What do regions like Champagne, Burgundy, Chablis, Tuscany, Alsace, the Loire, Saint-Emilion in Bordeaux, and Chateauneuf-du-Pape all have in common? They've all got soils that are variously described as chalky, decomposed limestone, and calcareous. In chemical terms, all are high in calcium carbonate, the basic building block of marine life.
So too does much of the Paso Robles AVA, particularly the sub-AVAs of the Adelaida District, Willow Creek District, Templeton Gap, El Pomar, and Santa Margarita Ranch. In all these regions, if you find a road cut, the rocks will be chalky and white, and if you dig into them you'll find marine fossils, from fish scales to oyster shells to whale bones. Yes, ten million years ago, our part of Paso Robles was under the Pacific Ocean. This makes our land, in geologic terms, relatively young. When they make their way to the surface, the rocks are creamy white and surprisingly lightweight:
What Are Calcareous Soils?
Calcareous soils are formed from the crushed up and decayed shells and bones of sea creatures. These layers settle down to the bottom of shallow oceans and, depending on how much heat and pressure they're subjected to, can be as soft as talc or chalk, or as hard as limestone or even marble. Of course, in order for plants to be able to access the calcium carbonate, it needs to be friable: soft enough for roots to penetrate. This means that even when you hear about a region having "limestone soils" the value to the plants isn't in the limestone itself, but in areas where the limestone has decayed into smaller particles.
From a grapevine's perspective, it doesn't really matter if the calcareous soils come from the erosion of limestone (as in Burgundy) or whether they never quite got heated and compressed enough to become rock (as in Paso Robles). The net impact is the same. There are four principal reasons why these soils are so often good for wine quality.
In winter, the calcareous clay absorbs moisture,
turning dark. Note the roots that have pene-
trated between the layers of clay.
Benefit 1: Water Retention & Drainage
Calcium-rich clay soils like those that we have here have water-retention properties that are ideal for growing grapevines. Some water is essential for cation exchange -- the process by which plants take up nutrients through their roots. But grapevines do poorly in waterlogged soils, which increase the likelihood of root disease. Calcium-rich clay soils have a chemical structure composed of sheets of molecules held together in layers by ionic attractions. This structure permits the soil to retain moisture in periods of dry weather but allows for good drainage during heavy rains.
The porosity of our soils mean that they act like a sponge, absorbing the rainfall that comes in the winter and spring months and holding it for the vines to access during the growing season. We've done backhoe cuts in late summer, after it hasn't rained for several months, and while the top few feet of soil are dry, there's moisture in the layers six feet down and more.
At the same time, we never see water pooling around the vines. Part of that is that our whole property is hilly. But hillside vineyards in other regions still end up with standing water at the bottoms of the hills. We never do. That balance of water retention and drainage is ideal, and it allows us to dry-farm in the summer months of what is essentially a desert climate.
Benefit 2: Higher Acids at Harvest
We've had anecdotal evidence of calcium-rich soils producing wines with more freshness for years. At the symposium on Roussanne that we conducted last decade, producers from non-calcareous regions (from Napa to the Sierra Foothills to vineyards in eastern Paso Robles with alluvial soils) consistently reported harvesting Roussanne roughly half a pH point higher than those of us from calcareous regions like west Paso Robles and the Santa Ynez Valley. But the chemistry of why this was the case has only become clear in recent years.
It appears that the key nutrient here is potassium, which is central to the processes by which grapevines lower acidity in berries as fruit ripens. High calcium levels displace potassium in the soils, inhibiting this chemical process and leaving more acidity at any given sugar level. Of course, this can be a challenge. I have friends in other parts of Paso Robles whose pH readings are so low at the sugar levels that we like to pick at (say, 22-24° Brix) that they have no choice but to wait for higher sugars. This can produce wines that carry massive levels of alcohol. But in moderation, it's a wonderful thing. I'm grateful that (unlike in many California regions) we can let malolactic fermentation proceed naturally, producing a creamy mouthfeel without unpleasantly high alcohol levels. In much of California, the higher harvest pH readings mean that they have no choice but to stop the malolactic bacteria from working to preserve the sharper malic acids in the finished wines, for balance.
The calcium-rich layers of the mountain behind
the winery shine bright white in mid-summer
Benefit 3: Root System and Vine Development
Unlike cereals and other annual crops that have shallow root systems, grape vines have deep root systems. This means that the composition of the deeper soil layers is more important for vine health and wine character than that of the topsoil. It also means that amending the soil (by, for example, liming to add calcium) is less effective than is natural replenishment of essential nutrients from deeper layers.
Grapevine roots are remarkable. They can penetrate dozens of feet into soil in their search for water and nutrients, and they continue to grow throughout the vines' lives. This means that the physical properties of the soil are important: a hardpan layer through which roots cannot penetrate can have a serious negative impact on a vine's output. Calcareous clay's tendency toward flocculation (soil particle aggregation) creates spaces in which water can be stored. In addition, the softness of these soils means that as they dry out, they shrink, creating fissures through which roots penetrate to where more residual moisture can be found. As they get wet, they expand again, opening up yet more terrain for the vines' roots to access. This process repeats itself annually. In our vineyard we've routinely found grapevine roots ten feet deep and deeper in experimental excavations.
Benefit 4: Disease Resistance
Finally, there is evidence that calcium is essential for the formation of disease-resistant berries. Calcium is found in berries in its greatest concentration in the skins, and essential for the creation of strong cell walls and maintaining skin cohesion. However, if calcium is scarce, plants prioritize intracellular calcium over berry skin calcium and berries are more susceptible to enzyme attack and fungal diseases.
Where Are California's Calcareous Soils?
When my dad and the Perrin brothers were looking for a place to found the winery that would become Tablas Creek, calcareous soils were one of three main criteria they were looking to satisfy (the others were sun/heat/cooling and rainfall). But they quickly realized that soils like these are rare in California, except in a crescent of land in the Central Coast between the Santa Cruz Mountains to the north and Lompoc to the south. The portion of this this area that is on the western slope of the coastal mountain ranges is too cold to ripen most Rhone varieties. The western and southern pieces of the Paso Robles AVA, on the eastern slopes of the Santa Lucia Mountains, are home to the state's largest exposed calcareous layers, and it's largely because of this that in 1989 we bought property here.
There's a great story about how they went about finding soils. As they tell it, they decided that it was a lot cheaper and faster to look at road cuts than to hire backhoes and dig their own. They looked for the better part of four years around California without finding soils that excited them. Until they were driving along Peachy Canyon Road one afternoon in 1989, saw one of the many switchbacks where CalTrans had dug into the hillside to make the roadbed, and pulled over to see if the white rocks that they noticed were really what they'd been searching for. The composition looked right, the fossils looked right, and they then brought over a French geologist to confirm their impressions. They put in an offer on the property where we are now later that year.
We've thought since the beginning that finding calcareous soils would be a key to making great wines. Learning the science behind why only underscores the importance that the vineyard's founders put on this search.
Thanks to Dr. Thomas J. Rice, Professor Emeritus of Soil Science at Cal Poly, for pointing me in the right direction on some of the trickier geology questions. See also:
- "Calcium in Viticulture: Unraveling the Mystique of French Terroir, Part I and Part II" by Valerie Sexton, Wine Business Monthly. Originally published September/October 2002.
- "Effects of juice pH and potassium on juice and wine quality, and regulation of potassium in grapevines through rootstocks (Vitis): a short review," by S. Kodur, Department of Agricultural Sciences, La Trobe University, Bundoora, Victoria, Australia, 2011.
- "Effect of rootstock on must and wine composition and the sensory properties of Cabernet Sauvignon grown at Langhorne Creek, South Australia," by R. Gawel, A. Ewart and R. Cirami. Wine & Viticulture Journal, January 2000.