Controls on soil formation:

1) Parent material

- source of the weathered material

- soil formed on actual rock (called bedrock) is termed residual soils.
- soils formed on unconsolidated sediment (loose) are termed transported soils because they formed on parent material that had been brought in and deposited by things like gravity, water, wind, etc.
- parent material affects rate of weathering
- quartz-rich rocks will weather slower than an olivine-rich rock.
- soil development on unconsol. sediment will progress faster because the unconsol. sediement is already weathered (unlike bedrock).

2) Time

- soils need time to evolve. The longer a soil has had time to form, the thicker it'll be. Soils need time to evolve before they are washed away.

3) Climate

- most important control on the formation of soils.

- variations in temp and precip determine which process will dominate: mechanical or chemical weathering. And the rate of weathering.
- Hot & Wet = thick layer of chemically weathered soil in same time as: Cold & Dry = thin mantle of mechanically weathered debris.

4) Plants and Animals

- supply organic matter to soils

- bog soil is almost entirely organic, while desert soil lacks organics.
- plants supply acids which increase the weathering/soil forming process.
- microorganisms like fungi, bacteria alsi help
- end product of organic decay is called humus.

5) Slope

- steep slopes encourage washing away of soil, so they tend to be very thin.

- flats produce thick, dark, humus-rich soils due to the retaining of water and organic debris.
- slope orientation: north slopes get less sun in the north hemisphere (less plants)

Soil Profile:

O horizon:

- organic rich
- upper part is plant litter such as loose leaves - still recognizable
- lower part is decayed organic matter - humus

A horizon:

- largely mineral matter
- up to 30% humus
- high biological activity
- water passing down thru the A carries out fine particles - eluviation
- water also dissolves soluble inorganic soil components - leaching

B horizon:

- called the subsoil
- material removed from A is deposited here - zone of accumulation
- accumulation of clay fom A enhances the retention of water here.

- hard clay accumulation is termed hardpan.

- OAB together is called the solum, a place where soil forming processes are active, and living plants and animal activity is confined.

C horizon:

- partially altered parent material and very little, if any, organic material.
- still easily identifiable as altered parent material.
- regolith.
- below C is bedrock.
Horizons can be less or well developed.
- less are immature soils.
- more are mature soils.

Soil Types:

- Pedalfer:

- Fe and Al rich.
- Forest or grassy vegetation
- sandy, light-colored

- Pedocal:

- CaCO3
- Drier climate soils, generally grasslands & brush.
- white color

- Laterite:

- Hot and wet climates, tropics
- deep soils
- bacteria removes organic, no humus
- that's why burn/slashing ruins soils and makes people move on and burn/slash more trees: deforestation.
- leaching removes calcite and silica, leaving oxidized Fe and Al
- if parent rocks is low in Fe, the soil will be Al-rich: Bauxite.
- gives the soil a red color.

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