clay, refers to a particular type of mineral found in some rock types and in soil, and also a description of sediment or soil which is made up of particularly small particles. See soil texture and geology. Soils described as ‘clays’ have a high content of clay minerals, but may also contain fine particles of calcium carbonate (in soils formed on limestone) and quartz (in soils that have been weathering over a long period of time). Clay-sized particles interact with soil organic matter to form soil structure. Different clay minerals predispose to variations in the stability of a soil’s structure. For example, kaolinite clays tend to support stable structures whereas montmorillonite clays, which show marked swelling when wet and shrinkage on drying, may cause structural instability. Mica-type clay minerals can hold significant amounts of potassium cations within their structures, which can be slowly released on exchange with other cations in the soil water. Clay can be important in vineyard subsoils because of its water-holding capacity, as in parts of pomerol, for example.