The Twelve USDA Soil Orders Entisols Inceptisols Mollisols Alfisols Ultisols Gelisols Vertisols Aridisols Spodosols Histosols Andisols Oxisols

Gelisols 9% worldwide 7% US Soils with permafrost or cryoturbation (mixing from frost heaving) Cold or frozen much of the year Can be organic or inorganic soils

Entisols Young, weakly developed soil Typically NO B horizon 16% Worldwide 12% US C Young, weakly developed soil Typically NO B horizon (in some cases, can have a very coarse-textured, poorly formed Bw) Typical profiles: C AC OC OAC Common on ridgetops, recent deposits (rivers, beaches, ash…), urban areas

Inceptisols Relatively young soils – have B horizons 10% Worldwide 9% US Relatively young soils – have B horizons More developed than Entisols but less developed than any other soil order (soil doesn’t meet the criteria of other orders)

Andisols Developed from volcanic material <1% Worldwide 2% US Developed from volcanic material A horizons that are rich in Al-humic materials Secondary minerals are predominately amorphous Range in development from moderate to well-developed

Aridisols Dry soils; arid to semi-arid environments 12% Worldwide 9% US Dry soils; arid to semi-arid environments Typically less plant growth, A horizon with less OM Can have many types of B horizons, but Bk’s are common

Mollisols 7% Worldwide 22% US Have a ‘mollic’ A horizon (>50% Base Sat., with good physical properties) Usually under grasslands VERY fertile soils Profiles may be: A B_ C A C or even A E B_ C

Alfisols Has a Bt horizon Moderate Base Sat. (< 50% and > 35%) 10% Worldwide 15% US Has a Bt horizon Moderate Base Sat. (< 50% and > 35%) Lower fertility than Mollisols, but still excellent soil Common under deciduous forests, grasslands A, E, Bt1, Bt2, C A, Bt1, Bt2, BC

Ultisols Old, highly weathered soil, acidic Low Base Sat. (< 35%) 9% Worldwide 10% US Old, highly weathered soil, acidic Low Base Sat. (< 35%) Has a Bt horizon Lower fertility than Alfisols Common under forests Develop in wetter temperate areas or tropical areas with lots of leaching O, A, Bt1, Bt2, Bt3, C

Oxisols Most highly weathered, old soils 8% Worldwide <1% US Most highly weathered, old soils Laterization – has an oxic B horizon Oxides prevalent, kaolinite common A horizon low in organic matter Common in tropical areas with high rainfall, stable landscape

Spodosols Has a Spodic horizon (Bs, Bhs) 3% Worldwide 3% US Has a Spodic horizon (Bs, Bhs) Illuvial accumulation of Fe, Al, O.M. Usually has an E horizon Acidic, lower fertility Found in cooler, temperate regions, areas of high leaching, coarse textured soil O, E, Bhs, Bs, C

Vertisols High content of shrink/swell clay in the upper 1m of soil 2% Worldwide 2% US High content of shrink/swell clay in the upper 1m of soil Montmorillonite Soil expands and contracts Cracks open in dry season, material falls in and cracks shut in wet season—mixes soil Subhumid to semi-arid areas Deep A horizon

Histosols Organic Soil (little mineral material) O horizons 1% Worldwide 1% US Organic Soil (little mineral material) O horizons Found in waterlogged areas of any type of climate: Wetlands When drained, can be fertile but difficult to manage due to poor physical properties Oa, Oe, Oi