Chapter 2Soil Origin and Development

Pedon - a small section or body of soil for studying soil characteristics... typically 3’ x 3’ x 5’

Over time, soils form distinct layers known as “horizons”

Weathering - the breakdown of rock by physical and chemical means

Physical weathering involves the breakdown of rock by temperature, water, wind and plants. Particle size is reduced without changing the chemical composition.

Chemical weathering involves alteration of the chemicalcomposition of rocks and minerals.

Types of Physical Weathering

ice wedging

root growth

exfoliation

gravity

H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-

water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion

4Fe+2 +3O2 --> 2Fe2O3

ferrous iron + oxygen combine to form ferric iron oxide (hematite)

2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K(OH) potassium feldspar in acidic water hydrolyses to kaolinite + quartz + potassium hydroxide

Types of Chemical Weathering

Dissolution - usually happens to limestone or marble because they contain calcite

Oxidation - oxygen that is dissolved in water reacts with minerals that contain iron

Hydrolysis - usually happens in silicate minerals, turning them into clay minerals

Soil Profile

A soil profile is a vertical cross-sectionof a soil. It is divided into anumber of distinct layers, referred toas horizons.

The horizons are normallydesignated by symbols and letters.

Topsoil - upper or A

horizon

Subsoil - middle or B

horizon

Parent Material - lower or C

horizon

O Horizon• uppermost, organic layer of soil • made up mostly of leaf litter and humus (decomposed O.M.)

A Horizon• topsoil…seeds germinate and plant roots grow here• dark-colored, made up of humus mixed with minerals

E Horizon• eluviation (leaching) layer…light in color• made up mostly of sand and silt• may not always be present

B Horizon• subsoil…known as “zone of accumulation” (illuviation)• contains clay and mineral deposits (iron, aluminum oxides, and calcium carbonate)

C Horizon• regolith…consists of slightly broken-up bedrock

R Horizon• unweathered rock (bedrock).

Soil Horizons

Organisms: vegetation, animals, microbes, etc.

Topography (landforms)

Soil Forming Factors

Time Climate

Parent Material

Soil Forming FactorsSoil characteristics develop as a result of their origin and environment.

Soil begins as a particular rock material and develops slowly over many years. Each soil is a product of a combination of the following factors:

Parent Material - the material from which a soil forms

talusAgents of transport:

• Water • Glacial Ice• Wind• Gravity• Volcanic Deposits

eolian deposits

glacial till

alluvial fans

Organisms - plants, microbes, soil animals, and humans

Microbes such as bacteria and fungi assist in the decomposition of plant litter. This litter is mixed into the soil by soil animals such as earthworms and beetles.

Roots contribute dead tissue to the soil, bind soil particles together and redistribute and compress soil.

Topography - the lay of the land…includes slope, aspect, and elevation

Climate - determines the nature (physical, chemical or biological) and rate of weathering that acts on parent material to form soil. The most important elements of climate for soil formation are precipitation and temperature

Climate governs the rate and type of soil formation and is also the main determinant of vegetationdistribution.

Both moisture and temperature influence evaporation, which can affect leaching.

Temperature determines the rate ofchemical and biological decay.

Time - the length of time required for a soil to form depends on the intensity of the other active soil forming factors of climate and organisms, and how topography and parent material modify their affect

Additions - plant material and animal byproducts, nutrient-rich dust, air pollutants, etc.

Losses - leaching, gases (N2) escaping into the atmosphere, etc.

Translocations - movement of materials within the soil layers due to capillary action, earthworms, etc.

Transformations - the altering of soil materials by physical or chemical weathering, organic matter decay, etc.

Soil Processes are responsible for the changes to soils over time

Soils are complex and dynamic systems in which many processes are taking place:

l

Rocks and Minerals - the parent material for most soils

• igneousformed when magma (molten rock deep within the earth) cools and hardens.

• metamorphicformed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure.

• sedimentaryformed from particles of sand, shells, pebbles, and other fragments of material that gradually accumulate and harden into layers over a long period of time.

Importance of Rock Types

Different rock types contain different minerals which in turn contain the nutrients essential for plant growth and development

igneous metamorphic sedimentary

granite K+ Ca+2 basalt Mg+2 Fe+2

marble Ca+2 gneiss K+ Ca+2 Mg+2 Fe+2 schist K+ Mg+2 Fe+2

sandstone Ca+2 limestone Ca+2 Mg+2 shale K+