• Terroir-based differences in wine quality are frequently attributed to the soil composition. The word terroir comes from the Latin root for earth (terre). Many believe that qualities of the soil are primarily responsible for differences in wine quality. The composition of the soil, from clay to limestone, in which vineyards are planted plays a large role in determining the vine's access to water and nutrients.

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  • Soil is more than just dirt. Soil is a complex system of decomposed rocks that have been enriched over time by decomposed organic matter. Bedrock is the deepest soil layer, while topsoil is at the surface and has the highest content of nutrient-rich organic matter. Between the bedrock and topsoil, there are many different layers, or horizons, of different types of soils that are classified based on their textures, mineral components and particle size.

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  • Along with the size or texture of the particles in the soil, the structure of these particles influences the flow of water and air in the soil, and ultimately nutrient uptake. A soil’s structure refers to its tendency to aggregate or stick together. The structure of the soil depends on the type of clay and the amount of calcium present. Montmorillonite clay has good structure, while kaolin clays do not. Calcium also helps provide structure to the soil, while sodium breaks it down. The effect of the soil structure is measured by its permeability. Matrix permeability refers to the ability of water to drain through the microscopic pores between the soil’s particles. Mass permeability measures the rate of flow.

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  • Soil permeability determines the field capacity or total amount of water that can be held by a given vineyard. Deep, well-drained soils can hold the same amount of water as shallow, dense soils. The overall water capacity should be fairly low, about a three-week supply. More than that and the foliage will continue to grow after veraison, or the start of ripening, slowing ripening and reducing grape quality. Soils can be encouraged to drain by adding mechanical drainage to a vineyard or by planting tap-rooted cover crops whose roots create channels through which water can drain.

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  • Water availability is the single most important element in vine growth. Soil water dissolves minerals from the soil and also makes them available to the vines. These nutrients play an important role in vine growth, but it is much less important than many believe. Nitrogen is the most important nutrient, and potassium, magnesium, phosphorus, sulfur and calcium are also important. These are known as macronutrients.

    Nitrogen is responsible for the growth of all green matter in the vine. A nitrogen deficiency can lead to a stuck fermentation, one that does not continue through to dryness, or the formation of foul-smelling hydrogen sulfide in the wine. Normally, nitrogen is provided through the decay of plant material. Natural sources include vine cuttings and cover crops planted between the rows. Synthetic sources include ammonium sulfate and ammonium nitrate among others. Nitrogen can be added to the grapes after crushing in the form of di-ammonium phosphate (DAP) to prevent stuck fermentations and hydrogen sulfide formation.

    Potassium is a macronutrient crucial to vine metabolism. Potassium is sometimes added in the form of potash or potassium sulfate, but an excess of potassium can lead to low acidity, producing wines that are out of balance. Phosphorus and magnesium are important in photosynthesis, but these macronutrients are seldom deficient. Sulfur, which acts as an antifungal agent, is added to vines on a regular basis, as a result, sulfur is never deficient in vines. Calcium is another necessary element that can be added in the form of slaked lime or ground limestone to highly acidic soils.

    Micronutrients (such as boron, manganese, iron, copper, zinc and molybdenum) are present in much smaller quantities. These aid in various aspects of vine growth from hormone and chlorophyll production to photosynthesis. When necessary, these micronutrients can be added by spraying the foliage in comparatively tiny amounts, in some cases, as little as a couple of ounces per acre.

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  • The acidity (pH) of the soil is more important to vine growth than soil nutrients. Alkaline (low acid) soils produce wines that are high in acidity, while acidic soils, those rich in organic matter, for example, can inhibit root growth.

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  • Clay
    Clay is the smallest and heaviest soil particle. Soils with high clay content retain water well because of the many surfaces of the tiny, compact particles. Clay stays colder than looser-textured soils, causing the grapes to ripen more slowly. Vines planted in clayey soils produce wines that are higher in acidity and more tannic. These soils are also more fertile because of their ability to retain water, as well as the nutrients transferred to the vine through the water. Merlot and Chardonnay flourish particularly well in clay soils because they ripen fairly easily. Grapes that ripen more slowly like Cabernet need the hydric stress that more well-drained soils provide to produce optimum ripeness.

    Silt and Sand
    The term silt describes the particles next largest in size. Soils that are a mixture of clay and silt are called loess, a type of soil found in Germany and Austria. Sand is the largest-sized particle, which makes for the most loosely-textured soils. Sandy soils hold the least amount of water and are often too dry to produce high-quality grapes. Sandstone is a type of rock composed of bonded sand. It is variable in density, and the quality of the wines made from it vary along with it. Sandstone that contains the mineral group potassium feldspar can produce wonderful wines. This type of sandstone is typical of Germany's Nahe and Rheinhessen regions.

    Silt and sand lack the mineral-rich water of clay. This accentuates stress on the vine by reducing the amount of available nutrients and water, which accelerates ripening. These types of soil are also warmer because they reflect a greater proportion of the sun's rays.

    Loam is a combination of clay, silt and sand. It provides a fertile soil rich in organic matter and nutrients and holds water well. For this reason, it causes vigorous canopy or leaf growth in vines, which can delay ripening and shade fruit, lowering quality. Clay mixed with loam can produce even greater vigor. High yielding vines that provide grapes for low-cost commercial blends, as in California's Central Valley, do particularly well in these soil types.

    Gravel is largely composed of quartz. Soil that is composed mainly of gravel or pebbles is called conglomerate. A high proportion of gravel in a soil enables it to drain freely and hold heat well. These characteristics are most useful for late ripening grapes such as Cabernet, but can cause water stress in varieties such as Merlot.

    Soils take on different characteristics depending on the bedrock from which they were formed. The best soils for growing grapes feature some sort of limestone at the base. Limestone is composed of the fossils of ancient marine life, compounded since the Jurassic age. This type of soil is high in calcium, which gives it good structure, but limestone can sometimes form an impenetrable layer that blocks root development. Soils with chunks of limestone are described as calcareous. Marl is a crumbly mixture of limestone and clay. Calcareous marl is typical of the Côte d'Or in Burgundy, where it provides the very best soil type for Chardonnay.

    Chalk is a special type of limestone soft enough for roots to grow through and stays fairly cool despite the temperature in the vineyard. Chalk provides a near-perfect amount of water to vines because it drains well, while holding enough water for nourishment. Because of the calcium content, chalk produces alkaline soils that helps the vine produce wines that with high acid content. Chalk provides a low-vigor environment for growing grapes that limit growth of the canopy, helping the fruit to ripen. Pure chalk soils are fairly rare, and are only widely found in Champagne, Jerez and Cognac. In Champagne, heavy clay caps the chalk bedrock. Because the chalk is so low in nutrients, it is necessary to add compost to the soil. Lignite (a type of peat known as cendres noirs) was traditionally used to impart nutrients, although the modern solution is to use compost from trash sites in surrounding villages, and as far away as Paris, identifiable by the blue remnants of common trash bags that have been ground up with the compost (boues de ville).

    Granite and Schist
    Granite and schist are igneous rocks formed in the earth's core. When weathered, they produce a loosely textured soil that drains well and has low fertility. This type of soil is common in the northern Rhône Valley, Portugal's Douro and in Beaujolais. Gamay and Syrah thrive in this type of soil. The crystalline structure of schist splits easily into layers. Schist is found in Côte Rôtie, Alsace and alongside the granite of the Douro. Grapes grown in schist often produce wines with delicate fruit, floral and highly defined mineral aromas. Granite and schist are major components in the soils of the Priorat, Bierzo and Ribeira Sacra regions in Spain.

    Shale and Slate
    Shale is a type of layered, hardened clay that shares many of the properties of clay. Shale that has been subject to heat and pressure forms slate, which is a relatively hard, layered rock. It retains both heat and moisture well, and for this reason it is celebrated for providing perfect conditions for ripening Riesling in Germany's Mosel.

    Volcanic Soils
    Volcanic soils are produced both from the lava of volcanoes (mostly basalt) as well as the igneous rocks (pumice and tufa) spewed from erupting volcanoes. The volcanic soils of the Napa Valley have a high mineral content but hold little water. As a consequence, they are fairly low vigor in nature. Volcanic soils elsewhere in the world can produce variable soils, depending on the area's native igneous rocks. Notable examples are some areas of Beaujolais, Italy's Basilicata region and regions within Greece.

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