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
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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 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 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.
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 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.
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 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.
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.