginine, an important yeast food, could be enhanced or suppressed depending on the management practice. The role of specific amino acids in the production of wine aromatic compounds has been examined in some detail recently (Front. Microbiol., doi: 10.3389/fmicb.2017.02554), opening up a new aspect of the soil/wine composition link.
In a recent article, Professor Robert White from the University of Melbourne addressed ‘The value of soil knowledge in understanding wine terroir’ (Front. Environ. Sci., doi: 10.3389/fenvs.2020.00012). This comprehensive article expresses succinctly the main issues in the soil/terroir relationship, summarising the present state of knowledge and posing questions that need to be answered to enhance our understanding, particularly the provision of methodologies to map terroirs and to monitor changes over time.
Bordeaux has long argued that the terroirs of its regions play a critical role in determining each region’s wine characters. Professor Cornelis van Leeuwen and his team from the Bordeaux Institute of Agricultural Sciences have been at the forefront in providing a strong scientific basis to the terroir concept. The scientific basis is a major step forward for, as the comedian Stephen Colbert once quipped ‘What’s the use of science if it cannot justify marketing claims!’ The Bordeaux group has reviewed soil-related terroir factors (OENO One 2018, vol. 52, pp. 173–88) and extended a part of this review to summarise the measurement and management of the soil effect in terroir expression (IVES Tech. Rev., doi.org/10.20870/IVES-TR.2020.4484). Spatial distribution of soil resistivity (as a surrogate for soil temperature), vine water status and vine nitrogen status can be mapped across a vineyard using 8–10 sampling points per hectare. Enhanced terroir expression can then be achieved by matching planting material and management techniques that are specific to the site. Expect more on this strategy as technology evolves.