Range & Forest Ecology: Soils What is soil? (…from the latin solum meaning floor) An independent...

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.


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).


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).


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.


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.


Continued…

Parent material is “the state of the soil system at time zero for a particular set of soil-forming environmental factors.”

Rocks &

Minerals

1)In Place

2)Transported

Agent

Gravity

Water

Ice

Wind

Parent Materials

Residual – Less than ½ of earth’s surface

Colluvial – Gravity-induced

Alluvial – Flood plainMarine – Fluctuating sea levelLacustrine – Lake deposit

Glacial – Till, outwash

Aeolian - Loess


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.


• 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

Soil Profile Development – continued…


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.


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. A1-A2-A3


O Horizon

1. Organic materials above mineral soil (litter layer)

2. Oi - slight decomposition3. Oe - medium decomposition4. Oa - highly decomposed


A Horizon

1. Humus accumulation in mineral soil2. Darkened color


E Horizon

1. Zone of maximum eluviation; loss of organic matter, clays, and/or Fe-oxides

2. No humus accumulation = lighter color


1. Illuviated- accumulation of clays (silicate, Fe & Al oxides), O.M., CaCO3, ...

- usually, from above (A or E) some from sides or below2. Complex in highly developed soils

- numerous subdivisions, e.g. color, lime, structure, etc.

B Horizon


1. Unconsolidated material2. Little affected by weathering processes

C Horizon


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.

Very Coarse SandCoarse SandMedium Sand

Fine SandVery Fine Sand

Diameter (mm)2.0 – 1.01.0 – 0.50.5 – 0.250.25 – 0.10.1 – 0.05

0.05 – 0.002less than 0.002

Soil Separate

Sand

SiltClay



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).



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.


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.


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.


Organic Matter & Biota

Exchangeable ions Surface adsorption

Solid phases & Minerals

Nutrient Uptake by Plants

Soil Solution

Soil Air

Rainfall, Evaporation, Drainage,

Addition of Fertilizer


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


pH range in soils

pH vs Availability of Nutrients

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

Strongly Acid Strongly AlkalineMed. Acid

Slightly. Acid

Very Slightly

Acid

Very Slightly Alkaline

Slightly Alkaline

Med. Alkaline

Phosphorus

Potassium

Calcium

Magnesium

Sulfur

Iron

Manganese

Boron

Copper and Zinc

Molybdenum

Nitrogen


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).

Ca++

Mg++ K+

H+

Al+++

NH4+

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

Al+++

Mg++Mg++

NH4+

K+

K+

H+

H+

Mineral particleOM fraction (Humus)


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


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.

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Range & Forest Ecology: Soils What is soil? (…from the latin solum meaning floor) An independent...

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