A soil is said to be alkaline when the measured ph is above about 7. A pH above about 8.5 usually implies a content of free lime or chalk (usually described as calcareous soil; see also limestone), or too high a content of soluble salts (including common salt, sodium chloride). In the latter case, such soils are described as saline. When these soils are leached, their pH can increase to 9 or more due to the hydrolysis of sodium ions (see cation exchange) and the organic matter can disperse, forming a black alkali soil. Saline and black alkali soils should definitely be avoided for viticulture. However, some of the best vineyard soils in cool climates are associated with limestone or chalk.