Alongside climate and grape variety, this is one of the most important factors influencing wine quality. The various soil types have developed over millions of years through the physical and chemical weathering of rocks and the humification of organic matter. During physical weathering over extremely long periods, natural forces such as wind, water, heat, cold and frost initially cause the rock formations to break down mechanically into rubble and gravel. Significant temperature contrasts, friction and shear forces, as well as frost action caused by frozen water, play a decisive role in this process.

Grape variety: genetic characteristics, varietal character, aromatics, maturation, physiological ripeness
Site/vineyard: soil, geology, climate, topography, water balance, terroir, suitability for viticulture
Vintage: weather conditions throughout the growing season (from winter dormancy to leaf fall)
Cultivation: vine training, leaf management, thinning, plant protection,yield, grape ripening, grape harvest
Wine-making: Vinification, ageing, bottling, bottle ageing, maturation,drinking maturity

Weathering processes

Various chemical weathering processes, such as oxidation, dissolution and acid attack, break down the mineral lattice structure of the rocks. In the process, minerals that are readily soluble in water, such as carbonates (inorganic salts and organic esters of carbonic acid) and sulphates (salts and esters of sulphuric acid), are dissolved first. As a result, the rock breaks down very slowly into gravel, sand, silt or clay. Every type of rock, even the hardest marble, granite or quartz, will eventually crumble to dust, although this may take many millions of years.

Conversion of organic substances

Organic substances from plant remains, animal remains such as worms, insects and small animals of all kinds, as well as dead microorganisms such as algae, bacteria and fungi, are converted into humus. In the process, nitrogen compounds (nitrates, ammonium) essential for plant growth, as well as other nutrients, are released. Fungi and bacteria play the main role in the decomposition of organic residues such as wood, leaves, roots or animal carcasses. Soil mites are important because of their shredding feeding activities. Earthworms play a crucial role in loosening the soil, mixing it and forming stable clay-humus complexes, which are produced in the earthworm’s gut and excreted as faeces. These contribute to the structural stability of the soil, can bind readily water-soluble nutrients and thus make them available to plants for longer.

Soil horizons

Every soil consists of soil horizons (soil layers) with specific properties. They are almost always horizontal and can be identified in the soil profile (a vertical section of the soil in an excavation). The sequence of these horizons is the key criterion for determining the soil type. From top to bottom, a soil is divided into an O horizon (organic soil horizon) – also known as the H-L-O horizon (peat formed from plant remains and litter) – and a three-part mineral horizon comprising the A horizon, B horizon and C horizon.

Symbols

Deep mechanical tillage causes the horizons to become mixed. Depending on the climate and the effects of erosion, the A or B horizon may be absent or only marginally developed. The individual horizons are designated by symbols. The main symbols are written in capital letters, whilst the supplementary symbols (features resulting from soil formation or pedogenic characteristics) are written in lower-case letters following the main symbol:

Main symbols

• O = organic topsoil horizon (excluding peat); O stands for ‘organic’
• A = mineral topsoil with a living, humus-rich layer
• B = mineral subsoil with a layer low in humus, consisting of fine-grained material weathered into sand, silt or clay
• C = slightly altered parent material with physical weathering
• G = semi-terrestrial horizon influenced by groundwater; G stands for groundwater
• S = terrestrial subsoil influenced by standing water, S from ‘standing water’
• M = horizon consisting of deposited soil material; M from ‘migrare’ (to migrate)
• P = mineral subsoil horizon consisting of clay or clay marl rock
• R = mixed horizon resulting from soil-mixing measures = drainage ditches, R from ‘Rigolen’
• E = mineral soil horizon consisting of laid sod

Additional symbols

• h = humus
• p = ploughed, p for plough
• l = leached (clay-depleted; the finest clay mineral particles are displaced into deeper soil horizons = t)
• e = eluvial (leached from lava; ‘e’ from ‘eluvial’ = washed out, acid-bleached)
• t = clay-enriched
• v = weathered

Horizon types

An Ae horizon is a zone, often greyish in colour, beneath the humus-rich topsoil. It is formed by severe soil acidification and the associated migration of complex iron-humus compounds. Beneath this lies a leaching zone, an illuvial horizon, which is enriched with the leached materials from the Ae horizon. Depending on the predominant humus or iron compounds, a distinction is made between Bh (h = humus) and Bs (s = standing water) horizons. The majority of the vine’s root system is found at a depth of 20 to 50 centimetres (horizons A and B), although this varies greatly depending on the soil type. Very old vines can develop roots extending to a depth of 15 metres or more.

Vine root system: 1 = grafting point, 2 = lateral roots, 3 = rootstock, 4 = foot roots

Soil horizons in viticulture

The horizons have generally already been mixed together through soil cultivation (subsoiling = loosening of the soil). Bedrock, parent material, soil cultivation, fertilisation and the water balance – with a balanced ratio between water-holding capacity and drainage – together with the local climate (microclimate or site-specific climate) define the vineyard site and give each vineyard site its typical and distinctive character of origin. The duration of the growing season, the orientation of the exposure (sunlight), the local microclimate on the slope, the prevailing soil conditions, the humus and lime content, and the water supply all influence the choice of the most suitable grape varieties.

Terroir

The frequently used catchphrase ‘Wine quality is primarily determined in the vineyard (and can only be improved to a limited extent in the cellar)’ can be found on many winemakers’ websites and is 100 per cent accurate. The geologist James E. Wilson aptly writes in his book *Terroir – The Key to Wine*: ‘The soil is the soul of the vine’.Today, the direct link between rock, grape variety and wine character is only marginally evident due to the widespread use of rootstocks that often have shallow root systems, combined with heavy application of mineral fertilisers and the adoption of new oenological methods in the cellar. In the vineyards of the past, which were mostly fertilised organically, with their old vines planted ‘ ungrafted ’ and often with roots reaching deep into the rock, this relationship was certainly much more pronounced.

In France, the importance of the interplay between climate, rock, soil, location, microclimate and grape variety was recognised very early on and, through the concept of ‘terroir’, was elevated, so to speak, to a philosophy. The terroir, together with the grape varieties best suited to it, is defined under wine legislation when wine-growing regions are classified as Appellation d’Origine Protégée (AOC/AOP). This represents a clear contrast to the philosophy in, for example, Germany and Austria, where great (and sometimes excessive) importance is attached not to the vineyard site, but primarily to the grape variety and the single-varietal vintage wines produced from it. However, a shift in thinking has already begun.

Soil composition

In terms of wine quality, it can be a great advantage if the vines are forced to drive their roots as deep as possible into the ground due to stony soil. It is the soil’s ability to act as an ion exchanger – that is, to exchange nutrient salts in the soil solution for the protons (H+) and anions (OH–) released by the plant – that makes it possible for the roots to be supplied with essential nutrients and trace elements in the first place. The minerals absorbed by the roots are reflected in the total extract of a wine.

Vine requirements

The vine requires around twenty essential trace elements and the primary nutrients for its growth in order to thrive as well as possible. As a perennial crop, however, it is less dependent on fertile soils than annual crops. There are vineyards with very poor soils where high-quality wines are produced. However, this does not mean that the fewer nutrients available, the better the wine quality.

The picture shows the Moselle loop near the villages of Leiwen and Trittenheim in the federal state of Rhineland-Palatinate (Germany), as seen from the Zummethöhe viewing point at 250 metres above sea level.

Nutrient requirements

A deficiency of nitrogen and amino acids in the must can hinder the yeasts during fermentation and result in fermentation faults such as UTA (atypical ageing tone). The balanced composition of nutrients in the soil, the availability of water and nutrients, as well as the soil structure and root penetration are important factors in determining a soil’s suitability. Nutrient deficiencies can be identified through plant or soil tests using the EUF method and, where necessary, rectified through fertilisation. A comprehensive classification or assessment of soil quality for agricultural use, and specifically for viticulture, is carried out by means of soil evaluation.

Calcareous soils

On calcareous soils with pH values above 8, the high calcium content in the soil hinders the uptake of other doubly positively charged ions such as nitrogen compounds, magnesium or the trace elements boron, iron, manganese or zinc; this can lead to calcium chlorosis or other physiological deficiency symptoms, even when nutrient levels in the soil are normally sufficient. Particularly at the start of the growth cycle, the nitrogen content (in the form of nitrate and ammonium) in the soil should be sufficient.

pH value

As a general rule, basic (alkaline) soils with a high pH value above 8 (for example, limestone, chalk and marl soils, which usually have a high calcium and magnesium content) produce wines with higher acidity, whilst acidic soils with low pH values between 6 and 4 (for example, granite and quartz sand) result in wines with lower acidity. Experiments involving increased potassium applications have shown that vines respond by producing higher levels of malic acid. To balance the increased influx of positive potassium ions, the plant produces negatively charged acid anions (malic acid). However, (regardless of acidity levels determined by the vintage or ripening conditions) other factors naturally also contribute to the acidity of the wine.

Vineyard soil

Good vineyard soil should be rather poor, of medium to deep depth, well-aerated, well-drained and not compacted; it should be nutrient-rich but not too rich, not too high in humus but rich in mineral components. Loose and stony soils with deposits of limestone, gravel and chalk have good drainage. In contrast, dense, heavy soils consisting of loam, clay and sand tend to become waterlogged or have poor drainage.

The best conditions for growing vines are found on slopes with good exposure (the direction of the midday sun), ideally facing south, as in late summer the sun’s rays strike at an almost vertical angle, thereby maximising the amount of sunlight received.”

The best spot on a slope is the wind-sheltered concave centre (belly, navel, kidney), where the highest temperature sums are reached and the soil is usually well-drained. Soil colour also plays an important role, as dark soils absorb the sun’s heat more quickly and thoroughly, whilst light-coloured soils reflect light, meaning that such soils do not warm up as quickly or as much. The suitability of an area for viticulture is known as ‘viticultural suitability’, which can be determined on the basis of a set of criteria.

Soil types – from Alberese to volcanic rock

The term ‘soil type’ refers to different manifestations of soil which, as a result of the processes of pedogenesis (soil formation), have developed consistent characteristics in the form of soil horizons and thus exhibit a similar stage of development. Whilst the soil type describes the appearance of a soil as a result of soil formation, soil types (also known as soil texture or grain size) are distinguished according to the grain-size composition of the mineral soil substance. The main soil types are sand, silt, clay and loam.

Alberese
Italian term for weathered sandstone with a high proportion of calcium carbonate (limestone) in Tuscany, which is particularly prevalent in the central and southern parts of the Chianti region. See ‘Limestone’ below.

Alluvium/Alluvion (alluvial soil) 
Sediment (loose material) carried and deposited by water. Alluvium is also a term used to describe the Holocene, the most recent geological epoch, which began at the end of the last ice age around 10,000 years ago and continues to the present day. Alluvial soils are fine-grained, highly fertile soil types that form in the floodplains and estuaries of rivers. They consist of soil particles that have been washed in and then deposited.

Depending on the settling rate of the soil particles carried in the water and the flow velocity of the floodwater, they consist of clayey silt, silt, sand or, in riparian areas with high flow velocities and intense erosion dynamics, of gravel and boulders. Despite their predominantly stony and sandy composition – as is the case, for example, in the Médoc region of France – these soils are very well suited to viticulture. The secret of the vineyards there lies in the clay lenses deposited during various floods and covered with sand and gravel within the alluvial gravel terraces, which are capable of storing water. The vine roots actively seek out such clay layers in their search for water.

Amphibolite
A rock that is mostly black, ranging from grey to dark green, formed by the metamorphic transformation of basalt (see below) under conditions of high pressure and temperature. It consists of up to 50 per cent of minerals from the amphibole group, such as hornblende (see below) or chermakite, as well as up to 40 per cent of other minerals such as garnet and quartz, and ores such as magnetite and pyrite.

aeolian
Phenomena caused by the wind, named after the Greek god of the wind, Aeolus. Aeolian transport dislodges fine material such as loess, silt or clay from unconsolidated rock and carries it over long distances by the wind. Aeolian weathering refers to the erosion of rock by wind-driven grains of sand and fine gravel, which acts in a similar way to a sandblasting machine. 

arcose
This geological term describes a pink to reddish, coarse-grained sandstone with a high feldspar content, which occurs primarily in dry, arid areas. It transitions into the coarser-grained granitic rocks.

Alluvial soils
Soils formed from river deposits that are periodically flooded. These are found, for example, in the floodplains of the Danube, Moselle and Rhine. When they are no longer flooded, they develop into brown soils and parabrunsoils. These soils are generally nutrient-rich, biologically active and fertile.

Basalt 
An alkaline igneous rock (solidified magma) consisting of feldspar, hornblende, olivine and magnetite, formed during the melting of the Earth’s mantle. It contains a high proportion of lime and soda and is rich in minerals. This hard, slowly weathering rock forms good soils and produces wines with pleasing acidity. It is particularly well suited to white wines made from Chardonnay, Grüner Veltliner, Pinot Blanc, Sauvignon Blanc and Welschriesling. Such soils are found on the Moselle and the Middle Rhine (Germany) and in Styria (Austria).

Pumice (pumice stone, pumice rock)
This porous, glassy volcanic rock is formed by gas-rich volcanic eruptions, during which the lava is foamed by water vapour and carbon dioxide. Chemically, it is no different from other lava, but is considerably lighter due to the trapped air. The colour varies from black through grey to white.

The term ‘Bimstuff’ refers to the grain size, with at least 75 per cent volcanic ash. Pumice soils have good water-retention properties and are very well suited to viticulture. It is found throughout the Greek island of Santorini, which was formed by a volcanic eruption. Obsidian is similar to pumice, but contains considerably less carbon dioxide. See also under ‘Canava’ and below under ‘Volcanic rock’.

Blue slate 
See below under ‘Slate’.

Boulbènes
A term commonly used in Bordeaux to describe a very fine, siliceous soil. It is found, for example, on the plateau of the French region of Entre-deux-Mers.

Brown earth
These A-B-C soils develop primarily on rocks that are low in lime but rich in bases, such as granite, gneiss, greywacke, clay slate and clayey sandstone. They formed under humid climatic conditions from humus-rich topsoils on low-lime silicate rock (Ranker) with deciduous and mixed woodland.

The characteristic brown colouration in the B-horizon is caused by iron oxides formed during the chemical weathering of iron-bearing silicates. The acids released by tree roots contributed significantly to the deep weathering of the B-horizon. The lime content, stone content and water balance of brown soils can vary considerably. Depending on their composition, these can be excellent soils for viticulture.

Parabraunerde differs from Braunerde in that clay particles have been transported from upper to deeper layers. This is a process that occurs during soil acidification. As lime is leached out, the cementing lime structures disappear, so that the released clay particles are washed down...