In my last post Mineral muddle, I wrote about Alex Maltman’s new book on geology, soils and the vine. His main concerns are geology and soils. But along the way he explains how a vine gets the nutrients it needs. Reading this book reminded me that these basic processes are rarely explained outside of scientific circles. As a result, we are more inclined to believe wild claims about vines taking up minerals from rocks and somehow those minerals affecting the taste of the final wine. Vines needs tiny amounts of minerals to function well. But what are these minerals and how do they obtain them?
What are vines made of?
Since the late 1800s we have known that, as Maltman puts it, vines are ‘not made of soil but, in a way, of sunshine, air, and water.’ He continues:
‘Sunshine provides the energy for vine leaves to derive carbon and oxygen from carbon dioxide absorbed from the air, to split water taken in through the roots into hydrogen and oxygen, and to manufacture from these three elements an astonishing number of carbohydrates compounds. And it is these three, along with water, that largely constitute vines, grapes, and wine.’ (p. 166)
But for photosynthesis to work and to result in healthy plant growth, the vine also needs a certain number of minerals, which a vine can take up from the soil. Thus the questions are, do they come from the soil or mineral bedrock; and what are the key mechanisms?
A really basic but important point which needs making is that vines grow in and get nutrients from soil, not from rock which is inert and can’t be processed by the vine. And soil is a complex mixture of weathered rock and organic content, i.e., rotted vegetation. Humus, the souce of most of the mineral nutrients a vine needs, is a very complex substance in its own right but its origin is vegetable, not (geological) mineral. Weathered rock contributes to the structure of soil (vital for water-holding and drainage as we saw in the previous post), rather than most of the nutritional content a vine needs.
Nitrogen is the most important nutrient that a vine needs and it is obtained not from rock or even soil, but ultimately from the air. However, the vine cannot access nitrogen directly from the air which surrounds it. Bacteria take in atmospheric nitrogen and turn it into ammonia gas. This is then converted by other bacteria into a soluble nitrate anion (a negatively charged ion) which can be taken up by vine roots.
Two ways to feed the vine
There are two ways in which a vine can take up the minerals it needs. It can do this, first, directly via the mycorrhizal fungi which live in the roots of vines and, second, by the ion exchange process.
Mycorrhizal fungi, beneficial organisms, live symbiotically in the roots of the vine and other plants. They can absorb nutrients from the soil and transmit them to the vine. For example, the fungi can transmit phosphorus to the vine, resulting in better shoot growth. The fungi can also transfer nitrogen from decaying cover crops to the vine as they create an in-soil network between the roots of vines and of cover crops. In exchange the fungi can take carbon from the vine or other plant. If you want to know more about these fungi, click here.
Second, vines can take up minerals by the ion exchange process. To put this in another way, vines have no teeth or digestive system. The minerals must be soluable in water, and be present in a form in the soil which makes them available to the vine. Vines access the minerals by the rather formidable sounding cation exchange capacity, CEC. Simply put, free electrons of, for example, potassium or iron adhere to the surface of a clay or humus particle which attracts and stores cations (positively charged ions). Hydrogen ions pumped out of the vine can displace the cations on the clay or humus particle as they are only loosely held by an electrical charge. As a result the previously stored cations (in our example potassium or iron) can now be absorbed by the vine roots (Maltman, p. 27 with a nice diagram which some of us older readers could have done with being larger!)
This where a possible major confusion can arise. These nutrient minerals (nitrogen, potassium, iron, etc) are not the same minerals that make up rocks and soils, geological minerals. Most of the the nutrient minerals, are in fact provided by humus, decomposed organic matter, and a form of clay known as montmorillonite. So from the point of view of vine nutrition, the nature of the geological rock it happens to be growing over is not that important, though rocks that have cracks or are porous can be an advantage if the vine’s roots need to go deep to search for water (not minerals!) This helps to explain why the same variety can grow in very different soils around the world. As Maltman says elsewhere: ‘Pinot Noir, for example, obviously thrives in the thin, calcareous soils of the Côte d’Or, France, but it does also in the thick alluvium of Bio Bio, Chile, the schists of Otago, New Zealand, as well as both the basaltic and the sandy soils of the Willamette Valley, Oregon.’
Edging towards the flavour of wine
Nearly all the geological minerals a vine can take up are tasteless and odourless. So, what ever it is we are tasting in wine, it won’t be the direct flavour of rocks. However, some minerals, especially chlorine, can form compounds that have strong aromas, flavonoids and terpenes from example. But that takes us on to the winemaking process.
The process of making wine, principally fermentation, further radically alters the composition of wine. (That is the subject for another explainer post.) It removes some elements and adds others. Maltman’s conclusion is: ‘Consequently, the proportions of mineral nutrients in a finished wine bear only a complex, indirect, and distant relationship to the geologic minerals in the vineyard.’ (p. 176) And note, that was a small relationship with nutrient minerals, not geological minerals.
When a vine puts down deep roots it is looking for water and securing itself physically. Obviously without roots a vine would simply fall over and die. It is not taking up minerals from the rock. As we have seen, the rock is not in a form that the vine can access anyway. And the nutrient minerals that a vine needs are contained in the weathered rock and mostly in the humus which is in the top layer of soil.
So does geology and soil type not matter? Geology does matter, not least because if all of the earth was made up of rocks that weathered impossibly slowly there would be no soil and therefore no vegetation or vines. There are very few vines growing in marble quarries! Soil type matters a lot but not principally through the geology which contributed to it. What matters far more is water-holding capacity, drainage and the nutrient and CEC status due to the amount of humus and clay which is present in it.
Terroir continues to matter
Terroir is critical to wine but it is much more than soil type. It is the sum of all the factors in a locality which contribute to the growth of the vine and therefore the final wine. Climate is the most important, but there are many other factors too – aspect, prevailing winds and of course multiple human interventions (choice of variety, training systems, canopy management, addition of composts or chemical fertilisers and many more). To return to the Pinot Noir example above which grows well on a range of soils. Most Pinotphiles are persuaded that the greatest wines come from the Côte d’Or and the temptation is to attribute that directly to its geology. The argument from terroir should not isolate one factor. It has to take into account the climate, latitude, aspect and all the human factors, not least 800 years of experience of growing vines on that slope.
Next – but it may be a while yet: what happens when yeast transforms grape juice into wine and where does the flavour come from?