Chapter 4

Vineyard Site Selection

Soils for Grapes

Soil affects grapevine productivity and wine quality, but soil, like climate, it is made up of many components including physical, chemical, and biological properties. All of these variables may ultimately affect vine growth and wine quality, but precise relationships are not well characterized for all such variables. Furthermore, the influences of vineyard management, climate, varieties, and rootstocks as well as variation winery practices may easily obscure the unique soil contributions to wine quality. For these reasons, it is difficult to describe an ideal soil for a vineyard.

USDA Soil Survey Maps

To begin identifying a soil type for a particular vineyard site the United States Department of Agriculture (USDA) soil survey maps should be referenced. Hardbound soil surveys for each county can be found at National Resource Conservation Service (NRCS) and usually at state cooperative extension offices, local libraries, or online. A soil survey report reveals the kinds of soil classes that exist in the county (or other area) covered by the report at a level of detail that is usually sufficient for vineyard site selection. The soils are described in terms of their location on the landscape, their profile characteristics, their relationships to one another, their suitability for various uses, and their needs for particular types of management.

Mapping Soil Electrical Conductivity

In addition to soil survey data, commercial services are available to develop “real time” field maps illustrating spatial patterns of soil variability within a parcel of land. The equipment used to develop soil electrical conductivity (EC) maps consists of Geonics EM38 electromagnetic sensors (See Figure 4.6), Veris electrical conductivity sensors, global positioning systems (GPS), and global information system software (GIS). GPS and GIS are discussed in more detail in Chapter 34, Precision Viticulture.

Electrical Conductivity

The electrical conductivity of soils varies depending on the amount of moisture held by soil particles. Sands have a low conductivity, silts have a medium conductivity, and clays have a high conductivity. Consequently, EC correlates strongly to soil particle size and texture.

Evaluating Soil Properties

Over the years, soil scientists have used EC to measure soil salinity. However, soil EC measurements also have the potential for estimating variations in soil physical properties where soil salinity is not a problem, including mapping soil types; characterizing soil water content and flow patterns; assessing variations in soil texture, compaction, organic matter content, and pH; and determining the depth to subsurface horizons, stratigraphic layers or bedrock, among other uses.

Excavating Soil Profiles to Evaluate Soils

Once an area has been identified as potentially suited for growing grapevines, a detailed soil survey is carried out, often by digging pits with a backhoe. A thorough site evaluation uses a series of excavated soil profiles in different areas of the vineyard. As a rule of thumb, it is advisable to dig at least one backhoe pit per 20 acres at a vineyard site and possibly more if evidence indicates a high degree of soil variability. Information from soil surveys, electrical conductivity mapping, and knowledge of other crops grown on the land can be used to determine more precisely where to dig pits.

Preplant Soil Sampling

Preplant soil sampling is critical for profitable wine grape production. For this reason, it is important to thoroughly assess soil properties before the vineyard is established. The costs for an in-depth assessment of soil properties are much lower than the costs accruing from planting a vineyard at an inappropriate site. Soil sampling before planting serves four major purposes:

Evaluating Physical and Chemical Soil Properties

Grapes can grow successfully in a wide range of soils, although they do have some key soil requirements for good growth without excessive intervention. These are: good drainage (roots require good soil oxygen), appropriate range of pH, reasonable rooting depth, moderate to good soil water holding capacity, moderate fertility, and minimal soil salinity. Soils that are limiting in some of these characteristics can be improved by modifications such as installing drainage tiles, deep ripping of soil to break up restrictive layers, irrigation, lime application to modify soil pH, and appropriate fertilization.

Physical Properties

Viticultural soils have traditionally been chosen by their physical properties. Moisture storage and drainage are considered important for good grape production, especially in areas where irrigation is unavailable or not permitted. Shallow, poorly drained soils tend to be susceptible to waterlogging and moisture deficiency with only average fluctuations in rainfall.

Chemical Properties

While soil type has long been appreciated as a wine quality determinant the direct influence of soil chemical properties on must and wine quality is a contentious area. Traditionally vineyard sites are picked on soil physical properties rather than chemical ones.

Precision Viticulture

Typically vineyard site selection involves many different sources of printed information such as soil survey manuals, topographic maps, aerial photographs, vegetation surveys, flood maps, hydrology maps, soil analysis, and property surveys to name a few. Each source contributes an important characteristic to the final decision. However, it is a challenge to keep track of all this information at once, to understand the interrelationships, and to correlate multiple data sources at single locations. The development and implementation of precision viticulture has made it possible to better manage and analyze different levels of input by combining the global positioning system (GPS), geographic information systems (GIS), and remote sensing technologies.

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