Order

The highest, most generalized category of soil within several soil classifications, and in particular the Soil Taxonomy system of the U.S. Department of Agriculture, where it is further divided into sub‐order, great group, sub‐group, family and series. Several of the soil units of the WRB classification are equivalent to orders in the USDA system, though others are equivalent to lower categories. See the individual articles on the 32 WRB soil types.

The classic, 1938 Soil Classification used by the U.S. Department of Agriculture, divided soils into three orders:

• Zonal soil: soil typical of a large area or zone, with characteristics that reflect the regional variables of climate and vegetation. There were two major types of zonal soil: (i)Pedalfer. A group of soils, lacking calcite in the solum, and with a weathering trend typical of humid climates, leading to enrichment in Fe and Al at the expense of Si. In the extreme, this leads to the formation of acid soils of low nutrient...