Geology, soils and wine quality in Sonoma County, California
W.H.(Terry) Wright
Professor of Geology
Sonoma State University
Box 279, Forestville, California 95436
USA
wrightw@sonoma.edu.
SUMMARY
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
1
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.
INTRODUCTION
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
2
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
3
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.
File: TD#6.cjespaper.terroirs/bu 2/6/16
4
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
5
surface, pebbles and rocks can absorb heat during the day and promote
indolent slow cooling in the evening.
Surface Processes
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
6
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)
File: TD#6.cjespaper.terroirs/bu 2/6/16
7
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.
Growth Curve of Wine Grape Plants
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
8
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.
File: TD#6.cjespaper.terroirs/bu 2/6/16
9
__Sonoma County has virtually no limestone, so natural calcium in
Sonoma comes from mineral weathering of volcanic rock. The lack of naturallyoccurring 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.
Chemical balance
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
10
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).
File: TD#6.cjespaper.terroirs/bu 2/6/16
11
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).
SONOMA COUNTY GEOLOGY, SOILS AND WINE
__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
File: TD#6.cjespaper.terroirs/bu 2/6/16
12
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.
File: TD#6.cjespaper.terroirs/bu 2/6/16
13
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 NapaSonoma 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
File: TD#6.cjespaper.terroirs/bu 2/6/16
14
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
15
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).
Taste of Terroir at Dehlinger Winery
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
16
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
17
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
18
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
19
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
20
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
21
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.
CLOSING COMMENTS
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
File: TD#6.cjespaper.terroirs/bu 2/6/16
22
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.
ACKNOWLEGEMENTS
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 _______.
REFERENCES
Atwater, Tanya, 1970, Implications of plate tectonics
for the Cenozoic tectonic evolution of western North
America. Bull. Geol. Soc. Amer., v. 81, p. 3513-3536.
File: TD#6.cjespaper.terroirs/bu 2/6/16
23
Blake, M.C., Jr., Graymer, R.W., and Stamski, R.E., 2002, Geologic
Map and Map Database of Western Sonoma, Northernmost Marin
and Southernmost Mendocino Counties, California: United States
Geological Survey, Miscellaneous Field investigations, no. 2402.
Cass A. 1999. What soil factors really determine water availability to
vines. The Australian Grapegrower and Winemaker Annual
Technical Issue 426a:95-97.
Haynes, S.J., 1999, Geology and Wine 1. Concept of Terroir and the Role of
Geology; Geoscience Canada, v. 26, p. 190-194.
Hubbard, S and Rubin, Y, 2004, The quest for better wine using geophysics:
Geotimes, August, 2004.
Meinert, L.D., and Busacca, A.J., 2000, Geology and Wine 3. Terroirs of the
Walla Walla Valley appellation, southeastern Washington State, USA;
Geoscience Canada, v. 27, p. 149-171.
Miller, V.C., 1972, Soil Survey, Sonoma County California: U. S. Department of
Agriculture, 188p, plus maps and tables.
Pomerol, Charles, 1989, Wines and Winelands of France: Geological
Journeys: Orleans, Editions du B.R.G.M., 370 p.
File: TD#6.cjespaper.terroirs/bu 2/6/16
24
Porter, Michael, 1996, Deficit irrigation in theory and practice: Practical Winery
and Vineyard, July-August, p 43-48.
Porter, Michael, 1994 (unpublished), Summary of chemical characteristics of
some Sonoma County Vineyards: Report 1, Michael Porter, vineyard
consultant, 3 p.
Porter, Michael, 2003, (unpublished) Effect of Mg content on wine grape
quality: Report 5, Michael Porter, vineyard consultant.
Smart, 1995, Smart Viticulture: Practical Winery and Vineyard, Nov/Dec 1995.
Swinchatt, J, and Howell, D.G., 2004, The Winemaker’s Dance, exploring terroir
in the Napa Valley: U.C. Press, Berkeley, California, 229 p.
Wilson, J.W., 1999, Terroir: The Role of Geology, Climate, and Culture in the
Making of French Wines: Mitchell Beazley, London, U.K., 336 p.
.
Wagner, David, and Bortugno, E.A., 1982, Geologic map of the Santa Rosa
Quadrangle, California: California Geological Survey, Regional Geologic Map
Series, Map 2A.
Wright, Terry, 2002, Diverse geology/soils impact wine quality: Practical
Winery and Vineyard, September/October 2002, p. 40-49.
Young, Gregg, 2001, Quality first in vineyard and orchard production: Gregg A.
Young, CPAg, 78 p.
File: TD#6.cjespaper.terroirs/bu 2/6/16
25
Figures
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)
File: TD#6.cjespaper.terroirs/bu 2/6/16
26
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)
File: TD#6.cjespaper.terroirs/bu 2/6/16
27