Reproduced from Year 5, Issue #29 of Spirito di Vino Asia...
Soil, more than just a patch of dirt…
As we saw in our last column, three main factors influence the style and quality of wine: grape variety, human contributions and the overall growing conditions, including heat, sunlight, water, and nutrients. This column attempts to describe how soil impacts these conditions and why it is often considered the most important element of “terroir.”
First, we need to remember that each grape variety has its own sets of specific requirements and behaviors at various stages of its growth cycle. With the right balance of sunlight, heat, water, and nutrients, the vine can yield good fruit to produce wines of acceptable quality. Any imbalances in these four conditions will negatively or positively influence the results. The key is to match variety to site in the first place, and the soil becomes important only by how much it impacts these conditions.
The growing season starts in spring with budburst, when the mean daily temperature must exceed 10 degree Celsius. A cool soil such as where clay dominates, can delay this important phase and, conversely, warm soil, where sand and gravel prevails, can speed up ripening overall. During fruit maturity, in late summer, the freshness accumulated overnight by large rocks lying around the vineyard can help delay the ripening process and help grapes accumulate precious aromatic precursors without losing acidity. Warmth or coolness of soil is not only affected by soil components such as sand or clay, but also by its humus content, depth, the ability to drain, and its overall make-up.
The factors influencing heat absorption also regulate water availability. The important issue about water is that it must be available in varying amounts at different stages of the vine growth cycle. In early spring when the vine grows canes and leaves, the vine needs plenty of water to encourage maximum vegetal development. In the middle of summer, however, at the period when grapes change color from bright green to red or yellow, a period called veraison, mild water-stress is preferable to stimulate the plant to focus its energy on the development of good quality fruit rather than on leaves and canes. After harvest, the vine needs water again to accumulate reserves in its wood, the key for the next annual growth cycle. Fertile soils are water rich throughout the entire season and encourage vegetation to grow. Poor and free draining soils hold little water and roots have to dig deep to access it. The last type of soil is often preferred in regions where rainfall tends to be unpredictable in the early autumn, when grapes are about to be harvested. The impact of rain in deep soil is less important than in shallow soil because the overall root mass absorbs less water from the rain which minimizes dilution of its sweet juice and loss of the aromatic precursors carefully accumulated during the growing season.
Deep soils are also favored to help the grapevine access the optimal amounts of nutrients at key stages of its growth cycle. The most important minerals are nitrogen, phosphorous and, potassium but it does not need large quantities for its health. Too much of these minerals encourages the plant to grow too vigorously and too little causes leaves to lose their chlorophyll and so hinders ripening. The right quantity of nutrients is especially needed during the flowering and pollination of the fruit, and to avoid any problem with millerandage or coulure – difficult fruit set - which would ultimately reduce the overall yield.
Soil composites can be particularly effective in reflecting sunlight from unusual angles, especially for vigorous grape varieties that tend to overgrow their canes and leaves, thus producing too much shade and so delaying ripening. In a cool climate such as the Mosel in Germany, the sunlight reflected on its slate soil helps to maximize the efficiency of the plant's photosynthesis and therefore encourage optimal ripening.
Soil is a very important factor in viticulture. However, it is only significant for the vine insofar as it impacts on sunlight, heat, water, and nutrients. A complex subject indeed.