Terroir-based differences in quality are often attributed to the soil. The word terroir comes from the Latin root for earth (terre), and many believe that qualities of the soil are primarily responsible for differences in wine quality.
Soil is more than dirt. It is a complex system of decomposed rocks enriched with decomposed organic matter. It begins with bedrock, forming layers (horizons) that change as one moves closer to the topsoil at the surface, which is richest in nutrient-rich organic matter. There are many different types of soil, the composition of which depends on the way in which it was formed. These soil types are usually classified according to their texture, or the proportion of different sized particles that make up the soil.
Clay exactly is the smallest type of soil particle, thus producing the heaviest type of soil. Soils with a high clay content hold water well because of the many surfaces of the tiny clay particles. These soils are colder than looser-textured soils, causing the grapes to ripen more slowly, and tend to produce wines that are higher in acidity and more tannic. They are also more fertile because of this water retention and the nutrients transferred to the vine through this water. Merlot and Chardonnay flourish particularly well in clay soils because they ripen fairly easily. Grapes that ripen more slowly, like Cabernet, need the water 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, giving the most loosely-textured soils. They 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 in the Nahe and Rheinhessen.
Silt and sand lack the mineral-rich water of clay, and are usually composed of materials such as quartz that vines cannot absorb. This can accentuate stress on the vine from lack of nutrients and water, advancing ripening. These types of soil are also warmer, reflecting a greater proportion of the sun’s rays.
Loam is a combination of clay, silt and sand. It provides a rich and fertile soil because it is high in organic matter and nutrients and holds water well. For this reason, it causes vigorous canopy growth in vines, which can delay ripening and shade fruit, lowering quality. Clay mixed with loam can produce even higher vigor. High yielding vines providing grapes for low-cost commercial blends do well in these soil types, for example in California’s Central Valley.
Gravel is often composed largely of quartz. Soil that is composed mainly of gravel or pebbles is called conglomerate. A high proportion of gravel in a soil causes 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 also take on different character according to the bedrock from which they were formed. Among the best for growing grapes feature some sort of limestone for a base. Limestone is produced the fossils of ancient marine life, laid down over long periods of the Jurassic age. This type of soil is high in calcium, because the acids in the organic matter break down the limestone. This gives the soil good structure, but can form an impenetrable layer that blocks root development. Soils with chunks of limestone are described as calcareous, and marl is a crumbly mixture of limestone and clay. Calcareous marl is typical of the Cote d’Or in Burgundy, where it provides the best soil type for Chardonnay.
One special type of limestone is chalk. Chalk is soft enough for roots to grow in and fairly cool. It provides a near-perfect amount of water because it drains well, while holding enough water to nourish the vines. Because of the calcium content, it produces alkaline soils. This alkalinity causes the vine to produce wines that have fairly high acid content. Because of these factors, chalk provides a low-vigor environment for growing grapes that limits 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 compost the soil. Lignite (a type of peat known as cendres noirs) was traditionally used for this purpose, although a more modern solution was compost from the dumps of the area (and as far away as Paris), identifiable by the blue remnants of common trash bags ground up with the compost (boues de ville).
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 Rhone Valley, the Douro in Portugal and in Beaujolais. Gamay and Syrah thrive in this type of soil. The crystalline structure of schist splits easily into layers. It is found in Cote Rotie, Alsace and alongside the granite of the Douro. Grapes grown in schist often produces wines with delicate fruit and floral aromas.
Shale is a type of hardened clay that splits into layers, which shares many of the properties of clay. Shale that has been subjected to heat and pressure forms slate, which is a relatively hard, layered rock. It holds both heat and moisture well, and for this reason it is celebrated for providing perfect conditions for ripening Riesling in the Mosel.
Volcanic soils are produced both from the lava of volcanoes (mostly basalt) as well as the rocks that are blown out of the vent (pumice and tufa). Those 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, according to the igneous rocks common to the area.
Along with the size or texture of the particles in the soil, the structure of these particles is important because it influences the flow of water and air in the soil, and thus nutrient uptake. The structure of soil 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 in the soil. Montmorillonite clay (see above) has good structure, while kaolin clays do not, and calcium also helps provide structure to the soil (while sodium breaks it down). The effects of the soils structure are measured by the permeability of the soil. Matrix permeability refers to the ability of water to drain through the microscopic pores between the soil particles, and mass permeability refers to the total rate of flow.
This permeability of the soil determines the field capacity of a particular vineyard site, which is the 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 important consideration for drainage is that overall water capacity is fairly low – about a three-week supply. More than that and the foliage will continue to grow after veraison. This slows ripening and reduces quality. Soils can be encouraged to drain through the addition of mechanical drainage to the vineyard or by planting tap rooted cover crops whose roots create channels through which the water can drain.
The main impact of the above considerations is that they determine water availability to the vine, and water is the single most important element in vine growth. Soil water also dissolves minerals from the soil, also making them available. These nutrients do play an important role in vine growth, but it is much less important than many people 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, and a deficiency can also 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, and natural sources include vine cuttings and cover crops planted between the rows. Synthetic sources include ammonium sulfate and ammonium nitrate among others. It can also 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 necessary to vine metabolism. It is sometimes added in the form of potash or potassium sulfate, but an excess of potassium can lead to low acidity, giving wine that is out of balance. Phosphorus and magnesium are used in photosynthesis, but these macronutrients are seldom deficient. Sulfur is added to the vines regularly because it serves as an anti-fungal agent. Because of this use, it is never deficient in vines. Calcium is another necessary element that is seldom deficient. It is added in the form of slaked lime or ground limestone in very acid soils.
Micronutrients are elements present in much smaller quantities. They include boron, manganese, iron, copper, zinc and molybdenum. They aid in various aspects of vine growth, from hormone production to the production of chlorophyll and photosynthesis. When necessary they are usually added in the form of sprays applied to the foliage in comparatively tiny amounts – as little as a couple of ounces per acre in some cases.
The acidity (pH) of the soil is perhaps more important to vine growth than soil nutrients. Alkaline (low acid) soils produce wines high in acidity, while acid soils (soils rich in organic matter, for example) can inhibit root growth and cause the vines to absorb too much copper.
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Wine Regions of the World
