Soil, Water, Sunshine, and the Concept of Terroir

If we look at a vineyard, it’s very tempting to assume that what we see at the surface simply continues on downward. Maybe it does, but most soils vary with depth, and the surface can be quite unrepresentative of down where the work is done, of the materials that surround the vine roots. That’s why these days vineyards are peppered with soil pits. Normally, immediately below the surface of the ground is the topsoil, the most fertile part, from which vines get most of their water and nutrients. Below this is increasingly compact, commonly clayey material, subsoil, in which relatively little grows. If we continue downward, sooner or later we hit bedrock, for every vineyard sits on bedrock, at some depth or other. Unlike many plants, vine roots can probe many meters downward into the subsoil and even penetrate fissures in the bedrock, particularly if there’s a need to seek out supplementary water. The way soil varies with depth is called its profile. The variations in physical and chemical properties may be gradual, or in discrete layers, referred to as soil horizons, an arrangement sometimes called a duplex soil. A hypothetical example of a layered soil profile is shown in Figure 10.1, and Figure 10.2 gives an example of how a property can vary with depth. The overall depth of a soil above bedrock is termed its thickness. In vineyards, this can be anywhere from as little as 20 centimeters, such as at Auxey-Duresses in the Côte d’Or, to alluvium on plains such as California’s Central Valley that is measured in hundreds of meters. Even where bedrock has weathered in place to yield the overlying soil, its effects (Figure 10.3) can only be very generalized, because of all the permutations of climate, landform, biology, history, and so on, that influence soil profiles. Granite, with its coarse grains and high content of feldspar and quartz, both of which are fairly stable minerals physically, tends to yield sandy, well-drained soils. They are often pale colored, like the parent rock, though in places with a higher manganese content, such as parts of Barolo and Beaujolais, they can have a bluish tone.