Alex Maltman, Vineyards, Rocks, and Soils. the wine lover’s guide to geology, Oxford, 2018. My question is: can this book begin to clear up the mineral muddle that wine commentators have got into? 

In John Szabo MS’ otherwise excellent book, Volcanic Wines (2016), there is a particularly alarming example of poor logic. It comes when talking about the derivation of wine flavour and texture from the geology and soils of the vineyard. He cites the studies commissioned by the highly respected Olivier Humbrecht MW of the dry extract of wines from the famous, volcanic soil-based Rangen Grand Cru.  These wines showed 20% more dry extract than the next highest example in the studies.  It was also judged to be the most ‘mineral’ in sensory analysis, with which I have no problem.  The conclusion is: ‘These studies lend some scientific credence to what has been known for a thousand years: the wines of the singular terroir of Rangen are indeed unique, due, in no small measure, to the volcanic origins of its soils.’

That Rangen is unique is of course fine. It is the assertion that this is ‘due … to the volcanic origins of the soils’ that is the problem.  (The ‘in no small measure’ betrays a sense of doubt, otherwise well suppressed.)  The problem is a) there is no explanation of why we should make a connection between the soils of the vineyard and the wine and b) a complete omission of all the points in the process between soil and final wine.  These include the vine variety, the root stock and how it interacts with the soil, the climate in the vineyard, the ways vines take up nutrients and process them, the way the vineyard has been managed, the multiple choices that are made in the winery about how the wine is made, the complex and transforming business of fermentation and the ageing of the wine. Why is it that it is the origins of the soils that determines the outcome? Further, what is the relation between any soil type and dry extract? Lastly, this is a particularly alarming example because by citing a couple of academic studies and using the technical language of dry extract gives the conclusion an air of plausibility it does not deserve. Mineral muddle over the flavour of wine in a rather sophisticated way. 

Why does one wine taste different from another? There are many issues here but perhaps the most intriguing is the place (or not) of rocks or more widely geology. Virtually every wine writer in the last 20 years has appealed to geology or at least to soil types to explain the character of a wine. The ‘oyster shell’ or ‘chalkiness’ of Chablis is put down to the former seabed that underlies Chablis, the ‘slatey’ taste of a young Mosel Riesling to the slatey vineyards in which it is grown. In the past descriptors tended to refer to fruit or vegetable characters. Viognier was peachy, Sauvignon Blanc was asparagus and greengage. But if you want your wine to be really sophisticated, desirable and expensive, then it is now de rigueur for it to be mineral.

This is strange. We don’t normally attempt to eat rocks! And usually we don’t think that the taste of soils or rocks is a desirable thing. Nobody says, this beef tastes fantastic because it tastes of the rock formation which lies below the field in which the cattle were raised. Why is mineral muddle attractive in wine but not other fields? 

And in any case all this language is metaphorical. There are very few people who think that there is actual asparagus in Sauvignon Blanc. My hunch is that the appeal to geology stems from the attempt to say something definitive and impressive sounding about the complex taste and sensation of individual wines. If you try describing wines using only fruit metaphors you will quickly discover that ‘lemon + some sort of apple + possibly peach/apricot’ will apply to a huge range of white wines. So what is distinctive about this single wine? Is there some magic factor which determines its taste or sensation? An appeal to the geological can work wonders here! 

I can’t help wondering if the rise of the language of minerality was a response from European winemakers and commentators in the face of the powerful fruitiness of new world wines. Your Marlborough Sauvignon Blanc may hit you over the head with its powerful gooseberry and passionfruit tastes but our Pouilly Fumé is restrained and flinty – and therefore much more sophisticated. And, it tastes like that because the vines are grown in unique flinty soils that can’t be replicate elsewhere. Even today, the website of the Pouilly-Fumé winemakers in the Loire makes this connection explicitly: ‘The characteristic aromas [of wine made from grapes grown on flinty soils] suggest spices and minerals (gun flint)’ (website of the winemakers of the Pouilly-Fumé). So, ultimately, this is a terroir argument taken to extremes – the extreme of the bedrock.  

Into this field wades Alex Maltman, retired professor of Earth Sciences, vine grower and wine lover. He has produced what I think is the first full-length book to be written about ‘vineyards, rocks and soils’ written by a professional geologist. As such it is a first-rate contribution to clear thinking about what does … and especially, what does not … affect the taste of wine. The book is very clearly written and the points made are well illustrated by reference to vineyards around the world. It is abundantly clear that the undergraduates of Aberystwyth University were extremely well served by an extremely able and lively teacher.

There are two very quick take-aways in this book. When you read about wine or listen to a producer talking about soil and/or minerality, check for the following:

1. if an appeal is made to a soil type or geological formation, is it followed by an explanation of how that affects first the vine and then the resulting wine? In the world of wine implying a connection between geology and wine style is rampant … and often without any connecting argument. 

Here is a rather measured example: ‘The soils in Bolgheri have a great variety in a relatively limited area. There are alluvial soils, of fluvial origin, with round pebbles deposited by the ancient waterways.’ That is all very interesting, but the question is, so what?

2. If a particular quality in a wine is attributed to the geology of the region, is any explanation given? The example from Pouilly-Fumé above is typical of this sort of manoeuvre. But no explanation follows.

In short, most of the speech we use about geology and wine relies on unsupported inference. 

What the wine lover needs to know about this book is that 140 of its 230 pages are devoted to a clear, concise and accessible account of how rocks are formed and how they are classified. This, therefore, is not your average wine book. But it is well worth investing in your geological education, or skim reading, to get the wider picture. The section which will intriguing wine lovers is on vineyard soils, nutrition and the wider concept of terroir. The author has done a great job in summarising the current scientific understanding of how vines grow and in particular, how they get the miniscule amounts of minerals that they need. Maltman has cleared the ground when it comes to mineral muddle, though no doubt there is more to be said. 

Along the way, in the main chapters of this book there are very polite but well argued swipes at fashionable but unfounded notions in the world of wine writing and promotion. The chapter on igneous rocks includes, of course, volcanoes. Wine writers have seized on this subject and claimed that wines from volcanic soils have some sort of commonality. But there are at least two problems. From this book we learn that volcanic rocks have enormous variation in their chemical and physical properties. Secondly, ‘…its worth recalling that volcanic soils are formed from the same kinds of chemical elements and the same kinds of geological minerals as other rocks …’ (p. 68)

Similarly, Maltman warns us, we should be careful of grouping together the various types of rock referred to as schist. To a lay person, schist sounds like a particular rock but it is class of rocks with a common physical characteristic, it breaks up into irregular shapes: ‘a coarse-grained metamorphic rock which consists of layers of different minerals and can be split into thin irregular plates’ as we can learn just from looking at Wikepedia. So it makes little sense, at least in terms of understanding, to form L’Association Terroirs de Schistes for such diverse places as Alsace and Collioure and claim there is a commonality to the wines.

Another strength of the book is that it provides concise explanations of what the vine does need and, less commonly available, how it accesses water, sunlight, carbon dioxide and common and trace minerals. As a result, much of the argument is in effect why a wine does not taste of the geological formation over which the vine is grown. But along the way the attentive reader will pick up the basics of the science of how the vine gets the nutrition it needs. I will write a second piece about this.

The yet more complex story of why wine tastes as it does would be another entirely different book. And that would be completely dominated by the chemistry of fermentation.

If geology is rarely a determinative factor, the physical properties of soil matter enormously to the vine. The key properties that are important are:

• a soil’s ability to hold water – this is why even small amounts of clay are very valuable as it holds water due to its tiny grain size
• excellent drainage – poor drainage makes the soil difficult to work and, more importantly, the water excludes the oxygen, a key requirement for the creation of humus. In turn, it is the humus which provides most of the mineral nutrients (not geological minerals) that a vine needs.

This aspect, the water-holding and drainage capacities of soils could have helpfully been dealt with in more detail in this book and as a main topic. We learn of the importance of this with regard to the shallow terra rossa soils of Coonawarra, saved by the water-holding limestone beneath it, but that is just one example. The key to the much-prized limestone-derived soils or bedrock may simply be that it strikes the perfect balance between water-holding and drainage for successful viticulture. 

Alex Maltman’s book is an excellent guide to a field that the wine world likes to talk about with scientific-sounding authority which in most cases is completely spurious.

P.S. There is a short version (or at least a relatively short version) of Maltman’s work freely accessible here.

My next post follows directly on: How does a vine get the nutrients it needs?