Digging in the dirt: what makes a good soil for grape growing?
Wine is arguably one of the most valuable products to depend on its regional identity. Its geographical and geological origins undoubtedly make a difference and the soil the grapevine grows in is a key part of that.
Soil is defined in the Oxford Companion to Wine 2015 edition as, 'Mineral material at the earth's surface formed by weathering of underlying bedrock or transported sediments which form the parent material of soil.' Soil itself is enriched with decomposed plant and animal remains that form organic matter often called humus.
Soil performs some incredibly useful functions - first as a physical anchorage for plants to put down their roots but also as a source of water and the nutrients a plant needs to sustain life (in addition to the production of glucose and oxygen via photosynthesis).
Soil & the concept of terroir
Historically soil has been closely linked with terroir, especially in the old world, but increasingly the new world is also interested in discovering what makes a good vineyard soil using techniques like precision viticulture, soil mapping and analysing leaf nutrient status.
What makes for good vineyard soil?
Scientific opinion currently is that it is a soil's physical characteristics that are most influential and of these physical attributes, water supply is the most important feature. A good vineyard soil will be well-drained enough that roots do not become waterlogged but is also able to store enough water to provide a sufficient supply over the growing season, even if rainfall is irregular. Of course, where irrigation is in use, the water storage part is not so critical but a lot of the world's vineyards are still dry farmed and depend on nature's bounty for enough water.
Other important aspects of soil to consider include its structure (so the size of the particles that make it up and how easy it is for water to drain and roots to penetrate); depth (shallow soils and impermeable bedrock can leave roots vulnerable to drought); aeration (better for microbes and fauna like earthworms and decomposition of organic matter); nutrient status and even colour (for instance dark soils absorb sunlight better than pale ones and can then radiate warmth back to the plant at night).
Soil fertility is a critical factor and it's well known that very fertile soils are not regarded as good for quality winemaking. There's no real link between actual minerals or flavour compounds from the soil (and the hot topic of 'minerality' is one for another article) but fertility levels do affect how the plant grows and the quality of the fruit it produces.
Nitrogen is particularly important and a high level of nitrogen compounds mean a fertile soil which in turn usually means a vigorous vine. This tends to mean lots of leaves which can end up with too much shade on the fruit so poor colour, green flavours, excess potassium and high pH. Or it can mean over-cropping and lack of concentration and flavour.
The best wines come from the poorest soils
Globally, it's common that the best wines come from less fertile soils where vigour is lower and crops naturally more concentrated. On the other hand, a lack of nitrogen in the fruit can mean stuck ferments and problems like reduction (usually due to smelly sulphur-containing compounds like hydrogen sulphide and mercaptans).
Some of these problems can be managed by adding compost or fertiliser, or yeast nutrients to the ferment, or on the other hand by growing cover crops to use up some of the excess nitrogen. Organic matter contributes to available nitrogen but also adds water-holding capacity to dry sandy soils, while on heavier clay soil it helps to give friability and aeration.
Which soils produce the best quality wines?
Certain soil types are more commonly associated with high-quality wine. Chalk is a soft, crumbly and very porous type of limestone (based on the mineral calcium carbonate) and is usually very pale in colour. It's valued in viticulture because it is well drained but the pores in the subsoil rocks also have high water storage capacity, and because it's porous, roots can easily penetrate down to this subsoil. Chalk soils tend to be low in fertility as well, which is another benefit as it's easier to add nutrients than reduce them. Champagne is the most famous example though southern England also has good chalky soils.
Limestone has the same mineral base of calcium carbonate but is harder so roots can only penetrate through cracks. Some particularly famous vineyard soils are limestone derived - the rusty red Terra Rossa soils of Coonawarra and Croatia overlay limestone. One downside of high levels of limestone is that the soils can be alkaline which may cause a condition called chlorosis where the leaves turn yellow and can't photosynthesize effectively. It's most commonly caused by lack of iron (uptake of zinc, manganese and boron can also be an issue) and the main solution is planting on lime tolerant rootstocks like Fercal or 41B.
Clays are made up of very fine particles of clay minerals (Phyllosilicates) which are hydrates (ie. have water within the crystal structure). As such, clays have good water-holding capacity, though this can mean they get waterlogged or can be cold (wet soils take longer to warm up). Clays often contain other groups of minerals too so the famous marl of the Côte D'Or is a mixture of clay with high calcium carbonate content.
Loess is an accumulation of clay and silt, usually deposited by the wind and is good for vines as it's porous and easy to grow through. Tokaj is a region famous for its loess.
Tuff is a fine-grained volcanic rock, formed from ashy material blown out by volcanoes. It's relatively soft, easily weathered into soil and quite handy for digging cellars such as in Hungary's Eger region, but not to be confused with the limestone Tufa.
Shale is fine-grained sedimentary rock that weathers into clay-rich soils while schist and slate are metamorphic rocks with distinct planes that fracture into sheets. Schist is coarser than slate and can be well drained, especially if the planes are tilted towards vertical such as in the Douro and Central Otago.
Anyone seriously interested in wine must have heard of the legendary slate soils of the Mosel - so well drained that even phylloxera can't survive here.
Gravel soils work well in the Médoc - because it's a damper maritime climate, it is important that heavy rains can drain away from roots and then in drier periods roots have to dig deep to find water.
Soil can cause all sorts of problems too - acid soils can result in too much aluminium or copper which can damage vine growth, as can excess salinity (salt). This is a notable issue in Australia where many of the soils were once sea. And of course soil provides a home for vine root pests like phylloxera and nematodes.
Research into other factors affecting terroir
Another complication that is only just starting to come to light is the influence of biological activity in the vineyard and winery on terroir and wine flavour too. A research paper in Oregon (Bokulich et al 2016) has found distinct characters for AVAs (American Viticultural Areas), and even individual vineyards, linked to differing populations of microbes in both vineyards and winery. And in Croatia, Radić et al (2012) have shown that that even different weed species in the vineyard are associated with different mycorrhizal associations in vine roots.
This is yet another complication in the picture of terroir and how the soil and site affect what we taste. (A Mycorrhiza is a symbiotic relationship - one that benefits both - between a fungus and a plant. The fungus colonises the plant cells and obtains carbohydrates while at the same time improving the plant's uptake of water and nutrients, especially phosphate which most plants can't easily obtain. This type of symbiosis is incredibly ancient and clearly beneficial as 92% of all plant families, including grapevines, form such relationships).
This is only a tiny snapshot of the complexities of choosing and managing soil for growing the best grapes. Luckily for many growers, centuries if not millennia of wisdom help, while science can often come to the rescue in newer regions. There's no doubt soil is important to making good wine, but it is also very clear that there's no simple answer to the question of what makes good vineyard soil.