Gotowa bibliografia na temat „Soils South Australia Barossa Valley”

Weathering of rocks is the crucial first step in making vineyards possible. For where the debris produced by weathering—the sediment we met in Chapter 5—becomes mixed with moist humus, it will be capable of supporting higher plant life. And thus we have soil, that fundamental prerequisite of all vineyards, indeed of the world’s agriculture. So how does this essential process of weathering come about? Any bare rock at the Earth’s surface is continually under attack. Be it a rocky cliff, a stone cathedral, or a tombstone, there will always be chemical weathering—chemical reactions between its surface and the atmosphere A freshly hewn block of building stone may look indestructible, but before long it will start to look a bit discolored and its surface a little crumbly. We are all familiar with an analogy of this: a fresh surface of iron or steel reacting with moisture and oxygen in the air to form the coating we call rust. In his “Guide to the Lakes” of England, William Wordsworth put the effects of weathering far more picturesquely: “elementary particles crumbling down, over-spread with an intermixture of colors, like the compound hues of a dove’s neck.” A weathered rock is one that is being weakened, broken down. The rock fragments themselves are further attacked, which is why stones in a vineyard often show an outer coating of discolored material, sometimes referred to as a weathering rind (Figure 9.1; see Plate 22). If the stone is broken open, it may show multiple zones of differing colors paralleling the outer surface of the fragment and enclosing a core of fresh rock. Iron minerals soon weather to a powdery combination of hematite, goethite, and limonite, and the rock takes on a reddish-brown, rusty-looking color. The great example of such weathering in viticulture is the celebrated terra rossa, but the rosy soils in parts of Western Australia and places further east such as McLaren Vale and the Barossa Valley are also due to iron minerals. Several Australian wines take their names from this “ironstone.”

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.

Rainbow Beach is a small town on the coastal dunes of eastern Australia, near Brisbane. I had travelled there to meet with some colleagues to sample soils from the vast coastal sand dunes that surround the area. It might seem an unusual place to visit to collect soil, but a unique sequence of soils has formed in the sand dunes, which differ greatly in age. As you move inland from the sea, the soils get progressively older and deeper, and more weathered and nutrient-poor. The youngest soils are shallow, having only just started to form in recent sand dunes, whereas the oldest soils are around half a million years old and can reach 25 metres deep. These are among the oldest, deepest, and most weathered soils that I have sampled, and what I recall most vividly is how stunted and sparse the vegetation was that grew there, reflecting their struggle to grow in such ancient, weathered soil. The soils of Rainbow Beach are by no means the oldest on Earth. Hidden beneath ice sheets in Greenland, scientists recently discovered a soil that was 2.7 million years old, a remnant of the fertile tundra that covered the area before the ice sheets came. And scientists working in South Africa recently discovered a soil, now compacted in rock, that is 3 billion years old. One of the most fascinating things about soil is that it is incredibly diverse; soils vary enormously across continents, countries, and from valley to valley and field to field. Even within a small patch of land, such as a field, forest, or vegetable garden, the underlying soil can vary considerably. Over distances of metres, it might differ in its texture and depth, and in its pH, being acid in one patch of a field and neutral in another. Soils also vary greatly in the diversity of living organisms that live within them. I will go into more detail about the diversity of soil life later in this book; but for now suffice to say that it is vast. Soils also change with time.