Part 1: Soil Principles

Vineyard geology – the rocks and soils in which the grapevines are rooted – pervades the world of wine. To illustrate the point, the picture below is a collage of wine labels – all of which bear geological terms. The back-labels on wine bottles also may mention geology, as in the following extracts: “our wine originates from limestone soils”; “the chateau is on sandy limestone from the Cretaceous period”; “the vines grow on argilo-calcareous soils with sea-shell fossils” and, most impressively, “our vineyard has Triassic and Jurassic sediments on undulating Proterozoic granulite and migmatite with numerous dolerite dykes”.

But it is in the wine press where wine-geology references really abound. It is de rigeur to at least mention the geology in vineyard descriptions, and commonly to assert how it influences the finished wine. Some even claim that the vineyard geology can be tasted in the wine: “you can taste the volcanic ash of nearby Vesuvius”; “wine allows me to taste soil and bedrock”; “a “graphite or schisty-ness flavor which I identify as coming from the soil of the Priorat”; “in Brouilly, there are veins of blue granite nuanced in the wines”.

Of course, all plants are influenced by the site and soil where they grow – every farmer or gardener knows that - but only with wine (despite the extensive post-harvest processing of the fruit) are the connections taken this far. And the fact is that much of the basis for this degree of interaction is anecdotal and subjective: the scientific justification is mixed.

This article attempts to review the situation, summarising present scientific understanding of vineyard geology and what it might bring to wine. It is in two halves. The first part outlines the general principles and the second part explains some of the wealth of geological terms and concepts that are so often met with in the world of wine.

A collage of wine labels illustrating the attraction of geological terms for wine names.

Historical: why do soils seem so important for vines and wines?

Back in the Middle Ages, the Burgundian monks were busy consolidating their newly granted monastic lands – and planting vineyards. They knew that vines grew better on some sites than others, and legend has it that the monks even tasted the soils to find which would give the best tasting wine. And why not? The vines were obviously taking up water from the soil and with it, presumably, everything else that they needed to grow. The vines and the resulting wine had to come from matter in the soil. The local climate affected the ripening of the grapes and to the monks there was a spiritual dimension to it all, but the idea that the vineyard soil was central to wine flavor seemed self-evident. Wine was made of the soil.

The idea was to become entrenched. It was all part of the patrimony of France, and the creed was handed down through the generations. Most soils come from the underlying bedrock so that by the time the appellation contrôlée system was introduced it seemed natural to involve the bedrock geology in delimiting preferred vineyard sites.

Today, to some it still seems self-evident. Wine writers have certainly embraced the sheer romance of the notion. Moreover, through bestowing an obvious “sense of place”, the idea provides immeasurable marketing value. After all, vineyard geology is one of the few things that cannot easily be replicated elsewhere. Also, in addition to the centuries of accumulated anecdote, there’s the fact that many classic wines, the Grand Crus and the like, come from sites with a particular geology. So – in a neat bit of cyclic reasoning – the bedrock is still supposed to be crucial. And it is certainly being perpetuated in the new fashion of tasting minerals in wine. “Minerals” and “minerality” are suddenly, apparently, the most commonly used wine descriptors  – and now being extended to specifics, such as saying wines have a graphite, granite or limestone minerality. Those Burgundian monks would certainly be pleased with their legacy!

Modern science, however, gives a somewhat different view. For we now know that vines, like all other plants, are not made of soil. Rather, they are made, in a way, of sunshine, air and water. By the late 1800s scientists were talking about photosynthesis. And today, ten Nobel Prizes later, we know pretty well how plants carry out their growth. So we are now in a position to put the folklore to one side and look at what the evidence says about how important vineyard soils really are for vines and wines.

Scientific: what do rocks and soils bring to vines and wines?

Grapevines, like all land plants, use sunlight to extract carbon dioxide from the air and combine it with water to produce sugars. From these, all the various carbohydrate compounds that make the vine are manufactured. In other words, sunlight-driven photosynthesis, not soil, makes the vines. The soil, however, is still very relevant: much of the vital water is obtained from it, drawn up through the vine roots. And we know of sixteen elements that are essential for the carbohydrate reactions to take place, almost all of them coming from the soil, dissolved in the soil water. (The availability or otherwise of these elements is generally referred to as the fertility of the soil.)

Most people know at least vaguely about such mineral nutrients, seeing as all animals, including humans, also require such “essential minerals”. In fact, this would seem to be the basis of the fashionable notion of tasting minerals in wines - minerals well known to exist in soils and assumed to have been transmitted through the vine to the finished wine. (However, my article in the forthcoming Spring 2013 Edition of Practical Winery and Vineyard Journal will document why the perception of what we are now calling “minerality” in wine cannot literally be the taste of vineyard minerals).

Regarding these nutrient minerals, let us first note that:

1. They are derived from  - but are not the same as - the geological minerals that make up the rocks, stones and physical framework of vineyard soils;

2. They are needed by the vine in only very small quantities: parts per million or less;

3. Most soils have sufficient nutrient reserves to meet the unusually modest requirements of grapevines, and vinegrowers can easily correct any inadequacies;

4. The vine roots do not passively accept whatever the soil water has dissolved in it but self-regulate, as far as they can, their nutrient uptake.

Hence the relations between the (minuscule) inorganic content of finished wine and geological minerals in the vineyard is complex and distant. That is one reason why it has proved so difficult to find a reliable way of chemically “fingerprinting” the provenance of wine. Even so, the nutrient supply is essential and so deserves a closer look.

Nutrients

Of the sixteen essential nutrients, a few are needed by the vine in relatively large proportions. Hence they are known as macronutrients. But note the word “relatively”: even these are measured in concentrations of only parts per million (milligrams per kilogram).

Nitrogen is particularly important in that small variations in its uptake affect the amount of vegetative growth of the vine and the way the yeast metabolises the must. Although, of course, air is mainly nitrogen, most of the vine’s nitrogen comes from the organic part of the soil, the humus, as does most phosphorus and sulfur. The latter, together with chlorine, can also be airborne and taken up through stems and leaves.  Of the other macronutrients, calcium, magnesium, and potassium are derived wholly from the soil. Calcium is important because it influences the pH (acidity/alkalinity) of the soil and hence the availability of other nutrients, enhancing it for some but reducing it for others.

The remaining nutrients, such as iron, manganese, zinc and copper, are also derived from the geological minerals that make the soil. The essential amounts are tiny (parts per billion) so these are called micronutrients, or sometimes trace elements. Excess of them can cause toxicity.  A common problem here is that although the vine roots attempt to govern the uptake of each nutrient, some elements are chemically sufficiently alike to “fool” the mechanisms so that too much is absorbed of one to the detriment of the other. A number of vine diseases result from this.

Grapevines try to develop extensive root systems, extending laterally and vertically for a metre and in some cases much more - a strategy of exploiting (relative to many other plants) large volumes of soil at a low root density. Some growers try to artificially increase the root density believing it may enhance any signature of the soil in the wine. Much of the root skeleton becomes established in the first few years of growth although small “feeder” roots continue to grow. Even then, the humus-based macronutrients will be derived from the topmost part of the soil and the other nutrients are usually available from not far  beneath. Any deeper roots are chiefly seeking water.

It follows from this that the belief that old vines have deep roots and therefore provide something extra to wine lacks scientific justification. Similarly, there is no basis for the common assertion that a complex geology leads to complex flavors in the wine. Vine roots take up their required balance of nutrients irrespective of the source, although vineyards located on a single rock type are more likely to suffer from nutrient paucity.

Vine roots have to absorb the mineral nutrients in soluble form. And this is the crux of the difference between geological and nutrient minerals: the latter are dissolved, and most consist of single elements, whereas the former are insoluble complex compounds. As an example, various transformations are needed to extract soluble magnesium (Mg⁺⁺) from the geological mineral pyroxene, a typical formula for which would be (Ca, Na), (Mg, Fe, Al) (Si, Al)₂O_6. If that looks complicated, then that’s the point. Most geological minerals are complex! And so are the processes by which the nutrients are made available.

This is because the constituent elements are firmly bonded within the geological minerals, and can only become released after various processes of chemical “attack” we call weathering. Water and air, and the impurities they contain, trigger chemical reactions with the bedrock minerals, changing their nature and eventually, through a complex range of processes, tending to release some of the elements in dissolved form. Then, if circumstances are right, they may be transported to the vine roots for absorption.

Here’s an example. (The names of the rocks and minerals involved will be explained in the second part of this article, on vineyard geology). All vines require potassium and in the Lodi district of California they acquire it primarily from the region’s granite bedrock. But that simple statement hides a whole sequence of processes. In outline, the granite has to begin physical disintegration in order to expose its constituent minerals to weathering.  One such mineral is muscovite (which contains potassium), and on exposure to water, weathering starts to convert it into various clay minerals such as vermiculite. But although vermiculite (the same stuff you can buy in garden centers or for loft insulation) consists of loose, tiny grains and hence presents large surface areas that increase exposure to further reaction, most of the constituent ions remain firmly locked in the new minerals. Continuing reactions attempt to convert the material into other, different clay minerals, but the potassium may still be “fixed” in them. In fact, it has been estimated that although a typical vineyard may have plenty of potassium in the geological minerals, as little as 2% might actually be available to the vines.

Eventually, varieties of clay minerals may be formed that are able to yield the potassium from their surfaces, but only provided it can be swapped for some other suitable element that happens to be in the water adjacent to the mineral surface. If this does happen, finally the potassium that originated in the bedrock is released in dissolved form. But even then, transporting it to the vine roots and setting up the chemical gradients needed to trigger absorption involves further complex mechanisms. Natural vine nutrition is all very intricate, prolonged and variable.

Organic Content of Soil

All soil has some degree of organic content. It is this that makes fragmented rock a true soil. After all, the Moon is covered by rock debris and dust but it has no soil. However, this distinction between rock and soil is unusually blurred in the case of vineyards as vines can exist in thin, exceedingly stony soils.

Typical soils consist of a physical framework, usually geological minerals derived from some parent rock, with spaces called pores that contain some combination of water, oxygen and other gases, and organic matter, living or dead. The living material ranges from worms, lice, mites and the like down through nematodes and protozoa to invisible bacteria. Particularly important for vines are fungi, and especially the filamentous growths called mycorrhizae, which can live in partnership with the vine roots. They can extract directly, without the need for dissolving, certain elements from the surface of rock and yield them to the vine, in exchange for carbon. Such processes can be important in marginal soils, such as those poor in phosphorus.

While too much organic matter can lead to imbalanced nutrition, and especially excess vigour from surplus nitrogen, it is now fashionable in viticulture to strive for a healthy, living soil. This is partly a backlash to the sterile soils resulting from past decades and more of carelessly applied agrochemicals, but also a realisation that healthy soils tend to be well structured, easier to manage and relatively resistant to disease. Such thinking is the basis of so-called organic viticulture.

Strikingly stony soils (composed of a pale-weathering igneous rock called andesite) at Tokaj, Hungary.

Physical aspects

In addition to the nutritional aspects outlined above, there are physical factors of the soil that are relevant to vines and wines. For starters, fundamentally it is the resistance of the bedrock to erosion that determines the lie of the vineyard land and all the climatic variations that stem from that, e.g. with altitude, air flow, slope angle and aspect, together with the tendency to break down to make soil.

Over time, soils tend to become thicker and finer, and nutrients may be progressively leached away faster than they are replenished. On hillsides gravity tends to move soils downslope. On plains and valley floors the loose soil debris may have been brought large distances, typically by rivers to give alluvium. Some research has suggested that soil depth can influence wine character: shallower soils can give better balanced wines with fewer vegetal notes.

The color of the soil can affect thermal properties, which can be relevant in more marginal areas of grape ripening. Pale-colored soils such as the white, chalky albariza soils of the Jerez region, Spain, reflect the heat of the day and increase light reflection. In contrast, the dark soils, walls and embankments of the Ahr district of Germany, one of the most northerly in Europe, are able to ripen red grapes through absorbing heat for re-radiation at night.  

The texture of the soil can affect resistance to compaction, by treading or by machinery, and the ease of root penetration. Some soils can be deep overall but have what is called a duplex structure, where an upper layer of friable (crumbly) soil, possibly quite thin, overlies a hard lower layer which is impenetrable to vine roots.

Most importantly, however, the soil determines how rainfall is absorbed and stored, to be made available to the vine roots. A great deal of both practical experience and scientific research documents how this water behavior is crucial to vine development, grape ripening, and even, ultimately, to wine character. Two properties of a soil are fundamental to how it interacts with water, what are technically called its porosity and its permeability. The porosity expresses what proportion of the soil is space available to be filled by water; the permeability represents how well the spaces are connected.

Obviously a high porosity is desirable for storing water but it is no good if the pores cannot be accessed. A high permeability facilitates rainfall percolation in a wet period but also allows the water to rapidly drain away. So a good soil balances the two. The properties largely stem from the shape and size of the grains that are forming the mineral framework of the soil. The coarsest soils normally encountered in vineyards are gravels.  They offer excellent drainage but normally need some additional way of reserving water. Gravels are important in areas of New Zealand’s South Island, like Marlborough, and on the North Island, in the Gimblett Gravels. They are especially noted in the Médoc, where, among others, Châteaux Lafite-Rothschild, Haut-Brion, Latour and Mouton-Rothschild are all located on mounds of gravel.

Particles measuring around 2mm or so across are sand, finer ones are called silt, and the very finest clay. A mixture of all three is termed loam. In general, coarser (larger) and round grains as in sands allow relatively high porosity and permeability; with irregular grains it depends on how they fit together. In addition, a suction force operates between the pore-water and the mineral grains, which is greatest in clays. Hence, together with other reasons, clay soils have the lowest permeability.

Some say that the most blessed vineyard sites have these water properties in exact, natural balance. Château Pétrus in Pomerol is an example. Perched atop a gravel mound providing excellent drainage there are nevertheless lenses of clay at depth, storing water from winter rains for the deepest roots. In Portugal’s arid upper Douro region it is well established that the preferred vineyard sites are on schist rather than granite bedrock. Why? Intrinsic weaknesses in the schist happen to be oriented vertically – ideal for root penetration and percolation by the winter rain. In contrast the granite bedrock is too strong for root access and, lacking fissures, rainwater just runs off it. 

The striking terra rossa of Coonawarra, Australia (typically only half a metre thick) is justly famous but the key to its quality is probably the drainage and storage offered by the underlying fissured limestone. Certain esteemed vineyards at Montalcino and Poggibonsi in Tuscany give low yields and concentrated wines because their calcareous soils have just sufficient clay content to ameliorate drought. Accumulations of particles that were transported by wind – the material called loess – can have a good balance between porosity and permeability. Loess underlies some of the most desirable sites in the Walla Walla Valley of Washington and Central Otago, New Zealand.

Those then, are the principles. Taken together it may seem they go some way to restoring the pre-photosynthesis importance of vineyard soils. And clearly both nutrition and water supply are crucial. There is, however, a major proviso. That is, in most of the world’s vineyards these soil factors are artificially manipulated. Fertilisation takes care of nutritional needs and irrigation governs water supply. The natural situation presents the starting point but then the various factors are overridden as appropriate, hence reducing the importance of the natural soil. 

The town of Chablis from the Les Clos vineyard. All the Grand Cru vineyards of Chablis lie on the relatively steep, SW-facing “scarp”
slope of an escarpment due to the slightly SE-inclines strata of calcareous sedimentary rocks.

Pebbles of mixed lithology in a river channel of the ancestral Columbia River, Newhouse Winery, Snipes Mountain, Washington.

Manipulation of vineyard soils

Earth-moving has long been carried out at vineyards, to a greater or lesser extent. In olden days, down-slipped soil was carted back up a vineyard slope; terraces were built by hand. These days, it is normal for a new vineyard to involve major earth-moving machinery. Drains are installed. In past times, pale soils in cool latitudes of Europe were darkened by the judicious sprinkling of coal dust and soot. Today, planting inter-row cover crops is fashionable for various reasons, negating the role of soil color. Impenetrable soil layers or tough bedrock is attacked by machines known as rippers, to improve vine-root penetration. The list goes on, but the major interventions involve fertilisation and irrigation.

Most modern viticulturalists assess the vineyard soils for nutritional balance and correct it if necessary. The vines may show visual symptoms that all is not well, such as the leaf-yellowing of chlorosis, due to deficiencies in nitrogen, zinc. etc. but arguably it is better practice to anticipate problems through routine analysis. A whole sub-science has arisen on grapevine pathology: how best to carry out analyses, interpret them and selectively apply remediation. It has even become automated to some extent. Nutrient anomalies are detected by the remote sensing involved in precision viticulture and corrections included with the irrigation water, so-called fertigation.

It is axiomatic to many that wine flavor is enhanced by vines that have had to endure a degree of “water stress”. Science has now determined a range of parameters quantifying the required water values for this, for different varietals and soil types, and a range of sensors are available to monitor the data in soils and vine roots so that irrigation can be carried out with precision. Moreover, the timing of the irrigation regime can be adjusted in order to achieve certain desired outcomes. For example, water stress applied between bud burst and flowering can reduce cluster numbers, if this is desired; between fruit set and veraison some water stress can decrease berry size, with associated quality increase.

Of course, there are those who eschew these practices as far as possible, avoiding irrigation by “dry-farming”. An important reason for this is to attempt to restore the role of the natural soil and to enhance, as it is often put, the enigmatic “sense of place”. But usually some nutrient input, albeit organic, is needed.  And a host of other decisions have to be made both in the vineyard and during the winemaking so that this remains a debatable proposition. One study showed that even with minimal interventions in a classic region like the Mosel, Germany, wine quality (as reflected by price and land values) fluctuates through time depending on the winemaker of the time.

For the majority of the world’s vineyards, at least some degree of intervention is necessary simply to maintain production. In fact, an important limiting factor in some of the world’s developing vineyard areas, such as Bio Bio in Chile and the Yakima Valley in Washington, is the restricted access to irrigation water. Soil factors can be left to nature if desired, with its attendant vagaries, but for most some manipulation is beneficial, and can be employed to whatever extent the winemaker’s philosophy allows. 

Concluding remarks

From the above, it is clear that soil properties are highly relevant at least to vine behavior. But with so much artificial manipulation it may seem debatable to what extent the continuing preoccupation in wine writings and marketing with the effect of geology is justified, in the vineyard let alone on the finished wine. It is however, likely to continue: it is all very romantic and the fact is that soil ripping and fertigation does not make good copy. There is, however, a wild card lurking in here.

As explained above, the mineral nutrients essential for vine growth are needed in certain, small amounts, typically within a restricted range of values. Too little gives deficiencies and too much can lead to vine problems. But do variations within the known, narrow ranges have any effect? At present, science does not know. But they could be pivotal.

It is well established that very small amounts of metallic elements such as copper, iron and zinc can affect certain organic reactions, such as enzyme metabolism and yeast activation. Hence, conceivably, the course and progress of vine growth and vinification could be influenced. That is, tiny concentration variations in the nutrients of geological origin could in complex and in circuitous ways be influencing wine character and flavour. At present this is largely speculation but future research along these lines could finally provide a scientific basis for the hosts of anecdotal evidence. Note that in this idea the nutrient minerals themselves are still in minuscule amounts and virtually flavorless: they themselves are not tasted. The vineyard soil is not tasted in wine. Nevertheless, the idea would provide some justification for continuing the Burgundian legacy!

-Alex Maltman, Professor, Institute of Geography and Earth Sciences, University of Wales at Aberystwyth, United Kingdom