The influence of soils on grape vine growth

The Douro (Portugal) and Carso (Friula-Venezia Giulia, Italy) wine regions are separated by the land mass that is Portugal and Spain, the full width of the Mediterranean Sea, and the Italian land mass but they have in common a lack of a "native" soil.  Even with this challenge, both regions still manage to produce high-quality wines.  I will detail the challenges presented to both regions and the solutions of choice but will precede that discussion with some background on soils and their role in the production of quality wine.

According to Christopher Bargman (Geology and wine in South Africa, Geoscientist 15(4), April 2005), soil is the major influence on the growth of the vine plant as it provides: (i) a supply of water; (ii) anchorage in the ground; and (iii) a source of nutrition.  According to education.mhusa.com, " soil is more than just dirt."  It is, instead, "... a complex system of decomposed rocks that have been enriched over time by decomposed organic matter."  The classic soil profile is shown below.

Source: westone.wa.gov.au

Adequate amounts of the appropriate nutrients are required to support proper growth of the vine, fruit development, and fruit maturity.  The table below shows the mineral requirements of the vine plant, the role of each mineral, acceptable ranges of each mineral in the soil, and the impact of mineral deficiency on the vine.

Source: Compiled from LGRGP.org and others

The key factors in nutrient availability are soil type, soil composition, and root structure.

The soil type dictates the quality of nutrients present and the adequacy of water drainage.  Our soil-type page shows the different types of soil that are of interest to the viticulturist.  The optimal soil type has a moderate content of low cation exchange capability (CEC) clay (Clay minerals act as harbors for nutrients because the positive ions of the nutrients are trapped by the negative charge of the clay minerals.  The abundance and types of minerals determine whether the clay is classed as low- or high-CEC.).

Soil composition affects root growth and development and the availability of nutrients for soil uptake.  Areas of interest here are soil pH, texture, and drainage.

Soil pH is a measure of the acidity (3.5 - 6.5) or alkalinity (7.4 - 9.0) of soil which, through its influence on nutrient solubility and micro-organism activity, affects the number and types of nutrients in the soil. Soil pH between 6 and 7 is considered optimal for vine plant growth as most of the needed nutrients and micro-organisms are available in that range.  Alkaline or acidic soils can be chemically treated to bring them closer to optimal.

Soil texture refers to the nature, size, shape, orientation, and arrangement of particles.  In our soil-type page we showed that sand, silt, and clay have standalone properties which are transformed when the soils are combined.  Clay forms flexible elastic bridges between soil particles to maintain soil structure and preserve porosity.  Pebbles and rocks in clay-rich soils break up the soil, providing pathways for water and root penetration.  Deep, rich soils will provide high-vigor growth and large, watery grapes.

Drainage Water is needed to transport nutrients to the vine plant roots.  Sandy, well-drained soils with little or no clay mineral content may provide inadequate amounts of nutrition, leading to reduced grape quality and a vegetal taste in the wine. Soils which retain too much water will exclude the oxygen needed for the nitrogen cycle.

Root Structure The roots of the vine plant: i) anchor the vine; ii) absorb water and nutrients; iii) store nutrients that nourish the plant during dormancy; and iv) produce hormones that control plant functions. The vine deploys a three-part structure to meet these varied needs.  First, quick-growing, short-lived roots deployed close to the surface are tasked with moisture collection.  Second, subterranean roots provide the anchoring function.  The principal roots are tasked with nutrient delivery and storage.  According to UCDavis, about 60% of the root structure of a vine plant can be found in the first two feet of the surface but individual roots can grow as deep as 20 feet depending on soil permeability, the level of the water table, and the rootstock variety.

Now that we understand the role of soils in the wine grape environment, we can go on to the next step in the Douro and Carso wine regions comparison.