Humus, the dark, living heart of soil, is essential to healthy vineyards and vibrant wines. So why is it so often left out of the story? In this excerpt from Taste the Limestone, Smell the Slate, geologist Alex Maltman challenges the romantic focus on vineyard rocks and reminds us that what really feeds the vine is something far earthier.

‘The rain quit and the wind got high,’ sang Woody Guthrie, ‘a black old dust storm filled the sky.’ For John Steinbeck: ‘The sky was darkened by the mixing dust, and the wind felt over the earth carried it away.’ The awful 1930s Dust Bowl on the American prairies is burned into our consciousness, but similar though less storied catastrophes have happened and continue to happen in other places around the world.

Such calamities come about essentially because the soil has lost its ‘goodness’, that is, its humus, the key to a healthy soil. Humus, decayed organic material, is in practice the main source of the minerals that the grape vine takes up and, as some would have it, reach the wine for us to taste. But when did you last see humus mentioned in a vineyard description or in a wine-tasting note? Wine lovers don’t really want to read about rotted insects and decayed plants so, well, it’s just ignored. In other words, we have here an ‘elephant in the room’ (defined in my dictionary as a huge topic that is uncomfortable for people to talk about and so is ignored).

The humus in a soil becomes depleted because of overcropping, which eventually leads to crop failure. If this is accompanied by periods of drought the exposed soil soon becomes crumbly and is easily blown around. Hence dust bowls and the like. A current viticultural example is the overcropping resulting from unchecked demand for Prosecco, with consequent soil erosion problems. One recent study took the volume of wine produced in a year and the amount of soil being lost annually and calculated that a single bottle of Prosecco was costing 4.4 kilogrammes (nearly 10 pounds) of lost soil.

Most farmers and gardeners know that you can’t keep extracting crops without putting ‘goodness’ back into the soil. Synthetic fertilizers will provide a temporary nutrient fix but organic matter such as manure or compost will in addition improve the texture and microbial life in the soil. Hence that’s probably what most people mean by soil ‘goodness’.

In nature, soil nutrients are recycled. In a woodland, say, at the end of the growing season some of the vegetation, including the nutrients it has been using, is discarded. It falls to the ground where it’s processed by the little creatures that live there into leaf mould, more generally called compost. Eventually, as the decay is completed, the material becomes the non-living, dark spongy matter we call humus. (Pronounced hew-mus. Note the single ‘m’. With two, it’s the chickpea spread that to humans is much nicer to eat.) And not only does the humus contain the recycled nutrients, they are easily accessible for the next season’s growth.

In agriculture, however, as each year’s crops are harvested the nutrients they contain are extracted from the system – the cycle is broken. So with successive harvests the nutrient content of the soil declines. Vines have unusually modest needs but even so the grower will need to make sure adequate nutrients are there in the soil, and accessible. This may involve (apart from using artificial fertilizer, which will do nothing for the soil condition) letting vine vegetation, weeds or cover crops decay in place, or spreading modest amounts of compost, pomace or other organic matter onto the soil.

Slowly weathering
The chemical elements that can act as nutrients are locked inside rocks and the geological minerals that make the soil framework and so are inaccessible to plant roots. It’s weathering that progressively liberates them. But this involves a whole series of intricate processes that take place slowly – very slowly. We’ve all seen old gravestones, perhaps hundreds of years old, weathered such that the epitaph is becoming hard to read. In fact, weathering rates are typically measured in thousands of years: hence the processes are simply unable to produce each year a fresh suite of nutrients. And that’s where humus comes in.

Because of the recycling explained earlier, humus is rich in nutrients, and in fact it’s the only source for plants of vital nitrogen. Moreover, humus is able to release the nutrients (through what is technically called cation exchange) much more readily than any geological mineral. So although weathered rocks are the fundamental, primordial source of nutrients, in practice, season after season, much of the nutrient provision comes not from the vineyard geology but from the soil humus.

Unromantic reality
Wine is almost wholly composed of carbon, hydrogen and oxygen; the inorganic content – the ‘minerals’ in a wine – is typically no more than 0.4 percent and most of that is potassium. Despite this tiny proportion, those minerals get much mention in popular wine writing, often with at least an implication that their presence is a result of vineyard geology: ‘the granite soil offers minerals which impact the mineral flavours of the wines’; ‘the vine transmits the minerals all the way from the stones around its roots to the final wine’; ‘a very mineralized taste coming from the special mineral richness of the volcanic soil’, and the like.

But as I discuss in Chapter 3, of that tiny amount of inorganic material in the wine a substantial proportion will have come from sources other than the ground. Nevertheless, a proportion may have originated in the soil and survived through to the finished wine: my argument here is that it almost certainly originated in the humus.

Flint, slate, graphite and the rest might be charismatic and fun to read about in wine magazines, but bacteria, decayed vegetation, rotted creatures, and so on? They’re never mentioned. In other words, the real source of most of the soil-derived nutrients, the minerals in a wine, is never acknowledged in tasting notes. Humus is, as I’ve suggested, something of an elephant in the room.

Taken from Taste the Limestone, Smell the Slate by Alex Maltman published by Académie du Vin Library.