Trade in agricultural inputs, particularly fertilizer (crude and manufactured), pesticides, farm machinery, and genetic material, has been increasing over the last 25 years. Countries with either natural resources (for example, potash, phosphate rock, natural gas, or sulphur) or well-developed manufacturing infrastructure and heavy commitment to research and development have come to be major agricultural input exporters. Much world trade in agricultural inputs occurs in regional markets, often within the same continent, although some nations, such as the United States, ship to more than 100 countries. The dependence of U.S. farmers on imports of agricultural inputs is growing. Although the United states still enjoys a positive aggregate trade balance in these inputs, that balance has declined somewhat with increasing imports of manufactured fertilizers and farm machinery.