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);