Wednesday, May 30, 2012

The science of viticulture: Soils and vineyard site selection

The vineyard site is a key variable in the production of high-quality grapes, which is, itself, a key requirement for the production of high-quality wine.  In the absence of an existing vineyard, the viticulturist has to seek out a site on which the wine grapes will be planted.  I have detailed the climatological and topographic requirements for such a vineyard in recent posts and will devote this instance to soils, the final -- and second only to climate -- site-selection requirement.

According to Wolf and Boyer (Vineyard Site Selection, Virginia Cooperative Extension), the best vineyard soils "permit deep and spreading root growth" and provide a moderate supply of water year-round.  Mark Chien (Soil and Site Selection Considerations for Wine Grape Vineyards, Pennsylvania State University) posits that wine grapes do best in moderately fertile soils that are unsupportive of vigorous vine growth.  What are the soil characteristics that will permit "deep and spreading root growth" and year-round access to water?  Those characteristics are presented in descending order of importance in the table below.

The most important requirements, according to the table, are internal water drainage and water-holding capacity.  Geologic permeability (the capability of a porous rock or sediment to permit the flow of fluids through its pore spaces -- is seen by Wolf and Boyer as perhaps the most important consideration in a candidate vineyard's soil.  Chien sees well-drained soils as a  common denominator among all great vineyard sites.  These soils "strike a balance between adequate depth and drainage and water-holding capacity" and vines deployed therein will have adequate water access during the summer and can rapidly drain water from the soils in the event of rainfall during the grape-ripening period.  Vineyards sited on convex land patterns are preferable to those on concave landforms in that the former shed surface water while the latter import water as well as soils which erode from higher ground.

Vineyard soil fertility is one of those cases where more is not necessarily better. Adequate amounts of the appropriate nutrients are required to support proper growth of the vine, fruit development, and fruit maturity.  Fertile soils are generally rich in organic material and moisture.  In that grapevines are naturally vigorous, vines grown in highly fertile vineyards will produce abundant canopies and fruit but the fruit will be mediocre because of limited access to the sun and the vine having spread its resources too thinly. Most high-quality vineyards are sited on low-/moderate-fertility soils.

The next soil feature mentioned in the table is effective rooting depth.  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 root structure to meet these varied needs.  First, quick-growing, short-lived roots deployed close to the service 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.

Soil texture refers to the nature, size, shape, orientation, and arrangement of particles.  In the soil-type page I show 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.

Soil pH is a measure of the acidity (3.5 to 6.5) or alkalinity (7.4 to 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 6.0 and 6.8 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 treated to bring them closer to optimal.

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 and others

The key factors in nutrient availability are soil type, soil composition, and root structure.  Soil type dictates the quality of nutrients present and the adequacy of water drainage.  The 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.  The roots of the vine plant, among other things, absorb water and nutrients and store the nutrients that nourish the plant during dormancy.  The principal roots of the plant are tasked with nutrient delivery and storage.

By this time the viticulturist will have been exposed to enough information to select a vineyard site which will have a better than even chance of producing high-quality grapes.  All of the pertinent questions about climate, aspect, slope, elevation, and soils have been raised and a path to high quality has been carved out.  The next task will be setting up the vineyard on the selected site.

© Wine -- Mise en abyme

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