The Twelve USDA Soil Orders

Entisols

Inceptisols

Mollisols

Alfisols

Spodosols

Histosols

Andisols

Oxisols

Ultisols

Gelisols

Vertisols

Aridisols

Gelisols

Soils with permafrost or cryoturbation (mixing from frost heaving)

Cold or frozen much of the yearCan be organic or inorganic soils

9% worldwide 7% US

Entisols

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 OACCommon on ridgetops, recent deposits (rivers,

beaches, ash…), urban areas

16% Worldwide12% US

A

C

Inceptisols

Relatively young soils – have B horizonsMore developed than Entisols but less developed

than any other soil order (soil doesn’t meet the criteria of other orders)

10% Worldwide 9% US

Andisols

Developed from volcanic materialA horizons that are rich in Al-humic materialsSecondary minerals are predominately amorphousRange in development from moderate

to well-developed

<1% Worldwide 2% US

Aridisols

Dry soils; arid to semi-arid environmentsTypically less plant growth, A horizon with less OMCan have many types of B horizons, but Bk’s are common

12% Worldwide 9% US

Mollisols

Have a ‘mollic’ A horizon (>50% Base Sat., with good physical properties)Usually under grasslandsVERY fertile soilsProfiles may be: A B_ C

A C or even A E B_ C

7% Worldwide22% US

Alfisols

Has a Bt horizon Moderate Base Sat. (< 50% and > 35%)Lower fertility than Mollisols, but still excellent soilCommon under deciduous forests, grasslandsA, E, Bt1, Bt2, C A, Bt1, Bt2, BC

10% Worldwide15% US

Ultisols

Old, highly weathered soil, acidicLow Base Sat. (< 35%)Has a Bt horizonLower fertility than AlfisolsCommon under forestsDevelop in wetter temperate areas or

tropical areas with lots of leachingO, A, Bt1, Bt2, Bt3, C

9% Worldwide10% US

Oxisols

Most highly weathered, old soilsLaterization – has an oxic B horizonOxides prevalent, kaolinite commonA horizon low in organic matterCommon in tropical areas with high

rainfall, stable landscape

8% Worldwide<1% US

Spodosols

Has a Spodic horizon (Bs, Bhs)Illuvial accumulation of Fe, Al, O.M.Usually has an E horizonAcidic, lower fertilityFound in cooler, temperate regions, areas of high

leaching, coarse textured soilO, E, Bhs, Bs, C

3% Worldwide3% US

Vertisols

High content of shrink/swell clay in the upper 1m of soilMontmorilloniteSoil expands and contractsCracks open in dry season, material falls in and cracks shut in wet season—mixes soilSubhumid to semi-arid areasDeep A horizon

2% Worldwide2% US

Histosols

Organic Soil (little mineral material)O horizonsFound in waterlogged areas of any type

of climate: WetlandsWhen drained, can be fertile but difficult to manage

due to poor physical propertiesOa, Oe, Oi

1% Worldwide1% US