Soil is the common name for the loose material laying on top of the bedrock. If we exclude purely organogenic soils, such as peat, soil material consists of smaller particles that have come of rock. Thereafter, the soil material has been deposited where we now find it. One can classify soils in many ways, one is according to deposition mechanism.
Alluvial soils have been deposited from running water. Depending on the waters velocity and the size of the particles, some will follow the water, others will tumble on the streambed while others will loose their speed and come to rest. As conditions vary, the deposition rates will vary and consequently alluvial soils will often exhibit distinct layers. Most soils in the vinyards of Alsace are alluvial, but one can distinguish three categories; the hillsides (most terroirs), the river beds (e.g. the lieux-dits Clos des Capucins, Herrenweg and Harth) and parts of the Alsace plain.
Colluvial soils are formed when soil particle slowly move downhill by gravity. Examples of terroirs where colluvium are important are on Grands Crus Schoenenbourg, Brand and Steingrubler.
Sea sediments are deposited in almost stagnant water, such as giant river beds. In this environment, even the smallest particles may form sediment. The result is deep, heavy, and fertile soils. The Alsace plain is one example.
The soil particles are divides into specific fractions:
One should distinguish between clay particles (defines by size), clay minerals (defined by molecular structure) and clay soils (defined as soils with more than 30% clay particles).
Vines need water and heat.
Water is a critical issue for vines and obviously supplies by the soil. Ideally, the soil should be well drained, but not prone to drying out. The coarser the soil structure is, the more easily it dries out. In science, there are two properties that are uses to quantify this; the field capacity (gravitational water holding capacity) and the wilting point (micro capillary water holding capacity).
Clay soils has a high field capacity due to large void space and high capillary forces. However, clay soils tend to be to wet some periods of the year, but if they dry out to the wilting point the plants roots will have problems to extract water. Pure clay soils are therefore seldom ideal for vines.
Pure sandy soils have a very low field capacity and will often contain very little water. Dry years, the vines will struggle and the ripening process will be delayed. When it is time for harvest, the must may lack the complexity achieved under normal condition.
Hence the ideal composition is a mixture of sand (to provide drainage), clay particles (to provide nutrients) and organic material (to provide habitat for micro organisms).
Alsace is located quite far north where the micro climate is essential to provide good ripening conditions under the pale autumn sun. Hence the soil must stay warm in the autumn, which depends on three factors.
First, the solar energy influx is essential. A vineyard with an inclination of 30-60 degrees, and has an orientation of south-east to south will have receive a maximum of solar flux per square meter.
Secondly, the soils´ ability to absorb the sunlight and transform it into heat will differ. Light soils will reflect sunlight, while dark soils will absorb and transform the light into heat that will stored and re-radiated during the night and hence maintain a high temperature during the dark hours.
Finally, a heavy soil rich in water requires more energy to warm up than a light, dryish soil. Light, clay soils are thus referred to as "cold soils", while dark, sandy soils are referred to as "warm".soils.
In terms of wine growing, the variety most prone to rot is Pinot Gris. Hence, it performs best on warm, fairly sandy soil. Such terroirs are Grands Crus Pfingstberg (Orschwihr) and Gloeckelberg (Rodern). Terroirs where teh vines may find it difficult to cope with drought include Grands Crus Brand (Turckheim) and Schlossberg (Kientzheim/Kaysersberg).