It is difficult to imagine how Hong Kongers will celebrate World Earth Day next week. Other than few keen hikers, most of us show little interest in our dirt. Ask about soil around Central and you’ll get a blank stare – even though heaps of it is poised just up the hill. And that’s why I’ve always been amused that winemakers hoping to market their wines on our shores spend inordinate amounts of time discussing soil. Few in the audiences have ever cupped dirt in their hands, let alone considered its composition, drainage capacity and mineral delivery.
The dirt in one of Burgundy’s top vineyards, Clos de Vougeot, is considered so precious that vineyard workers are required to scrape it from their shoes before heading home each night. While at Burgundy’s grand cru Montrachet vineyard a few months ago, I scooped up a souvenir spoonful of soil and, as though I were a teenager shoplifting a pack of cigarettes, my host urged me to quickly hide the illicit clods in my handbag before the workers and winemaker took note. The marketing emphasis on vineyard soils evolved primarily through the influence of the French wine community who believe that soil composition has a profound impact on a vine’s development and the resulting wine’s flavour profile. In large part, French vineyards are delineated and classified based on their soil types, the other main factor being exposure to sunlight. Actually, grape vines grow well in most soils, presuming their tap-roots can wend their way deep into the soil to access minerals and water reserves not always available near the surface. Yet centuries of vineyard observation revealed that sandy, chalky, clay or limestone soils deliver distinctly different wines and are best partnered with specific grape varieties. Because vines are one of the few crops that excel on difficult and infertile soils, many mainland officials have encouraged vineyard cultivation, reserving dark fertile soils for food production, not alcoholic beverages.
Vines do not like wet feet and so when it comes to site selection, a soil’s drainage capacity is critical. Thus many of the world’s top growing sites are on rocky, stony soils: riesling excels on sliding heaps of shist and slate along Germany’s Rhine River. Port is grown on hills so rocky that dynamite is used to blast the slate into new vineyard sites. Bordeaux is so pebble-laden that one of its top districts, Graves, is a translation of the word gravel. Stones are not a requirement for quality vine growth, but they help contribute to soil ‘’friability’’ or crumbliness, assisting in drainage and aeration of the soil.
A soil’s mineral composition has a direct impact on grape quality, whether affecting vine health during the growing season or the ultimate chemical composition of the ripened grapes. For example, too much nitrogen and the vines excitedly overproduce. Too much lime and the leaves turn a dejected yellow. Too much soil potassium and a wine’s acidity will be dizzily unbalanced.
Another soil consideration is its ability to absorb, reflect or deflect heat. The hand-sized ‘’pudding stones’’ of the Rhone Valley absorb heat by day which helps ripen the grapes in the evenings. The black basalt in New Zealand’s Wairau Valley helps keep the vines warm during cool nights, whereas the blindingly white soils of southern Spain’s sherry producing Jerez region, keeps the soils cool. This effect is seen when walking on Indonesia’s southeastern hot volcanic black sand versus Borocay’s sugar white beaches. If you are not planning a special ecological observation for World Earth Day, then at least give soil a thought when lifting your glass.