Winegrowers in Sonoma County, California, are learning to use knowledge of soil properties to produce world renowned wines. Highly variable soil texture, mineralogy and chemistry reflect the complex underlying geology, which dictates soil characteristics. High quality grapes grow only under certain optimum soil conditions, including a balance of nutrients with a Ca:Mg:K ratio by weight of about 6:1:1, and clays with a low cation exchange capacity (CEC). Low CEC clay is also desirable for minimum water retention and slow nutrient transfer to grape vine plants.
__The highest quality grapes grow on sandstones of the Wilson Grove Formation, and extrusive rhyolitic lavas/volcanic ash of the Sonoma Volcanics. These formations tend to produce soils that are close to perfectly balanced in nutrient content, and have low cation exchange capacities. Alluvial deposits produce soils of variable quality and suitability for wine grape growth, owing to their variable source composition and chemistry. Soils developed on or from Franciscan Complex bedrock also may be suitable for wine grape growth, but may contain the magnesium- and nickel-rich mineral serpentine which can cause magnesium imbalance in the soil, or nickel toxicity. Because magnesium is a highly mobile chemical element, its presence in alluvium can change soil suitability for wine growth both downslope and downstream from magnesium sources such as serpentine-bearing bedrock. Franciscan greywacke sandstone produces ideal soils for quality grapes in high coastal climate zones.
Some modification of local soil conditions (e.g. addition of lime, or nutrients) can be undertaken to improve local soil conditions for wine grape growth and/or rootstocks can be chosen to match soil characteristics to overcome soil deficiencies or other problems. In Sonoma County, considerations of geology and soil are critical in the production of high-quality wines. Increasingly, the world class wines of Sonoma County are a product of careful study of the soil and climate aspects of terroir, both of which combine to make this a special place for winegrowing.
Quality winemaking starts in the vineyard. Starting from the 1500s, the French experimented with different plantings in different areas and, through trial and error, commonly found the perfect match between grape variety and environment (e.g. Pomerol, 1989; Wilson, 1999). In Sonoma County, California, the factors that define a quality vineyard are being revealed, one by one. Whether the wines are the flavourful Chardonnays of Carneros, the wonderfully fruity Zinfandels of Dry Creek, or the rich, deep berry flavours of Russian River Pinot Noir wines, winegrowers have been working to perfect the final product by matching varietal and rootstock to a number of factors in the vineyard environment as the French learned to do so many years ago. Winegrowers now realize that, in order to produce a superior wine, they need to be part plant physiologist, part soil scientist, and part meteorologist and climatologist to achieve their goal.
The concept of terroir incorporates the notion of site and location which includes all factors that work together to create a region with particular characteristics to match the needs of wine grapes that will produce quality wine. These factors start with the rock and resulting soil through geologic processes, continue through climate and vineyard practice and the winemakers art, and end with the consuming public. Now many vinophiles are aware of the scientific basis for our experience with terrior (e.g. Haynes, 1999; Meinert and Busacca, 2000). In this paper we consider, for the Sonoma County AVA (American Viticultural Area), the major geologic factors of terroir: the parent rock and the soil mineralogy and texture, and their importance in quality grape growth and wine production.
Unfortunately, many winegrowers in California (and elsewhere) sometimes cater too much to the major “end member” of terroir, the consuming public. Wine columnists and reviewers ___ seem to prefer big, aromatic Cabernets above everything else. This preference has tended to change the wine making process to get higher rating numbers, to some degree masking the important effects of terroir.
GEOLOGY AND TERROIR
__The foundation of terroir is the underlying geology, including bedrock and the soils that develop from bedrock. We have all heard vineyard legends about the relationship between certain rock types, soils, and grape quality. On examination of the geology of some representative terroirs in the Sonoma County AVA, we can explain the evolution and character of the soils and their effect on wine quality. When combined with winegrower experiences with these terroirs, knowledge of soil character and evolution is proving to be extremely useful information for all.
Rocks and Soils
Rocks represent all kinds of chemistry as reflected in the crystals or minerals of which they are made, and the textures which describe the physical relationships among minerals. The dominant chemical component of rocks found at the surface is SiO2(= silica), more commonly known as the mineral quartz. Quartz is composed of a covalently-bonded molecule which is among the most durable of natural materials: it does not break down easily and is inert chemically. Most sandy soils contain quartz. Other minerals with silica compounds involved in different structures have weaker bonds and most commonly are a mixture of magnesium (Mg), calcium (Ca), potassium (K), sodium (Na) and other minor elements which can be released to the environment by mechanical or chemical weathering, and thus can provide the nutrients essential to the growth of wine grape vines. This “big four” group of nutrients (Mg, Ca, K, Na) in various combinations is an important factor in determining the contribution of soil chemistry to the production of flavourful wine grapes. Below we will explore how these nutrients affect vine plants and what combination and ratio of nutrients works best for premium winegrowing in Sonoma County.
The mineral content of rocks is also important in determining soil texture - the nature, size, shape and orientation and arrangement of particles - which in turn dictates the root growth depth and the ability of the vine plant to obtain required nutrients. Access to nutrients is largely a function of water availability in the soil, as it is water that is needed to transport nutrients to vine plant roots. Clay minerals play a critical role here, as they can retain water and act as harbors for nutrients because of their cation exchange capacity (CEC). The abundance and type of clay minerals determines CEC. Kaolinite and illite are low CEC clays while Montmorillinite has high CEC. Nutrient ions, which are dominantly electrically positive cations, are trapped by negative fringe charges on clay minerals and humus (decayed organic matter). Clay minerals and humus have large surface areas per unit weight, which make them effective nutrient harbors.
A sandy, well-drained soil with little or no clay mineral content and thus low CEC may result in a local deficiency of nutrients, reducing wine grape quality producing vegetal taste in wine. A clay-rich soil with a high CEC may have locally available nutrients, but can also cause the roots to be immersed in water (have wet feet), thus excluding oxygen which is needed for the nitrogen cycle and other processes that feed the vine plant. Deep rich soils create high vigor growth producing large watery grapes. A moderate content of clay minerals with a low CEC seems to be optimum, with just enough textural and nutrient benefits, and water, to keep the grapes growing through the growth stage, and naturally slacking off after growth stops and ripening begins (Fig. 1). Mike Porter, a Sonoma County vineyard consultant, has ranked soils according to CEC, and invariably finds that a low CEC (3-14) in sampled Sonoma County vineyards begets the highest quality grapes (Porter, 1994, unpublished).
Clay and humus also control or modify the physical properties of the soil. They may form flexible elastic bridges between soil particles to maintain soil structure and preserve porosity, even after being compacted by heavy equipment. Pebbles and rocks in the soil seem to be a major factor in water supply: in clay-rich soils, pebbles and rocks tend to break up the soil, providing avenues for water percolation and root penetration. If present on the vineyard surface, pebbles and rocks can absorb heat during the day and promote indolent slow cooling in the evening.
The first geologic process ___in soil formation is the breakdown of minerals in place at the surface. This can happen by the physical fracturing and separation of minerals or mechanical weathering: or by chemical change – chemical weathering - instigated by dilute acids in water from the atmosphere.
Follow the history of a soil: the parent material breaks down by mechanical weathering, falling from a cliff or eroded from a streambed and forming fragments of various sizes including silt, sand and gravel. As these fragments accumulate on more or less level surfaces, water attacks them with various dilute acids and this rearranges the molecules, ejecting some ions and adding new ones, and changing the structure. Quartz maintains its chemistry and structure, but a close and very abundant cousin, feldspar (K, Ca, Na, Al silicates) can be easily broken down chemically and altered to yield clay minerals, among the most common and important components of soil. Chemical weathering releases nutrient cations with type and abundance dictated by the original chemistry of the parent rock. Organic material derived from decayed plant matter accumulates over time, and adds nitrogen (N) and humus to the soil along with phosphorous (P) and sulphur (S), necessary nutrient and structural components in the growth of wine grape plants. Humus and clay minerals act as harbors for nutrients, then water in the soil carries nutrients to the vineplants.
Over long time spans, a winnowing effect takes place with descending water carrying clays and other fines downward and creating layers of different composition called the soil profile. In general, the upper (A) horizon is residual soil with quartz and organic material. The next lower horizon (B) is where fines and chemicals accumulate, and is commonly rich in clays and chemically precipitated minerals. The lowest is the bedrock itself (C) underlying the soil. Because the layers vary in thickness and composition, test backhoe pits provide a study surface 4-5’ deep. Chemical testing and visual description of textures at the various levels are part of the field study of soils.
In arid or seasonally arid areas like Sonoma County, a combination of physical and chemical weathering takes place, and soils during different seasons may have different properties dictated by the moisture content of the soil. The high variability of rock types in Sonoma County makes for many different soil types and changing soil properties, a major challenge for the winegrower and soils consultant. It is sometimes claimed that there is more soil variety in Sonoma County than in all of France!
Another complication for winegrowers is the “active geology” of California. Tectonic uplift, faulting, and down-warping valleys are all going on before our very eyes on time scales faster than the formation of soils. Down-slope movements, sheetwash, flooding, uplifting river terraces, rapid erosion and transportation of materials all happen continuously, and further complicate soil formation and distribution. Many areas have hybrid soils - transported soils made up of material brought from some ridge by a mudflow or landslide. In such settings, unfavourable ions, especially magnesium, can be transported easily and can contaminate soils far from the serpentine source rocks which are the most common source of Mg in Sonoma County. Soils take many thousands of years to develop a stable profile, whereas hardly a storm passes through Sonoma County without major rearrangement of surface materials in a matter of days. A recent treatment of processes in Napa County is in Swinchatt and Howell (2004)
Grape vine roots can penetrate to depths of 30 feet (~10 m) into the soil and rock below to reach water. Vine roots are opportunistic: they follow layers with more water and nutrients, seeking out pockets of sand or gravel with higher water content. Nutrients are transferred by complex processes through the root into the cells of the vine plant (Wilson, 1999) but need to be transferred by water: no water, no growth. Alfred Cass (Cass et al, 2003) has developed a theoretical concept he calls “Total Available Water” (TAW) which can be used to determine the best rootstocks and varietals for any given soil.
French viticulturists have long recognized and named the stages of grape growth (Fig. 1). First there is a period of growth, through bud break and flowering to a point - “arrêt” - where growth stops. From this point on ripening begins, there is no more growth of foliage or grape, and the plant “coasts” until final ripening occurs and “veraison”, literally the “time of sale”, is reached. This coasting period is a time when little water is needed, and many dry-farmed vineyards use this as a time of slow ripening ”hang time” which, given the right climate, will produce premium grapes. Continuing irrigation at this stage (Fig. 1) will keep the vine plant growing, producing large grapes with a high skin/juice ratio, increasing vigour but lowering the quality of the grapes considerably. Deep, nutrient-rich, water-charged soils will have the same effect as with excess rainfall or over-irrigation. With progressively less water supply, either tailing off naturally or with less irrigation, grapes will stop growing at just the right time, lowering the skin/juice ratio and giving the desired rich, flavourful product. 2 hypothetical curves, upper at 10% clay, and lower at 5% clay, illustrate the effect of clay content on water supply and grape quality. Lower clay, generally indicates lower water supply, lower CEC, less vigor, and 8-15% seems to be a good amount. Very low clay (<5%) promotes too rapid drainage, a shortage of nutrients and lower quality grapes.
Of course, many more factors are involved in the growth of high quality grapes, including sunlight, leaf density, temperature variations, heat retention, natural rainfall, etc. Yet in many ways the geological setting remains a key aspect __ of terroir.
The bedrock of many great wine regions of France is limestone, which is composed primarily of the mineral calcite, CaCO3. This mineral weathers chemically to supply calcium ions to the soil solution. It also makes the soil less acid, raising pH. Higher pH enhances nutrient availability of calcium and magnesium (Wilson, 1999). Calcium also tends to aggregate clay, that is, it allows clay minerals to combine with organic matter to form clay aggregates - clay grains stuck together in sand-size particles - allowing deeper root growth and more water retention in clay-rich soils. This structure improves permeability and enhances water retention. A > 6:1 ratio of Ca:Mg promotes good soil structure, aeration and drainage for the vine plant (Young, 2001). The limestone soils of Bordeaux have ratios of 10:1 Ca:Mg. Too much nitrogen can, however, impair uptake of calcium with many deleterious effects.
__Sonoma County has virtually no limestone, so natural calcium in Sonoma comes from mineral weathering of volcanic rock. The lack of naturally-occurring limestone means that many soils are acidic, and thus it is a common practice to add lime or gypsum to increase pH and calcium levels in vineyard soils in Sonoma County. Lime is a short-term solution, and must be reapplied every few years, whereas gypsum tends to penetrate more deeply, lasting longer.
Experience with many grape varietals in different soils has given birth to the concept of “chemical balance”: that the essential nutrients have to be present in certain ratios for optimum wine quality. The major nutrients are nitrogen, calcium, magnesium, potassium, sulphur, phosphorous and other micronutrients. The vine plant needs these for successful plant growth and wine quality, but only in certain ratios are they most beneficial to optimum plant vitality and growth as summarized in Young (2001).
The most important key seems to be the relationship between Ca, Mg, and K. The best soils for wine quality have high calcium content and potassium greater than magnesium in lower ratios, typically, 6:1:1, Ca:Mg:K. Numbers derive from soil chemical analyses done in labs that use standard methods developed by the Soil Conservation Service and U.C. Davis. Numbers are parts per million which show true weight of elements. Bordeaux soils have ratios of 10:1:1 and produce superb wines.
Table 1 shows a list of geologic formations, generalized soil series chemistry and characteristics for the Sonoma County AVA. Data comes from Mike Porter (1994, unpublished) and Young (2001). Comments are included on the suitability of particular bedrock and soil units for quality winegrowing in Sonoma County AVA. Table 1 shows that soils developed from sandstones/volcanic ash of the Wilson Grove Formation, as well as extrusive rhyolitic lavas/volcanic ash from the Sonoma Volcanics, provide the proper nutrient balance and lowest CEC, and accordingly are correlated with high wine quality.
Magnesium Content in Soil
A recurring problem in Sonoma County soils is high magnesium content relative to potassium content. Magnesium, a very mobile element, is a product of chemical weathering of serpentine, a common rock type in this area. Mg is mainly derived from rocks of the Franciscan Complex and its erosional products (Table 1). Magnesium is a necessary nutrient for wine grape vines as it aids in uptake of other nutrients, and helps build the chlorophyll molecule. It also acts as a “glue” that aids in building the granular structure necessary for drainage and oxygen content of soils. (Young, 2001) In high concentrations, however, magnesium can seal the soil structure, causing poor water drainage. If it is present in high abundances relative to potassium (K/Mg ratios <0.5) it upsets the water uptake mechanism of the vine plant, making the vine appear to need water when it does not need water. Low K:Mg ratios can also give resulting wines a “grassy” or “vegetal” flavour, highly undesirable in premium wines (Porter, personal communication, 2003). Serpentine soils, and alluvial fans and river sediments downstream from serpentine outcrops, commonly have this problem. These soils also tend to have poor structure because they are commonly clay-rich, adding to the chemical insult that results from excess magnesium.
Potash (K) infusions can ameliorate problematic Mg-rich soils, but many occurrences of serpentine also have nickel as a trace element, which is toxic to vine plants and just about everything else (Daniel Roberts, pers. comm., 2004). Rootstock choice can lower the effect of Mg, as some rootstocks are more tolerant of excessive amounts of Mg than others. Another successful tactic is to divert surface water from serpentine sources around (away from) vineyards. This approach has been used successfully in Guenoc Vineyard, Lake County, California (James Richmond, personal communication, 2003).
__Despite geologic complexity __Sonoma County is the location of some of California’s very best wines. Local winegrowers are learning to work with highly varied soil conditions which combined with the superb climate produces world-class wines. For our purposes we can regard Sonoma County geology as ambidextrous; the San Andreas fault serves as the dividing line between two entirely different geological entities (Figs. 2, 3). The __ schematic transect in Figure 2 shows the different rock units and their geological ages. Figure 3 shows the general rock types in plan (map) view, with the boundaries of Sonoma County appellations superposed thereon. Table 1 lists the bedrock geologic formations and their overlying soil series, typical soil characteristics, and quality of wine grapes.
Rocks West of the San Andreas Fault
West of the San Andreas fault the rocks are mostly ancient granites with overlying sand and gravel. In Sonoma County granite is only exposed on Bodega Head, a location too near the cold waters of the Pacific Ocean to be of much use for viticulture, although if the local climate was warmer planting there might be able to emulate vineyards in the granite soils of Northern Côtes du Rhône, France. __ Granite originated as molten igneous rocks deep in the earth, and cooled about 100 million years ago. These granites formed at least 345 miles south and were moved laterally northward along the San Andreas Fault to their present location by tectonic movement over the last 29 million years, related to plate tectonic processes that have been so important in creating modern California (Atwater, 1970, Swinchatt and Howell, 2004).
To the east, gently sloping terraces perch above the coast like a staircase. These are young erosional surfaces with a thin covering of sand and gravel that originated on old beaches and beneath the waves offshore, and currently are being pushed up by continuing pressures along the San Andreas Fault. Some of these surfaces have been uplifted to more than 1,000 feet (~300 m) above sea level. They are mute evidence of shorelines formed during Pleistocene time, but now uplifted high above their birthplace. These surfaces provide sandy soils suitable for growing grapes, and cold-weather varietals (pinot noir, chardonnay) yield excellent wine at high elevations. However, most of this area is too close to the ocean and thus too cold for viticulture.
Rocks East of the San Andreas Fault
Here a rich smorgasbord of rock types (Figs. 2,3) represents a long and complicated geological history. The result is a high diversity of soil types, each type providing its own conditions of texture, structure, and nutrients. Fortunately the climate here is perfect for wine grape growing, with long, warm days and cool nights, so the combination of soils and climate make Sonoma County ideal for growing a variety of great wines. Foggy mornings in the Russian River Valley of North Sonoma County (Fig. 3), Cazadero valleys and the central coast area make this an ideal climate for Pinot Noir, Chardonnay and other cool weather varietals.
Franciscan Complex: Deep Oceans and Subduction
The oldest bedrock occurs on steep slopes and high hills and mountains in northern Sonoma County. Called the Franciscan Complex from exposures near San Francisco, it is an eclectic collection of different rock types that date back as far as 150 million years but can include rocks as young as 40 million years. Mostly of oceanic origin,_the Franciscan Complex includes sea-floor marine sediments along with iron-rich igneous volcanic, plutonic, and metamorphic rocks. These rock types originated far offshore and became mixed together by faulting at a fundamental plate tectonic geologic boundary called a subduction zone, where the ocean floor tectonic plate dives under the continental edge. For illustration and further discussion, see Swinchatt and Howell (2004).
One result of complex interactions at subduction zones is “mélange”, the French word for mixture, complex rocks with textures resembling “rocky road” ice cream. Harder rocks resistant to faulting are like the nuts, marshmallows, and chocolate chips, whereas softer, weaker rocks, deformed almost plastically along faults, are like the ice cream matrix. The pattern for Franciscan Complex rocks on Figure 2 shows this relationship.
Franciscan Complex mélange is particularly well-exposed along the Sonoma County Pacific coast and on high ridges between Bodega Bay and Gualala. _ There are also Franciscan rocks in slopes and ridges above the Russian River (appellation 5, Fig. 3), southern Dry Creek (2, Fig. 3), and the crest of the ridge running from north of Carneros in a band along the Napa-Sonoma County border, through Knights Valley (3, Fig. 3) and along the high ridges east of Alexander Valley (1, Fig. 3). Similar __rocks in association with layers of sandstone, conglomerate, and iron-rich igneous rocks underlie large areas in the ridges west and east of Dry Creek Valley. These igneous rocks and the associated sandstones and conglomerates are part of the ocean floor that was scraped off onto the continent during subduction; an ophiolite complex designated part of the Great Valley Sequence. The soils of these areas are sandy or pebbly when developed on sandstone and conglomerate. Red pebbly clay has developed from weathering in-place of iron-rich igneous rocks, as particularly well displayed on the west side of upper Dry Creek Valley on Bradford Mountain, where superb red wines are born..
Rock distribution in the Franciscan Complex is random, as are the soils that come from their breakdown. Locally there are veins of serpentine which form magnesium-rich soils with potentially toxic nickel levels; blocks of hard sandstone, which form sandy, clayey soils; and iron-rich igneous and metamorphic rocks, which result in red, clayey soils. Nutrient ratios vary greatly, with a natural emphasis on high magnesium and low potassium occurrences (Table 1). The cation exchange capacity of__clay minerals (CEC) is moderate to high.
The recent land rush to find quality vineyard sites in Sonoma County has resulted in vineyard development of slopes on Franciscan Complex rocks and their overlying soils. The varied soils in these areas present new and sometimes formidable challenges for vineyard development and management. The best areas for development will be on soils developed from marine sediments, primarily greywacke sandstone and shale. Places to avoid are areas of Franciscan mélange with serpentine and other high magnesium-content soils, commonly with high clay content.
Wilson Grove Formation: Shallow Seas
In west-central Sonoma County the landscape consists of rolling hills and relatively gentle slopes near Sebastopol (Fig.3). This topography is typical of areas underlain by the Wilson Grove Formation, a fine-grained, shallow marine quartz sandstone that formed in an embayment of the ocean three to five million years ago during Pliocene time (Table 1). It sits unconformably on top of deeply eroded Franciscan rocks (Fig. 2) and ranges up to 1,000 feet (~300 m) thick. Fossil clam shells demonstrate its marine origin.
The sandy loam soils of the Gold Ridge-Sebastopol series (Table 1) form as a direct result of breakdown of rocks of the Wilson Grove Formation. The low ridge running from Forestville to Sebastopol and south to Cotati is the classic terroir of this association, now being recognized as prime land and climate for Pinot Noir and Chardonnay grape varietals. In some areas, the sandy loam soils have small, rounded pebbles of vari-coloured rocks which appear scattered about the surface and are mostly eroded from the underlying Franciscan Complex. Layers of volcanic ash and pumice are interspersed with the sandstone. Similar rocks and soils occur in the northwestern part of Sonoma County, capping ridges north to Annapolis and providing sandy soils for high Sonoma Coast vineyard sites, prime land for Pinot Noir (Peay and Annapolis vineyards).
Dehlinger Winery is located six miles north of Sebastopol (Fig. 3), and has vineyards developed on soils derived from the underlying Wilson Grove Formation. Winery owner Tom Dehlinger and vineyard manager Marty Hedlund have mapped soils by examining surface soils, using an auger for samples below the surface, and observing vine vigour. Aerial photos of vine vigour late in the season vividly show the boundaries between different soil types (Fig. 4). The vineyard has been divided into blocks based on soil types as reflected in vine vigour, and the blocks are harvested individually. Dehlinger and Hedlund have painted the stakes different colours to delineate the different areas of ripening. Smart (1995) has cited this vineyard as a prime example of viticultural management of terroir factors.
Dehlinger vineyard has two basic soil types. The most flavourful grapes for Pinot Noir, according to Dehlinger and Hedlund (pers. comm., 2003) come from reddish Altamont (Sebastopol) hilltop soils that have ~18 inches (~0.5 m) of silty sand with some pebbles underlain by a red clay layer with a golden sand below. Soil characteristics include medium permeability and good water retention, obviating the need for irrigation. High calcium, balanced magnesium and potassium, and a very low cation exchange capacity make this an ideal soil. They let the water stress come on naturally as harvest time approaches. The soil is naturally acid, so they have treated it with lime and gypsum to increase calcium content, neutralizing pH and allowing more efficient nutrient transfer. Octagon soils (top of hill), Altamont soils (red on upper slopes), and Gold Ridge soils (deep silty and sandy) vineyard blocks are all planted with Pinot Noir, same clone, same rootstock: only the soil is different. Soil permeability is highest in the Gold Ridge silty, sandy soils, and lower in the Altamont and Octagon soils, which contain clay layers or a moderate amount of clay.___All wines from these vineyard blocks are extremely flavourful with deep, lingering berry flavors. As would be expected, high vigour areas have lower quality with vegetal tastes, and are not included in premium wines.
A terroir tasting with Tom showed the colour and “nose” of Pinot Noir wines from the Gold Ridge sandy soil vineyard blocks is lighter than those from the other blocks noted above, with a garnet and orange colour and herbaceous with sweet fruit hints of strawberry and sage. The Altamont soil vineyard wine is deeper in colour and higher in tannin with blackberry, plum, cherry, and jammy flavors, and no hint of herb. The Octagon soil wine also has no herbal qualities and is dominated by a rich, complex berry nose and taste, with more tannin than the others.
Dehlinger and Hedlund believe that soil permeability and clay content are the primary reasons for these differences (pers. comm., 2003). X-ray diffraction studies of samples from different blocks confirm that the clay is a low CEC kaolinite, and that in moderate amounts (10-15%), affords the perfect soil combination for their highest quality wine in the Octagon soil block. Considering consistent wine rating scores in the mid-90s and the entire vintage is sold out to those lucky enough to be on a mailing list, the care Dehlinger and Hedlund take with their soil differences and related factors certainly has paid off.
Sonoma Volcanics: Volcanos in Eruption
If you visited the Sebastopol area (Fig. 3) three to six million years ago, you would be floating in the Wilson Grove Sea, but your attention would be dominated by erupting volcanoes to the east. From Mount St. Helena south to San Francisco Bay, and extending from the east side of the Santa Rosa plain to the east side of Napa Valley, numerous volcanic cones and fissures erupted dark lava and were the site of explosions which formed thick piles of white volcanic ash. One explosion flattened a forest of redwoods and quickly buried them in ash, to be revealed today as fossil trees at the Petrified Forest near Calistoga. The slopes of Sonoma Mountain, the west side of the ridge that extends north of Sonoma to Mark West Creek, Mount St. Helena, and the east ridge of the Napa Valley, are all eroded Sonoma volcanic rocks.
Benziger Family Winery Flavour Blocks
Soils of the Sonoma Volcanics are highly variable in type, thickness, and extent. At Benziger Family Winery, Mike Benziger and Alan York have used soil and slope aspect to divide their vineyard in Glen Ellen into “flavour blocks,” all managed individually. York describes their concept of “biodynamic farming” of different terroirs, where they use internal soil organization for each block, managing the vines to match soil conditions. The volcanic parent material dictates soil type. On the east side of Benziger’s vineyard, bedrock of clay and iron-rich volcanic breccia and lava flows predominates. Soils here have moderate clay content, which provides a slow, steady supply of nutrients and water up to the end of the growing season. The soil is deep here, which translates to high vigour. Closer row spacing and canopy management are used to lower vigour, increasing vine stress and concentrating flavours. This type of soil typically has abundant calcium, high magnesium relative to potassium, and moderate to high CEC (Table 1). Benziger’s southwest block has sandy, well-drained soils with little clay, developed from Sonoma Volcanic ash. In vineyards with this type of soil there is little humus and few charged clay particles to attract and harbour nutrients. Water drains rapidly through the soil, carrying most nutrient ions below root level and producing natural nutrient and water stress. These soils typically have perfect chemical balance and very low CEC, making them ideal for wine grapes.
Rivers and Present Erosion
Since Sonoma volcanic activity subsided about three million years ago, the dominant geologic processes have been uplift and river erosion, both of which continue today. Pressures along the San Andreas Fault and along parallel faults inland actively push ridges up and down-drop valleys, creating new settings for erosion to occur (see Swinchatt and Howell, 2004 for great diagrams). Older river deposits include sand mixed with pebble and cobble layers capping ridges near Glen Ellen and named for that town. Glen Ellen Formation cobbles also cover the hills north of Mark West Springs in a band that extends intermittently to Knights Valley (appellation 3, Fig. 3). Volcanic ash mixed with Glen Ellen gravels weathers to give moderate clay content in some areas. Soil chemistry varies greatly, depending on the source of sediment. Younger alluvium tends to have very high calcium, high magnesium in areas downstream from serpentine outcrops, and low potassium. The best quality settings for wine grape growth seem to be located on older alluvium deposits like the Glen Ellen soils (Felta, Huichicha, Cotati soils) where time has been a factor to lower Mg content by water movement.
The rolling hills of the Santa Rosa Valley and north to the ridge between Windsor and the Russian River, as well as the hills south of Highway 12 in the Carneros district, are underlain by older alluvium. The Russian River established a meandering course to the Pacific Ocean while the area was still relatively flat. It established a path parallel to faults in the Alexander Valley, but flowed west to the ocean across a low relief plain down regional slope, over the uplifted deposits of the Wilson Grove Sea. In the last three million years, as uplift increased, the river cut down as the mountains rose up, forming its direct path to the ocean in a steep canyon. As the landscape evolved, the main valleys developed by down-warping between faults. As the ridges pushed up, the debris of erosion filled the valleys with sediment. Streams, lakes and swamps occurred in the bottoms of the valleys, while the sides were covered with alluvial fans of material swept out of the mountains by streams (Figure 5). The interplay of uplift, fan development, and down-cutting by the Russian River formed a number of terraces or “benches” along the sides of the valley between river bottom and bedrock highlands. This activity continues today. Alluvial fans tend to have coarse, rocky soils mixed with sand and clay. Soils developed along the flood plains of creeks and rivers tend to be sandy, but locally may have a high percentage of clay. Because the alluvium in the benches is much older than the river bottomland, Mg has been leached from the soil, creating some of the most perfectly balanced soils in Sonoma County. The Allan ranch vineyard here produces some of the most sought after pinot noir from labels such as Williams-Selyem, Gary Farrell and Rochioli.
Flood plains can have soils high in magnesium, especially in the Clear Lake clay soil series (Table 1). Contamination of soils by magnesium is obvious in the Valley of the Moon, between Santa Rosa and Kenwood. Streams emanating from the Mayacamas mountains to the east flow over extensive serpentine exposures, resulting in magnesium-rich soils. __
A cross-section of the west side of the Russian River Valley across Westside Road near Allen, Hop Kiln and Rochioli vineyards shows the relationship between terraces and river floodplain (Fig. 5). The wines of this area reflect both soils and topography. The upper bench, an old alluvial fan with thinner soils, produces lighter, more delicate Pinot Noir with cherry flavors dominating. This soil is sandy and pebbly, and the site is stressed naturally by wind and lack of water. Grapes at the river level are harvested six days later than those of the bench. Soil is deep and fertile with a pebble over-wash from creeks. Pinot Noir from this level is darker, more intense, and more fruity because of the longer, cooler growing season and the steady supply of nutrients through the soil.
Clay: A key aspect of Sonoma County Soils and Vineyards
Clay content of the soil in moderate amounts is critical for water retention and nutrient supply and the production of deep flavors. Either a small amount of low CEC clay like kaolinite, or some clay-rich layers, will suffice. Pebbles also contribute to quality in very clay-rich soils by providing avenues for water movement and root penetration.
Soil Effects on Wine Quality: Terroir Tasting at Iron Horse Winery
We found the perfect comparison for soil effects on wine quality at Iron Horse Vineyards in the Green Valley appellation in Forestville (appellation 6, Fig. 3). With the guidance of Forrest Tancer, owner/partner, and David Munksgard, winemaker, we selected two different vineyard sites planted with Pinot Noir, Block Q and Block N,. The rootstock, clone, exposure, trellising, and climate are identical for each site, as is the elevation at 120 feet a.s.l. (~40 m) in a sheltered valley, east facing slope. The two sites__ have different soils, with bedrock of Franciscan Complex sandstones overlain by Josephine soils underlying Block Q, and Wilson Grove sandstone overlain by Gold Ridge soils underlying Block N. The clay content is markedly different, with Franciscan Josephine soils having 42% clay in Block Q, and Wilson Grove Gold Ridge soils at only 20% clay in Block N. During several terroir tastings general opinion was that both wines were excellent with the Block Q wine deeper in colour and taste than wine from Block N, with deep cherry colour, berry and cherry cola flavor, and other lingering flavours, a moderately complex wine. The Block N wine, in contrast, has light cherry colour, light berry, softer tannins and a bigger nose; Block N wine is more elegant and complex in general than wine from Block Q. Thus we can conclude that isolating the soil factor in this tasting provides a convincing demonstration of the effect of soil characteristics on wine quality.
The geology of Sonoma County creates great diversity in terroir, particularly in soils, with a resulting diversity in wine flavors. A word of warning: published soil survey maps can be inaccurate, and accordingly each actual or potential vineyard area should be mapped individually for geology and soils distribution. The geologic maps published by the California Geological Survey (e.g. Wagner and Bortugno, 1982) and the U.S. Geological Survey (Blake et. al., 2002) show accurate distribution of bedrock. This is important, because soil variation, as noted above, is closely related to bedrock occurrence in Sonoma County. Experience with soils in operating Sonoma County vineyards demonstrates that a sampling density of about 5-10 pits per acre is necessary to determine vineyard soil character and quality. New methods of geophysical mapping with Ground Penetrating Radar may prove to lead a revolution of high-tech vineyard soil mapping (Hubbard and Rubin, 2004). Geographic Information System (GIS)- and Global Positioning System (GPS)-based studies make the task of discovering and mapping soil character and distribution easier, but a lot of field work remains to be done to map soil type and distribution, in both existing and potential vineyards. As winemakers learn more about geology and soil in California and elsewhere, they will be able to take further advantage of these factors to produce even more world-renowned wines.
I thank the many Sonoma County winemakers and others who gave freely of their expertise, opinions and time, especially Mike Benziger of Benziger Family Winery. The manuscript benefited from reviews by _______.
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Figure 1 Plot of water supply in arbitrary units against time in months over a typical growing season in Sonoma County, California. Also shows effects of clays for soil with ~40% clay, and irrigation. Note the growth stages indicated, from bud break (D) to maturity (V).
Figure 2 Geologic transect of Sonoma County from Bodega at the Pacific Coast in the west to the Mt. St. Helena–Calistoga area in the east; intermediate locations shown at top. Table 1 provides additional information on rocks and soils (based on Wagner and Bortugno, 1984, Blake et.al, 2002)
Figure 3 Sonoma County American Viticulture Area (AVA) geologic and appellation map. Solid lines delineate bedrock geologic units (Franciscan Complex, Sonoma Volcanics, etc.); dotted lines delineate ten smaller scale AVAs (= appellations) as numbered on the figure. Map base from Wagner and Bortugno, 1984, Sonoma County Grape Growers, 2000.
Figure 4 Aerial view of Dehlinger Vineyard near Sebastopol, showing the effects of soils variation on vigour on vine growth. Green areas (dark) are high vigour soils, with higher water retention. Air photo by Marty Hedlund just before harvest (September, 1999).
Figure 5 Cross-section of Alluvial deposits, Westside Road intersection with Sweetwater Springs Road, west-central reach of the Russian River near Rochioli-Hopkiln wineries. (Modified from Wright, 2002)
Table 1 Sonoma County geologic formations, soils and soil characteristics, with general comments on the effects of bedrock and soil units on vineyard suitability and performance. (Wright, 2002; Young, 2001)