Grapes are cultivated on a very wide range of soils from gravelly sands to clays that can be of high or poor fertility.
The heaviest crops come from vines on deep fertile soils that allow rooting to 3- 5m or more. However, the quality of the fruit for winemaking using traditional vineyard practices is usually better on shallower, less fertile soils. On these, poorer soils, vine growth is less strong and ripening starts earlier and is a slower process. Thus, under these more stressful growth conditions, the fruit is firmer, of better balance, and the wine produced has a richer aroma and flavor.
Generally, one should avoid very heavy clays or very shallow soils, poorly drained sites and those with a high concentration of salts. Soil pH is also important, although vines will tolerate pH’s in the range of 4.5 to 8.5.
Soil pH will affect the availability of nutrients to the vine. In cool climates, vines grown in soils above pH 7.3 are prone to iron chlorosis.
Nutrient availability in the soil depends upon a range of factors including the nutrient concentration and ratio in the soil; rooting depth; soil water supply and the use of cover crops, mulches or cultivation.
Lack of available water during early growth results in wilting of leaves and succulent shoots. At the first sign of drought, normally rapidly growing tips become more grayish green, like the mature leaves.
Tendril die back and drop is also a useful indicator of early drought stress. Drought or restricted moisture availability during rapid berry enlargement will restrict berry size and have a direct effect on yield and quality. Wine grape yield is largely influenced by the amount of water available for growth between flowering and veraison. This has been confirmed in trials in Spain using moderate amounts of water.
Irrigation can also improve wine grape quality by ensuring that vines maintain active growth, especially in drier climates.
In the same Spanish trials, irrigation had no effect on °Baumé in white grapes, but reduced °Baumé in red wine grapes. The pH of the wine was also reduced, but there was no effect on wine color. Recent trials by Yara Iberia have shown similar effects.
However, it is important not to use excessive irrigation as too much water can lead to loss in quality; monitoring soil moisture is essential. A slight shortage in water just as maturity is approaching can hasten ripening by limiting shoot growth and reducing berry size by shrinkage. This increases the sugar content in the grape and also boosts wine color by increasing anthocyanin content.
In general, for maximum productivity, vineyards require between 400-1350 mm (4000 to 13500 m3/ha) of available water, per year, depending upon climate, soil, variety, etc. Thus, irrigation and fertigation is increasingly practiced in many countries, especially where rainfall is limiting vine growth.
Soil water management is critical to good quality wine production. When soils are too dry, yield and quality is reduced. However if soils are waterlogged for prolonged periods the result is root death, restricted vine growth and poor wine quality.
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