chapter 2 soil origin and development. pedon - a small section or body of soil for studying soil...
TRANSCRIPT
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:
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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+