Terroir, water status, minerality and earthy aromas
The soil is one of the aspects that determine the ‘terroir’ of wine. But exactly what effect does the soil have on wine? Is there actually a direct link between the soil on which the vine grows and the earthy and ‘mineral’ aromas in the wine?
Terroir is a commonly used term to indicate the unique character of a wine. The term “terroir” includes more than just the soil on which the vine grows. It also includes the location of the vineyard, the climate and even the (traditional) actions of the winegrower in the vineyard such as a certain type of pruning or soil maintenance. This set of factors influences the ripening and characteristic aroma’s of the grapes and produces a wine that (allegedly) cannot be copied at another location. It is however difficult to appreciate the contribution of an individual aspect of terroir – in this case the soil. Nevertheless, there are a number of clear characteristics of the soil that have an effect on wine quality.
Poor and rich soils
The extent to which a vine can grow and absorb nutrients depends on the pH of the soil and the amount of organic material. The pH determines how easily the absorption of minerals can take place. By minerals is meant here the essential elements without which organisms cannot live such as phosphate, nitrate, calcium, magnesium, potassium, sodium and the trace elements. A neutral or slightly basic soil is ideal for the absorption of these nutrients. Clay and organic matter in a richer soil determine the concentration of cations such as Ca2+, Mg2+, K+ and Na+ in the soil moisture1. These ions are dissolved and can therefore be absorbed by the roots. Poor sand and rock soils do not have this cation exchange between solid matter and soil moisture. On these soils, plants are for the uptake of these elements almost entirely dependent on humus. Microorganisms break down the humus into these inorganic minerals, a process that is, not coincidentally, called mineralization. For all soil types – either the rich clay soils or the poor sandy soils – humus is the most important source of nitrogen and phosphate compounds.
In addition to dissolved minerals, there are also solid soil minerals, for example limestone or a feldspar pebble. Weathering of these rocks can ensure the release of minerals through the formation of clay minerals such as kaolinite. However, this process is much too slow (geological time scale) to be important for a continuous supply of nutrients for the grapevine.
“The soil has an effect on the aromas that are being developed”
A poor stony soil is often preferred for growing wine grapes. Due to the limited presence of water and nutrients in these soils, the grapes go to the ripening phase earlier. As a result, the aromas mature faster, the grapes get higher sugar levels and the amount of acids drops earlier. A fertile soil with many nitrogen compounds, on the other hand, is less suitable for viticulture. Higher nitrogen levels result in more vegetative growth of the grapevine, and grapes with less concentrated aromas1.
Drainage and soil aeration
The physical properties of the soil such as the structure, depth, and drainage determine the water status of the soil. Proper aeration of the soil stimulates soil life, and therefore also the availability of nutrients that are released for the grapevine. The water status is directly related to the type of soil and has a greater influence on wine quality than the chemical composition of the soil2-4. This is due to the fact that the amount of water in the soil determines the absorption rate of the nutrients5. A small amount of water is necessary for the absorption of nutrients from the soil and for the photosynthesis in the leaves. However, a small water shortage during the growing season, just before the véraison, can be beneficial. It is in fact a signal to the plant to stop the vegetative growth and to invest more energy in the ripening of the grapes. This provides grapes with more sugars, colors and aromas. However, prolonged water stress – from either too little or too much water – is disadvantageous and can cause a disruption in the absorption of nutrients and / or the arrest of photosynthesis2.
The ideal soil for viticulture should therefore contain good drainage, but should also retain sufficient water to bridge dry periods. Sandy soils have very good drainage, but retain virtually no water. Clay soils on the other hand have very good water retention, but poor drainage. The winegrower can adjust these characteristics of the soil for example by keeping enough humus in the vineyard on sandy soils, and by ensuring sufficient drainage on clay soils. The latter can be achieved, for example, by sowing deep-rooted herbs and flowers between the rows and/or by preventing mechanical soil compaction by heavy machinery. Limestone, or limestone-clay soils have a better water status, and are often the desired soils for a vineyard. The limestone retains sufficient water, and cracks in the limestone ensure a good drainage and prevent an abundance of water.
Vineyards in arid regions can use irrigation systems to prevent a water shortage and therefore water stress. Irrigation of the vineyard, however, bypasses the specific soil characteristics, which cancels out the “terroir” effect of the soil. Irrigation of vineyards is therefore often strictly regulated in Europe for the production of quality wine, but is often permitted outside of Europe. Nevertheless, vineyards that use an irrigation system nowadays often cause a controlled slight water stress2.
It has been outlined above that, given the weather conditions, the soil type determines the growth of the vine and the ripening of the grapes via its water status and the availability of nutrients. The soil therefore also has an effect on the aromas that are being developed. To compare the effect of the soil type, all other factors that determine the terroir (climate, location and actions in the vineyard) must be comparable. This is virtually impossible, and therefore only a limited number of studies was able to compare the effect of different soil type on aromas in the grapes. For example, in Portugal clay and calcareous soils with a good water status produce sparkling wines that contain more varietal aromas – floral and fruity aromas – than grapes that come from sandy soils. Further, a study in Spain shows that clay soils produce grapes with more and more ripe fruit aromas and fewer vegetal aromas than sandy soils. Furthermore, in the Bordeaux, grapes from a well-drained gravel soil have lower methoxypyrazine concentrations than grapes from limestone or clay soils. Methoxypyrazines are responsible for the spicy, green, vegetal aromas in wine such as those of bell peppers, asparagus and beans, and are typical for grape varieties such as Sauvignon Blanc and Cabernet Sauvignon4.
Therefore, the choice of soil for a particular grape variety can reinforce certain aromatic characteristics of the grape. However, this does not mean that a grape variety necessarily has a preferred soil. Other factors of the “terroir” can also influence the aromatic characteristics and are often more determining for the ripening of the grapes. For example, a warmer climate, a lower yield or a higher canopy also ensure more mature fruit aromas. Nevertheless, the added effect of the soil is important if you approach the limits of what is possible in the vineyard (i.e. for the production of top quality wines).
Soil aromas in the wine
A good soil ensures that the grapevine can absorb nutrients from the soil for the growth and ripening of the grapes. But do the vines really absorb everything that is in the soil, and does that end up in the wine glass? Wine descriptions refer to “Chablis with aromas of flint and layers of minerals”, “earthy aromas”, “the scent of shells” or “the scent of wet stones”. Are these inventions of creative wine writers with a reference to the soil on which the wine was produced? Or is there really a connection between the soil and these aromas in the wine?
“Minerals have no taste”
Licking a rock or stone, whether it is granite, slate, or limestone, does not give any odor or taste sensation. Apart from a difference in structure that one can feel with the tongue, a piece of granite is indistinguishable from a piece of slate or a pebble from the Maas valley. The rocks in the soil only receive cation exchange capacity after weathering. And, because vines cannot break open silicates, a direct transfer of the “minerality from the soil” via the grapevine into the glass is out of the question. The concentrations of minerals in the soil moisture are determined by humus and clay. Moreover, the plant takes it up selectively. Scientific evidence for a direct link between the minerals in the soil and the so-called minerality and earthy aromas in the wine is therefore lacking. The Chardonnays with “characteristic aromas of flint” from the Chablis come from limestone soils, without any flint. The wine descriptions therefore do not correspond to the soil on which the wine originates. What does one taste and smell when it comes to earthy aromas, wine with “minerality”, or tones of shells and flint?
READ NOW ALSO: Terroir provides stress, and that you can taste!
Aromas of organic compounds
Organic compounds – molecules derived from (dead) organisms – end up in the wine and come from the vineyard, or are produced by the yeast during alcoholic fermentation. Organic compounds such as 2-methyl isoborneol and geosmine are produced by algae, bacteria and fungi present in the vineyard and give aromas of plowed soil and wet stones. These organic compounds are associated with these scents because they are also released in the air (and therefore can be smelled), for example when the land is plowed, or because they splash from the stones during a rain shower. They have a very low perception threshold and can at higher concentrations be characterized as a wine fault.
Aromas of flint and shells, however, come from a whole range of organic sulfur compounds. These can often arise from a (slightly) reductive fermentation caused by a shortage of nitrogen in the must. This lack of nitrogen occurs in particular with grapes grown on low-nitrogen (stony) soils1,6,7. So the earthy and “mineral” aromas that are present in the wine really exist. However, they do not come from the soil minerals, but from organic compounds originating from the vineyard or produced during vinification.
No direct link
Although some wine writers, winegrowers and wine sellers would like to make you believe otherwise, there is no direct link known between the soil minerals and the earthy, “mineral”, aromas in the wine. It is a beautiful (sales) story in which the wine is directly related to the terroir and the soil on which it is produced. However, the soil does influence the taste of the wine. Due to its chemical and especially physical properties, it partly determines the ripening of the grapes and the development of fruity and vegetal aromas.
1. Maltman A. Minerality in wine: a geological perspective. Journal of Wine Research. 2013. http://dx.doi.org/10.1080/09571264.2013.793176
2. Van Leeuwen C, et al. Vine water status is a key factor in grape ripening and vintage quality for red Bordeaux wine. How can it be assessed for vineyard management purposes? Journal International des Sciences de la Vigne et du Vin. 2009;43:3,121-134 https://doi.org/10.20870/oeno-one.2009.43.3.798
3. Ubalde JM, Sort X, Zayas A, Poch RM. Effects of soil and climatic conditions on grape ripening and wine quality of cabernet sauvignon. Journal of wine research. 2010;21:1,1-17 http://dx.doi.org/10.1080/09571264.2010.495851
4. González-Barreiro C, Rial-Otero R, Cancho-Grande B, Simal-Gándara J. Wine aroma compounds in grapes: a critical review. Critical Reviews in Food Science and Nutrition. 2013;55:2,202-218 http://dx.doi.org/10.1080/10408398.2011.650336
5. Spangenberg JE, Zufferey V. Changes in soil water availability in vineyards can be traced by the carbon and nitrogen istope composition of dried wines. Science of the Total Environment. 2018;635,178-187 https://doi.org/10.1016/j.scitotenv.2018.04.078
6. Rodrigues H, et al. Sensory and chemical drivers of wine minerality aroma: an application to Chablis wines. Food Chemistry. 2017 http://dx.doi.org/10.1016/j.foodchem.2017.03.036
7. Parr WV, Maltman AJ, Easton S, Ballester J. Minerality in wine: towards the reality behind the myths. Beverages. 2018;4(4),77 https://doi.org/10.3390/beverages4040077