SOILS AND WINE GRAPES OF THE NAPA VALLEY AVA* Paul W. Skinner, Ph.D. Soil Scientist Terra Spase, Inc. Introduction Soils have long been associated with the distribution of quality vineyards and the production of premium quality grapes and wines in both new and old world viticultural areas (Wilson, 1998, Coombs and Dry, 1988, Johnson, 2001). The gravelly soils of Bordeaux, the rocky alluvial fans of the Rhone and Alsace, the slate soils of the Mosel, the chalk soils of Champagne, and the terra rosa soil of Australia, are all well known examples of soils associated with premium viticultural areas and wine grape production. Soil type has also played an important role in past vineyard developments within the Napa Valley AVA (American Viticultural Area). Increasingly, soil type is influencing the planting of premium wine grape varietals within the Napa Valley AVA, as growers consider its effect on vine rootstocks, spacing, trellis configuration, and overall vine vigor. The soils of the Napa Valley AVA exhibit a wide range of unique characteristics that allow for the production of many different premium grapes and wines. This report will examine properties that characterize the soils of the Napa Valley AVA and will attempt to elaborate upon their connection to the existing potential for the production of premium quality wine grapes there. A brief review of some of the report’s findings follows below. Chapter I. The Formation and Description of the Soils in the Napa Valley AVA The soils of the Napa Valley AVA are complex expressions of the climate, biology, topography, and geology that developed within the area over a long period of time. There are a total of 33 soil series mapped within the boundaries of the Napa Valley AVA (NRCS Soil Survey, 1977). A soil series is the basic unit of soil classification and consists of soils that are alike in all major profile characteristics except the texture of the surface layer. These 33 soil series represent 6 of the 12 soil orders that comprise modern soil taxonomy. This fact alone describes an incredible diversity of soils within a relatively small area. The soils of the Napa Valley AVA are both varied and variable throughout the region. Texture, depth, drainage, organic matter, and rock fragments all exhibit considerable ranges across the Napa Valley AVA soil landscape. The mosaic of soil series reflects the complexity of the factors of soil formation which are present in the Napa Valley AVA and which set the area apart from many other grape growing regions of the world. Chapter II. The Distribution of the Soils in the Napa Valley Appellations Overview There are currently fourteen individual appellations within the Napa Valley AVA. This section provides detailed maps showing the distribution of soil types and their characteristics within these fourteen appellations. Individual maps of soil series distribution, elevation, shaded relief, depth to bedrock, drainage class, hydric soil status, estimated soil organic carbon, and a NDVI (normalized difference vegetation index) are included. Three individual AVA offer interesting snapshots of the region. Carneros The soils of the Carneros region form a patchwork across the landscape. The variations in soil characteristics make for abrupt transitions from premium Chardonnay to Merlot and Pinot noir vineyards within relatively short distances. Pinot noir is typically planted on low vigor potential soils including the Forward and Bressa series that have some type of restriction (depth, clay layer, high % of gravel or rock in the subsoil, etc.). Higher vigor potential soils with less depth or nutrient restrictions such as the Haire and Diablo soils are often planted to Chardonnay and Merlot. Yountville Soil drainage classes are predominantly well drained in the western part of the appellation and reflect the need for supplemental irrigations during the summer growing season (Figure 1). A significant portion of the soils in the eastern portion of the Yountville AVA are somewhat poorly to poorly drained, suggesting the need for subsurface drainage during the springtime growing season while a small percentage of soils in the central portion along drainages, streams and rivers are considered hydric. Mt. Veeder The majority of soils within the Mt. Veeder Appellation are derived from sandstone and shale or volcanic material. However, the variability of soil depth, texture, slope, and aspect across soil types is quite large. As a result, vineyards in the Mt. Veeder Appellation are often challenging to manage, and low yielding but capable of producing premium quality fruit. Many of the older vineyards were planted in the 1970’s on steeply sloping soils using cut and fill terraces, St. George rootstock, and a standard bilateral cordon and California sprawl or T trellis. This planting system, although practical and cost-effective at the time, resulted in some undesirable soil erosion and vine growth that was highly variable. As a result, this system has been replaced with vineyard plantings in the 90’s that parallel the slope without using terraces in order to decrease the potential for erosion during the winter months. The newer vineyards also utilize phylloxera and drought resistant rootstocks, closer spacing, and vertical shoot positioning to achieve a more balanced vine growth. Chapter III. Soil Effects on Grapevine Growth and Production in the Napa Valley AVA Overview Soils exert control over the growth and production of grapevines in the Napa Valley AVA via two major mechanisms. One is through the influence of soil depth and texture, (% sand, silt and clay) on the water holding capacity of the soil. Soils in the Napa Valley AVA range from loamy sand to clay textures with a range in water holding capacity from 0.36 inches to 2.62 inches per foot of soil respectively. Soil depths range from 10 inches to more than 72 inches in many of the major grape growing areas of the Napa Valley AVA. Water Status of Napa Valley AVA Soils The total amount of water stored during the winter months of rainfall is a function of both soil texture and soil depth in the soils of the Napa Valley AVA. The stored water is considered the primary reservoir available to grapevines during the growing season. Several Napa Valley AVA soils are capable of storing enough water during the winter months to supply grapevine water requirements for an entire growing season. These are the Bale, Pleasanton, Cole, and Yolo soils that have supported dry farmed or non- irrigated vineyards since the turn of the century. Vine nutrition and soil fertility of Napa Valley AVA soils It is interesting to note that although most of the soils in the Napa Valley AVA differ in chemical and physical characteristics compared to many common soils in major vineyard regions in Europe, vines growing in the upland areas of the Napa Valley AVA are subject to similar magnesium and phosphorus nutrient deficiency symptoms as some important grape growing areas of Germany, Italy and France. These deficiencies can have direct and important effects on vine leaf area and cluster and berry development (Skinner and Matthews 1990). Soil vigor potential The concept of soil vigor potential was developed by Terra Spase to quantitatively describe the combined effects of up to 20 different soil physical and chemical attributes of soil, and their effect on vine growth, water status, and production parameters. Soil vigor potential data can be used to help in the selection of block layouts, vine rootstocks, vineyard spacing and trellis configurations, irrigation system design, and the application of preplant soil amendments. On high vigor potential soils until recently it has been the practice in the Napa Valley AVA to increase vine and row spacing, select high to moderate vigor rootstocks, and utilize a divided canopy trellis systems with a drip irrigation system. Summary The soils of the Napa Valley AVA are diverse in their origin, extent, and influence on the grapes grown there. With over 30 soil series and six of the twelve soil orders present, significant differences in the production and quality of Napa grapes could be expected. Soil series in the Napa Valley AVA differ with regard to their parent material, age, topography, climate and biology. From the deep alluvial soils of the valley floor to the shallow rocky upland soils derived from volcanic and sedimentary parent materials, the soil series of the Napa Valley AVA have direct effects on vine root growth, water uptake, and vine nutrition. Information in this report establishes a scientific basis for understanding the diversity of soils that occur within the Napa Valley AVA Significant evidence, both analytical and empirical, exists to establish strong links between that diversity and the range and quality of the region’s grape varieties. *Excerpt from: Soils and Wine Grapes in the Napa Valley AVA 233 pages, 137 color maps and images Available from: Terra Spase, Inc. 345 La Fata St. Suite D, St. Helena, CA 94574 ph 707-967-8325, Fax 707-967-8596 Email: [email protected], web: www.terraspase.com Literature Cited Coombs, B. G. and Dry, P.R. editors. 1988. Viticulture. Volume 1. Resources. Winetitles. Adelaide, South Australia. Johnson, H. and Robinson, J. 2001.The World Atlas of Wine. Octopus Publishing Group LTD.2-4 Heron Quays, London. NRCS Soil Survey. 1977. The Soil Survey of Napa County, California. U.S. Dept. of Agriculture. Skinner, P.W. and Matthews, M. A. 1990. A novel interaction of magnesium translocation with the supply of phosphorus to roots of grapevine (Vitis vinifera L.). Plant, Cell and Environment 13:821-826. Wilson, J. E. 1998. Terroir. The Role of Geology, Climate, and Culture in the making of French Wines. Univ. of California Press. Berkeley and Los Angeles, California. .