TERROIRS OF SONOMA COUNTY, CALIFORNIA; GEOLOGY, SOILS AND WINE QUALITY IN THE VINEYARD

The main factors in wine quality are water supply and history, soil chemistry and nutrient supply, all dictated by clay content and type and original parent material. The key to understanding these factors is the term balance meaning that factors in certain ratios are critically important in wine quality. In Sonoma County a wide variety of geological settings and climates make this an ideal area to study the effects of Terroir factors.

Clay is important both as a harbor of nutrient cations and to retain water to feed the vineplant during various growth stages. Deep rich soils, with high CEC Montmorillinite clay, many nutrients, and abundant water will produce vigorous growth, and watery tasteless grapes. Highest quality grapes are found in soils of moderate clay content, dominantly low CEC Kaolinite where the nutrients are low and water supply is slow. This allows the grapes to balance to conditions and create small grapes with highly concentrated flavors. Dehlinger Winery has mapped vigor difference in Sebastopol soils which correlates with clay content, with low vigor/higher Kaolinite clay soils providing the highest quality grapes.

The best soils have balanced chemistry. In Sonoma County, the Franciscan Complex contains serpentine, which contributes magnesium to soils from weathering and transportation in stream systems. Mg inhibits vineplants uptake of K which regulates water uptake, so what looks like water stress is commonly high Mg soils. Wine quality is low, acidic and vegetal in flavor The important balance is between Ca which should be present in much greater amounts than K which should be equal Mg. Adding lime to the surface, or in drip irrigation, or drilling root-deep holes and packing in Potash to increase K are two amendations that have worked. Soils developed on Rhyolite ash and Wilson Grove Formation, eroded from ash have natural perfect balance and are similar to Bordeaux. Slope side vineyards and older stream terraces have good balance, because the Mg is leached out with time, while valley floor soils commonly are high in Mg where it accumulates.

Field tests and Terroir tastings demonstrate that geological knowledge is critical in telling if an area will produce a $10 bottle of wine or a $100 bottle of wine.