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Soils and Weathering

Definitions

Weathering: disintegration and decomposition of rock at or near the surface.

Soil: combination of sediments and decayed organic matter (humus) and empty pore space.

Erosion: incorporation and transportation of material by a mobile agent, usually water, wind, or ice.


Weathering

1. Mechanical/Physical weathering: physical breakdown of rock into smaller pieces with composition identical to source rock

2. Chemical weathering: Chemical alteration or dissolution of minerals

Major controls on weathering:

Cold, dry favors physical

Hot, wet favors chemical


Soils -"A critical sustaining structure for plants, animals, and human life." (Elemental Geosystems)

1.Definition

2. Soil profile - a model of soil horizons. Below is from Elemental Geosystems

Horizons - continuum from organic to mineral content the deeper you go.
O - organic matter from plant and animal litter enters from above
A - organic matter mixed with humus, fertility retained by soil colloids
E - eluviation - leaching of clay, iron, aluminum; little organic matter
B - illuviation - accumulation of materials leached from above
C - weathered bedrock - mineral matter enters from below
R - parent material - bedrock or unconsolidated material

3. Soil Formation

Formation processes of water:
i.water percolating down - moves and removes soluable material, leaching
ii.evaporation makes water soak upwards causing minerals to precipitate (salinization)
 
Controls on Soil Formation:
Dynamic
- Climate - water and energy
weathering rates faster in warm, moist climate
more rainfall, more leaching and more vegetation
 
- Plants and Animals
plants and animals are a source of organic matter (humus)
organic acids enhance chemical weathering
micro-organisms (bacteria, fungi) break down organic matter

 

Passive
-Parent Material -
Residual Soil - type of bedrock (granite? limestone? sandstone?)
Transported Soil - alluvial soils laid down by water
Volcanic Soils - andisols - parent material lands on top of the soil.

 

 
Topography - Slope and angle - soil poorly developed on steep slopes and piles up in cracks, angle determines sun exposure
 
-Time - soils generally take millenia to develop, but can be eroded away in a few years

 

Human component - agriculture and development
 

 

4. Examples of soil orders and processes as they relate to climate
 

Example of a soil from a Tropical Rainforest (Af) climate
Oxisol - tropical rainforest soils - very fragile and infertile
tropical humid - intensly leached, practically no humus, low cation exchange capacity,
laterization - enriched in Fe and Al hydroxide (bauxite)

Example of a soil from a Desert/Dry (Bwh) climate
Aridosol - fertile soils of the Western US
desert, arid grass, brush
alkaline/basic soil - high cation exchange capacity - quite fertile, but fragile - little organic matter
calcification - caliche deposits in B and C horizons
salinization - precipitation of minerals to the top of the soil
basic soil

Example of a soil from a Mediterranean / Mesothermal (Csb) climate
Alfisol - most widespread of the soil orders - moderately weathered forest soils
Aluminum and iron color the A horizon yellowish-brown
 
Example of a soil from a Humid continental / Microthermal (Dfb) climate
Spodosol - soil of northern coniferous forests
temperate humid; Eastern US.
acidic and sandy soil - less fertile, heavy eluviation - especially under a coniferous forest.
podzolization - organic rich in A horizon, iron and aluminum in B horizon (red brown)
acidic soil

Example of a soil from a Tundra / Polar (Et) climate
Inceptisol - weakly developed young soils (many kinds), inherently infertile
lack of maturity, weathering just beginning
eluvial soil without any illuvial horizon

 
5. Soil Properties - determine soil tilth
 
soil color - suggests composition and chemical make-up (can be misleading)
 
soil texture - size and organization of mineral particles in the soil (sand, silt, clay content)
 
soil structure - the arrangement of soil particles (shape/size of peds)
 
soil consistence - consistency and cohesion (product of texture and structure)
 
soil porosity - porosity, permeability and moisture storage capacity
 
soil moisture - field capacity - water available for plants
 

 
6. Soil fertility - the ability to support plants
 
chemical behavior related to the clay-humus complex - attracts cations (CEC)
 
life - lifeforms in and on top of a soil system are vital in the generation and maintenance of soils.
 
soil properties - determine soil tilth and fertility - balance of air, water, and nutrients
 

 
Soil, Weathering and Mass Movement Links