SOIL

Topsoil is of primary importance to the vine because it supports most of its root system, including most of the feeding network. Subsoil always remains geologically true. Main roots penetrate several layers of subsoil, whose structure influences drainage, the root system’s depth, and its ability to collect minerals.

The metabolism of the vine is well known, and the interaction between it and the soil is generally understood. The ideal medium in which to grow vines for wine production is one that has a relatively thin topsoil and an easily penetrable (and therefore well-drained) subsoil with good water-retaining characteristics. The vine does not like “wet feet”, so drainage is vital, yet it needs access to moisture, so access to a soil with good water retention is also important. The temperature potential of a soil, its heat-retaining capacity, and its heat-reflective characteristics affect the ripening period of grapes: warm soils (gravel, sand, loam) advance ripening, while cold soils (clay) retard it. Chalk falls between these two extremes, and dark, dry soils are obviously warmer than light, wet soils. High-pH (alkaline) soils, such as chalk, encourage the vine’s metabolism to produce sap and grape juice with a relatively high acid content. The continual use of fertilizers has lowered the pH level of some viticultural areas in France, and these are now producing wines of higher pH (less acidity).

 

THE MINERAL REQUIREMENTS OF THE VINE

Just as various garden flowers, shrubs, and vegetables perform better in one soil type as opposed to another, so too do different grape varieties. Certain minerals essential to plant growth are found in various soils. Apart from hydrogen and oxygen (which are supplied as water), the most important soil nutrients are nitrogen, which is used in the production of a plant’s green matter; phosphate, which directly encourages root development and indirectly promotes an earlier ripening of the grapes (an excess inhibits the uptake of magnesium); potassium, which improves the vine’s metabolism, enriches the sap, and is essential for the development of the following year’s crop; iron, which is indispensable for photosynthesis (a lack of iron will cause chlorosis); magnesium, which is the only mineral constituent of the chlorophyll molecule (lack of magnesium also causes chlorosis); and calcium, which feeds the root system, neutralizes acidity and helps create a friable soil structure (although an excess of calcium restricts the vine’s ability to extract iron from the soil and therefore causes chlorosis).

 

GUIDE TO VINEYARD SOILS

To the wine amateur, the details of geology are not always important; what matters is how soil affects the growth of vines. If one clay soil is heavier or more silty, sandy, or calcareous, that is relevant. But there is enough jargon used when discussing wine to think of mixing it with rock-speak.

Acid soil Any soil that has a pH of less than 7 (neutral). Typical acidic soils that are acidic due to their parent rock include brown or reddish-brown, sandy loams or sands, volcanic soils, and any igneous or silicate-rich soil. Neutral soils can become acidic from too much humus or acid rain. Acid soils are low in calcium and magnesium, with negligible amounts of soluble salts and reduced phosphorous availability.

Aeolian soil Sediments deposited by wind (eg, loess). Albariza White-surfaced soil formed by diatomaceous deposits, found in southern Spain.

Alberese A compact clay and limestone found in the Chianti region. Albero Synonymous with albariza.

Albian A type of schist found in Maury, Roussillon.

Alkaline soil Any soil that has a pH of more than 7 (neutral). Typical alkaline soils include chalk and any calcareous soils.

Alluvial deposits (noun – alluvium) Material that has been transported by river and deposited. Most alluvial soils contain silt, sand, and gravel and are highly fertile.

Aqueous rocks One of the three basic rock forms (see Rock). Also called sedimentary or stratified.

Arenaceous rocks Formed by the deposits of coarse-grained particles, usually siliceous, and often decomposed from older rocks (eg, sandstone).

Arène A coarse, granitic sand ideally suited to the Gamay, arène is found in the Beaujolais region.

Argillaceous soil This term covers a group of sedimentary soils, commonly clays, shales, mudstones, siltstones, and marls.

Argovian marl A chalky, clay-like marl found in many parts of the Côte des Beaune.

Arkose A red, Triassic sandstone consisting of feldspar, quartz, and clay minerals, arkose is often found in the Côtes d’Auvergne and parts of Beaujolais (eg, St-Amour).

Aubuis Found in the Touraine district of the Loire and highly rated for Chenin Blanc in Vouvray and Montlouis, aubuis is a stony mix of permeable, fertile, calcareous clays that are said to be well suited to white grape varieties.

Barro A similar soil to albariza but brown in colour, sandier, and with less diatomaceous content. While Palomino grapes are grown on albariza soil, barro is reserved for Pedro Ximénez grapes.

Basalt material This accounts for as much as 90 per cent of all lava-based volcanic rocks. It contains various minerals, is rich in lime and soda, but not quartz, the most abundant of all minerals, and it is poor in potash.

Bastard soil A Bordelais name for medium-heavy, sandy-clay soil of variable fertility.

Bauxite As well as being a valuable ore mined for aluminium production, bauxite is found in limestone soils of Coteaux de Baux-de-Provence.

Block-like soil Referring to the soil structure, “block-like” indicates an angular or slanting arrangement of soil particles.

Boulbènes A Bordelais name for a very fine siliceous soil that is easily compressed and hard to work. This “beaten” earth covers part of the Entre-Deux-Mers plateau.

Boulder See Particle size

Calcareous clay Argillaceous soil with carbonate of lime content that neutralizes the clay’s intrinsic acidity. Its low temperature also delays ripening, so wines produced on this type of soil tend to be more acidic.

Calcareous soil Any soil, or mixture of soils, with an accumulation of calcium and magnesium carbonates. Essentially alkaline, it promotes the production of acidity in grapes, although the pH of each soil will vary according to its level of “active” lime. Calcareous soils are cool, with good water retention. With the exception of calcareous clays (see above), they allow the vine’s root system to penetrate deeply and provide excellent drainage. Carbonaceous soil Soil that is derived from rotting vegetation under anaerobic conditions. The most common carbonaceous soils are peat, lignite, coal, and anthracite.

Chalk A type of limestone, chalk is a soft, cool, porous, brilliant-white, sedimentary, alkaline rock that encourages grapes with a relatively high acidity level. It also allows the vine’s roots to penetrate and provides excellent drainage, while at the same time retaining sufficient moisture for nourishment. One of the few finer geological points that should be adhered to is that which distinguishes chalk from the numerous hard limestone rocks that do not possess the same physical properties.

Clay A fine-grained argillaceous compound with malleable, plastic characteristics and excellent water-retention properties. It is, however, cold, acid, offers poor drainage, and, because of its cohesive quality, is hard to work. An excess of clay can stifle the vine’s root system, but a proportion of small clay particles mixed with other soils can be advantageous.

Clayey-loam A very fertile version of loam, but heavy to work under wet conditions, with a tendency to become waterlogged.

Cobble See Particle size Colluvial deposits (noun – colluvium) Weathered material transported by gravity or hill-wash.

Crasse de fer Iron-rich hard-pan found in the Libournais area of France. Also called machefer.

Crystalline May be either igneous (eg, granite) or metamorphic.

Dolomite A calcium-magnesium carbonate rock. Many limestones contain dolomite.

Entroques Type of hard limestone found in Burgundy (eg, Montagny).

Feldspar or Felspar One of the most common minerals, feldspar is a white- or rose-coloured silicate of either potassium-aluminium or sodium-calcium-aluminium and is present in a number of rocks, including granite and basalt.

Ferruginous clay Iron-rich clay.

Flint A siliceous stone that stores and reflects heat and is often associated with a certain “gun-flint” smell that sometimes occurs in wines, although this is not actually proven and may simply be the taster’s auto-suggestion.

Gabbro A dark, coarse-grained igneous rock found in Muscadet.

Galestro Rocky, schistous clay soil commonly found in most of Tuscany’s best vineyards.

Glacial moraine A gritty scree that has been deposited by glacial action.

Gore A pinkish, decomposed, granitic arenaceous soil found in Beaujoalais, St Joseph, and Côtes Roannaise.

Gneiss A coarse-grained form of granite.

Granite A hard, mineral-rich rock that warms quickly and retains its heat. Granite contains 40 to 60 per cent quartz and 30 to 40 per cent potassium feldspar, plus mica or hornblende, and various other minerals. It has a high pH that reduces wine acidity. Thus, in Beaujolais, it is the best soil for the acidic Gamay grape. It is important to note that a soil formed from granite is a mixture of sand (partly derived from a disintegration of quartz and partly from the decomposition of feldspar with either mica or hornblende), clay, and various carbonates or silicates derived from the weathering of feldspar, mica, or hornblende.

Gravel A wide-ranging term that covers siliceous pebble of various sizes that are loose, granular, airy, and afford excellent drainage. Infertile, it encourages the vine to send its roots down deep in search of nutrients. Gravel beds above limestone subsoils produce wines with markedly more acidity than those above clay.

Greensand A dark greenish coloured, glauconite-rich sand of Cretaceous origin found in some vineyards in southeast England. Greensand is used as a water softener, which is ironic considering that it is found over chalk subsoil, known for its hard water.

Greywacke Argillaceous rocks that could have been formed as recently as a few thousand years ago by rivers depositing mudstone, quartz, and feldspar. Commonly found in Germany, South Africa, and New Zealand.

Gypsum Highly absorbent, hydrated calcium-sulphate that was formed during the evaporation of sea-water.

Gypsiferous marl A marly soil permeated with Keuper or Muschelkalk gypsum fragments, which improve the soil’s heat-retention and water-circulation properties.

Hard-pan A dense layer of clay that forms if the subsoil is more clayey than the topsoil at certain depths. As hard-pans are impermeable to both water and roots, they are not desirable too close to the surface but may provide an easily reachable water-table if located deep down. A sandy, iron-rich hard-pan known as iron-pan is commonly found in parts of Bordeaux.

Hornblende A silicate of iron, aluminium, calcium, and magnesium, it constitutes the main mineral found in basalt and is a major component of granite and gneiss.

Humus Organic material that contains bacteria and other micro-organisms that are capable of converting complex chemicals into simple plant foods. Humus makes soil fertile; without it, soil is nothing more than finely ground rock.

Igneous rock One of the three basic rock forms (see Rock), igneous rocks are formed from molten or partially molten material. Most igneous rocks are crystalline.

Iron-pan A sandy, iron-rich hard-pan.

Jory A volcanic soil, primarily basalt, which is in turn a hard and dense soil that often has a glassy appearance. One of the two primary soil types found in Oregon’s Willamette Valley, particularly on the lower foothills, such as the Dundee Hills, where Pinot Noir excels.

Keuper Often used when discussing wines in Alsace, Keuper is a stratigraphic name for the Upper Triassic period and can mean marl (varicoloured, saliferous grey, or gypsiferous grey) or limestone (ammonoid).

Kimmeridgian soil A greyish-coloured limestone originally identified in, and so named after, the village of Kimmeridge in Dorset, England. A sticky, calcareous clay containing this limestone is often called Kimmeridgian clay.

Lacustrine limestone A freshwater limestone that forms at the bottom of lakes. Lacustrine-limestone soils have been found on Pelee Island and the Niagara district of Ontario, Yakima Valley in Washington, and Quincy in the Loire Valley.

Lignite The “brown coal” of Germany and the “black gold” of Champagne, this is a brown carbonaceous material intermediate between coal and peat. Warm and very fertile, it is mined and used as a natural fertilizer in Champagne.

Limestone Any sedimentary rock consisting essentially of carbonates. With the exception of chalk, few limestones are white, with grey- and buff-coloured probably the most commonly found limestone in wine areas. The hardness and water retention of this rock vary, but being alkaline it generally encourages the production of grapes with a relatively high acidity level.

Loam A warm, soft, crumbly soil with roughly equal proportions of clay, sand, and silt. It is perfect for large-cropping mediocre-quality wines but too fertile for fine wines.

Loess An accumulation of wind-borne, mainly silty material, that is sometimes calcareous but usually weathered and decalcified. It warms up relatively quickly and also has good water-retention properties.

Machefer See Crasse de fer

Macigno Hard grey-blue sandstone found in the Chianti region.

Marl A cold, calcareous clay-like soil (usually 50 per cent clay content) that delays ripening and adds acidity to wine.

Marlstone Clayey limestone that has a similar effect to marl.

Metamorphic rocks One of the three basic categories of rock (see Rock), this is caused by great heat or pressure, often both.

Mica A generic name encompassing various silicate minerals, usually in a fine, decomposed-rock format. Millstone Siliceous, iron-rich, sedimentary rock.

Moraine See Glacial moraine

Mudstone A sedimentary soil similar to clay but without its plastic characteristics.

Muschelkalk Often used when discussing wines in Alsace, Muschelkalk is a stratigraphic name for the Middle Triassic period and can mean anything from sandstone (shelly, dolomitic, calcareous, clayey, pink, yellow, or millstone) to marl (varicoloured or fissile), dolomite, limestone (crinoidal or grey), and shingle.

Oolite A type of limestone. Oolith A term used for small, round, calcareous pebbles that have grown through fusion of very tiny particles.

Palus A bordelais name for a very fertile soil of modern alluvial origin that produces medium-quality, well-coloured, robust wines.

Particle size The size of a rock determines its descriptive name. No handful of soil will contain particles of a uniform size, unless it has been commercially graded, of course, so all such descriptions can only be guesstimates, but it is worth noting what they should be, otherwise you will have nothing to base your guesstimates on. According to the Wentworth-Udden scale, they are: boulder (greater than 256mm), cobble (64mm–256mm), pebble (4mm–64mm), gravel (2mm–4mm), sand (1⁄16mm–2mm), silt (1⁄256mm–1⁄16mm) and clay (smaller than 1⁄256mm). Notice that even by this precise scale, Wentworth and Udden have allowed overlaps, thus a 1⁄16mm particle might either be sand or silt and, of course, sub-divisions are possible within each group, as there is such a thing as fine, medium, or coarse sand and even gritty silt.

Pebble See Particle size

Pelite Fine-grained clayey-quartz sedimentary rock found in Banyuls.

Peperite Limestone or marly rock found on Madeira and along Idaho’s Snake River Valley that has been ejected by volcanic activity and is literally “peppered” with tiny peppercorn-like grains of basalt.

Perlite A fine, powdery, light, and lustrous substance of volcanic origin with similar properties to diatomaceous earth.

Perruches Very stony, flinty clays combined with silica, perruches soils warm up quickly and are said to be why Sauvignon Blanc grown on them have a flinty taste.

Phtanite Dark-coloured sedimentary rock bearing stratas of quartz crystals, found in Savennières and Coteaux du Layon Platy soil Referring to the soil structure, “platy” indicates a horizontal alignment of soil particles.

Porphyry A coloured igneous rock with high pH.

Precipitated salts A sedimentary deposit. Water charged with acid or alkaline material, under pressure of great depth, dissolves various mineral substances from rocks on the sea-bed, which are then held in solution. When the water flows to a place of no great depth or is drained away or evaporates, the pressure is reduced, the minerals are no longer held in solution and precipitate in deposits that may be just a few centimetres or several thousand metres deep. There are five groups: oxides, carbonates, sulphates, phosphates, and chlorides.

Prism-like soil Referring to the soil structure, “prism-like” indicates a columnar or vertical arrangement of soil particles.

Pudding stones A term used for a large, heat-retaining conglomerate of pebbles.

Quartz The most common and abundant mineral, quartz is the crystalline form of silica. It is found in various sizes and in almost all soils, although sand and coarse silt contain the largest amount. Quartz has a high pH, which reduces wine acidity, but quartz that is pebble-sized or larger, stores and reflects heat, which increases alcohol potential.

Red earth See Terra rossa

Rock A rock may be loosely described as a mass of mineral matter. There are three basic types of rock: igneous, metamorphic, and sedimentary (or aqueous or stratified).

Ruedas Red sandy-limestone soil found in the Montilla-Moriles region of Spain.

Ruffe A fine-grained, brilliant-red sandstone soil rich in iron-oxide, ruffe is found in parts of the Languedoc region of France, particulary the Vin de Pays des Coteaux de Salagou.

Safres A sandy-marl found in the southern Rhône Valley. Saibro A decomposed red tufa soil that is highly regarded in Madeira.

Sand Tiny particles of weathered rocks and minerals that retain little water but constitute a warm, airy soil that drains well and is supposedly phylloxera-free.

Sandstone Sedimentary rock composed of sand-sized particles that have either been formed by pressure or bound by various iron minerals.

Sandy-loam Warm, well-drained, sand-dominated loam that is easy to work and suitable for early-cropping grape varieties.

Schist Heat-retaining, coarse-grain, laminated, crystalline rock that is rich in potassium and magnesium but poor in nitrogen and organic substances.

Scree Synonymous with colluvium deposits.

Sedimentary rock One of the three basic rock forms (see Rock), it includes arenaceous (eg, sandstone), argillaceous (eg, clay), calcareous (eg, limestone), carbonaceous (eg, peat, lignite, or coal), siliceous (eg, quartz), and the five groups of precipitated salts, (oxides, carbonates, sulphates, phosphates, and chlorides). Sedimentary rocks are also called aqueous or stratified.

Shale Heat-retaining, fine-grain, laminated, moderately fertile sedimentary rock. Shale can turn into slate under pressure.

Shingle Pebble- or gravel-sized particle rounded by water-action. Siliceous soil A generic term for acid rock of a crystalline nature. It may be organic (such as flint) or inorganic (quartz) and have good heat retention, but no water retention unless found in a finely ground form in silt, clay, and other sedimentary soils. Half of the Bordeaux region is covered with siliceous soils.

Silt A very fine deposit, with good water retention. Silt is more fertile than sand but is cold and offers poor drainage.

Slate Hard, often dark grey (but can be any colour between brown and bluish grey), fine-grain, plate-like rock formed under pressure from clay, siltstone, shale, and other sediments. It warms up quickly, retains its heat well, and is responsible for many fine wines, most notably from the Mosel.

Slaty-schist A sort of half-formed slate created under lower temperature and pressure than fully formed slate.

Spiroidal soil Referring to the soil structure, “spiroidal” indicates a granular or crumb-like composition of soil particles.

Steige A type of schist found on the north side of Andlau in Alsace, it has metamorphosed with the Andlau granite and is particularly hard and slaty. It has mixed with the granitic sand from the top of the Grand Cru Kastelberg and makes a dark, stony soil. Stone This word should be used with rock types, such as limestone and sandstone, but is often used synonymously with pebble.

Stratified rock One of the three basic rock forms (see Rock); also called sedimentary or aqueous.

Terra rossa A red, clay-like, sometimes flinty sedimentary soil that is deposited after carbonate has been leached out of limestone. It is often known as “red earth”.

Terres blanches Steep Kimmeridgian marls in Sancerre.

Tufa A limestone concretion that forms via water dripping through gaps in limestone, tufa is typical of the soil of Orvieto, Umbria, and is also found in Montalcino, Tuscany, as well as the Langhe region of Piedmont.

Tuff Rocks formed by fractured or water-bound material ejected by volcanic activity, tuff drains well and is found in Taburno, Campania, in Italy; Balatonfüred-Csopak, Balatonfelvidék and Balatonboglár around Lake Balaton in Hungary; and the Galilee region of Israel, particularly Upper Galilee and the Golan Hights.

Tuffau A buff-coloured, sandstone-rich, otherwise chalky limestone as found in the Loire, particularly around Touraine, and used in the construction of many of its châteaux.

Volcanic soils Derived from two sources, volcanic soils are lava-based (the products of volcanic flow) and vent-based (material blown into the atmosphere). Some 90 per cent of lava-based rocks and soils are comprised of basalt, while others include andesite, pitchstone, rhyolite, and trachyte. Vent-based matter has either been ejected as molten globules, cooled in the air, and dropped to earth as solid particles (pumice), or as solid material and fractured through the explosive force with which it was flung (tuff).

Willakenzie A silty clay-loam colluvium, this is one of the two primary soil types found in Oregon’s Willamette Valley.

Originally published in The Sotheby’s Wine Encyclopedia. © Tom Stevenson