Chapter-1-Lesson-3.pdf

Soil Sci 1 – Principles of Soil Science Lesson 3 - Soil Formation and Development Soil Formation – formation of the soil with special reference to the processes and soil forming factors responsible for the formation of the true soil from parent ma...

Soil Sci 1 – Principles of Soil Science Lesson 3 - Soil Formation and Development Soil Formation – formation of the soil with special reference to the processes and soil forming factors responsible for the formation of the true soil from parent material Soil Development – the changes in the soil profile brought about by natural processes of leaching, translocation of colloids, accumulation of organic matter, and continued rock and mineral weathering. www.ctahr.hawaii.ed Soil profile – vertical face of the soil that can be exposed for example, u Figure 6. The soil profile by digging a pit or in a road cut; includes all the layers (horizons) from the surface down to the parent material. (Figure 6) Soil solum – part of the soil profile that contains the roots or influenced by plant roots. Soil horizon – a layer of soil approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and number of organism present, degree of acidity. Figure: soils.usda.gov Pedon - a smallest volume that can be called a soil. (Figure 7) Figure 7. Pedon 5 Factors of Soil Formation (Jenny, 1941) Soil = f(clorpt) Climate Organisms/vegetation Parent material Topography/relief time In short CLORPT 1. Climate (c) - primarily precipitation and temperature - significantly affects the rate of weathering 1. Temperature - Influences the types of living organisms - Influences the rate of chemical reactions Chapter 1: Soil: A Natural Resource |page 10  Soil Sci 1 – Principles of Soil Science - Affect on physical weathering. Ex. Freeze-thaw cycles, exfoliation 2. Rainfall - Influences in chemical; reactions that leads to the degradation of the parent material 2. Living Organisms (Biotic Factors) (lo) - Both macro and microorganisms. Microorganisms – example: fungi produce acids that leads to the dissolution of rocks Higher organisms – plants/ animals/ humans Take note: Climate and Living organisms are considered as active factors of soil formation. 3. Relief/Topography (r) - Refers to the configuration of the land surface Topography – related to the landform Relief – topographic position 4. Parent Material (p) - is the precursor of soil. - it determines the mineralogy and hence the nutritional status of the soil. Example: SiO2 (quartz) – does not contribute to the fertility status of the soil. KAl(Mg,Fe)3Si3O10(OH)2 – (Biotite) – contributes K, Mg and Fe General Categories of Parent Materials: 1. Organic parent materials: - Organic residues on the surface from many generations of vegetation - Common in wet environments and in very cold environments where microbial decomposition is very slow. 2. Inorganic parent materials - Rocks and minerals Classification of Parent Materials: 1. Residual/Sedentary - Rocks and minerals that are original to a location that gave rise to a soil. 2. Transported - Moved by wind, water, ice or gravity Chapter 1: Soil: A Natural Resource |page 11  Soil Sci 1 – Principles of Soil Science Transported Parent Material 1. Alluvial – Transported by rivers/streams 2. Collovial – Transported by gravity 3. Eolian – wind deposits 4. Lacustrine – lake deposits 5. Marine – sea or ocean bottom deposits 6. Glacial – deposited by melting glacier Weathering - the totality of chemical, physical and biological processes resulting in the degradation of the parent material and subsequent formation of the soil. Two types of weathering processes: 1. Physical processes (Disintegration) 2. Chemical processes (Decomposition) Physical weathering - breaks down of rocks into smaller fragments. - No change in chemical composition a. Temperature (e.g. frost wedging, thermal weathering-exfoliation) - expansion and contraction - freezing and thawing of water b. Wind and water effects (e.g. salt weathering) c. Force exerted by plant roots; action of animals (e,g, bioturbation) Frost wedging - Freezing/thawing pf water in fractures/disintegration of rocks Thermal weathering - Expansion and contraction due to heating and cooling Salt weathering - Favored by dry conditions – warm and cold (arctic) arid climates - It is due to the growth and expansion of various salts crystals Chapter 1: Soil: A Natural Resource |page 12  Soil Sci 1 – Principles of Soil Science Bioturbation - Disintegration resulting from plants and animals - Burrowing invertebrates (e.g. earthworms, ants, termites and vertebrates (mammals) – turn over huge amounts of regolith material which via attrition reduces particle size - Roots - penetrate rocks and weathered mantle and force apart material for water access - Tree fall – transfer subsurface rock and regolith to surface Role of Physical Weathering - It reduces rock material to smaller fragments that are easier to transport - Increases the exposed the surface area of rock, making it more vulnerable to further physical and chemical weathering. Chemical Weathering - Transformation/ decomposition of one mineral into another - Example: hydration, hydrolysis, acid-solution, dissolution-reduction-oxidation reactions - All these chemical processes involve the conversion of a more complex substance to fewer complex products and the availability of water is vital for them to take place. Hydration - The chemical bonds of the mineral are changed as it interacts with water Hydrolysis - The reaction of any substance with water - Hydrogen ion attacks and replaces other positive ions - Silicate minerals decomposed by water due to hydrolysis produces clay, soluble salt, silica 2KAlSi3O8 + 2(H+ + HCO3-) + H2O Al2Si2O5(OH)4 + 2K+ + 2 HCO3- + 4SiO2 K-Feldspar Kaolinite Dissolution - Aided by small amounts of acid in the water - Soluble ions are retained in the underground water supply Chapter 1: Soil: A Natural Resource |page 13  Soil Sci 1 – Principles of Soil Science Oxidation - Chemical reaction where compound or radical loses electrons - Important in decomposing ferromagnesian minerals - Iron (Fe) minerals rust when exposed to water and oxygen - Oxidation: loss of electron from element - Iron oxidation produces a. Hematite (Fe2O3) : reddish brown rust b. Limonite [FeO(OH)] : yellowish rust 4Fe3+ + 3O22- 2Fe2O3 Biochemical Weathering - Microbes & vegetation (rhizosphere) release organic acids – facilitate hydrolysis minerals -complex ions within the mineral and help their release - E.g. K release from biotite Changes Accompany Rock Weathering Color – rock color from grey to red or yellow hues due to oxidation of iron (Fe2+ to Fe3+) Density – removal (decrease) or addition (increase) of material; collapse (decrease or dilation (increase) of original material. Composition – mineralogical and chemical change towards more stable forms – solubility of elements, mineral susceptibility and secondary mineral types Fabric or texture – change from rock fabric to soil fabric (development of new structures) Factors that Affect Weathering: 1. Composition of the rock/mineral 2. Physical condition of rocks/minerals (e.g. particle size) 3. Environmental factors (other factors of soil formation: climate, living organisms, topography/relief Chapter 1: Soil: A Natural Resource |page 14  Soil Sci 1 – Principles of Soil Science Products of Weathering Mineral Residual Products Material in Solution Quartz Quartz grains Silica Feldspar Clay minerals Silica K+, Na+, Ca2+ Amphibole Clay minerals Silica (hornblende) Limonite Ca2+, Mg2+ Hematite Olivine Limonite Silica Hematite Mg2+ 5. Time (t) - Time that the parent material had been subjected to the active factors of soil formation Chapter 1: Soil: A Natural Resource |page 15