Soils 205-90

Lecture 20- Weathering, and soil forming factors

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Soil formation

A. Weathering Processes

1. Mechanical or Physical Weathering

(a) Temperature

u heat and cool = expansion and contraction

u freeze-thaw

(b) Abrasion

u water - sorting and reworking

u glacial ice

u wind

(c) Disintegration = smaller units

u no secondary minerals

2. Chemical Weathering

(a) Forms secondary minerals

u dissolution and precipitation

(b) Water and dissolution

u hydrolysis = reaction with water

KAlSi₃O₈ + H₂O ® HAlSi₃O₈ + K⁺ + OH^-

solid

solid

solution

2 HAlSi₃O₈ + 11 H₂O ® Al₂O₃ + 6 H₄SiO₄

solid

solid

solution

u hydration = addition of H₂O

Al₂O₃ + 3 H₂O ¬¾® Al₂O₃· 3H₂O

hydrated solid

¾(hydration)®

¬(dehydration)¾

(c) Acid solution weathering

u CO₂ + H₂O ¬¾® H₂CO₃ ¬¾® H⁺ + HCO₃^-

carbonic acid

Ö dissolves materials

CaCO₃ + H₂CO₃ ¬¾® Ca(HCO₃)₂

lime (insoluble)

calcium bicarbonate (soluble)

u organic acids formed during O.M. decomposition

u S and N oxidation

S ¾® H₂SO₄

NH₄⁺ ¾® HNO₃

u increased H⁺ = more rapid weathering

(d) Oxidation and reduction

u release and oxidation of Fe

3 MgFeSiO₄ + 2 H₂O ® H₄Mg₃Si₂O₉ + SiO₂ + FeO

Olivine

Serpentine

Ferrous oxide

FeO + O₂ + H₂O ® 4 FeOOH

Ferrous oxide

Geothite

 

u repeated wet/dry cycles = redoximorphic features

- red-yellow coloration (staining)

- bleaching

- mottles

B. Soil Forming Factors

Cl - climate

O - biotic (living organisms)

R - topography (relief)

P - parent material

T - time

"soils result from the effects of climate and biotic activity, modified by topography, acting on parent material over time"

S = f(Cl, P, O, R, T)

S = soil properties and f( ) = "function of"

1. Climate - Cl

(a) temperature and precipitation

(b) rates of biochemical processes

(c) arid vs humid climates

Ö arid = little leaching of salts and lime

Ö humid = leached of salts and lime

(d) low vs high temperature

Ö low T - slow O.M. decomposition

= O.M. accumulates in surface

Ö high T - rapid O.M. decomposition

= rapid nutrient cycling

 

2. Biotic - O

(a) influenced by climate

(b)

grassland	vs	forest
- high bases		- low bases
- humus in mineral soil		- little humus accumulations
	¯	¯
	conifers	deciduous
	lower bases	higher bases
	lower O.M.	more O.M.

3. Topography - R

(a) slope and aspect

(b) modifies climate and biotic

palouse hills

® N

eroded hilltops

- drier	- wetter
- less leached	- highly leached
- weaker development	- strongly developed
- well drained	- poorly drained
- less clay movement	- high clay movement

 

4. Parent material - P (see section 19)

(a) limits some properties

- sands
- lime
- clay

(b)

basalt	vs	granite
high bases		low bases

5. Time - T

recent

1. little profile development

2. weakly weathered

3. no clay movement (illuviation)

4. "young" soils

5. "inherited" characteristics (P)

vs

aged soils

1. well-developed horizons

2. weathered - secondary minerals

3. translocated clay (illuviated)

4. Cl, O, R influenced

5. "acquired" properties