Soil is the most interesting, and often most misunderstood, component of viticulture and is one of the main factors, along with climate, that determines the quality of the grapes at harvest. Having an understanding and working knowledge in geology as well as the study of soils, known as pedology, and the study of how soil affects wine grapes, known as edaphology, is crucial to the successful harvest of wine grapes and the production of quality wine.
Soil Types for Rootstocks & Varieties
Choosing a soil for wine vines is tricky, because the type of soil needs to work for both the vine and the rootstock. Wine rootstocks are often different from the scions they are grafted onto, as rootstocks may be chosen for their disease resistance, a practice that became standard after The Great French Wine Blight - we explain this in our "How The Great French Wine Blight Changed the Global Wine Industry" feature. Rootstocks can be very adaptable to different types of soils, including soils that are acidic, alkaline or even have high salinity.
In general, rootstocks need a porous, well-draining soil that allows the roots to grow and create a stable foundation. There are many ways to define soil types: a soil type can be defined by its texture, consistency, depth, color or its organic composition, such as sand, clay, alkalinity, acidity or how the soil is layered. Soil is made up of different layers that include topsoil, subsoil, hardpan and bedrock, the depth and composition of each layer contributes to determining what the soil type is.
Sandy Soil Drains Well
Soils that contain at least 50% sand, are referred to as sandy soils, sandy soils have a coarse or porous texture and drain water very well, even in regions with a lot of rainfall. Tempranillo grows best in deep soils with high quantities of sand, like the Ribera del Duero region of Spain, where the deep soils have a porous structure and are well draining.
Chardonnay Grow Healthy in Clay Soils
Clay soils have a very fine texture and depending on the amount of clay in the soil, can become very dry and hard. Typically, soils with less than 25% clay are unable to maintain moisture while soils with 25% to 40% clay are able to transport moisture and maintain healthy layers, soils with more than 40% clay can become compacted especially in the presence of too much water, these layers are common compositions fro hardpan soil layers. Chardonnay is adaptable to most soil types, but grows particularly well in deep soils, like those of Burgundy, France, known for its high clay soils.
Avoid Alkaline Soil for Viticulture
Alkaline soils refer to clay soils that have a Potential of Hydrogen or pH of more than 7. These soils do not have a good structure that allows for transportation of water and nutrients. Alkaline soils can inhibit the plants ability to absorb copper, iron and zinc, weakening the plant. Alkaline soils are found in areas that have large deposits of sodium carbonate.
Sauvignon Blanc, one of Acid Soil's few Admirers
Acid soil is soil with a pH of less than 5. High acidity in the soil can create nutrient deficiencies in grape vines, as the acidity makes iron precipitate phosphorus unavailable to the plant due to the high amounts of free aluminum, which can also stunt root growth. While many grape species do not thrive in acidic soil, Sauvignon Blanc seems to grow best in shallow silty soils with high acidity like the dry stony soils of the Wairu Valley in New Zealand.
Siliceous Soil Makes up 50% of Bordeaux's Viticultural Soil
Siliceous soil is a composite soil with moderate porosity that is made up of humus and small crystal-like rock particles. The rock particles absorb heat from the sun during the day and ensure the roots maintain an even temperature overnight. The Bordeaux region in France contains an enormous viticultural surface area consisting of siliceous soil; over 50% of its soil is of this type. Here, the Cabernet grape flourishes due to the soils ability to hold it’s structure and drain water, making the roots grow deep to seek nutrients.
Humus isn’t a type of soil, but it is very important to soil. Humus is aged or mature compost, and, at its best, is made up of organic materials that are as broken down as they can get, which makes it different from compost, as compost is in the process of becoming humus. Humus is often observable in topsoils and has a very dark color and a light spongy texture.
Sangiovese Favors Silty Soil
Silt soil has a granular variable size between clay and sand, and is moderately porous. Silty soil is high in quartz and feldspar. Sangiovese grows best in these shallow, limited soils like those found in the Tuscany Region of Italy where the soils found on the foothills and slopes of the southern Tuscan mountains are limestone rich. The area features a very diverse landscape that favors Sangiovese as covered in our "The Scenery and Sangiovese of Tuscany" article.
Merlot Fares Well with Chalk or Calcareous Soil
Chalk or calcareous soils contain high amounts of calcium carbonate on top of a limestone base, and these soils are very alkaline with a pH between 7 and 8. The soil porosity can vary from very coarse with limestone rocks present in the soil to a very fine clay soil, although, chalk soils that contain moderate to high levels of clay can be nutrient-rich and retain some moisture. Merlot grows best in soils that have good moisture-holding capacity, like those found in the Languedoc-Roussillon wine region of France, located at the foothills of the Pyrenes Languedoc-Roussillon, the soil is high in limestone and chalk. This area is favored for its red and white wines alike as covered in our feature on Roussillon, France.
Loamy Soil is Essential to Pinot Noir, Especially in Willamette Valley
A loamy soil is one that typically has balanced quantities of sand, silt and clay, but can be defined as sandy loam if it has more sand, or clay loam if it contains more clay. Pinot Noir grows best in less deep sandy loam or clay soils, like those found in the south and southeast facing rocky hills sides of Oregon’s Willamette Valley.
Soil, It’s a Balancing Act
The amounts of each organic compound in the soil gives the soil its specific Cation Exchange Capacity or CEC. The CEC of soil is its ability to hold and transfer nutrients to wine vine roots. The two compounds that maintain CEC the best are clay and humus. Soils that are primarily clay and humus are very nutrient rich while soils with more sand or silt have moderate to low CEC. However, even soils with high amounts of clay and humus may have a reduced CEC due to viticulture intervention, organisms in the soil, water content and drainage.Often the soils in wine regions combine many soil features that produce the most flavorful wine, which is the case for Zinfandel grapes grown in Italy. Zinfandel grapes grow best in low to moderately fertile soils like those found in the Apulia Region of Italy, where the soil is a remarkable red due to the high iron oxide in a clay loam soil also high in limestone.
Why Soil is So Important to Viticulture, Terroir and Varieties
The type of soil that grapes grow in plays an important role in the development of the vine, the size of the grape and the amount of sugar in the grapes, however, soils that are typically considered healthy aren’t necessarily the best for wine grape production. Wine grapes, grown in soils that produce vigorous growth in the vine, typically produce grapes with light colors, low sugar and bland flavor, as the nutrients go into the vine, not the grape. These grapes can then undergo a late harvest, which allows the grapes to shrivel slightly on the vine, reducing water content and concentrating color and flavor, this is referred to as hang time. This hang time, however, is in the midst of a debate between vintners and winery owners, as grapes are sold by the pound, shriveled grapes, while in some species produce exceptional flavor, have lost most of their water content, and thus, weigh less. The less they weigh, the profit vineyards will make from them. But that's a conversation best left to another article, like our "The Hang Time Debate: The Vintner's Battle Piece".
Wine grape vines are unlike other agricultural products, because they produce the best wine grapes when the vine experiences stress. Water stress will develop high tannins in the skin of the grapes and give astringency to the wines made from those grapes. As we know that wine vines perform better under stress, we begin to see how blended soils, can provide the best growing conditions for wine vines. When vines are stressed, the flavor compounds become concentrated, which in turn, will produce wines that have exceptional flavors. Stress in wine vines, should not be mistaken for deprivation of water or essential nutrients, it simply means that these resources are not readily available to the vine, so the vine can focus on sending these resources to grape growth and development.
When vineyards are choosing the varieties to grow in their vineyard, they have to choose sites with soil that are conducive to providing the best conditions combined with the climate in the region for the grape variety.