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The effects of soil on the taste of wine
GSA TODAY | MAY 2016 Gregory J. Retallack, Dept. of Geological Sciences, University of
Oregon, Eugene, Oregon 97403, USA, gregr@uoregon.edu; and
Scott F. Burns, Dept. of Geology, Portland State University,
Portland, Oregon 97207, USA, burnss@pdx.edu
ABSTRACT
The conventional wisdom of vintners is that alkalinity, and thus
less sour and more rounded taste, are enhanced in wine and
grapes challenged by low-nutrient soils. A common thread here is
pH, an objectively measurable variable that is both a part of wine
taste and a proxy for soil fertility. The role of low-pH soils is
supported by metadata on Oregon wines from different soils in
the Willamette Valley of Oregon, USA, which show significant
inverse correlations between minimum pH of the soil and pH of
finished Pinot Noir wine. There is also a direct correlation
between depth of clayey horizons and pH of the finished wine.
The minimum pH of these soils is near the base of the clayey (Bw
or Bt) horizon and is inversely correlated with depth of the clayey
horizon. Low soil pH is found in thick middle Pleistocene soils of
bedrock (Jory, Willakenzie, Laurelwood, and Bellpine soil series)
and high soil pH in thin soils on late Pleistocene and Holocene
Missoula Flood deposits and loess (Hazelair, Woodburn, and
Chehulpum soil series). Similar relationships are found between
soil pH or depth and the pH of grapes at harvest, which is lower
and more varied than pH in finished wine. These relationships are
especially notable in years of good harvest, but obscured by wine-
making techniques in years of poor harvest. Good harvest years
are not necessarily vintages esteemed by wine connoisseurs, which
are more strongly correlated with low October precipitation.
INTRODUCTION
The effects of soil on wine are a key component of the French
concept of gôut de terroir (taste of soil), first codified in 1905 legis-
lation of Appellation d’origine contrôlée (Trubek, 2008). The
French concept of soil at the time was less scientific than romantic
and political, as revealed by Emile Zola’s (1888) famous novel La
Terre (The Soil). An expanded concept of wine terroir, including
local climate and winemaking traditions, can be traced back to
fifteenth-century Burgundian monks, but the naming of local
wines after localities and comparisons of their relative quality
were recorded in ancient Rome, Greece, and Egypt, back about
4,980 years ago (McGovern, 2003). Nevertheless, it has been diffi-
cult to find a scientific justification for judgements like the
following quotation: “The sandy soil will, in general, produce a
delicate wine, the calcareous soil a spirituous wine and the
decomposed granite a brisk wine” (Busby, 1825, p. 11). In contrast
is the opinion of Maltman (2008, p. 1), “The notion of being able
to taste the vineyard geology in the wine—a goût de terroir—is a
romantic notion that makes good journalistic copy and is mani-
festly a powerful marketing tactic, but it is wholly anecdotal and
GSA Today, v. 26, no. 5, doi: 10.1130/GSATG260A.1.
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in any literal way is scientifically impossible.” Maltman particu-
larly decries descriptions of wines as tasting “slaty” or “earthy.”
Between these extremes of literally tasting soils in wine and the
implausibility of transferring tastes from soil to wine are
numerous studies documenting soils’ effects on wine quality
(Imre et al., 2012; Costantini et al., 2012; Burns, 2012), including
this study. Here we address the conventional wisdom of vintners
that low-fertility soils produce more profitable wines (Goode,
2014) in a case study of pH as a proxy for soil fertility compared
with the pH of Pinot Noir wines produced from that soil in the
Willamette Valley of Oregon, USA.
Wine within the usual pH range of 3.4–3.8 tastes pleasantly
fresh, brisk or tart, but with too much acid it can be as sour as
vinegar, and too little acid leaves it flat and prone to spoilage
(Goode, 2014). Wines are 80%–90% water and 0.1%–20% sugar,
with pH determined by a balance between 0.3%–1% acids
(tartaric, malic, citric, lactic) and mildly alkaline alcohol
(8%–20% ethanol, glycerol), organic compounds (0.3%–1%
flavor compounds, such as anthocyanins, tannins, and flavo-
noids), and mineral cations (0.1%–0.3% potassium, sodium,
calcium, and magnesium; Jackson, 1994). Pinot Noir wine has
more than 800 distinct organic compounds, which determine
aroma, color, and flavor (Fang and Qian, 2005).
Soil pH is a convenient proxy for fertility. Moderately acidic
soils (pH 4.5–5.8) are low in plant nutrients (Ca2+, Mg2+, Na+, K+)
because the exchange complex has high amounts of non-nutrient
cations (H+, AlOH2+, Al(OH)2+). Moderately alkaline soils (pH
8–10) have growth-limiting salts and moisture deficits. The most
fertile soils for plant growth are between these extremes
(Retallack, 2001).
GEOLOGICAL AND PEDOLOGICAL BACKGROUND
Oregon’s Willamette Valley is a tectonic forearc basin on the
convergent margin of the northwestern United States, dividing an
uplifted subduction complex of the Coast Range from active
andesitic volcanoes of the Cascade Range. Much of the valley is
underlain by volcaniclastic marine sandstones and siltstones
ranging in age from Eocene to Oligocene, but parts of the region
were overrun by middle Miocene Grande Ronde and Wanupum
Basalts of the Columbia River Basalt Group (Yeats et al., 1996).
Large areas of the valley floor are covered by Willamette silts from
the 15–18 ka Missoula Floods (O’Connor et al., 2001; Allen et al.,
2009). These three geological elements determine three main
kinds of soils planted in vineyards (Moore, 2002; Burns, 2012):
(1) middle to late Pleistocene soils on Miocene basalt, (2) on Eocene-
Oligocene sedimentary rock, and (3) late Pleistocene soils on loess
or alluvium (Fig. 1).
The thick Jory silt loam was named the state soil of Oregon for
its importance to the wine industry (Oregon State Legislature,
2011). It includes two distinct varieties developed on bauxitic
laterite and basalt. At the type locality of Jory Hill south of Salem,