Ebook understanding vineyard soils part 2

4 Where the Vine Roots Live I work the earth to put oxygen into it  Quote by Jean Michel Deiss, wine maker, in Andrew Jefford (2002), The New France: A Complete Guide to Contemporary French Wine Soil Structure In chapter we discussed 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 determining root growth is a soil’s structure, the essential components of which are as follows: • Spaces (collectively called porosity) through which roots grow, gases diffuse, and water flows readily • Water storage 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 concepts here are porosity, aeration and drainage, water storage, aggregation, and soil strength, each of which is discussed in turn Porosity If it were not for the action of forces associated with the growth of plants, animals, and microorganisms, and physical forces associated with water and its movement, the elementary soil particles—clay, silt, and sand—would simply 99 100 understanding vineyard soils pack into an unconsolidated, disorganized heap As a result of these forces, soil particles are organized into larger units called aggregates, between and within which there is 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 m3/m3 (also expressed as 50%–60%) In subsoils, where there is little organic matter and usually more clay, the porosity is typically 40%–50% Box 4.1 describes a simple way of estimating soil porosity Box 4.1 Estimating Soil Porosity A simple equation for porosity, expressed as f (phi), is r f   rb p (B4.1.1) In this equation, rp (rho p) is the average density of the soil particles, assumed to be 2.65 Mg/m3 The term rb (rho b) is the soil’s bulk density, which ranges from less than Mg/m3 for soils rich in organic matter to 1.0–1.4 Mg/m3 for well-aggregated loamy soils, and to 1.2–1.8 Mg/m3 for sands and compacted subsoils Thus, for a loamy soil with a rb value of 1.33 Mg/m3, we have f  1 1.33  0.5 2.65 (B4.1.2) giving a porosity of 0.5 m3/m3 or 50% To measure a soil’s bulk density, take five to six intact cores with steel cylinders, preferably at least cm in diameter and 6–10 cm deep Trim the soil so that the dimensions of the soil core are the same as the cylinder and the soil volume is easily calculated Dry the cores in an oven at 105°C and weigh each core to obtain the weight of oven-dry (o.d.) soil The bulk density rb of each core is calculated from the equation rb  Weight of o.d soil in the core Volume of soil core (B4.1.3) Calculate the average value of rb for the several samples taken Note that soil bulk density in the rows and mid rows is likely to be different because of compaction by wheeled traffic in the latter where the vine roots live 101 Total porosity is important because it determines how much of the soil volume roots and water 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 (figure 4.1) 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 function Figure 4.1 A vineyard soil with swelling clays showing cracks on drying Table 4.1 Relationship among Pore Size, Formative Forces, and Function Pore diameter (μm) 5000–500 500–30 30–0.2