Clay Soils: Water Retention and Body in Wine

The mineral part of a soil is made of sand, silt and clay particles, and their relative percentages give the soil its texture. Clay is the smallest class, under 0.002 mm, against 0.002 to 0.05 mm for silt and 0.05 to 2.0 mm for sand. Clay particles are like ants: extremely small and tightly packed. Because fine clay particles pack closely together, clay soils have the smallest individual pores, yet their overall pore volume is greater than that of sandy soils, which allows them to retain more water and nutrients.

• Sand measures 0.05 to 2.0 mm, silt 0.002 to 0.05 mm and clay less than 0.002 mm; particles larger than 2 mm, such as gravel, count as coarse fragments
• The largest clay particle is more than 1,000 times smaller than the smallest sand particle
• Clay holds onto plant nutrients far better than sand and stores significant amounts of water in its fine pores
• The USDA soil texture triangle identifies 12 textural classes based on the percentages of sand, silt and clay

The clay soils of Western Oregon date back 10,000 to 14,000 years: at the end of the last ice age, massive floods filled what is now the Willamette Valley, and very fine clay particles took much longer to settle out of the floodwaters than sand and silt, so clay accumulated in parts of the valley. Bourgogne lay beneath a warm, shallow sea between 200 and 250 million years ago, which left marine marl and limestone deposits in the subsoil; numerous geological movements during the Tertiary era then allowed people to create a patchwork of thousands of plots in which clay, limestone and marl terrains were combined into the Climats.

• About 30 million years ago, the formation of the Alps created folds, hollows and humps, and the collapse of the Bressan rift created the relief of the Côte and the Hautes Côtes
• In the Willamette Valley, clay accumulated in parts of the valley while sand and silt settled further north
• From a viticultural point of view, the most important soil types in Rioja are calcareous clayey soils, ferrous clay soils and alluvial soils

Clay's water-holding cuts both ways. Sandy and clayey soils may contain similar total amounts of water at a given time, but clay retains a larger portion of it in small pores and on particle surfaces at much higher tension, which makes it more difficult for roots to extract; sands hold water at lower tension but store far less plant-available water because they drain quickly. Soils containing a lot of clay are usually cooler because they absorb and retain water so well, which can be extremely useful in drier climates. Bordeaux averages 950 mm of rainfall a year, so the soil there needs good drainage, and soil that gets too wet adds a further cooling influence on the vines in an already cool climate. One Bordeaux trade body links the styles to the soils: clay-rich soils retain moisture and are associated with muscular, full-bodied wines, limestone and gravel drain well and are associated with brighter acidity, and sand is associated with lighter, more elegant wines.

• Finer-textured soils with more clay have greater water-holding capacity but can drain more slowly, which results in more surface water
• Clay soils are more vulnerable to compaction, although sandy soils can also be susceptible under improper management
• Clay soils harden when dry and become sticky and heavy when wet
• Fine-textured clay soils can be irrigated less often, but the water must be applied slowly to prevent runoff and reach the root zone

As of 2026, Pomerol has 813 hectares and 138 winegrowers, with Merlot occupying 80% of the vineyards; the soils of the plateau are a mixture of clay-gravel and sandy gravel, with density varying from one plot to the next, over a subsoil rich in iron oxide known locally as crasse de fer. Unlike St-Emilion, Pomerol has no limestone but gravel, sand and clay, and its high terrace has a greater percentage of gravel and clay for regulating the water supply to the vine. At the top of the plateau, heavy blue clay rises to the surface, forming what is known as the Pétrus 'buttonhole', and James Lawther MW credits these water-retaining clays with helping provide the deep colour, power and volume typified by Château Pétrus. In Bourgogne, Pinot Noir tends to produce light and sophisticated or powerful and full-bodied wines depending on the proportion of limestone, stone content and clay in the plot, while Chardonnay prefers more clayey marly limestone soils. In Rioja, calcareous clayey soils are yellow-ochre, poor in organic matter, rich in calcium and, because of the area's irregular topography, difficult to drain; ferrous clay soils are reddish-brown, usually deep and very compact because of their high clay content.

• The island, or buttonhole, of clay on Pomerol's high terrace covers 20 hectares, with nearly 11 hectares of it inside Pétrus, and the Moueix regime feels strongly that the colder clay soil needs Merlot
• The high clay content fans out eastwards through Vieux Château Certan, L'Evangile and La Conseillante and on into Château Cheval Blanc in St-Emilion
• Merlot thrives on Bordeaux's clay-limestone and gravel soils
• Rioja's ferrous clay soils predominate in the middle hills and in the higher areas separating the alluvial terraces of Rioja Alta and Rioja Oriental

There are three forms of soil water. Gravitational water fills the large pores right after heavy rain or irrigation and drains downward under gravity, so plants cannot use it; capillary water, held in small and medium pores, is the plant-available water that remains and supplies most of the water for plant growth; hygroscopic water is a thin film held so tightly on particle surfaces that plants cannot extract it, even under severe stress. Available water capacity, the water held between field capacity and the permanent wilting point, is low in sands and moderate in loams, while clays combine high total water with high tension, so only a portion is available. Clay also holds onto plant nutrients far better than sand.

• Field capacity is reached after gravitational water drains, when the remaining water is held by capillary forces and can be easily extracted by plants
• At the permanent wilting point, the remaining water is held so tightly to soil particles that roots can no longer extract it, even though moisture is present
• “No amount of sand added to clay soil will change its texture,” in the words of Linda Brewer of Oregon State University
• Gypsum's calcium ions help clay particles aggregate and improve structure

One Bordeaux trade body links soils rich in clay with muscular, full-bodied wines. Merlot, which thrives on Bordeaux's clay-limestone and gravel soils, has a supple, rounded, velvety texture, with notes of cherry, plum, fig and violet. In Bourgogne's more clayey marly limestone soils, the regional wine bureau credits the clay with helping produce breadth in the mouth, characteristic of the region's great white wines. In Pomerol, Lawther credits the water-retaining blue clays at the top of the plateau with helping provide the deep colour, power and volume typified by Château Pétrus.

• Merlot tends to produce wines with generous alcohol and moderate acidity
• In Bourgogne, Pinot Noir tends to give light or full-bodied wines depending on the proportion of limestone, stone content and clay where it grows
• In the same Bordeaux framing, limestone and gravel drain well and are associated with brighter acidity, and sand with lighter, more elegant wines, in contrast to clay's full-bodied style

Primary sources: https://extension.oregonstate.edu/catalog/em-9544-soil-plant-water-relationships, https://extension.oregonstate.edu/news/clay-soil-challenges-solutions-oregon-gardeners, https://extension.oregonstate.edu/catalog/em-9694-ponding-plugging-pugging-how-care-wet-spring-soils, https://www.wsetglobal.com/knowledge-centre/blog/2021/august/24/how-does-geography-affect-a-wine-s-style, https://www.wsetglobal.com/knowledge-centre/blog/2022/july/14/what-makes-high-quality-bordeaux-wine, https://www.bordeaux.com/en/designations/saint-emilion-pomerol-fronsac/pomerol/, https://www.bordeaux.com/en/grape-varieties/merlot/, https://www.bordeaux.com/en/mag/blog/the-art-of-the-blend/, https://www.bourgogne-wines.com/wine-and-terroir/our-natural-assets/geology/the-birth-of-bourgogne-wines/the-birth-of-bourgogne-wines-thousands-of-years-of-history,2479,9399.html, https://riojawine.com/en-gb/blog/geography-and-terroirs-of-rioja/. Last verified 2026-10-04