loam, the ideal soil for the growth of most plants, consisting of a balanced mixture of clay, silt, and sand (see soil texture). With enough organic matter, loams have a friable, crumby structure (see soil structure). These desirable characteristics are enhanced where calcium is prominent among the ions bonded to the clay particles and organic matter. A good loam has a high capacity to store water and plant nutrients but, unlike stiff clay, is not close textured enough to impede the free drainage of water. Rich, loamy soils can encourage excessive vigour in vines, however, particularly in cool to mild climates with ample rainfall, so loams (which exist in almost all regions) are not always ideal for viticulture.