Cropland, when it is allowed to lie idle in either a tilled or untilled condition for a year or more, is called fallow. The soil is allowed to rest in order to encourage an abundance of the microscopic organisms necessary to fertility, and to build up nutrients and moisture. Where such methods as brush fallowing, green fallow, or false bedding are practiced, the land is left untilled for as long as a decade, and grasses, weeds, and shrubs are left uncontrolled until plowed under as a way of restoring vitality to the soil. In low-rainfall areas, like the short-grass plains of western North America, acreages of croplands are seeded every other year. During fallow years they’re tilled to form a sealing dustcoat on the surface. The tilling also keeps the land bare of weeds that absorb moisture. Leaving the land fallow encourages blowing soil in drought years, notorious examples being the Dust Bowl of western Kansas during the 1930s and, recently, agricultural areas of northern China. It also contributes to desertiﬁcation. Yet the practice is still extensive, as can be seen in photos taken over the Golden Triangle northeast of Great Falls, Montana, where alternating strips of wheat and bare ground reach to the horizon.