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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.

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.

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.

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.