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:
- Chemical stability
- Rock structure
- Climate
Cold, dry favors physical
Hot, wet favors chemical
Soils -"A critical sustaining structure
for plants, animals, and human life." (Elemental Geosystems)
1.Definition
- combination of sediments and decayed organic matter (humus) and empty
pore space
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)
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- 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)
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- soil structure - the arrangement of soil
particles (shape/size of peds)
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- 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
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- Soil,
Weathering and Mass Movement Links