Week 12 Flashcards
Soil
Soil: uppermost layer
of the earth’s crust or
lithosphere
– Dynamic natural
material made up of
fine mineral and
organic particles
- Pedology: soil
science, study of
soils
Soil Components
Solid Components
- Mineral fraction
- Organic matter
Pore spaces
- Soil air (gases)
- Soil water
Soil Profile
- vertical section of soil
- horizon
- distinct horizontal layers
- differ in physical, chemical and organic composition and physical properties (texture, structure, colour)
Soil Horizons
O = Organic horizons
- derived from organic parent material
- peat in wet ecosystems (e.g. bogs)
Organic horizons
L = leaf litter
F = fibric material
H = humus
Mineral horizons
Ah = A horizon, rich in humus
= dark, nutrient rich
Ae = A horizon, leached
= grey, few nutrients
B = modified horizon
= deposits from Ae
C = parent material
= essentially unaltered
Formation Factors “CLORPT”
Soil = f (Climate + Organisms + Relief + Parent Material + Time)
CLimate - temperature and precipitation
- controls physical/chemical weathering
Organisms - organic matter, biotic influences
Relief - topography, aspect, elevation
Parent Material - composition, structure, texture acidity and nutrients
Time - change, dynamic (slow)
Soil-Forming Processes
- Enrichment (Addition)
= material addition (mineral or organic deposition)
e.g., addition leaf litter at soil surface - Removal = losses
a) surface erosion
– materials removed from LFH or A horizons
b) leaching
– materials are dissolved in water
– solution is pulled down by gravity
- Translocation = movement between
horizons
a) eluviation
- downward transport of fine particles
- leaves sand and silt
- results in Ae horizon
b) illuviation
- accumulation of clay,
humus and aluminum
and iron oxides in B
horizon - Transformation
a) mineral alteration
- chemical alteration of 1° to 2° minerals
b) humification
- decomposition of organics into humus - Biological Processes
- Human Impacts
Soil Classification
soils are classified using a key and
analysis of physical properties of the soil profile
Soil Classification
soils are classified using a key and
analysis of physical properties of the soil profile
Mineral Fraction
Classified according to changes in
physical or chemical composition over
time
* Primary minerals
* relatively large, unaltered compounds,
same as bedrock
* Secondary minerals
* relatively small, result of chemical
weathering or mineral alteration
** determines soil texture and structure **
Soil Texture Classes
Texture classes:
- combinations of sand,
silt, and clay
- field – hand texturing
- lab – determine % + soil
texture triangle
Soil Structure
- bound by colloids to form “peds”
- structure describes the shape
when individual grains
aggregate - Four main structure classes
- Granular
- Platy
- Blocky
- Prismatic / columnar
Organic Matter
Partially decayed remains of plants and animals
Humus = very fine organic matter,
source of nutrients
Soil colour = composition and chemistry
- = organic content
- white/pale = silica and aluminum oxides
- reds and yellows = iron oxides and
minerals
black
Soil Air
Important for plants/organisms and water movement
Compare soil air to atmosphere:
- CO2 greater in soil air
- O2 less in soil air
Air space depends on:
- soil porosity
- amount of soil water filling pores
Soil Water
Types of Soil Water
a. gravitational water
- water removed by gravity when soil is saturated
b. capillary water
- water that resists gravity by ‘clinging’ to soil particles via capillary tension (force)
storage capacity
- amount of capillary water, available for plant use
- depends on soil texture and organic matter (OM)
- highest storage capacity = fine texture + abundant OM
wilting point (wp)
- critical water storage level for plants
- below wp»_space;high capillary tension»_space; water not
available
- depends on texture:
fine texture = high capillary tension = high wp
If we add equal amounts of water to equal volumes of a
fine versus coarse textured soil, in which soil will a
plant wilt first?
The plant will wilt first in the fine textured soil.
The fine textured soil has large surface area and
small pores, and water binds to the surface
with strong capillary tension.
The water is more difficult for plants to extract.
