USDA soil taxonomy

The classification was originally developed by Guy Donald Smith, former director of the U.S. Department of Agriculture's soil survey investigations.

They are, from most general to specific: order, suborder, great group, subgroup, family and series.

Soil properties that can be measured quantitatively are used in this classification system – they include: depth, moisture, temperature, texture, structure, cation exchange capacity, base saturation, clay mineralogy, organic matter content and salt content.

The above soil orders in sequence of increasing degree of development are Entisols, Inceptisols, Aridisols, Mollisols, Alfisols, Spodosols, Ultisols, and Oxisols.

[6] The great group categories are divided into three kinds of soil subgroups: typic, intergrade and extragrade.

These properties are not developed or expressed well enough to cause the soil to be included within the great group towards which they grade, but suggest similarities.

For some soils the criteria also specify the percentage of silt, sand and coarse fragments such as gravel, cobbles and rocks.

The soil moisture regime, often reflective of climatic factors, is a major determinant of the productivity of terrestrial ecosystems, including agricultural systems.