Central Coast Wine Blogs

An interview with Thomas J. Rice: Author of 'Paso Robles: An American Terroir'

 

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.

 
 
An aerial view of Halter Ranch vineyards

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."

Cover Cropping: Naturally Maintaining Soil Health

 

This spring cover crop is an explosion of purple and yellow as mustard and pea vetch enjoy the soft rains and early warmth. 

Driving through wine country in the spring, you may see grass and clover growing between the vineyard rows.  Weeds?  Maybe, but more likely what you are seeing is a cover crop.  Many cover crops are annually reseeding grasses or clovers.  These are generally mown 1-3 times throughout the spring and the green thatch is left on the ground as a natural mulch and nutrient base for soil organisms.  In late spring or early summer, the crop is allowed to set some seed, and then the whole crop is plowed under so that it doesn't compete with the vines during the dry summer months.  Some vineyards, particularly those with erosion problems, may use perennial grasses which form a thick sward that holds the soil in place, and also provides traction for farm equipment in wet weather. 

  

Many vineyards utilize a 'natural mix' cover crop of rye, barley, clover, pea vetch,and mustard.  This ensures that no matter what the growing conditions are—hot, cold, wet, dry—the cover crop will have a healthy head start.  Diversity as insurance.

Annual cover crops are mown several times during the spring. Crews leave the mowings on the ground as mulch and green manure to improve the tilth, biodiversity, and moisture-retaining properties of the soil. The cover crop is then tilled under in the summer after it has gone to seed, and the ground is neatly ring-rolled. Bare summer earth prevents moisture competition while the grapes are developing, and keeps the vines warmer. A lack of summer vegetation also discourages gophers. The cover crop begins growing again during the fall rains.  The new growth prevents erosion, and by spring provides ready habitat and nectar for predator insects, when many insect pests are at their most prolific.

Mustard is known for its ability to absorb excess salts and return them to soil in organic mineral form.  Clovers and vetch are nitrogen fixers, and add high volumes of nitrogen-rich biomass back into the soil.  The cereal grasses—rye and barley—anchor the soil, are drought tolerant, and provide high quantities of lightweight fiber.  Blossoming plants like the mustard and vetch attract beneficial insects. 

By returning green matter back to the soil each year, vineyard owners maintain the diversity and health of the soil and its water retaining properties.