Farming begins with fertile soil. Without well-drained, nutrient rich agricultural soils, farmers could not grow the world's food supply. The United States Department of Agriculture classifies soils based on their mineral content, organic matter content and particle texture. Only a few soil classifications are ideal for farming; the best farm soils are a precious resource, covering only a small percentage of the world's land mass.
Mollisol soils comprise the ground under the grass plains of North America, Europe, Asia and South America. According to the USDA Soil Conservation Service, mollisol soil contains a high percentage of organic matter and is thus highly fertile. Mollisol soils also tend to have a deep topsoil layer, referred to as a deep A horizon by soil scientists. However, this deep topsoil layer was created by centuries of deep-rooted prairie grasses growing on the surface, according to the Michigan University publication "Dustbowl Legacies" by Sylvester and Gutman. Removal of the natural grasslands for agriculture in the United States without careful attention to soil conservation lead to a disastrous loss of topsoil referred to as the "Dustbowl" in the 1930s. With proper soil conservation techniques, mollisols can be the best farm soils for most crop applications. Approximately 7 percent of the world's land mass is comprised of mollisol soils.
Alfisol soils make up about 10 percent of the world's land surface, according to the USDA Soil Conservation Service. These soils form under forest cover, and are predominant in the fertile valleys and farmlands of the northeastern United States. Alfisol soils leach nutrients down from the surface layer of soil and forest humus, incorporating it into a deep underlayer of fertile, moist subsoil. Alfisol's rich nutrient and moisture content make northeastern farms and orchards highly productive despite the short growing season and harsh winters.
Andisol soils are highly productive farmland, but are found in less than 1 percent of the world's landmass. Andisols form in areas with prior volcanic activity that later experience significant rainfall which weathers and breaks down the volcanic glass into a high-nutrient soil. Because of the ash content, these soils can be very light and well-drained, forming a sandy-loam type farming base ideal for root crops as well as long-growing crops like coffee or bananas.