The Soils of the Marco de Jerez

It is common knowledge that the soils in which vines grow – not to mention the climate - make a huge difference to the character of the wine produced. Given the quality of the wines of the Marco de Jerez, the soils must be something special, and they are, but how did they come about?

We need to go back in time some 70 million years to the start of what geologists call the Tertiary period when the African tectonic plate started to shift north and by about 55 million years ago had pushed up and inverted sediments to form the rock of Gibraltar and other limestone outcrops. This had two effects. It caused a deep depression between the area to the north, behind the rock, and the Sierra Morena which is a much older and harder upthrust and runs on an approximate line from Huelva to Sevilla to Córdoba, or roughly along what is now the course of the river Guadalquivir. It also closed off what is now the Mediterranean causing it to dry up eventually, the modern Strait of Gibraltar only being created some 13 million years ago.

The Guadalquivir (or Betic) depression became an inland sea for millions of years during which time huge amounts of marine sediments were deposited consisting mostly of vast amounts of plankton and coral skeletons, shells and bones which were slowly compressed into limestone and chalk up to 100 metres thick. Continual tectonic pressure causing gentle uplift and rivers washing down vast amounts of sediment, along with falling sea levels from about 36 million years ago during the Oligocene period, caused the sea to gradually disappear. This left a clay and sand overlay which was gradually eroded over millions more years, helped by three tsunamis over the last 4,000 years, to form the varied soil and subsoil conditions prevailing today and expose the albariza. If the ice caps melt and sea levels rise again, the Jerez area may well disappear once again beneath the waves.

This chart shows the extent of Guadalquivir sedimentation (geocching.com)

The Sherry vineyards currently amount to almost 7,000 hectares in which there are three general soil types: albariza, barro and arena. Some two thirds of the vineyards are albariza with the barros and arenas making up the rest in roughly equal proportions, though albariza’s share will have increased a bit since the uprooting of excess vineyards. Only vineyards with enough albariza can be classified as Jerez Superior. Listed below are the more important soil types.

This is albariza (foto:Ramiro Ibanez)

Albarizas are without doubt the finest soils for Sherry production and are almost pure white as they contain up to 80% calcium carbonate, or chalk. They also contain clay, sand and silica as well as various minerals such as magnesium, iron, gypsum and of course marine fossils. These soils are low in nitrogen and organic matter making them less fertile, and yields are therefore lower. Various names are given to the albarizas according to their chalk content and composition:

Tajón (or Tejón): hard and compact with about 80% chalk

Tosca (or Tosca cerrada or antehojuela): contains at least 60% chalk and some fine sand and clay in thin layers, highly absorbent

Lentejuela: softer more powdery and absorbent with around 50% chalk and some clay and sand, very easy to work

Barrejuela (or Barajuela): around 50% chalk but streaked or even layered with ochre (ferric oxide mixed with clay) and easy to work

Lustrillo: chalk content of about 30-50% with some gypsum, sand and clay

Albarizas are the lowest yielding of the Sherry soils, but produce the best wine. They act like a sponge, absorbing huge amounts of rainwater, and during the hot summer months the surface dries over retaining the water below for the vine roots. The softness of this soil allows the vines to develop good root systems which can grow up to 6 metres deep, while the clay content ensures that the minimum amount of water can evaporate from the surface of the soil. Albariza is where the Palomino grape reigns supreme, though Pedro Ximénez is also grown in it, albeit usually on the lower slopes.

Barro means “clay” and is the main constituent of this soil which is mixed with up to 30% chalk and some sand. It tends to occur in dips in the landscape called “bujeos” and contains more iron, giving it a fairly dark brown colour, and more organic matter than albariza. It tends to crack in the heat of summer and is more prone to weeds, and while Palomino is grown here, the wines are coarser and fuller bodied, and the yields are higher.

Arenas means “sand” and this soil can contain over 70% of it, mixed with some chalk and clay, and the latter tends to bind it so the sand is not always loose. It is usually found near the coast and often overlies clay, and its content of iron oxide gives it a reddish yellow colour, while it has a fairly high content in aluminates and silica. The Moscatel grape is not as fussy as some varieties about soils, and is happy enough in most, but it has been very successful in the coastal sandy soils of Chipiona.

This map from the Consejo Regulador shows soil distribution. Marismas are marshes

Viticulture in Jerez: 1. Landscape, soils and climate

The Sherry production zone is in the north west of the province of Cadiz and north of the city, broadly between the rivers Guadalquivir and Guadalete, and consists of a triangle of towns. Jerez de la Frontera is about 20 km inland from the Atlantic, and the other two are coastal; Sanlucar de Barrameda in the north at the mouth of the Guadalquivir and opposite the famous marshy nature reserve called Coto Donana, and Puerto de Santa Maria in the South at the mouth of the Guadalete.

The landscape is low-lying, rolling country, much of it protected, dotted with small lakes and cork trees - not to mention lots of wind farms which are rarely overworked. Currently the Sherry vineyards cover 8,800 hectares and are located at modest altitudes from about 0 - 150 metres above sea level, generally facing south east. The land here was once under the sea and is made up of marine sediments, principally limestone and clay with lots of chalk, and sandy areas near the coast.

The chalky areas, known as "Albariza", which make up some 62% of the vineyard, are ideal for the Palomino, the main grape by far for Sherry production. The albariza has great properties as a soil, as it acts like a sponge absorbing the winter rains, and dries on the surface in summer, retaining that water. It can be incredibly muddy and slippery when wet and blindingly bright white when dry. Also it can be quite pure with up to 40% calcium carbonate, the rest being clay and silica from long extinct shellfish. Soils such as these are easy to work for humans, and easy for the vines to extend their root systems. Most of the famous "Pagos" or top vineyards, of which there about 300, are to be found on albariza.

The heavier clay soils, called "Barros", and the sandy coastal soils, called "Arenas" accommodate the less fussy grape, the Moscatel. It is happy on any old soil. The other important grape, the Pedro Ximenez - or PX - is generally found on albariza. Below left is a shot of albariza banked up to catch the maximum winter rain. This is called "aserpiado". In the spring, the soil will be levelled off again. Below right is a shot of the 3 soil types, showing just how white is the albariza.


     

The climate in these parts is another major factor in Sherry production. Jerez is close to parallel 36 north, and is actually not far from Africa, so the summers are hot, up to over 40C and dry with mild winters supplying about 600 litres per square metre of rainfall. This, along with dewfall from the sea is sufficient. The average annual temperature is 17.3 C and annual hours of sunshine amount to over 3,000. The nearby Atlantic moderates the weather, which is subject two two prevailing winds, the Poniente, a cool damp wind from the west, and the Levante, a hot dry wind from the east. Frost is a rarity, thank goodness. The weather varies a little from area to area. It can be a touch hotter in Jerez being further inland with a little less sea breeze.