Soil Horizons and the Soil Orders

O

organic matter, not mineral material (leaf litter)

A

topsoil, has accumulated organic matter but dominated by mineral material

E

Eluvial horizon, has lost clay/minerals/organic matter to underlying horizons

B

Illuvial horizon/subsoil, has gained clay/iron/minerals from horizons above

C

parent rock

R

solid rock

order of soil horizons

O, A, E, B, C, R

factors of soil formation

climate, organisms, topography, parent material, time

mollisols

-thick/dark A horizons with lots of organic matter

-dry-ish and cool climate

-grasslands

alfisols

-thin A horizon

-E horizon has lost clay to B-this process requires rainfall

-mostly forest soils=organic matter at surface

-mildly acidic

ultisols

-clay enriched B horizons

-much more weathered and acidic than alfisols

-warmer climate than alfisols

-much older than alfisols

-mostly formed under forest

spodosols

-thin A, distinct E and B

-all forest soils

-B horizon has accumulated Fe, Al, and organic matter (become mobile under acidic conditions), not clay

-reddish brown color

-wet climate

-sandy parent material, little clay

histosols

-lots of organic material

-peat/muck

-wetlands= soil is saturated

-lots of C

-drained and used for crops

entisols

-weakly developed- only have A and C

-B not developed because of parent material (maybe because sandy) and/or lack of time

inceptisols

-weakly developed but do have weakly developed B horizons (dont have clay or other materials)

-limited by time or parent material

-prone to steep slopes where erosion removes soil

aridisols

-well-developed- lots of time

-in deserts

-flat land, little erosion

-well developed B with calcite accumulation, or clay enriched sometimes