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

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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