For the last twenty years, Tom Rice has led a double life. As an energetic yet mild-mannered professor of soil science at California Polytechnic Institute in San Luis Obispo, California, Dr. Rice has supervised over 50 detailed senior projects and Masters theses which detail the soils and environment of the San Luis Obispo environs, including soil maps of well known vineyards in both the eastern and western regions of the Paso Robles AVA. The soil science program at Cal Poly, which Dr. Rice helped develop, consistently attracts the largest soil science undergraduate enrollment in the United States.
In his other life, Dr. Rice works as a private consultant and adventurer, braving cultural unrest to speak in Xi’an, the People’s Republic of China on watershed management; clambering through hot, rattlesnake hills to study the viability of planting vineyards over the oil fields of Texas, escorting scientists through the Mojave Desert, and researching the insidious incursion of mercury contamination in the Lake Nacimiento basin of coastal California. In September 1998, Dr. Rice presented a talk at the University of Florence, Italy, on “Vineyards on Limestone Soils in California and France.”
The book begins . . . “It was not a terribly auspicious beginning for a world class wine country. For brevity’s sake, we’ll only go back 20 million years. The Tertiary Period was when most of our landmass was forming. There followed a number of epochs; the Pliocene era (2 to 5 million years ago) was the California Coastal Ranges mountain-building era. The San Andreas Fault was fracturing during the Miocene era (5 to 24 million years ago) and it has been a real inconvenience ever since.”
An introduction describes the terroir as seen through the eyes of early ranching, farming and winegrowing families. Chapter one addresses environmental contrasts between the east side and west side, and general issues of water quality, wildlife habitat and native plants.
The remaining chapters are organized by region: Templeton Gap, Adelaida Hills, Salinas River Valley Terraces West, Estrella River Terraces, Salinas River Terraces Southeast, and the Creston/Shandon Area. Terraces West would include the San Marcos drainage area, and the Dusi Ranch vineyards. The Southeast Terraces are possibly two separate regions—Santa Margarita and Huerhuero River Terraces.
Dr. Rice adds, "Although there are not many wineries in that region, there are big vineyards, with lots of geologic and soil variation. This region encompasses vineyards as diverse as Wild Horse, Maloy O’Neill, and Chateau Margene. We once had redwood forests here, and huge flood plains which were uplifted into river terraces. There’s been a lot of undercutting as layers were forced underneath the plains, so in the high plateaus and orchards of the El Pomar region, the eroded tops of the highest points may be all calcareous shale, with underlying alluvium and a veneer of Paso Robles formation . . . sand, gravel, and clay."
I have been working on this book for twelve years; four years for my writing partner, Tracy Cervellone. She is handling the cultural history aspect of the book, and doing a fantastic job. We hope it will be a book that anyone interested in the region or winemaking will find interesting. I have been “digging holes” throughout Paso Robles for twenty years, and the book includes research from my own projects and students’ projects which I have supervised.
Would you describe the broad geological differences between the central coast and northern California? You mentioned previously that the San Andreas Fault curves off into the Pacific south of Napa, resulting in more granitic Continental shelf rock in Napa, whereas Paso Robles is almost entirely sea floor. Are there other differences between north and south California?
There is a huge geological difference between the two areas. Napa is part of the North American plate, which is an expansion zone, moving slowly southward; it features volcanic, gravel, and shale layers. The coastal splinter of California from the San Francisco Bay to Baja is a compression zone. It is being forced against the NA plate and is moving slowly north, grinding against the plate like cheese against a grater.
San Andreas is more than just an inconvenient fault. The surface terrain of western California is shaped by folding and faulting, but much of the shoreline is created as various layers of the Pacific plate slide under the North American plate. As these plates are pushed under the continent, shavings of ancient coral and marine layers curl upward, creating new California coastline, and simultaneously pushing against it. This effect is referred to as the San Andreas system, and is responsible for the parallel cracks and faults in western California.
The Pacific Plate is comprised of ancient seabeds and shallow coral beds. After being pushed up against the NA plate, often shallow seabeds formed again and evaporated on top of it, resulting in the rich marine layers and fossils we find in Paso Robles. The central coast region was once like a huge Puget Sound, with embayments of shallow ocean life formed by the grinding pressure between different directions (north, south, up, down) and populated with ancestral marine sealife and mammals like whales.
How would you compare Paso to other wine geo-regions throughout the world?
There are embayments in France and Italy similar to west Paso—in Tuscany, the Rhone and Bordeaux regions, even Burgundy. Also the southern Adelaida region of Australia. They are all mainly marine sediments that have weathered into soils.
In that case, are there differences between central coast and northern California soils?
In terms of soils, all of the California winegrowing regions have a variety of soils, ranging from alluvial river deposits and eroded sedimentary layers, to ancient calcareous seabed deposits. But Paso Robles has a stronger profile of Monterey and Paso Robles formations, which feature calcium carbonate (limestone) and calcium silicate (mudstone). Sonoma is on a small wedge of the Pacific plate, but it features more of the igneous, granite and quartz of the NA plate; and Napa has more volcanic diabase.
Napa soils have more moisture, and more weathered, acid soils with more organic matter. They have low to non-existent calcium carbonate. They have a typical pH of 5 to 7.
Paso Robles is drier and the soils have less biomass, less organic matter. They’re more alkaline, with a higher pH of 6 to 8.5.
In Napa, the northern exposures receive more rainfall and have more humus and riparian growth; they’re more heavily planted to vineyards, while the southern exposures are drier and more sparsely planted.
In Paso Robles, the western exposures are wetter and richer; the eastern hillslopes are drier and shallower.
How do soils in the two different regions react to rainfall and extreme temperatures?
Paso Robles soils have more porosity, due to their pre-calcareous structure. This structure holds the soil open and allows for moderate to extreme drainage. Irrigation practices and the loss of nutrients through leaching are often a prime concern for Paso Robles growers.
Alluvial soils pose a different problem. Clay is heavy and is often washed down into basins, fans and valleys. Clay has a “plastic” quality—when extremely wet it becomes like liquid plastic and it can “drown” vines because it does not allow a flow of oxygen to the roots. Essentially, in flooding conditions, the vines will suffocate. Conversely, when it dries it can shrink aggressively and crack. Shrinking clay has even been known to snap off roots. That may actually be a good thing, sort of a self-pruning mechanism, particularly in old vine vineyards. Fortunately, the interwoven pre-calcareous content throughout Paso Robles creates plenty of drainage. Napa has shallow soils, which are good for vineyards, but their alluvial and clay soils rest on solid volcanic and granitic beds, which obstruct downward drainage. Soil, rootstock and scion choices in each of these areas will be entirely different.
What are the basic differences and similarities between the terrains of east Paso vs. west Paso?
Soils planted to vineyards west of the Salinas River are mainly derived from the Monterey Formation, which is composed of shale, mudstone, siltstone and limestone. They are both calcareous (carbonate-rich) and siliceous (silica-rich).
Soils planted to vineyards east of the Salinas River are derived from a wider variety of parent materials. The river bench areas are mainly alluvial sediments from the Paso Robles Formation and have varying depths of clay, gravel and sand. Compacted subsoil horizons often restrict downward water flow and limit vine root growth. However, there are also many upland hillside soil areas on the east side which are derived from the Monterey Formation and resemble the soils on the west side of the river.
There’s more homogeny on the west side; soils on the east side are more diverse with no single dominant soil type.
What is your response to the charge that there is no limestone in west Paso Robles?
Limestone does exist throughout Paso Robles, but not in huge formations like boulders or cliffs as in parts of France. On the east side, it’s usually found at depths of five feet or more. These strongly effervescent lime layers exist closer to the water table and may take the form of a stickier subsoil. Western Paso Robles is mostly porcelainous (silica-containing) mudstone and calcareous shale, but there’s also dolomite and other carbonate rocks. Limestone layers in west Paso Robles exist, but in thin one or two-foot thick layers. Because of the more severe topography you can often find layers of limestone on eroded ridgetops, and the boulders you see dug up for walls are often chunks of limestone. They are sometimes displaced and sandwiched between porcelainous beds. Parts of Paso Robles have been so folded they look like an omelet.
What are the black soils found in streaks in the eastern El Pomar district?
I suspect these shallow streaks of rounded, black stones and earth are simply carbon-rich layers formed by old, but not ancient, lakebeds and streambeds.
A winemaker recently said that high acid, low pH soils (6.1 to 7.0) result in high acid, low pH wines. Do you know if this is true?
This is a somewhat simplistic correlation. I think it is probably related more closely to levels of potassium. High acid soils are often associated with low potassium as well as low pH, and this would probably create wines with higher acidity.
Deep clay soils, which exist in both Paso Robles and northern California, have higher moisture content and anecdotally lower acidity, lower Brix and higher moisture in the grapes. Shallow, rocky soils result in higher acidity and higher Brix, lower moisture, and lower yields. Therefore, it’s likely that you’ll get both higher alcohol and higher flavor in shallow soils.
If you were going to plant a vineyard and you could choose any site in Paso Robles, where and what would you choose to plant?
It would be so hard to choose! There are so many good places, and such diversity. It’s just a question of matching soil properties with rootstock, choosing a scion to match the climate, and appropriate irrigation choices.