Geologist Alex Maltman’s recent article on Decanter’s website, entitled “Busting Wine Terroir Myths: The science of soil and wine taste”, contains much useful information, but several of his claims strike me as misleading. Maltman is a leading authority in the field, and author of the highly regarded  Vineyards, Rocks, and Soil: The Wine Lover’s Guide to Geology. As I am not a soil scientist, I have no basis for challenging the scientific claims he makes. My worries have more to do with the sweeping skeptical conclusions he draws and the central role of a straw man argument that does too much work in the article.

The main thrust of his argument is that climate, weather, and land topology play a much larger role in shaping the final product than does soil type. This may well be true and the evidence Maltman provides suggests as much. But for most people who believe terroir is central to wine quality, that would not be an utter surprise, and it would not diminish the argument for the importance of terroir. There are many definitions of terroir floating around and some people may mean “soil type” when using the term. But most wine writers and wine professionals include climate, weather, and land topology as central components in the concept of terroir. Maltman agrees with that broad definition but the rhetoric of his article suggests a broader assault on the concept than is warranted. There is a subset of “terroirists” who do focus specifically on soil type as a central input, but the idea of the importance of place conceptually doesn’t require it. If it turns out that weather is the main input, the sense of place would nevertheless be secure.

But my main worry is that giant straw figure perched in the middle of the text. Many years ago, wine writers often waxed poetic about mineral flavors in wine by suggesting they were directly transferred from the soil to the wine. As Maltman points out, this idea has been shown to be false. Most of the minerals taken up by the vines have no flavor and in any case there is no mechanism for such a transfer to take place. Some wines do exhibit flavors reminiscent of gravel, flint, wet stone, salinity, etc. But they are not a copy or representation of minerals in the soil. In other words, you’re not literally tasting soil when you sip on your Riesling.

My problem is that no serious wine educator or writer with a modicum of experience has made such a claim in many years. Soil scientists were routinely pointing out this fallacy 10-15 years ago, the wine press dutifully reported it, and the wine community has largely accepted the point. This is no longer part of the conversation. Maltman quotes someone without attribution making such claims:

We read, for example, that ‘the vine transmits its nutrients all the way from the stony soils to the final wine’ and ‘the vines sip on a cocktail of minerals in the vineyard soil, for us to taste in our wineglass’.

Read where? For controversial claims that support sweeping generalizations it would be nice to have a source attribution.

Professor Maltman makes several interesting claims that are worth attending to:

• Warm rocks radiating heat into the vineyard on cool nights are unlikely to have much effect on the grapes.
• Whatever mineral uptake there is by the vines, the rootstock rather than the cultivar would matter more.
• Organic material, including the microbiome, in the soils may play an essential role in explaining properties of the wine.
• Irrigation can overcome deficiencies in the water retention properties of the soil

These are all useful points, although that last claim is one that I have seldom heard discussed and seems to contradict the widely held belief that water retention properties of the soil are crucial.

But it seems to me Professor Maltman misses the forest while focused on the trees. He dismisses as mere anecdote testimony from winemakers and growers that soil type produces differences in wines. But the alleged connection between soil and wine properties is more than an occasional stray story. For centuries, winemakers and viticulturists throughout the world  have observed the connection between soils and what they taste in the wine. These are not mere anecdotes but a massive data set of correlations that must be explained. There clearly is something going on despite the fact that we don’t quite know how to explain it. Maltman writes that

Certain vine cultivars are often said to suit particular rocks: Chardonnay and limestone, Syrah and granite, for example. But much of this derives from the geology that happened to be where a cultivar first flourished; Syrah and Chardonnay thrive today in many soil types.

But the issue isn’t whether cultivars can grow well in various soils. The issue is whether various soils create different flavor and texture profiles when planted with suitable grapes. The claim that Pinot Noir and Chardonnay have interesting organaleptic properties when grown in limestone is not a casual, unreflective claim but an idea that has been widely accepted by thoughtful, attentive experts making  careful observations. Of course, these observations of a correlation by themselves don’t prove a causal connection between soil and wine properties. But they are more than just anecdotes. That well established correlation must be explained or explained away. Perhaps it is all about the weather. But if so that entails rather substantial, widespread, independent errors on the part of competent observers.

In summary, Professor Maltman seems to conflate the claim that perhaps terroir functions differently than we thought with the claim that terroir matters less than we thought.

It’s worth mentioning another assumption that informs Professor Maltman’s article. He seems to assume that if soil type is important to wine quality, the influence of the soil must be a direct causal influence. But nothing in the concept of terroir requires that assumption. It is conceivable, again without knowing the science,  that The influence may be indirect yet nevertheless substantial.

For example, Maltman writes:

All organisms require nutrients in particular proportions, but whereas animals like ourselves ingest them in bulk and have internal mechanisms (liver, kidneys etc) to sort and expel the excess as waste, plants such as vines regulate them on the way in.

How? Put simply, the vine has an armory of sophisticated mechanisms aimed at selecting and balancing its nutrient uptake as required, even varying it as the growing season progresses.

It would be interesting to know if variations in these sophisticated mechanisms that balance nutrient uptake are chemical reactions to variations in soil type. If so, might these chemical changes in the vine influence flavor? That would be an indirect influence of the sort I have in mind.

There is much about this topic we don’t know, but the fact that we haven’t discovered the mechanism is not by itself sufficient to warrant the conclusion that a correlation is spurious.