Range & Forest Ecology: Soils
What is soil? (…from the latin solum meaning floor)
An independent 3-dimensional natural body occupying the
earth’s surface and capable of supporting plant growth. Its
properties result from the integrated effect of climate and
living organisms acting upon parent material as conditioned
by relief over periods of time.
Range & Forest Ecology: Soils
3 Viewpoints
Pedological – soil is considered a “natural body”; emphasis is on
geologic history & soil formation process (time scale of 103 to 107
years), with minor emphasis on practical utilization.
Edaphological – consideration of soil from the standpoint of higher
plants; emphasis is on properties of soil as they relate to plant
production, e.g nutrient availability, slope, aspect, moisture,
texture (time scale of 100 to 102 years).
Ecological – consideration of soil as a component of an ecosystem;
emphasis on flows of energy, nutrient cycling dynamics, microbial
activity & decomposition (time scale of 101 to 102 years).
Range & Forest Ecology: Soils
Components of Soil (plant’s perspective)
1) Mineral grains – provides anchorage, pore space (H2O, air),
& nutrients on exchange basis.
2) Organic matter – plant & animal residues in various stages of
decomposition, soil organism exudates; can be source of
plant nutrients through a microbially-mediated cycling
process; enhances water storage & improves soil structure.
3) Soil water – solvent medium for nutrients required by plants.
4) Soil air – provides oxygen for cellular function & atmospheric
nitrogen for N-fixing organisms (atmosphere 78% N2).
Range & Forest Ecology: Soils
The development of soil can be thought of as occurring in
two phases…
1) Soil Genesis – the weathering of rock substrates by:
• Mechanical forces – expansion & contraction due to thermal flux;
erosional forces by wind & water; plant roots can exert sufficient
forces to cleave & exfoliate rock fragments.
• Chemical reactions – many; principal agent is percolating rain
water charged with CO2 (weak acid solution) which affects
carbonates (dissolution), feldspars & micas (hydrolyze to clays &
release cations); biological processes form organic acids.
Range & Forest Ecology: Soils
Continued…
2) Soil Formation – Hans Jenny (1941) characterized soil
formation (S) as a function of five independent variables:
climate (cl), organisms (o), topography (r), parent material (p),
& time (t).
S = ƒ(cl, o, r, p, t)
Organism include such elements as the soil microbial community,
litter inputs, vegetation type.
Parent material largely determines chemical characteristics of the
derived soils.
→ The interaction of organisms & parent material with climate
produce a soil with characteristic features.
Jenny, H. 1941. The factors of soil formation. New York: McGraw-Hill.
Range & Forest Ecology: Soils
Continued…
Parent material is “the state of the soil system at time zero for a
particular set of soil-forming environmental factors.”
Agent
1) In Place
Rocks
&
Minerals
Parent Materials
Residual – Less than ½ of earth’s surface
Gravity
Colluvial – Gravity-induced
Water
Alluvial – Flood plain
Marine – Fluctuating sea level
Lacustrine – Lake deposit
2) Transported
Ice
Wind
Glacial – Till, outwash
Aeolian - Loess
Range & Forest Ecology: Soils
Soil Profile Development – Soil formation is largely a
biochemical process, whereby:
•Organic material (plant roots, litter
inputs, soil organisms) mixes with
inorganic mineral fractions.
• Microbial activity accelerates
chemical weathering.
• Primary/original minerals are
transformed into secondary minerals.
• Resulting in development of
horizons with characteristic color,
textural & structural qualities.
Range & Forest Ecology: Soils
Soil Profile Development – continued…
• Regolith – Unconsolidated layer above hard,
unweathered, bedrock
• Solum – Upper portion of the regolith that
has been altered through biochemical and
physical processes. The material between the
solum and bedrock is referred to as the C
horizon. It is slowly changing into solum.
• Pedon – A 3-D sampling that displays the full
range of properties that are characteristic of a
soil (1-10 m2).
• Soil Profile – One vertical face of a pedon.
• Soil Horizons – Horizontal layers,
differentiated by color or texture, described
within a profile
Range & Forest Ecology: Soils
Soil Profile Development – typical horizons:
Master Horizons
O – Organic horizon (>20% OM by weight); partly or mostly
decomposed OM; found in wetlands, forest litter layers;
provides nutrients (nitrogen, potassium, etc.), aids soil
structure (acts to bind particles), enhances soil moisture
retention.
A – Mineral horizon with accumulated OM (<20%); typically
darker than other horizons.
E – Zone of maximum leaching (eluviation); loss of clays,
soluble OM, Fe oxides; common in forest soils; light colored.
B – Mineral horizon with accumulation (illuviation) from
above; usually contains most clays and fines; usually a more
dense layer.
C – Soil parent material, either in situ or transported; minimal
weathering and biologic activity.
R – Hard, unweathered bedrock.
Range & Forest Ecology: Soils
Soil Profile Development – continued…
• All master horizons will not
necessarily occur in a single soil.
When they do they are normally
found in this order
• Master horizons can be
subdivided using numbers, ex. A1A2-A3
Range & Forest Ecology: Soils
O Horizon
1.
2.
3.
4.
Organic materials above mineral
soil (litter layer)
Oi - slight decomposition
Oe - medium decomposition
Oa - highly decomposed
Range & Forest Ecology: Soils
A Horizon
1. Humus accumulation in mineral soil
2. Darkened color
Range & Forest Ecology: Soils
E Horizon
1. Zone of maximum eluviation; loss of organic matter, clays, and/or
Fe-oxides
2. No humus accumulation = lighter color
Range & Forest Ecology: Soils
B Horizon
1. Illuviated
- accumulation of clays (silicate, Fe & Al oxides), O.M., CaCO3, ...
- usually, from above (A or E) some from sides or below
2. Complex in highly developed soils
- numerous subdivisions, e.g. color, lime, structure, etc.
Range & Forest Ecology: Soils
C Horizon
1. Unconsolidated material
2. Little affected by weathering processes
Range & Forest Ecology: Soils
Physical Properties – Texture & Structure
1) Texture – Characterization of the solid inorganic phase of soils &
refers to the relative proportions of sand, silt & clay; called the soil
separates & constitute the “fine earth” fraction (diameter < 2 mm) of
the soil.
Soil Separate
Sand
Silt
Clay
Diameter (mm)
Very Coarse Sand
2.0 – 1.0
Coarse Sand
1.0 – 0.5
Medium Sand
0.5 – 0.25
Fine Sand
0.25 – 0.1
Very Fine Sand
0.1 – 0.05
0.05 – 0.002
less than 0.002
Range & Forest Ecology: Soils
Physical Properties – Texture & Structure
1) Texture – Continued…
• Rock fragments (diameter > 2 mm) are described separately on a
pedon description; modifiers are used when rock fragments occupy
more than 15 percent by volume, ex. gravelly sandy loam.
• Organic horizons (Oi, Oe, Oa) are described based on the degree of
decomposition (fibric, hemic, sapric, etc.).
1) Texture – Continued…
Particle Size Analysis
1. In the laboratory: mechanical analysis
- Sands are determined by sieving
- Silt and clay is determined based on settling time using
Stoke's Law
Texture – Continued…
2) Steps to determine soil texture in
the field by “feel” have been
developed; useful for a quick &
dirty estimate; requires practice
(see handout).
Range & Forest Ecology: Soils
Physical Properties – Texture & Structure
2) Structure – Soil structure is the way soil particles aggregate together
into what are called peds. Peds come in a variety of shapes depending
on the texture, composition, and environment.
Range & Forest Ecology: Soils
Physical Properties
2) Structure – Continued…
• Agents that bind aggregates &
promote stability: fungal
mycelia, microbial exudates, H
& Ca ions, & clays.
• Structural stability is the
capacity for the peds to retain
their shape, i.e. absorb water &
not disintegrate.
• Soil aeration, water movement,
& plant root penetration are
greatly enhanced by good soil
structure.
Range & Forest Ecology: Soils
Soil Chemistry: Important aspect is the relationship
between the ions (i.e. nutrients) (a) in soil solution,
(b) adsorbed on charged particles, & (c) as
constituents of mineral & organic fractions, which
tend towards equilibrium; dynamic interaction.
Range & Forest Ecology: Soils
Nutrient Uptake by Plants
Exchangeable ions
Surface adsorption
Soil Air
Soil Solution
Organic Matter
& Biota
Solid phases
& Minerals
Rainfall, Evaporation,
Drainage,
Addition of Fertilizer
Range & Forest Ecology: Soils
Soil Chemistry
The availability of plant nutrients are most directly influenced
by (1) soil pH, and (2) cation exchange capacity (CEC).
1) Soil pH –
– Strictly, measure of the hydrogen ion concentration in solution: log10[H+]; [H+] = concentration (activity) of H+
– Neutrality is pH = 7 (H+ = OH-)
– As acidity  , pH 
Range & Forest Ecology: Soils
pH range in soils
pH vs Availability of Nutrients
Med.
Acid
Strongly Acid
Very
Slightly
Acid
Slightly.
Acid
Very
Slightly
Alkaline
Slightly
Alkaline
Med.
Alkaline
Strongly Alkaline
Nitrogen
Phosphorus
Potassium
Sulfur
Calcium
Magnesium
Iron
Manganese
Boron
Copper and Zinc
Molybdenum
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5 10.0
Range & Forest Ecology: Soils
Soil Chemistry
2) Cation Exchnage Capacity (CEC) – Measure of the number of
negatively charged sites on charged particles that attract exchangeable
cations (via electrostatic charge).
Mineral particle
OM fraction (Humus)
Ca++
K+
Ca++
NH4+
H+
Al+++
Mg++
Ca++
Ca++
Al+++
K+
H+
Ca++
Ca++
Mg++
Mg++
NH4
+
H+
Ca++
K+
Range & Forest Ecology: Soils
Important CEC properties:
• Size of CEC determined by soil properties
(permanent & pH-dependent charge)
• Large quantity of nutrients can be held on
CEC (particularly clay)
• Provides rapid buffering for most cations
• Exchangeable ions held against leaching
• Exchangeable ions readily available through
cation exchange
Range & Forest Ecology: Soils
Summary
• Components of soil are mineral grains (anchorage & nutrients), organic matter
(nutrient cycling), water (solvent medium), & air (oxygen & nitrogen).
• Soil formation is the product of climate, organisms, topography, parent material,
& time.
• Soil profile development encompasses the interaction of the above factors to
develop characteristic soil horizons.
• Soil texture refers to content of sand, silt, & clay particles.
• The nature & arrangements of peds & aggregate stability are the most
significant characteristics of soil structure.
• The central concepts in soil chemistry are based on the equilibrium of ions in
solution.
•CEC is a measure of exchangeable cations on negatively charged sites in the
soil, & is influenced by the presence & kinds of clay minerals & humus.