View
11
Download
0
Category
Preview:
DESCRIPTION
Lecture about soil genesis and classification 2011
Citation preview
FRST 201: A BRIEF REVIEW OF SOIL GENESIS AND CLASSIFICATION
FRST 201: A BRIEF REVIEW OF SOIL GENESIS AND CLASSIFICATIONSuzanne Simard
1. Soil genesis
Pedogenic process = physical and chemical processes in soil
The essence of genetic soil horizons lies in the functions they perform rather than their morphology
Individual soil horizons are the working aggregates of a complex thermodynamic system
Two steps in soil genesis:
1. accumulation of parent material
2. differentiation of horizons
Differentiation of horizons in a profile occurs because of:
1. additions
2. losses
3. transfers
4. transformations
of organic matter, carbonates, sesquioxides (R2O3), silicates
these proceed simultaneously
Simple processes that proceed in all soils, which in turn are controlled by other factors (CLORPT):
Hydration
Oxidation/reduction
Solubilization
Leaching
Precipitation
Mixing
Cracking
Churning
Weathering
Decomposition
Mineralization
Volatilization
Some processes offset horizonation and others promote it Soils are is a balance of processes; a continuum
Usually a shift in simple processes, not a difference in genetic processes
Relative importance of each process operating in horizon differentiation not uniform for all soils. Contrasting example:DESERT SOILPODZOL
Small loss of soluble salts and carbonates Much greater loss of salts and carbonates
Downward transfer of salts and carbonates to deeper levels Greater leaching of salts and carbonates to ground water?
Small additions of organic matter Appreciable gains in organic matter
Limited transfers and transformations of clay minerals Limited losses of clay minerals
Limited transfers and transformations of sesquioxides Marked transfers of sesquioxides, organic matter
Some loss of silica
2. FOREST FLOOR
Uppermost soil horizons and Forest Floor: Usually enriched in carbon
Decomposition of organic matter by soil organisms producing enzymes Oxidation of carbon ( CO2 = soil respiration
Release of energy via oxidation
Forest Floor Horizons: Horizon types that may be present are:
L (litter): relatively fresh organic residues in which original structures virtually intact
F (fermented): dominated by partly decomposed organic materials. Original structures are breaking down and discolored, but still recognizable
H (humus): organic residues in advanced stage of decomposition. Original structures no longer discernable.
Ah: upper mineral soil enriched in organic matter.
Humus forms:Three taxa at the Order level: Mor, Moder, Mull
F horizon is diagnostic for Order identification
Groups: a formative element is added to the Order name. e.g., (Hemimor=F horizon>50% of F and H; Humimor=H>50% of F and H)
Phase: reflects a certain property that is not diagnostic for the taxa (e.g., Cryic Humimor)
Mor
F horizon matted
Distinct horizons with abrupt boundaries
Little intermixing with mineral
Fungal mycelium the dominant agent of decomposition (Fm)
F, H relatively thick, usually >2 cm
Nutrients tied up in forest floor horizons; mostly unavailable to plants, but is an important source of soil nutrients
Usually coniferous forest
Moder
F horizon loose (friable)
Insect droppings commonly present (Fz, Fzm) Fungi may be present
Soil fauna, bacteria and some fungi are agents of decompostiion
H horizon and surface soil horizon may be mixed (Ah)
F, H usually >2 cm thick
If Ah present, < 2 cm thick
Nutrients more available than with Mor (faster nutrient cycling)
Usually mixed coniferous/broadleaf forest
Mull
F horizon has almost non-existent pattern
Soil fauna and bacteria are agents of decomposition (Fz)
F and H horizons very thin (< 2cm)
F and H very mixed with surface mineral horzon (Ah) Ah > 2 cm thick
Nutrients are quite available to plants (rapid cycling)
Usually broadleaf forest or grassland
MINERAL SOILSProperties of a class:
1. diagnostic critera, e.g. color
2. accessory criteria, e.g., sesquioxide content
3. accidental characteristic, e.g., stoniness. May or may not co-vary with 1. Makes it seem like the data doesnt fit.
Mineral Soil Horizons:
A mineral horizon at or near the surface in zone of leaching or elevation of materials in solution or suspension, or site of maximum accumulation of o.m., or both.
Ah accumulation of o.m.
Ae eluviation of o.m., Fe, Al; coarser than underlying B
Ab buried horizon
Ap disturbed by mans activities
B enrichment of o.m., sesquioxides or clay, or by development of soil structure, or by change of colour due to hydrolysis, reduction or oxidation
Bh accumulation of o.m.; darker than C (Podzol);
Recommended