| Clay can be polarizing for growers – some love it, others don’t.
Clay can be a challenge for grapegrowers, but it can also help produce some of the world’s best wines.
Everybody knows clay: go for a winter walk and it sticks to your boots, your feet gettting heavier with every step.
Gardeners may curse its lack of drainage, its heaviness and solidity. Potters rely on it: without that plasticity, that ability to hold a thin and delicate shape, our plates and cups would be made of – what? Metal? Plastic? We take clay, and its remarkable properties, for granted.
Read anything about wine, and you’’d be forgiven for not knowing whether clay is a hero or a villain. On the one hand it doesn’t drain well, and vines like drainage; they hate to get their feet wet. But come a hot, dry summer, and Saint-Estèphe in the Médoc will be trumpeting its advantage over more gravelly appellations: our clay held the water, they’ll say; our vines weren’t stressed, unlike theirs, poor things. Clay soils tend to be colder, and warm up more slowly in spring, because they’re wetter. In Bordeaux they tend to be associated with earlier-ripening Merlot, which can deal with such things
So what is this stuff that we’re allowed to love and hate in the same breath?
There are different ways of defining clay, or describing it. One way is by the size of particle: the smallest particles are clay, 0.002mm in size at the most. That is really, really small. Somewhat bigger, and they’re silt. A bit bigger again and we have sand; and while you can see a grain of sand with the naked eye, you’ll need a pretty whizzy magnifier to see a particle of clay. Geologist Alex Maltman, in Vineyards, Rocks and Soils (Oxford University Press, 2018) says that if you imagined that a grain of sand was the size of a wine barrel, then an individual clay particle would be smaller than a coin.
If you describe soil in terms of particle size you go from sand up to gravel, and beyond. Looked at this way, the difference between the sticky stuff clinging to your boots, and the galets roulés of Châteauneuf-du-Pape, is merely one of particle size. But common sense might suggest that it’s not that simple. And clay, it turns out, is not simple at all: it is most remarkable stuff with extraordinary properties.
Breaking down clay
First of all, there are different kinds of clay. They’re different according to the clay minerals they contain, and that depends on the sort of rock from which they have weathered, since clay has to weather from rock. Granite, which contains feldspar and quartz, can weather to the clay mineral kaolinite; igneous rock can weather to montmorillonite, another clay mineral.
This makes a big difference if you’re a vine; vines, as we know, get their nutrients from the soil via cation exchange, a cation being an atom with a positive electrical charge. (Cations bind to negatively-charged particles in the soil, and can be exchanged for other positively-charged particles in the soil water.) Kaolinite has rather low cation-exchange capacity, so such soils are relatively infertile. Montmorillonite has high cation-exchange capacity, and is therefore fertile – one reason why the world’s volcanic soils have so much to offer vines. But again, what might appear a disadvantage can sometimes be an advantage: infertile soils can restrict vine vigor. With clay, the key words seem to be, “it all depends…”.
| The soils of Pomerol contain significant levels of clay.
Clay is formed in sheets. How these sheets are bonded at a molecular level, and how they’re arranged, gives rise to differing water-holding properties, but that clay you shake off your boots has a mind-boggling surface area. Maltman says that thanks to the sheet structure, and the minute size of clay particles, a gram of dry clay can have a surface area bigger than two car-parking spaces. Get your head round that if you can.
Water gets between these sheets, and what happens then affects vines. If it stays there, then it can be a valuable water supply for vines in a drought. But some clay minerals expand as they get warmer, forcing out the water. Swelling clays, which expand as they get wet, can block the natural drainage channels in the soil, where water can minutely seep down. Then you get waterlogging. You also get oxygen forced out of the soil, and that’s the real reason vines don’t like getting their feet wet – a swollen, waterlogged soil is short of oxygen.
Kaolinite soils drain well; it doesn’t swell much, so doesn’t block up the soil, but it can store some water. And, as we’ve seen, it doesn’t encourage vigor in vines. So it can be useful in vineyards. But it’s useful in other ways, too; those sunscreen sprays that some growers are using are made from kaolinite; it’s a fining agent, too (as is bentonite, another name for montmorillonite). And it’s ability to form strong but thin shapes means that this is the stuff fine china is made from. That’s where it was first reported – in China.
Montmorillonite, one of a group of clay minerals called smectites, gets its name from Montmorillon in western France. It absorbs water with alacrity, swelling enormously, with all the advantages and disadvantages this can have for vines. It tends to shrink and dry out in times of drought, however, so doesn’t necessarily have the water when vines need it most. Where can one find smectite? Tokaj has it; it weathers from the region’s many volcanoes. So do parts of Burgundy. So does Petrus, which seems to have more smectite than the rest of Pomerol.
The iron-rich smectite at Petrus, where the clay bulges up through the gravel, can lead to some hydric stress in the vines, though usually not too much. It swells, then shrinks and cracks. It is said not to get waterlogged. But the roots don’t go down very far in it because it’s so dense: one to two meters only. This helps to restrict vine vigor.
Latour has smectite near the surface, too, under a layer of gravel; there is a association with wine power when the clay is so high up. When it’s further down, with deep gravel on top, it may not give such oomph to the wines, but its water-holding properties are useful. Free-draining gravel on its own is great in a deluge, but not so good in a drought. Lafite, so different from Latour in style, has gravel over limestone.
Kaolinite and montmorillonite are not the only clay minerals; there are lots of them, all slightly different. There is illite, and chlorite, and vermiculite, and many, many others, on and on for a page or two, had we but world enough and time. A vineyard, typically, will contain lots of different clay minerals. Any assessment of the role of clay in a given vineyard is a complicated matter, and one that depends on what else is in the soil along with clay, as well as where the clay is.
On the Côte d’Or the clay is mixed with limestone. Smectite clays, which swell and shrink, are mostly found in the red grands crus, and are abundant on the mid-slope of the Côte de Nuits; the white grands crus often have kaolinite. There can be vermiculite and illite at the top of the slopes; there can be kaolinite further down. That limestone can weather to all sorts of clay minerals, depending on what was mixed in when the rock was formed.
Geologist Gary Jordan, who makes wine at Jordan Wines in Stellenbosch and will soon make wine at Mousehall on the chalky hills of Sussex, England, says that the clay content in both places is similar, at around 30 percent. In his Stellenbosch vineyards it is mostly derived from feldspar and mica minerals in granite, with kaolin in some places, derived from tin-bearing granite. In Sussex the clay is derived from mudstone and siltstone, and varies accordingly, and here it can shrink and swell quite dramatically.
The climate in the two places is different, however. “The drainage of soils with a high clay percentage is much more important in the cooler climate of England,” he says. “While Stellenbosch and London may have the same annual rainfall, Stellenbosch has most of it in winter. The growing season is when one has to worry about clay holding too much water.”
In Stellenbosch the cooler, south-facing slopes are more deeply weathered; the warmer north-facing slopes are more gravelly or rocky, but it’s the clay in the subsoil that provides moisture to keep the vines going through the hot, dry summers. “Without the clay the vineyards would suffer,” says Jordan.
“What has been a revelation, though,” he adds, “is how dry vines on chalk become, and in hot, dry English summers one is very grateful to have vineyards on some clay.”
Hot, dry English summers? With climate change, we need clay.