Ter­roir and taste

Do the rocks and stones in a vine­yard ac­tu­ally mat­ter? No, says ge­ol­o­gist Alex Malt­man. What­ever the notes say, we can’t taste ‘min­er­als’ in a wine

Decanter - - NEWS - Alex Malt­man is an Emer­i­tus Professor of Earth Sciences and au­thor of Vine­yards, Rocks & Soils: The Wine Lover’s Guide to Ge­ol­ogy (Ox­ford Univer­sity Press, May 2018)

I shoulD bE jump­ing for joy. For years I’ve taught, re­searched and gen­er­ally en­thused about ge­ol­ogy and its im­por­tance, and now my sub­ject is mak­ing head­lines in the world of wine. ‘soil, not grapes, is the lat­est must-know when choos­ing a wine,’ bloomberg tells me, for ex­am­ple. so why am I not full of joy? Well, be­cause as a sci­en­tist I have to fol­low the ev­i­dence, and this leads me to query this new pre-em­i­nence of vine­yard ge­ol­ogy.

of course, a link be­tween wine and the land has long been trea­sured as some­thing spe­cial. It even sur­vived the dis­cov­ery of pho­to­syn­the­sis – that vines and wine are not made from mat­ter drawn from the ground but al­most wholly of car­bon, oxy­gen and hy­dro­gen, ab­stracted from wa­ter and the air. The rocks and soils in which the vines grow are cer­tainly still part of the sci­en­tific pic­ture, but this pre-emi­nent role is some­thing new.

To­day there are restau­rants with wine lists or­gan­ised not by grape, wine style or coun­try of ori­gin, but by vine­yard ge­ol­ogy. Al­ice Feir­ing’s book The Dirty Guide to Wine urges drinkers to choose their wines by ‘look­ing at the source: the ground in which it grows’. There’s a con­sor­tium of grow­ers from such di­verse places as st- Chinian, Al­sace, Cor­sica and Valais that claims com­mon­al­ity of its mem­bers’ wines sim­ply be­cause their vines are grow­ing on schist – even though schist and the soils de­rived from it are in­cred­i­bly var­ied. The same could be said about the very fash­ion­able idea of (so-called) vol­canic wines.

Yet in none of this are we told what the ge­ol­ogy ac­tu­ally does, how a par­tic­u­lar rock brings some­thing spe­cial to the wine in our glasses. And our present sci­en­tific un­der­stand­ing makes it dif­fi­cult to see how this might hap­pen. The fact is that the claims are largely based on anec­dote: the science sug­gests that the vine­yard rocks and soils have more mod­est roles.

Ques­tion­able claims

so what are their ef­fects? Well, qui­etly in the back­ground the be­drock ge­ol­ogy sets the con­text by de­ter­min­ing the phys­i­cal land­scape. The re­sis­tance of dif­fer­ent rocks to ero­sion gov­erns where hills and plains de­velop, where we get favoured sites for vine­yards such as hill­sides and river val­leys. but the ma­jor di­rect con­tri­bu­tion of ge­ol­ogy, con­sis­tently con­firmed by re­search in var­i­ous parts of the world, con­cerns wa­ter sup­ply: pro­vid­ing de­cent drainage for the vines while stor­ing suf­fi­cient wa­ter for dry pe­ri­ods. It’s piv­otal to how grapes swell and ripen.

how­ever, many dif­fer­ent kinds of ge­o­log­i­cal ma­te­ri­als ful­fil this – grav­els in bordeaux, for ex­am­ple, gran­ite soils in the northern Rhône, chalk in Cham­pagne. More­over, grow­ers rou­tinely at­tend to any short­com­ings by insert­ing drains and, in most parts of the world, ir­ri­gat­ing. That is, the role of the nat­u­ral ge­ol­ogy is over­rid­den.

how the vine roots are warmed by the soil plays a role, but a par­tic­u­larly pop­u­lar claim is that the rock of some par­tic­u­lar vine­yard pro­vides an ad­van­tage through be­ing heated dur­ing the day and re-ra­di­at­ing warmth to the grapes at night. how­ever, the sci­en­tific data show that this ca­pac­ity varies little be­tween dif­fer­ing rock types – all of them do it, pro­vided the ground is bare – and that it’s not a very great ef­fect any­way. It’s prob­a­bly only sig­nif­i­cant in some cool-cli­mate ar­eas where the grapes are trained close to the ground. And any­way, there is a school of thought that finer grapes are pro­duced where night-time tem­per­a­tures are markedly cooler than dur­ing the day.

The fea­ture of vine­yard ge­ol­ogy most of­ten men­tioned re­lates to it sup­ply­ing the nu­tri­tion needed by grow­ing vines. It’s of­ten made to sound as though vines sim­ply soak up what­ever nu­tri­ents the lo­cal ge­o­log­i­cal ma­te­ri­als yield, and these are then con­veyed through the vine to the even­tual wine. 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’. some state­ments even sug­gest that the rocks them­selves are mak­ing it through to the wine, as in ‘the weath­ered Devo­nian slate is right there in your glass’. sadly – I sup­pose – sci­en­tific un­der­stand­ing of how vines grow means this kind of thing just doesn’t hap­pen. To ex­plain, let’s look at some as­pects of how vines and soils work.

‘It’s re­ally the root­stocks onto which vines have been grafted that in­ter­act with the soil’

El­e­men­tal con­cept

By nu­tri­tion, we mean the 14 el­e­ments that a vine needs – be­sides the car­bon, oxy­gen and hy­dro­gen – in or­der to grow. Most of them are met­als, things like potas­sium, cal­cium and iron, and in the first place these are locked inside the ge­o­log­i­cal min­er­als that make the rocks, stones and the soil’s phys­i­cal frame­work.

It’s easy to demon­strate that these nu­tri­ents have to be in so­lu­tion in or­der for the vine to ab­sorb them – just scat­ter­ing iron fil­ings, say, on the vine or on the ground doesn’t do much good. Vine roots sim­ply can­not ab­sorb solids. But a se­ries of in­tri­cate and com­plex weath­er­ing pro­cesses can re­lease some of these el­e­ments from the par­ent ge­ol­ogy, and en­able them to be­come dis­solved in the soil wa­ter that is ad­ja­cent to the vine roots.

But these pro­cesses are slow, too slow to be able to pro­vide each grow­ing sea­son with a re­place­ment set of nu­tri­ent min­er­als. And this is where hu­mus – de­cayed or­ganic mat­ter – comes in. Ev­ery farmer and gar­dener knows that they can’t keep har­vest­ing crops year on year with­out en­rich­ing the soil. With the un­usu­ally mod­est nu­tri­tional needs of a

grapevine, the hu­mus need only make a small frac­tion of the soil, but it has to be there.

Among other things, hu­mus is able to re­cy­cle nu­tri­ents, it’s in­ter­linked with or­gan­isms ben­e­fi­cial to the soil, and it is the only nat­u­ral source of es­sen­tial ni­tro­gen and phos­pho­rus, which are lack­ing in most rocks. The rock de­bris in the vine­yards of the Mosel, Pri­o­rat or Châteauneuf-du-Pape may look hope­lessly bar­ren, but around the vine roots there’s hu­mus.

So, to car­i­ca­ture the point a little, if you perceive a taste of min­er­als in your wine and say this is due to the vine­yard ground, then you should be think­ing not in terms of lime­stone, slate, gran­ite, etc, but of de­cayed vegetation.

On the up­take

An­other mat­ter of­ten over­looked is that even if the nu­tri­ents are avail­able in the soil pore-wa­ter, they are not nec­es­sar­ily ab­sorbed by the roots. 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. There is some pas­sive up­take of el­e­ments and the se­lec­tiv­ity mech­a­nisms are far from in­fal­li­ble, hence nu­tri­ent im­bal­ances can arise, but these are rou­tinely checked for by a con­sci­en­tious grower and cor­rected as nec­es­sary.

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. In any case, it’s re­ally the root­stocks onto which they have been grafted that in­ter­act with the soil. We may be au fait with the var­i­ous Caber­nets and Pinots, and even the dif­fer­ent clones of San­giovese and Mal­bec, but to many of us 140 Rug­geri, Kober 5BB, 1616 Coud­erc and the like are an alien world.

The nu­tri­ents are taken up by the vine be­cause they are es­sen­tial to its growth pro­cesses but, although it may seem a tru­ism to say so, their ac­tual source is ir­rel­e­vant. The vine does not care, so to speak, whether a par­tic­u­lar nu­tri­ent min­eral orig­i­nated in this or that ge­o­log­i­cal min­eral, in hu­mus or in a bag of fer­tiliser. Mag­ne­sium is mag­ne­sium ir­re­spec­tive of its source and does the same jobs.

The pro­por­tions of these nu­tri­ents change sub­stan­tially dur­ing vini­fi­ca­tion, though some of them may sur­vive through to the fin­ished wine. But the amounts are tiny: a typ­i­cal wine has only around 0.2% of in­or­ganic mat­ter in to­tal, and it’s pretty much taste­less any­way. Salt, sodium chlo­ride, is an ex­cep­tion, but grapevines try to pre­vent the up­take of sodium, and hence most wines con­tain less salt than the min­i­mum re­quired in or­der for us to de­tect it even in plain wa­ter. An im­por­tant point, how­ever, is that the pres­ence of these nu­tri­ents in wine can in­di­rectly af­fect a range of chem­i­cal re­ac­tions and thereby in­flu­ence our taste per­cep­tions. But these are com­plex and cir­cuitous ef­fects, a long way from vine­yard ge­ol­ogy dom­i­nat­ing wine.

Out of sight...

The ap­par­ent im­por­tance of vine­yard ge­ol­ogy has been bol­stered by the fact that we com­monly use ge­o­log­i­cal words to com­mu­ni­cate taste per­cep­tions, as metaphors. We may, for ex­am­ple, re­port a flinty taste in wine (es­pe­cially if we know there’s flint in the vine­yard ground!). But flint lacks any taste or odour, and be­ing a solid com­pound is un­avail­able to vine roots. We are prob­a­bly re­call­ing the smell pro­duced by strik­ing lumps of flint to­gether, which has a chem­i­cal cause ir­rel­e­vant to vine­yards. There are sim­i­lar

‘The vine has an ar­mory of so­phis­ti­cated mech­a­nisms aimed at se­lect­ing and bal­anc­ing its nu­tri­ent up­take’

chem­i­cal and bio­chem­i­cal ex­pla­na­tions for such per­cep­tions as an aroma of wet stones, tilled earth, sea-shells or a metal­lic taste.

Where iden­ti­cally made wines from nearby sites taste dif­fer­ently and the soil dif­fers, it’s easy to pounce on the soil as the ex­pla­na­tion. It’s right there, pal­pa­ble and fa­mil­iar. But there will be other fac­tors at play in a vine­yard, well known to in­flu­ence wine char­ac­ter but which are in­vis­i­ble and hence over­looked. Fine vari­a­tions in cli­mate, for ex­am­ple.

The land at the Fault Line Vine­yard at Abacela, in Ore­gon’s Um­pqua Val­ley, shows vari­a­tions in soil types over small ar­eas and sim­i­lar changes in the wines. Here, how­ever, the owners col­lected me­te­o­ro­log­i­cal data from 23 dif­fer­ent spots within the vine­yard, ev­ery 15 min­utes, for five years. This re­vealed un­ex­pected vari­a­tions in such things as in­ten­sity of so­lar ra­di­a­tion, and tem­per­a­tures dur­ing the ripen­ing pe­riod dif­fer­ing by nearly 5°C – all within this sin­gle vine­yard. On their con­clud­ing list of fac­tors that in­flu­enced grape ripen­ing, soil dif­fer­ences were not high.

Work in progress

There has been ex­cite­ment in sci­en­tific cir­cles in re­cent years about the pos­si­ble im­por­tance of mi­cro­bi­ol­ogy in the vine­yard, be­cause new tech­nolo­gies have re­vealed dis­tinct fun­gal and bac­te­rial com­mu­ni­ties at dif­fer­ent sites. What ef­fect this has for wine taste is at present un­clear, but since the king­dom of fungi in­cludes or­gan­isms such as the mould botry­tis and the yeast bret­tanomyces, it could be very im­por­tant. How­ever, per­haps be­cause all this is un­seen and it’s all tech­ni­cal stuff, lack­ing the ap­par­ent charisma of ge­ol­ogy, such things are avoided in most wine pub­lic­ity.

So in view of all this, is it enough just to make grand as­ser­tions about ge­ol­ogy with­out of­fer­ing a ba­sis? Say­ing, for in­stance, that an Aus­trian Ries­ling has ‘com­plex­ity be­cause of the slatey para-gneiss, am­phi­bo­lite and mica soils’ may sound im­pres­sive, but surely some in­di­ca­tion is needed on how this works?

Of course, it’s per­fectly pos­si­ble that science is miss­ing some­thing. And I will be de­lighted if some­one points out some sig­nif­i­cance of the vine­yard ge­ol­ogy that I haven’t con­sid­ered. I’ve long been try­ing to high­light how ge­ol­ogy un­der­pins so many things in our mod­ern lives; if I knew how I could do it for the taste of wine, then I’d be over­joyed.

Syrah vines on the gran­ite-rich soils of the La Chapelle vine­yard over­look­ing Tain l’Her­mitage in the northern Rhône val­ley, France

Châteauneuf-du-Pape galets

Pri­o­rat schist

Mosel grey slate

Cham­pagne chalk

Typ­i­cal of the St-Estèphe com­mune in Bordeaux, a vine­yard at Château Lil­ian Ladouys sits on gravel over clay and lime­stone sub­soils

Be­low: the Grand Hill sub­sec­tion of Abacela’s Cob­ble­stone Hill par­cel at its Fault Line Vine­yards in Ore­gon, where the ge­ol­ogy com­prises a com­plex mix of meta­mor­phic, sed­i­men­tary and vol­canic soils

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