soil acidity, or low soil ph, occurs where hydrogen cations (H+) predominate relative to those of the alkaline elements calcium, potassium, sodium, and magnesium adsorbed to the surfaces of clay and organic matter. Acidity is found mainly where the alkaline mineral elements have been leached out of the soil profile under heavy rainfall over extended periods, and their place in the soil’s cation exchange capacity has been taken by hydrogen ions.

In soils of less than pH 5, described as highly acid, exchangeable aluminium is released by slow decomposition of the clay and can inhibit grapevine root growth. Such soils are best avoided for viticulture; if this is not possible, then lime should be applied. The soil acidity of the médoc in Bordeaux, for example, promotes copper availability and consequent toxicity, wherever there has been a build-up of copper in the topsoil from spraying vines with bordeaux mixture.