The Living Soil

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.

Putting it All Together

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.

The Nutrition of Grapevines

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.

Site Selection and Soil Preparation

As outlined in chapter 1, “determining the site” in old established wine regions such as Burgundy, Tuscany, and the Rheingau has been achieved through centuries of acquired knowledge of the interaction between climate, soil, and grape variety. Commonly, vines were planted on the shallow soils of steep slopes, leaving the more productive lower terraces and flood plains for the cultivation of cereal crops and other food staples, as shown, for example, by the vineyards along the Rhine River in Germany. The small vineyard blocks of the Rhine River, the Côte d’Or, Valais and Vaud regions of Switzerland allowed winegrowers to dif­ferentiate sites on the basis of the most favorable combination of local climate and soil, which underpinned the concept of terroir. In much of the New World, by contrast, where agricultural land was abundant and population pressure less, vineyards have been established on the better soils of the plains and river valleys, as exemplified by such regions as the Central Valley of California, the Riverina in New South Wales, Australia, and Marlborough in New Zealand. Apart from the availability of land, the overriding factor governing site selection was climate and the suitability of particular varieties to the prevailing regional climate. In such regions, although soil variability undoubtedly occurred, plantings of a single variety were made on large areas and vineyard blocks managed as one unit. Soil type and soil variability were largely ignored. Notwithstanding this approach to viticulture in New World countries, in recent time winegrowers aiming at the premium end of the market have become more focused on matching grape varieties to soil and climate and adopting winemaking techniques to attain specific outcomes for their products. For established vineyards, one obvious result of this change is the appearance of “single vineyard” wines that are promoted as expressing the sense of place or terroir. Another reflection of this attitudinal change is the application of precision viticulture (see “Managing Natural Soil Variability in a Vineyard,” chapter 6), whereby vineyard management and harvesting are tailored to the variable expression of soil and local climate in the yield and sensory characteristics of the fruit and wine.

Where the Vine Roots Live

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.

What Makes a Healthy Soil?

Soil scientists used to speak of soil quality, a concept expressing a soil’s “fitness for purpose.” The prime purpose was for agriculture and the production of food and fiber. However, to the general public soil quality is a rather abstract con­cept and in recent years the term has been replaced by soil health. A significant reason for this change is that health is a concept that resonates with people in a personal sense. This change is epitomized in the motto “healthy soil = healthy food = healthy people” on the website of the Rodale Institute in Pennsylvania (http://rodaleinstitute.org/). One consequence of this change is an increasing focus on the state of the soil’s biology, or life in the soil, an emphasis that is expressed through the promotion of organic and biodynamic systems of farming. Viticulture and winemaking are at the forefront of this trend. For example, Jane Wilson (2008), a vigneron in the Mudgee region of New South Wales, is quoted as saying, “the only way to build soil and release a lot of the available minerals is by looking after the biology,” and Steve Wratten (2009), professor of ecology at Lincoln University in New Zealand has said, “Organic viticulture rocks! It’s the future, it really is.” This exuberance has been taken up by Organic Winegrowers New Zealand, founded only in 2007, who have set a goal of “20 by 2020,” that is, 20% of the country’s vineyards under certified organic management by the year 2020. The Cornell Soil Health Assessment provides a more balanced assessment of soil health (Gugino et al., 2009). The underlying concept is that soil health is an integral expression of a soil’s chemical, physical, and biological attributes, which determine how well a soil provides various ecosystem functions, including nutrient cycling, supporting biodiversity, storing and filtering water, and maintaining resilience in the face of disturbance, both natural and anthropogenic. Although originally developed for crop land in the northeast United States, the Cornell soil health approach is readily adapted to viticulture, as explained by Schindelbeck and van Es (2011), and which is currently being attempted in Australia (Oliver et al., 2013; Riches et al., 2013).