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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.

Chapter 3 gives examples of how grapevines, being woody perennials, have the potential to develop extensive, deep root systems when soil conditions are favorable. One of the most important factors governing root growth is a soil’s structure, the essential attributes of which are • Spaces (collectively called the pore space or porosity) through which roots grow, gases diffuse, and water flows • Storage of water and natural drainage following rain or irrigation • Stable aggregation • Strength that not only enables moist soil to bear the weight of machinery and resist compaction but also influences the ease with which roots can push through the soil The key attributes of porosity, aeration and drainage, water storage, aggregation, and soil strength are discussed in turn. Various forces exerted by growing roots, burrowing animals and insects, the movement of water and its change of state (e.g., from liquid to ice) together organize the primary soil particles—clay, silt, and sand—into larger units called aggregates. Between and within these aggregates exists a network of spaces called pores. Total soil porosity is defined by the ratio . . . Porosity = Volume of pores/Volume of soil . . . A soil’s A horizon, containing organic matter, typically has a porosity between 0.5 and 0.6 cubic meter per cubic meter (m3/m3)—also expressed as 50% to 60%. In subsoils, where there is little organic matter and usually more clay, the porosity is typically 40% to 50%. Box 4.1 describes a simple way of estimating a soil’s porosity. Total porosity is important because it determines how much of the soil volume water, air, and roots can occupy. Equally important are the shape and size of the pores. The pores created by burrowing earthworms, plant roots, and fungal hyphae are roughly cylindrical, whereas those created by alternate wetting and drying appear as cracks. Overall, however, we express pore size in terms of diameter (equivalent to a width for cracks). Table 4.1 gives a classification of pore size based on pore function.

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.

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.