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Soil is the living “skin” of the earth’s terrestrial ecosystem. Like skin, it is bom-barded by the sun’s radiation, wind, and rain and abraded by all manner of objects scraping its surface. Unlike skin, after an initial phase of weathering, soil develops primarily from the surface downward as plant and animal residues are continually added to the surface layer. These organic residues nourish a diverse population of organisms, feeding on the dead residues and on each other. In turn, they release mineral nutrients in an interminable cycle of growth, death, and decay, commonly called the carbon (C) cycle. Figure 5.1 is a diagrammatic representation of the C cycle in a vineyard. Green plants use energy from the sun to make carbohydrates, and subsequently proteins, lipids (fats), and other complex molecules, for their own growth and reproduction. As plant tissue matures, biochemical changes take place that lead to senescence; leaves yellow and eventually fall. In a perennial plant such as the grapevine, leaves are shed in winter but roots grow, age, and die all the time. Prunings may also be returned to the soil. Collectively, the above-ground plant material returned to the soil is called litter. Below ground, root fragments that are “sloughed off” and C compounds that leak from living roots constitute rhizo-C deposition. This below-ground C material is a readily accessible substrate (food) for microorganisms, which proliferate in the cylinder of soil surrounding each plant root, a zone called the rhizosphere. The rhizosphere is also the zone from which roots take up nutrients, as described in “The Absorbing Root,” chapter 3. When digesting and decomposing C substrate derived from litter and in the soil proper, organisms obtain energy and essential nutrients for growth. In so doing, in a healthy soil, they consume oxygen (O2) and release carbon dioxide (CO2) to the air below and above ground, thus completing the C cycle. Directly analogous to the process described for nitrogen (N) in chapter 3, C is said to be immobilized in the bodies of the soil organisms and mineralized when it is released as CO2.

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.

In reality, there can be no generic definition of an “ideal soil” because a soil’s performance is influenced by the local climate, landscape characteristics, grape variety, and cultural practices and is judged in the context of a winegrower’s objectives for style of wine to be made, market potential, and profitability of the enterprise. This realization essentially acknowledges the long-established French concept of terroir: that the distinctiveness or typicity of wines produced in individual locations depends on a complex interaction of biophysical and human cultural factors, interpreted by many as meaning a wine’s sense of place. As discussed in “Soil Variability and the Concept of Terroir” in chapter 1, because of this interaction of factors that determine a particular terroir, it is not surprising that no specific relationships between one or more soil properties and wine typicity have been unequivocally demonstrated. While acknowledging this conclusion, it is still worthwhile to examine how variations in several single or combined soil properties can influence vine performance and fruit character. These properties are: • Soil depth • Soil structure and water supply • Soil strength • Soil chemistry and nutrient supply • Soil organisms Provided there are no subsoil constraints, the natural tendency of long-lived Vitis vinifera, on own roots or rootstocks, to root deeply and extensively gives it access to a potentially large store of water and nutrients. In sandy and gravely soils that are naturally low in nutrients, such as in the Médoc region of France, the Margaret River region in Western Australia, and the Wairau River plain, Marlborough region, New Zealand, the deeper the soil the better. A similar situation pertains on the deep sandy soils on granite in the Cauquenas region, Chile. However, such depth may be a disadvantage where soils are naturally fertile and rain is plentiful, as in parts of the Mornington Peninsula, King and Yarra Valley regions, Victoria, Australia, and the Willamette Valley region in Oregon (see figure 1.11, chapter 1), because vine growth is too vigorous and not in balance.

Grapevines must have 16 of the 118 known elements to grow normally, flower, and produce fruit. These essential elements, listed in table 3.1, are also called nutrients and as such are divided into • Macronutrients, which are required in relatively large concentrations • Micronutrients, which are required in smaller concentrations Box 3.1 discusses the different ways of calculating nutrient concentrations in soil, plants, and liquid. Vines draw most of their nutrients from the soil, and so table 3.1 also shows the common ionic form of each element in soil. Ions, the charged forms of elements, are introduced in box 2.4, chapter 2. For example, carbonic acid (H2CO3), which is a compound of carbon (C), hydrogen (H), and oxygen (O), dissociates in water into the ions H+ and HCO3−. This is a chemical reaction that can be written in shorthand form as . . . H2CO3 ↔ H+ + HCO3− . . . The double arrow shows that the reaction can go either forward (to the right) or backward (to the left), depending on the concentrations of H+ and HCO3− relative Concentration (symbol C)a is the amount of a substance per unit volume or unit weight of soil, plant material, or liquid. For example, the concentration C of the element nitrogen (N) can be expressed as micrograms (μg) of N per gram of soilb, noting that . . . 1 μg N/g = 1 mg N/kg = 1 part per million (ppm N) (B3.1.1) . . . An amount is the product of concentration and weight. For example, the total amount of N of concentration C (measured in μg/g) in a soil sample of 100g is . . . 100C μg or 0.1C mg (B3.1.2) . . . Because all soil and plant materials contain some water, analyses are best expressed in terms of oven-dry (o.d.) weights. The o.d. weight of a soil sample is obtained by drying it to a constant weight at 105ºC; for plant material the drying temperature is 70ºC. The amount of a nutrient is often expressed per hectare (ha) of vineyard.