The Adelaida District's Limestone Layers Laid Bare

On Friday, Winemaker Neil Collins poked his head into the office we share, looking excited, and said, "Hey, you got a minute?" I did, and we hopped into the ATV and Neil piloted us south across the creek and along the south side of the reservoir the property's previous owner made back in the 1950s. He stopped near the dam, and we headed out on foot. What he had found was remarkable: that Las Tablas Creek had become an exhibit for the local geology.

This has been a banner year for Las Tablas Creek. After three years where it barely ran, the series of storms that we got in late December and early January got it flowing fast:

During a break in the storm Viticulturist Jordan Lonborg got down to shoot this video of Las Tablas Creek… and it’s moving fast, has filled up our reservoir and is now flowing through the spillway for the first time since early 2019! pic.twitter.com/mTWzT1o9hJ

— Tablas Creek (@TablasCreek) January 5, 2023

A week later, after another storm had dumped six more inches of rain in about 24 hours, it burst its banks and flowed over Adelaida Road just outside the winery, producing impressive enough footage that it made it onto several national networks as an illustration of the widespread California flooding:

All that water flowing down the creek and into (and out of) the reservoir changed the landscape in in visible ways. In the creekbed it's clear how high the water came because everywhere below that line it scoured away the topsoil and exposed the limestone layers underneath:

At the far end of the lake is a spillway through which the water flows once it has filled the reservoir. It's a remarkable illustration of the local geology. Most of the calcareous soils that underly the Adelaida District are soft, as much clay as rock, which has given rise to the (incorrect) theory that none of it is limestone¹. While it's a good thing that we don't have solid limestone underneath us, as limestone is too hard for vines' roots to break up or break through, there are bands of limestone that run throughout the region. The previous owner made use of one of these layers in the creation of the spillway, which follows the slanting descent of the layer from dam-level down to the original creekbed. Here are two views: on the left from above the spillway in late January, and on the right from below last week:

In both photos, though most clearly from below, you can see the many layers of softer rock that the water has eaten away over the years, while staying above the harder limestone layer. 

A side-stream that flowed into the creek showed another good example of the mix of harder and softer calcareous layers, and the step-like pattern that is repeated in creekbeds throughout the region:

While the softer layers crumble and decompose, the harder limestone bits stay in the topsoil. Most wineries remove them before planting anything, as otherwise they chew up tractors at an alarming rate. The rocks that are removed are the raw materials for the walls you see at Tablas Creek and around the Adelaida District:

All this rock is sitting there year-round, just a few feet below the surface. Thanks to the rain we've received (and continue to receive) this winter it's easier to see than ever. 

Footnote:

1. If you're interested into a deep-dive into the chemistry and geology of the calcareous soils out here, check out my 2020 blog Why Calcareous Soils Matter for Vineyards and Wine Grapes.

Why Calcareous Soils Matter for Vineyards and Wine Grapes

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.

Further Reading:
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.

Celebrating 11 New AVA's in Paso Robles

At long last, nearly seven years after it was submitted to the TTB (the Tax and Trade Bureau -- the office of the federal government that oversees wine regulation) we received news this week that the petition from the Paso Robles wine community to establish eleven American Viticultural Areas (AVA's) within the current Paso Robles AVA has been published for comments.  This is the critical step called a "notice of proposed rulemaking" at which the TTB (tasked, among its many other responsibilities, with protecting the public from misleading or confusing information about wine) has reviewed all the geological, climatological and historical information presented in the petitions and determined that they pass muster.  It doesn't mean that the region can start using them on wine labels this week, but it's an important validation of the proposed AVA's and boundaries, and the last step before final approval.  The map, as published for review, is below.  Click on the image for a larger version or here for the official PDF: 

Over the next 120 days, interested parties (which, in this case, means pretty much anyone) can submit a comment at the TTB Web site in support of or in opposition to the plan.  I'm hopeful that with all the hard science that went into the petitions, and the broad cross-section of the Paso Robles wine community that was involved in their submission, approval will be relatively straightforward.  The Paso Robles AVA Committee included 59 different grower and winery members -- including Tablas Creek -- from every one of the proposed AVA's. 

For the Paso Robles region, the publication for review of our AVA petition is an important and necessary milestone. Paso Robles is currently the largest un-subdivided AVA within California at approximately 614,000 acres. By contrast, the Napa Valley appellation (which includes sixteen AVA's delineated within its bounds) is roughly one-third the area at 225,000 acres. When the Paso Robles AVA was first proposed and approved back in 1983 it contained only five bonded wineries and less than 5000 planted acres of vineyard.  Big swaths of the AVA, including the area out near us, were largely untouched by grapevines.  In the last thirty years, Paso Robles has grown to encompass some 280 wineries and 32,000 vineyard acres.  This vineyard acreage is spread over a sprawling district roughly 42 miles east to west and 32 miles north to south.  Average rainfall varies from more than 30 inches a year in extreme western sections (like where Tablas Creek is) to less than 10 inches in areas farther east.  Elevations range from 700 feet to more than 2400 feet.  Soils differ dramatically in different parts of the AVA, from the highly calcareous hills out near us to sand, loam and alluvial soils in the Estrella River basin.  The warmest parts of the AVA accumulate roughly 20% more heat (measured by growing degree degree days) than the coolest; the average year-to-date degree days in the Templeton Gap since 1997 is 2498, while in Shandon far out east it's 2956.  This difference in temperatures is enough to make the cooler parts of the AVA a Winkler Region II in the commonly used scale of heat summation developed at UC Davis, while the warmest sections are a Winkler Region IV.  This is the equivalent difference between regions like Bordeaux or Alsace (both Winkler II areas) and Jumilla or Priorat (both Winkler IV areas). 

[A quick aside. The southern Rhone is classified as a Winkler III region... and the fact that our proposed Adelaida District is a transitional Winkler II/III jibes with our experience that the same grapes ripen here slightly later than they do at Beaucastel.]

Our region's diversity was well noted in the TTB's ruling.  In addition to the longhand descriptions of each region's soils, climate and topography, the TTB included side-by-side comparative charts -- unique, in my experience of AVA approvals -- that detailed why each new AVA was worthy of being distinguished from its neighbors.  I can't imagine anyone reading these petitions and concluding that there wasn't grounds for subdivision.

All this is not to say that Paso Robles doesn't share some important factors, and one important hurdle that the petitions had to clear was demonstrating that the region enjoyed sufficient macro-level similarity to remain an AVA.  The TTB's ruling recognized several characteristics that the entire region shares, including the 40-50 degree diurnal temperature variation, the relatively warm climate with limited incursion of marine air, and the moderate rainfall, less than the slopes of the coastal mountains but more than the arid Central Valley to our east. 

The AVA system is so powerful exactly because it has the flexibility to recognize macro-level similarities as well as important micro-level distinctiveness.  Think of France: that Burgundy shares overarching characteristics doesn't mean it's of no value to distinguish Chambertin from Meursault, or Chassagne-Montrachet from Volnay.  The appellation system, at its best, gives consumers both a broad-level understanding of what grapes will grow best and what character they should expect from the region's soils and climate. 

One risk in the creation of new AVA's within an existing one is that the existing AVA -- into the marketing of which the local wineries have invested enormous amounts of time and money -- will lose much of its significance as many wineries abandon that appellation name to make a name for their new, smaller one.  Happily, Paso Robles won't lose its identity -- or the accumulated marketing capital we've all built over the last three decades -- thanks to a conjunctive labeling law passed by the California assembly with the encouragement of the Paso Robles Wine Country Alliance in 2007. Conjunctive labeling means that wineries who choose to use one of the new AVA's will also be required to use "Paso Robles" as significantly.  This law was modeled on one passed for the Napa Valley in 1990 that has been widely credited with helping maintain Napa as the most powerful brand in American wine. 

The continued presence of Paso Robles on wine labels does not diminish the impact of having the different AVA's approved. These new AVA's will be a powerful tool for wineries to explain why certain grapes are particularly well suited to certain parts of the appellation, and why some wines show the characteristics they do while other wines, from the same or similar grapes, show differently. Ultimately, the new AVA's will allow these newly created sub-regions to develop identities for themselves with a clarity impossible in a single large AVA.

It's worth pointing out that no one needs to use the new AVA's.  Wineries who wish to continue to use only the Paso Robles AVA are welcome to.  And many will likely choose to do so as the new AVA's build their reputation in the market.  Not all the AVA's have a critical mass of established wineries, and it seems likely that a handful of the new AVA's will receive market recognition first, while the reputation of others will take time to build.  But I believe that it will be several of the currently less-developed areas that will benefit most in the long term, through the ability to identify successful winemaking models and build an identity of their own.  We shall see; having a newly recognized AVA is not a guarantee of market success, just a chance to make a name for yourself.  The cream will rise to the top, and consumers will benefit.

In which we dig ourselves a hole, on purpose

By Levi Glenn

It's been a little over a year since our purchase of our new parcel.  The property is just to our south: 150 acres of rolling oak woodland, a walnut orchard (now removed), and a fair amount of the creek from which we take our name. There are probably only sixty plantable acres and the rest will be left in its natural state. And while there's nothing visible above-ground yet, we're making progress toward planting this beautiful piece of land. The first stage was to find out what we have below-ground, and what we found confirmed our belief that this is indeed going to be a great piece of vineyard.

We knew there were rocks. Lots of rocks, but more importantly white rocks. Limestone rocks. Just how many of these rocks? How does one find out?  Invite 13 aspiring soil scientists come to your soon-to-be vineyard and dig a bunch of holes with a backhoe. Using this process, these students turned holes in the ground into this beautiful multicolored soils map:

Before I get too far along I would like to send out a big thank you to the Cal Poly Soil Resource Inventory 431 class of Spring 2012, along with the enthusiasm and guidance of Dr. Thomas J. Rice. They found a lot of rocks. (They also presented their findings to us in a professional and succinct manner that should make their professor and university proud.)

The main tool a soil scientist has is a soil pit. They dug 41 different soil pits -- typically straightforward holes in the ground 5-6 ft. deep -- across the new property. Grapevine roots can reach down 30 ft., but a 5-6 foot pit gets you the majority of the root mass. Then you assess the layers (technical term: horizons) in the soil. To give you a sense of how we use this data, let's look at one soil pit in the Calodo series. A photo is below, followed by its soil analysis.

The team identified three distinct horizons in the pit: Ap (the top 20 centimeters), Bk (the next 26 centimeters) and Crk (the next 44 centimeters). Below the Crk horizon the team found bedrock. Each horizon is identified by composition, color, texture, plasticity, and pH. Here are the details:

Ap— 0 to 20 cm (0 to 8 in.); gray (10YR 5/1) gravelly clay loam, very dark grayish brown (10YR 3/2) moist; moderate medium granular structure; moderately hard, firm, sticky and plastic; common very fine and fine roots; violently effervescent, many nodules (20.02% CaCO₃); slightly alkaline (pH 7.44); clear wavy boundary.

Bk— 20 to 46 cm (8 to 18 in.); gray (10YR 5/1) very gravelly clay, very dark grayish brown (10YR 3/2) moist; moderate medium granular structure; slightly hard, very friable, sticky and plastic; common very fine and fine roots; violently effervescent, many nodules(32.77% CaCO₃); slightly alkaline (pH 7.62); clear wavy boundary.

Crk— 46 to 90 cm (18 to 35 in.); fractured limestone (59.48% CaCO₃); moderately alkaline (pH 8.07).

If you're wondering about the term "violently effervescent", it refers to how a soil scientist tests for calcium carbonate, or CaCO3. When testing a soil for CaCO3 levels, you pour Hydrochloric Acid on the rocks and if they start to bubble, their calcium carbonate content is sufficiently high to qualify as limestone.

Summarizing the information above, you can see the increasing clay and CaCO3 concentration as you go down away from the surface, until you ultimately hit the bedrock. This continuum traces the transition from the surface -- where you're likeliest to find organic matter -- to bedrock, which is nearly 100% limestone.  Even better, most of the rock fragments are small pieces of calcareous shale that are easily broken apart by grapevine roots.

For us, the highlight of the above technical information is one number: the 59.48% CaCO3 in the Crk horizon. I have never seen another soil with this high a CaCO3 percentage. CaCO3 is the chemical composition for limestone, the white rock that is so well suited for wine grapes. [Read the Why limestone matters for wine grape growing post from 2010 if you'd like a refresher on its importance.] The Calodo soil series has the highest concentrations of CaCO3, and the Linne soil series also has high concentrations, but tends to be deeper with more clay. These two soils make up the main ridge on our new vineyard property, the teal and yellow colors on the soil map at the top of the page.

There are a total of 8 different soil types that the research team found. They vary widely, from rocky limestone to deep alluvial clays. This will allow us to match each soil type to different varieties. Grenache, for example, is capable of surviving in extreme drought conditions, which help to tame its often excessive vigor, so it's suited to rocky limestone-strewn hilltops like ours, pictured below. 

Roussanne on the other hand needs a little more nutrition and would prefer a little more moisture, so it will likely be suited to some of the flat lowlands (think the green lower-lying areas toward the outside of the propery) that have a more clay and better water retention. Ultimately this gives us more information to make better choices when it comes time to plant.

This ridge is first place we are going to plant on the new property. Grenache and Mourvedre are the most likely candidates. We typically assume that the tops of our hills produce the best grapes because of the low yields that the difficult, rocky soils enforce, but hilltops also have the advantage that they won’t freeze. Anywhere there is a slope, cold air drains downward, to be replaced by warmer air from above. Last year I recorded a 10 degree temperature difference from the top of this hill to the bottom. Planting should start in 2014 if all goes to plan. We will start with 5-10 acres and plant a little bit more each subsequent year.

The crew is eager to get started planting, but the day-to-day farming of this property will present its own challenges. We have already ripped the soil to break up compaction, but in doing so we brought an immense number of large rocks to the surface. Those had to be removed before we seeded the hill with cover crop. We know we'll continue to battle the rocks since any time we cultivate it brings more of them to the surface. But the sheer steepness of the property will be the hardest thing to deal with. With slopes from 25-45% on over half of the hill, it will take our most seasoned tractor drivers to tackle this terrain. You can see below the topographical map. The closer the contour lines are, the steeper the slope:

We were fortunate to have not just one soil expert but 14 of them to help us navigate the complexities of our new property. Thank you to Dr. Rice, all your students, and Cal Poly for putting so much time and effort into this project.

Soil Scientists: Samuel Bachelder, Gregory Beaudreau, Eric Boyd, Michael Founds, Laurie Fraser, Aaron Keyser, Jeanette McCracken, Stephen Nolan, Scott Pensky, Natalie Rossington, JaquelineTilligkeit

Soil Scientist and Lead Editor: Emilie Schneider

Project Leader: Thomas J. Rice, Ph.D., C.P.S.S.

Tablas Creek 101: Why We Are Where We Are

By Robert Haas

In the early 1970’s Jean-Pierre Perrin accompanied me when I was visiting my then Napa Valley suppliers of Vineyard Brands: Freemark Abbey, Chappellet, Clos du Val and Phelps.  We took a little tour of the valley and he remarked that it was extraordinary that, in a climate that seemed extremely apt, there were none of the grape varieties traditional to the Rhône.  It was this experience that was the beginning of the idea of doing it ourselves.  However, we were pretty busy developing our own businesses at the time and the idea, never forgotten, got shelved for a while.

In 1985, we decided to give it a try and began seeking a proper location.  We were looking for three things: a Mediterranean climate similar to that of the southern Rhône, enough annual rainfall to grow grapes without having to irrigate regularly, and maybe most importantly – certainly most restrictively – calcareous (chalky) clay soils similar to those of Beaucastel.

As we had noted in the 1970’s, much of coastal California has a classic Mediterranean climate.  But more specifically, we wanted an area with both a long growing season to ripen late-ripening grapes like Mourvèdre and Roussanne and moderating influence (like elevation, nighttime cooling or fog) that could keep the earlier ripening Rhone varieties like Syrah and Viognier from becoming heavy and flabby.  Paso Robles has the largest day-night swing in temperature of any wine growing region in North America, typically 45 degrees and often more.  (This week, we’ve had several days with 55 degree swings: daytime highs over 100 and nighttime lows in the 40’s!)  The proximity of the cold Pacific Ocean, which never gets much above 60 degrees even in mid-summer, and the dry summer climate that allows the day’s heat to radiate off at night provide cooling, while the relatively unbroken 3000-foot high Santa Lucia mountains protect the region during the daytime and allow it to warm up.

Even better, the further south you go in California, the later the onset of the winter rainy season.  We typically get our first serious winter rainstorm in the middle of November, two weeks later than our colleagues in Napa and a month later than those in Sonoma.  The extra weeks matter in cool years, and we have harvested into November five of the last seven vintages. 

The lower limit for dry-farming grapevines is about 25 inches of rain annually.  We get that much here thanks to our 1500-foot elevation and our location in the Santa Lucia foothills just 11 miles from the Pacific.  Pacific storms are pushed up into cooler air as they cross over the mountains.  This cooler air can carry less moisture, so the clouds drop it as rainfall.  Our average rainfall of about 28 inches is double what the town of Paso Robles receives just ten miles further east and 800 feet lower.  We figure we’re just about the most southerly location in California (and one of the warmest) to receive this much rainfall.

Finally, and we thought most importantly, we felt that chalky soils would give us, as they do in the southern Rhône Valley,  healthier vines with better water retention, better nutrient availability, healthier root system development and more disease resistance (for a full discussion of the qualities of chalky soils, see the 2010 blog post Why Limestone Matters for Grape Growing). We found our spot after four years of searching: 120 acres of rugged hilly terrain in the foothills of the Santa Lucia mountains.  We bought it and that was the birth of Tablas Creek Vineyard.

But why are there chalky soils here?  The story begins sixty-five million years ago, toward the end of the Cretaceous period, when the fearsome Tyrannosaurus Rex ranged the land.  At that time, the continental United States was largely covered by shallow seas, with only the Appalachian and Rocky Mountain ranges on dry land.  What are now the central plains and the southeast, and both the east and west coasts (including what is now Paso Robles) were under the ocean.  A map from the USGS shows it well:

The Pacific plate was then, as now, moving east and colliding with the North American plate, pushing up the land that now forms the western shore of the North American continent.  (A fascinating series of maps on the Paleogeography and Geologic Evolution of North America can be found at https://deeptimemaps.com/global-paleogeography-and-tectonics-in-deep-time/)

The Cretaceous period, starting about one hundred forty million years ago and ending sixty-five million years ago, was the earth’s most active period of chalk formation.  Chalk is principally calcium carbonate.  The circulation of seawater through mid-ocean sea ridges during the Cretaceous – a time when mid-ocean ridges were unusually active – made Cretaceous oceans particularly rich in calcium.  This richness stimulated the growth of calcareous nanoplankton, the tiny sea creatures whose calcium-rich skeletons settle to the sea floor and eventually accumulate to form chalk and its metamorphic relations limestone and marble. It was during the Cretaceous that the chalk cliffs of Dover and the chalky hillsides of Champagne and the Loire were formed; as were the chalky clay soils around Tablas Creek in west Paso Robles.

The seas began to recede toward the end of the Cretaceous and continued through the Tertiary, driven initially by the slower growth of mid-sea ridges and a cooler climate that would eventually trap large quantities of water in glaciers.  The sedimentary chalk-rich rocks that had been laid down in the Cretaceous were exposed throughout the middle portions of the United States.  In California, seismic activity pushed up calcareous soils in only a few places, principally along the Santa Lucia Mountain range, where these soils were folded and intermixed with older continental and volcanic soils.  Much of these soils are west of the coastal range, in climates too cool to ripen Rhone varieties.  It was our good fortune that one large exposed chalky layer was east of the coastal range, in west Paso Robles and Templeton. 

So, all three components come together here in Paso Robles.  Chalky soils sit at the surface.  We typically get enough rain to farm without having to irrigate.  And it warms up enough to grow the grapes we love, while staying moderated enough to keep them in balance.

Why Limestone Matters for Wine Grape Growing

[Editor's Note June 2020: I've written a new version of this blog, taking into account new research and correcting a few areas that I've come to believe were inaccurate or incomplete. Please see Why Calcareous Soils Matter for Vineyards and Wine Grapes.]

Winemaker and Vineyard Manager Neil Collins with a handful of broken calcareous rock

It has long been recognized that great wine regions such as Champagne, Burgundy, Chablis, the Loire and southern Rhône valleys, and Saint-Emilion in Bordeaux are rich with limestone.  Or, more precisely, these soils are rich in plant-accessible calcium carbonate, the principal chemical component of limestone, typically from decayed limestone outcroppings.  (Limestone itself is too hard for plants' roots to penetrate.) 

Limestone is 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.  When we were searching for a site on which to plant our vineyard, finding calcium-rich soil similar to that of Château de Beaucastel was a primary criterion. That calcium-rich soils were only found in the Central Coast focused our search in this area.  The west side of Paso Robles and Templeton is the state's largest exposed limestone layer, and in 1989 we bought our property here.

For all the anecdotal evidence of the superior qualities of calcium-rich soils, the science behind how calcareous soil influences grapevine health and the wines that come from them is still being explored.  It turns out that there are four principal reasons why these soils improve wine quality.

In winter, the calcareous clay absorbs moisture, turning dark.  Note the roots that have pene- trated between the layers of clay.

Water-retention capabilities
Calcium-based soils 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.

Soils that are less able to retain moisture must be irrigated.  Drip irrigation creates a funnel-shaped wet area in the topsoil immediately under the dripper.  As cation exchange cannot occur in the absence of moisture, the only roots that are taking up nutrients are those within the dripper zones.  And as those zones are in topsoil rather than in deeper soils, it is clear why French regulations prohibit the use of irrigation in the top terroirs in France: wines from those vineyards would not be able to express their terroir.

At Tablas Creek, we have become more and more convinced that dry farming is perhaps the most important aspect to producing wines of place.  And our calcium-rich soils mean we can dry farm, even though it almost never rains between April and November.

Cation exchange and berry pH
Calcium-based soils tend to be more basic than soils derived from other nutrients.  The soil pH of the calcareous layers at Tablas Creek tends to be around 8, much higher than the typical topsoil pH of between 5.5 and 6.0.  Research has shown that cation exchange is greater at higher levels of base saturation, perhaps because most of the minerals that grapevines require are at their most accessible when soils are more basic.  

High-calcium soils are also correlated with easier nutrient uptake. The nutrients a grapevine needs to thrive (magnesium, potassium, calcium, and sodium) are taken up at certain specific sites on the root hairs through the process of cation exchange. Negatively charged compounds in plant roots attract positively charged cations.  Calcium helps soil particles aggregate through a process called flocculation, which helps make available more cation exchange sites to a plant's roots. 

In low-pH soils, Hydrogen ions start to displace the ions of the four principal nutrients.  Only above a pH of 6.0 are all four nutrients readily available. So, calcium carbonate acts as a buffer (it has been used for centuries as an antacid) and counteracts the acid created in the breakdown of organic matter in topsoil.  The end result is a a pH level at which nutrient availability is at its highest.

Finally, there is increasing evidence that soils rich in calcium help maintain acidity in grapes late in the growing season.  A researcher at the University of Bordeaux linked the healthy cation exchange processes in soils rich in calcium -- and with enough water -- to higher grape acidity and lower wine pH.  And we have good anecdotal evidence of this property.  At the symposium on Roussanne that we conducted two summers ago, producers from eastern Paso Robles, where calcium-rich soils are much rarer and rainfall consistently less, consistently reported harvesting Roussanne roughly half a pH point higher than those of us on the calcareous (and wetter) west side.

The limestone-rich layers of the mountain behind the winery shine bright white in mid-summer

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.  Calcium's tendency toward flocculation (soil particle aggregation) creates spaces into which roots can penetrate and in which water can be stored.  This quality is particularly important with clay, and clays high in calcium tend to offer better soil structure and less mechanical resistance to roots than those without.  In addition, in long periods of dry weather, the clays dry out and crack, allowing roots to penetrate deeper into the soil where more residual moisture can be found. And even in our vineyard (relatively young in vineyard terms) we've found roots ten feet deep and deeper in experimental excavations.

In addition, soils where calcium is scarce -- like those where water is unavailable -- tend to show excessive exploratory root growth, and may have large, inefficient root systems that support small, relatively weak growth above ground.  And this makes sense: vines have a certain amount of energy to apportion between root growth, canopy growth and berry ripening.  If they are forced to invest more energy in searching for calcium there is less available for other tasks.

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.

We've thought since the beginning that limestone was a key to making great wines.  It's great to learn the science that underpins our belief.

Anyone who is interested in the more detailed science behind this article should read the two-part piece by Valerie Saxton in Wine Business Monthly, from which I drew heavily for this article.  You can find Part I and Part II online.