12 Soil Order Types

Oxisols (ox):

• Tropical soils

• Oxidized subsurface horizons: high iron and aluminum oxides (reddish / yellowish color)

• Heavy precip. Leaches soluble minerals from A horizon 

• Low pH, low fertility 

• Lack of distinct horizons

Aridisols (id)

• Desert soils 

• Largest single soil order (19% of Earth’s land surface)

• Shallow soil horizons 

• Low precip., infreq. Leaching

• Little vegetation, low org. Matter 

• Salinization: salts dissolved in soil water migrate to surface, despotited as water evaporates: damage / kill plants

Mollisols (oll)

• Grassland soils 

• Mollis = soft 

• Rich soils, high organic matter (high humus content) - dark surface horizon 

• Cover more area in continental US than any other soil order

Alfisols (alf)

• Moderately weathered (leached) forest soils 

• Most widespread soil order 

  • 5 suborders from equator to high latitudes 

• Semi-arid to moist (Mediterranean)

Soils of the Midwest: Continuum: Aridisols (west), mollisols (central), alfisols (east)

Ultisols (ult)

• Highly weathered (leached) forest soils 

  • High precipitation, subtropical (southeast US)

  • Ultimate weathering 

• Low pH, lower fertility 

• An Alfisol might evolve into an Ultisol 

• Reddish

Spodosols (od)

• Northern coniferous forest soils 

  • Northeast US, Northern Europe 

  • Rarely found in Southern Hemisphere

• Spodos = wood ash, grey color

• Acidic, highly weathered (leached), sandy 

  • E horizon: light colored, clay minerals leached out (eluviation)

Entisols (ent)

• Recent (young) 

• Undeveloped soils 

• Lack vertical development of horizons

• Occur in many climates

• Poor ag. soils (except river silt deposits)

• Form on active slopes, floodplains, poorly drained tundra, tidal mudflats, dune sands, glacial outwash plains

Inceptisols (ept)

• Inceptum = beginning 

  • Inception of B horizon 

• Weakly developed / minimal horizon development 

  • More developed than Entisols 

• Young, infertile 

• Moist, eluvial: some loss 

• Found in most climates

Gelisols

• Cold and frozen soils 

• Contain permafrost within 2m of surface

• Temps at or below 0^OC; soil dev. Is slow 

• Decomp. Is slow; store large amts of org. matter and carbon

• Thick O horizons 

Andisols (and) 

• Soil formed from volcanic parent materials (ash and glass) 

• High mineral content recharged by eruptions 

• Commonly found near or downwind or volcanoes: thick layer of ash deposited

Vertisols (ert) 

• Vert → invert 

  • Horizons invert: bring lower horizons near surface 

• Soils that shrink and swell (expandable clays)

  • Crack when dry / swell when wet 

• High in nutrients, good for farming

Histosols (ist) 

• Wetland soils (bogs, peat beds)

  • Saturated year-round 

• Accumulate thick org. matter (no permafrost)

• Form in former lakes or poorly drained depressions