In discussions of terroir rocks have exalted status. The journey downwards through the geological record is mostly a search for hard boundaries – chalk, basalt, slate – while the overlying burden of clay, silt and sand tends to get lumped together, as soil. Causative descriptions tend to bottom out when bedrock is struck with a few high profile exceptions, most notably at Château Pétrus in Pomerol, where a blister of smectited blue clay challenges the limestone-dominated underworld of French vineyards for the top spot.

As with the adding of a Grand Cru name to a village commune in Burgundy, the buffering properties of Pétrus’ subsoil are stretched, along with the truth, to cover all of Pomerol. According to the most recent en primeur messaging, the water retaining properties of clay spared the appellation the worst effects of drought in 2016, 2018 and 2020. The reality on the ground is more intricate: just as limestone and granite have differing permeabilities, so smectite and illite clay contrast hugely in the volume of water they can potentially make available to roots. In Bordeaux, Professor Kees van Leeuwen’s in situ comparison of three Médoc soils – sand and gravel, limestone and illite clay – found that water stress occurred most rapidly on the clay.

The malleable nature of clay belies its resilience. The Jurassic clays of Burgundy were deposited over 100 million years ago, while the stiff London clay has held onto its territory for 50 million years without the need of  a hard rock crust to protect it from the elements.

Clay’s durability derives from its being a geological cul-de-sac, lying, as it does, at the end of physical and chemical erosional processes. Granite is a commonly occurring igneous rock, but over time it decomposes, separating into its constituent light (quartz) and dark (feldspar/mica) elements. Quartz is highly resistant to both forms of  erosion, but the feldspar degrades into aluminium and silica, the molecular building blocks of clay.

The diversity of clay reflects the availability of other elements during its deposition – magnesium, sodium, potassium, calcium – but the fundamental structure bonds sheets of octahedra alumina and tetrahedra silicates into layers.

For the purposes of this article, I’m going to consider the water retaining properties of three different clay minerals or the crystal structures formed by these layers – a) kaolinite, b) illite and c) smectite.

Figure 1 - Kitch, 2011

(Figure 1 – Kitch, 2011)

Unlike gravel, sand and rock, clay has a net negative charge (-ve) which it seeks to neutralize by forming either strong ionic bonds with positively charged cations, or weak dipole bonds with water molecules (H+/OH-).

Smectite (c) and illite clays (b) are formed from two silica tetrahedra sheets and a single alumina octahedra sheet. The sheets  stack into layers three deep, which are separated from one another by an interstitial gap. The difference between illite and smectite clays is that in the case of the former,…

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