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Chapter 2 Soil Origin and Development

Chapter 2 Soil 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.

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Chapter 2 Soil Origin and Development

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  1. Chapter 2 Soil Origin and Development

  2. 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”

  3. 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 chemical composition of rocks and minerals.

  4. Types of Physical Weathering root growth exfoliation ice wedging gravity

  5. Types of Chemical Weathering Dissolution - usually happens to limestone or marble because they contain calcite H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3- water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion Oxidation - oxygen that is dissolved in water reacts with minerals that contain iron 4Fe+2 +3O2 --> 2Fe2O3 ferrous iron + oxygen combine to form ferric iron oxide (hematite) Hydrolysis - usually happens in silicate minerals, turning them into clay minerals 2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K(OH) potassium feldspar in acidic water hydrolyses to kaolinite + quartz + potassium hydroxide

  6. Soil Profile Topsoil - upper or A horizon A soil profile is a vertical cross-section of a soil. It is divided into a number of distinct layers, referred to as horizons. The horizons are normally designated by symbols and letters. Subsoil - middle or B horizon Parent Material - lower or C horizon

  7. Soil Horizons 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).

  8. Soil Forming Factors Soil 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 Time Climate Soil Forming Factors Topography (landforms) Organisms: vegetation, animals, microbes, etc.

  9. Parent Material- the material from which a soil forms Agents of transport: • Water • Glacial Ice • Wind • Gravity • Volcanic Deposits talus alluvial fans glacial till eolian deposits

  10. 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.

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

  12. 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 vegetation distribution. Both moisture and temperature influence evaporation, which can affect leaching. Temperature determines the rate of chemical and biological decay.

  13. 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

  14. Soil Processes are responsible for the changes to soils over time Soils are complex and dynamic systems in which many processes are taking place: 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. l

  15. Rocks and Minerals- the parent material for most soils • igneous formed when magma (molten rock deep within the earth) cools and hardens. • metamorphic formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure. • sedimentary formed from particles of sand, shells, pebbles, and other fragments of material that gradually accumulate and harden into layers over a long period of time.

  16. Importance of Rock Types Different rock types contain different minerals which in turn contain the nutrients essential for plant growth and development

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