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