GEOLOGY OF WINE 87 Factors 1–4 can enhance the temperature during the growing season There is also a slope factor during spring budding Drainage of cold air The woody stems of vines can withstand extremely low temperatures but, once buds start to break from the stem, the vine is vulnerable to damage by even modest frosts Bud-break is usually taken to be at 10 C but, depending on the variety of vine and the amount of nutriment stored from the previous season, budding may start at 3.5–7 C Keeping in mind that the dates of the ‘ice-saints’, 11–14 May, are long after bud-break in northern Europe, late frosts can be a serious problem The slope needed to provide a natural protection from still cold air depends on how severe the frost will be In the Napa Valley in California, a slope as little as 2.5 may be sufficient to allow the freezing air to drain away In Chablis, in northern France, a slope twice as steep as this may not be adequate to cope with the more severe frosts leaving the sediment, and hence the roots of the vine, waterlogged (Figure 3) For simple survival of a vine, waterlogging is a greater danger than a shortage of water, because the limited root surface area can be poisoned from a shortage of oxygen in solution If the vine has used all the oxygen in stagnant water, the root metabolism starts to form poisonous alcohols Clearly, it is essential that even the lowest roots of the vine are well above the top of the water table As the roots normally penetrate downwards to several metres, and can penetrate to 20 m, vineyards can only be successfully established many metres above the normal local water table In the Me´ doc, land drainage is used in vineyards which are closer to the Gironde Estuary In the Barossa, in South Australia, where low rainfall makes drip-irrigation helpful, it is essential that open reservoirs are many metres below the height of the vineyards themselves Water Balance Like all plants, vines need water in the ground However, without actually being a species of arid climates, its need for water is small As a result, vines will grow almost anywhere Nevertheless, to produce a good crop of grapes, the plant needs a small steady supply of moisture from the ground Without artificial supplies of water, e.g., by irrigation, the ideal drainage involves a high porosity for storing water, a low matrix permeability to stop it draining away, and a high mass permeability to ensure that excess rainwater drains away fast A facies with ideal drainage is chalk, a distinctive, very fine-grained limestone, which occurs in much of Champagne Typical chalks have a porosity of 35–45%, a matrix permeability of 2–6 mD, and a very high, but variable, mass permeability (normally more than 150 mD and can be several thousand millidarcy, i.e., more than a darcy) For the vine, such a high porosity and low matrix permeability mean that plenty of water is held in the pores of the sediment, which cannot escape under gravity because the pores are less than 30–60 mm However, plants can exert suctions equivalent to 100 m or more of water, i.e., the vines can extract water which cannot escape under gravity In addition, the low matrix permeability means that the chalk does not easily dry out However, when there is very heavy rain, the mass permeability becomes valuable: it ensures that the large volume of rainwater drains away without Figure Muscovite rich silty sand in a vineyard of Michel Torino, Cafayte, south south west of Salta, Argentina A reliable soil for vines: medium porosity with relatively high permeability, holding water for roots, but allowing it to drain away before all dissolved oxygen has been lost It will need some nitrogen for optimal growth The muscovite, although not a direct source of potassium itself, probably indicates that adequate potassium is present from the weathered muscovite and illite ß Jake Hancock