Table 1.

Principal characteristics and areas for the 12 orders of the USDA Soil Taxonomy system, with common subsoil constraints to root growth in representative suborders

Order	Description	Area (%)a	Common subsoil constraints
Alfisols	Soils from semi-arid to humid regions, typically developed under hardwood forest cover, with subsurface accumulation of clay	9.6	Hypoxia, hardness, temperature
Andisols	Developed from volcanic ash	0.7	Mn toxicity, low P
Aridisols	Desert soils without water for plants	12	Hardness, salinity/sodicity, low P
Entisols	Soils with minimal evolution, as in eroded or accumulation regions. No subsurface horizons	16.2	–
Gelisols	Permafrost (frozen soil) within 100cm from the surface	8.6	Temperature, low P, Mn toxicity
Histosols	Organic rich, generally in cold latitudes	1.2	Hypoxia, low P, temperature, low Ca
Inceptisols	Soils with weakly developed subsurface horizons	9.8	–
Mollisols	Thick dark surface horizon	6.9	Hardness, hypoxia, temperature
Oxisols	Soils from tropical regions, highly weathered, deep, and uniform profiles	7.5	Acidity (low P, low Ca, K, Mg, Al, and Mn toxicity)
Spodosols	Bleached horizon over grey-brown (spodic) horizon	2.5	Hypoxia, acidity (low P, low Ca, Al, and Mn toxicity)
Ultisols	Weathered soils with low base saturation (low fertility) in subsurface	8.5	Acidity (low P, low Ca, K, Mg, Al, and Mn toxicity), hypoxia, hardness
Vertisols	Shrink and swell soils, that is soils that exhibit temporal variability in volume	2.4	Hypoxia, hardness

^a Percentage of the total land area (1.3×10⁸ km²); rows sum to 86%—the remainder (~14%) is from rock and ice-covered regions (Wilding, 2000).