The seeming precision of the areas, of course, belies the vagaries of definition and inevitable uncertainties in the FAO's planetary compilations. The general changes in land use undoubtedly are correct although the definitions of land use in this and the preceding figure are not precisely the same. Figure 4.1.2 represents the three centuries of change that ushered in the state of Figure 4.1.1. Nevertheless, during three centuries cropland surely advanced and forest and woodland retreated. Because of rising yields, farmers in some countries today grow surpluses, driving down prices.
Some support comes in the form of incentives for farmers to idle cropland.
Figure 4.1.3 illustrates the consequent change in land use with a U.S. example. In 1992, government programs idled about one-fifth of U.S. cropland, undoubtedly much of it marginal and illustrating how land can be spared when yields rise on other land. The fraction of U.S. cropland idled by government programs in 1992.