Nutrients for Healthy Vines and Good Wines

The fertility of a soil refers to its nutrient supplying power. It is one of the most important soil factors affecting vineyard productivity, which is measured in tonnes of grapes per ha (or sometimes tons per acre). For viticulture, soil physical prop­erties, notably structure, aeration, and drainage are also very important determi­nants of productivity, as discussed in chapters 3, 6, and 7. Because vines are grown in permanent rows, and there are many cultural operations, soil physical prob­lems are often more difficult to ameliorate than problems of soil fertility. Soil fertility is assessed either by observing the condition of vines growing on a particular soil or by measuring the nutrient supplying power of the soil itself. The assessment should include recommendations on how to correct any problems identified. Thus, assessment of soil fertility can be made in two parts: 1. Diagnosis of nutrient deficiencies or excesses. The aim here is to identify which nutrients are deficient or in excess and the degree of deficiency or excess. An excess of a nutrient, which may create an imbalance with other nutrients, often leads to a nutrient toxicity. 2. Estimation of nutrient requirements. The goal here is to estimate how much of a limiting nutrient is required to achieve optimum growth or how to remedy a toxicity problem. Nutrient amendments can be made with fertilizers, manures, and composts, or by growing cover crops that include legumes. Visual symptoms are the signs that indicate a deficiency or excess of one or more essential elements in a plant. In the case of grapevines, such symptoms include chlorosis, stunted growth of shoots, necrosis of leaf margins, irregular fruit set, and small berries. Chlorosis is a generic term for leaf yellowing due to loss of chlorophyll. N deficiency typically causes an overall chlorosis of the leaves, but in other cases chlorosis occurs between the leaf veins (interveinal chlorosis). Some examples of visual symptoms are given in table 5.1 and figure 5.1.

Soil Quality in Vineyards

The soil must provide a favorable physical environment for the growth of vines—their roots and beneficial soil organisms. Some of the important properties con­tributing to this condition are infiltration rate, soil strength, available water ca­pacity, drainage, and aeration. Ideally, the infiltration rate IR should be >50 mm/hr, allowing water to enter the soil without ponding on the surface, which is predisposed to runoff and erosion. The range of infiltration rates for soils of different texture and structural condi­tion is shown in table 7.1. Typically, the soil aggregates should have a high de­gree of water stability so that when the soil is subjected to pressure from wheeled traffic or heavy rain, the aggregates do not collapse, nor do the clays deflocculate. Some of the problems associated with the collapse of wet aggregates and clay de-flocculation, and the formation of hard surface crusts when dry, are discussed in section 3.2.3. Pans that develop at depth in the soil profile, as a result of remolding of wet aggregates under wheel or cultivation pressure, can be barriers to root growth. Soil strength is synonymous with consistence, which is the resistance by the soil to deformation when subjected to a compressive shear force (box 2.2). Soil strength depends on the soil matrix potential m and bulk density BD, as illustrated in fig­ure 7.1. In situ soil strength is best measured using a penetrometer, as discussed in box 7.1. The soil strength at a ψm of −10 kPa (FC ) should be <2 MPa for easy root penetration and should not exceed 3 MPa at –1500 kPa (PWP). As shown in figure 7.1, when ψm is between −10 and −100 kPa, the soil strength increases with BD. The BD of vineyard soils can increase, particularly in the inter-row areas because of compaction by machinery, such as tractors, spray equip­ment, and harvesters. Typically, compaction occurs at depths between 20 and 25 cm and is more severe in sandy soils than in clay loams and clays (except when the clays are sodic; see section 7.2.3). Figure 7.2 shows the marked difference in soil compaction, measured by penetration resistance, under a wheel track and un­der a vine row on a sandy soil in a vineyard.

How the Soil Supplies Nutrients

Most plants need 16 elements to grow normally and reproduce. Some of these el­ements are required in relatively large concentrations, ideally >1,000 mg/kg (0.1%) in the dry matter (DM); these are called macronutrients. The others, called micronutrients, generally are required in concentrations <100 mg/kg DM (0.01%). Of the essential elements, C and O are supplied as CO2 from the atmosphere, whereas H and O are supplied in H2O from the atmosphere and water sources. Chlorine is also abundant in the air and oceans as the Cl_ ion. Winds whip sea spray containing Cl, Na, Mg, Ca, and S into aerosols to be deposited by rain on the land or as “dry deposition” on vegetation. Nitrogen as N2 gas in the atmo­sphere enters soil–plant systems primarily by “biological fixation” (section 4.2.2.1), although small amounts are also deposited as NH4+ and NO3­_ ions from the air. Cobalt (Co) is essential for biological N2 fixation in legumes and blue-green al­gae. For the remaining essential elements, the major source is minerals that weather in the soil and parent material. Another term frequently used is trace element, which can include both essen­tial and nonessential elements. A trace element normally occurs at a concentra­tion <1,000 mg/kg in the soil. There are three categories of trace elements: 1. The essential micronutrients Cu, Zn, Mn, B, and Mo, which are beneficial at normal concentrations in the plant (ranging from 0.1 mg/kg for Mo to 100 mg/kg for Mn) but which become toxic at higher concentrations. Iron is the only micronutrient that is not strictly a trace element. 2. Elements such as chromium (Cr), selenium (Se), iodine (I), and Co that are not essential for plants, but are essential for animals. 3. Elements such as arsenic (As), mercury (Hg), cadium (Cd), lead (Pb), and nickel (Ni), which are not required by plants or animals and are toxic to either group at concentrations in the organism greater than a few mg/kg. Trace elements in the soil are normally derived from the parent material. Ex­amples of concentrations of trace elements in soils derived from different parent materials are given in table 4.2.

The Vine Root Habitat

In the deep gravelly soils of the Bordeaux region, Seguin (1972) found vine roots at a depth of 6 m. Woody “framework roots” tend to be at least 30–35 cm be­low the surface and do not increase in number after the third year from planting (Richards 1983). Nevertheless, smaller diameter “extension roots” continue to grow horizontally and vertically from the main framework. They may extend lat­erally several meters from the trunk. These roots and finer lateral roots in the zone 10–60 cm deep provide the main absorbing surfaces for the vine. But in soils with a subsoil impediment to root growth, such as many of the duplex soils in south­east Australia (section 1.3.2.1), less than 5% of vine roots may penetrate below 60 cm (Pudney et al. 2001). Nor do vines root deeply in vineyards where irriga­tion supplies much of the vine’s water in summer. Plant roots and associated mycorrhizae (section 4.7.3.2) help to create soil structure. A desirable soil structure for vines provides optimal water and oxygen availability, which are fundamental for the growth of roots and soil organisms. The structure should be porous and not hard for roots to penetrate, allow ready exchange of gases and the flow of water, resist erosion, be workable over a range of soil water contents, allowing the seedlings of cover crops in vineyards to emerge, and be able to bear the weight of tractors and harvesting machinery with a min­imum of compaction. The quality of soil structure and its maintenance in vine­yards are discussed further in chapter 7. We might expect the soil particles described in chapter 2 simply to pack down, as happens in a heap of unconsolidated sand at a building site. However, if the sand is mixed with cement and water, and used with bricks, we can construct a building—a solid framework of floors, walls, and ceilings. This structure has in­ternal spaces of different sizes that permit all kinds of human activities. So it is with soil. Vital forces associated with the growth of plants, animals, and mi­croorganisms, and physical forces associated with the change in state of water and its movement act on loose soil particles.

Soil and Wine

The concept of terroir as a complex interaction among soil, climate, biology, and human intervention is introduced in section 1.1. The belief that the soil in a par­ticular vineyard imparts a distinctive character to the resulting wine is strong in Europe, but less so in the New World. The special character or personality of a wine may be confined to just one small block, less than 0.5 ha, for example, the “core block” within L’Enclos of Château Latour in the Bordeaux region (Borde­lais) of France. Alternatively, a special character may be attributed more widely to wines from an appellation (the commune Pauillac) or to a subregion such as the Haut-Médoc. But soil is very variable in the landscape (chapter 1), so that as the vineyard area increases, the character of a wine is less and less likely to show a dis­tinctive and defining influence of the soil. Soil variation, in combination with a variation in the mesoclimate (section 1.3.2), will mask a clear, intense expression of the underlying terroir. The grape variety, cultural practices, and the wine maker will then dominate the wine character. Thus, the true influence of terroir can only be satisfactorily studied for small areas. As pointed out in section 8.2.1, soil information is typically collected at a low sampling density over large areas to produce general-purpose soil classifica­tions. The resulting soil maps are necessarily of a small scale (e.g., 1:1,000,000), which means the information about small areas (1–10 ha) is unlikely to be very accurate (see box 8.1). Hence, intensive soil surveys, with at least 6 soil pits per ha, are necessary to study the soil factor in terroir when soil variation can be mapped at a large scale (1:5,000). Further, with more widespread use of precision viticulture technology, as discussed in section 5.3.5, the variation in specific soil properties (e.g., depth to an impeding B horizon and soil strength) can be mea­sured at intervals of about 2 m and mapped at a very large scale (>1:1,000). At a small scale (representing a large area), we can make generalizations, such as that soils on limestone or chalk in Burgundy, Champagne, and the Loire Val­ley in France are highly regarded for producing distinctive wines.

Site Selection and Soil Preparation

At the Pine Ridge winery in Napa Valley, California, a sign lists six essential steps in wine production. The first step reads . . . Determine the site—prepare the land, terrace the slopes for erosion control, provide drainage and manage soil biodiversity. . . . Determining the site means gathering comprehensive data on the local cli­mate, topography, and geology, as well as the main soil types and their distribu­tion. Traditionally, site determination was done using the knowledge and experi­ence of individuals. Now it is possible to combine an expert’s knowledge with digital data on climate, parent material, topography, and soils in a GIS format to assess the biophysical suitability of land for wine grapes. Viticultural and soil ex­perts together identify the key properties and assign weightings to these proper­ties. An example of an Analytical Hierarchy Process is shown in figure 8.1. In this approach, both objective and subjective data were pooled and evaluated to decide the suitability of land for viticulture in West Gippsland, Victoria. In this region with a relatively uniform, mild climate, soil was given a 70% weighting, and the important soil properties were identified as depth, drainage, sodicity, texture, and pH. But in other areas, with another group of experts, a different set of key prop­erties and weightings may well be identified. For example, a similar approach used in Virginia, in the United States, gave only a 25% weighting to soil and 30% to elevation (which affected temperature, a critical factor governing growth rate and ripening) (Boyer and Wolf 2000). This kind of approach can be refined to indicate site suitability for a partic­ular variety within a region of given macroclimate. For example, Barbeau et al. (1998) assessed the suitability of sites in the Loire Valley, France, for the cultivar Cabernet Franc, using an index of “precocity.” Such an index is related to the ability of the fruit to accumulate sugar and anthocyanins and to attain a favorable acidity.

Soil–Water–Vine Relationships and Water Management

Water is a prerequisite for vine growth. It is essential for photosynthesis and to maintain the hydrated conditions and cell turgor necessary for a host of other bio­chemical processes in the plant. As we saw in chapter 4, diffusion of nutrient ions to the root, and their movement by mass flow into the vine’s “transpiration stream,” both depend on water. The volumetric water content θ, defined as the volume of water per unit vol­ume of soil (section 3.3.2), indicates how much water the soil can hold. How­ever, to understand what drives water movement in the soil, we must understand the forces acting on the water because they affect its potential energy. The energy status of soil water also influences its availability to plants. There is no absolute scale of potential energy. But we can measure changes in potential energy when useful work is done on a measured quantity of water or when the water itself does useful work. These changes are observed as changes in the free energy of water, which gives rise to the concept of soil water potential. The derivation of the soil water potential ψ (psi) is given in appendix 7. Historically, the energy status of soil water has been described by a number of terms related to soil water potential, such as pressure, suction, or hydraulic head. These terms ψ and their units are explained in box 6.1. The terms and head will be used in this book. Several forces act on soil water to decrease its free energy and give rise to compo­nent potentials. These are adsorption forces, capillary forces, osmotic forces, and gravity. Adsorption Forces. In very dry soils (relative humidity, RH, of the soil air <20%), water is adsorbed onto the clay and silt particles as a monolayer in which the molecules are hydrogen bonded to each other and the surface. With an in­crease in RH, more water molecules are adsorbed by hydrogen bonding to those on the surface. The charged surfaces of clay minerals also attract cations, and the electric field of the cation orients the polar water molecules around the ion to form a hydration shell, containing 6–12 water molecules.

The Makeup of Soil

Minerals and organic matter comprise the solid phase of the soil. The geological origin of the soil minerals, and the input of organic matter from plants and ani­mals, are briefly discussed in section 1.2.1. A basic knowledge of the composition and properties of these materials is fundamental to understanding how a soil in­fluences the growth of grapevines. A striking feature of soil is the size range of the mineral matter, which varies from boulders (>600 mm diameter), to stones and gravel (600 to >2 mm diameter), to particles (<2 mm diameter)—the fine earth fraction. The fine earth fraction is the most important because of the type of miner­als present and their large surface areas. The ratio of surface area to volume de­fines the specific surface area of a particle. The smaller the size of an object, the larger is the ratio of its surface area to volume. This can be demonstrated by con­sidering spherical particles of radius 0.1 mm, 0.01 mm, and 0.001 mm (1 mi­crometer or micron, μm). The specific surface areas of these particles are 30, 300, and 3000 mm2/mm3, respectively. In practice, the specific surface area is mea­sured as the surface area per unit mass, which implies a constant particle density (usually taken as 2.65 Mg/m3). A large specific surface area means that more mol­ecules can be adsorbed on the surface. Representative values for the specific sur­face areas of sand, silt, and clay-size minerals are given in table 2.1. Note the large range in specific surface area, even for the clay minerals, from as little as 5 m2/g for kaolinite to 750 m2/g for Na-montmorillonite. Because specific surface areas are important, we need to know the size distri­bution of particles in the fine earth fraction. This is expressed as the soil’s texture. The types of minerals that make up the individual size fractions are also impor­tant because they too influence the reactivity of the surfaces. Both these topics are discussed here. All soils show a continuous distribution of particle sizes, called a frequency dis­tribution. This distribution relates the number (or mass) of particles of a given size to their actual size, measured by the diameter of an equivalent sphere.

Soil and the Environment

English has no exact translation for the French word terroir. But terroir is one of the few words to evoke passion in any discussion about soils. One reason may be that wine is one product of the land where the consumer can ascribe a direct link between subtle variations in the character of the product and the soil on which it was grown. Wine writers and commentators now use the term terroir routinely, as they might such words as rendezvous, liaison, and café, which are completely at home in the English language. French vignerons and scientists have been more passionate than most in pro­moting the concept of terroir (although some such as Pinchon (1996) believe that the word terroir has been abused for marketing, sentimental, and political pur­poses). Their views range from the metaphysical—that “alone, in the plant king­dom, does the vine make known to us the true taste of the earth” (quoted by Han­cock 1999, p. 43)—to the factual: “terroir viticole is a complex notion which integrates several factors . . . of the natural environment (soil, climate, topogra­phy), biological (variety, rootstock), and human (of wine, wine-making, and his­tory)” (translated from van Leeuwen 1996, p. 1). Others recognize terroir as a dy­namic concept of site characterization that comprises permanent factors (e.g., geology, soil, environment) and temporary factors (variety, cultural methods, wine­making techniques). Iacano et al. (2000) point out that if the temporary factors vary too much, the expression of the permanent factors in the wine (the essence of terroir) can be masked. The difference between wines from particular vineyards cannot be detected above the “background noise” (Martin 2000). A basic aim of good vineyard management is not to disguise, but to amplify, the natural terroir of a site. Terroir therefore denotes more than simply the relationship between soil and wine. Most scientists admit they cannot express quantitatively the relationship be­tween a particular terroir and the characteristics of wine produced from that ter­roir. Nevertheless, the concept of terroir underpins the geographical demarcation of French viticultural areas: the Appellation d’Origine Contrôllée (AOC) system, which is based on many years’ experience of the character and quality of individ­ual wines from specific areas.