Lecture 3B - Principles of Soil Science PDF

AGRI 51 PRINCIPLES OF SOIL SCIENCE  SOIL DEVELOPMENT AND FORMATION SOIL-FORMING PROCESSES 2  Soil Formation Synonymous to soil genesis The process of creating soil from “not soil”. Scientific name for “not soil” is parent material 3  Soil Forming Proc...

AGRI 51 PRINCIPLES OF SOIL SCIENCE  SOIL DEVELOPMENT AND FORMATION SOIL-FORMING PROCESSES 2  Soil Formation Synonymous to soil genesis The process of creating soil from “not soil”. Scientific name for “not soil” is parent material 3  Soil Forming Processes 1. Transformations 2. Translocations 3. Additions 4. Losses/Removals 4  Soil Forming Processes Diagrams illustrating some common additions, removals, translocations and transformations in soils, and how these are useful in interpreting soil development. After Dijkerman (1974) and Simonson (1978). 5  Soil Forming Processes Transformations Occur when soil constituents are chemically or physically modified or destroyed and others are synthesized Include: Weathering of primary minerals Decomposition of organic residues Synthesis of organic acids, humus, and other products Change in the size/ arrangement of mineral particles 6  Soil Forming Processes Transformations Parent materials are transformed into silicate clays through chemical and physical weathering These silicate clays and decomposition products recombine to form new minerals, such as hydrous oxides of iron and aluminum (or sesquioxides) 7  Soil Forming Processes Transformations Organic materials decompose and are incorporated into the mineral aspect of soils Other organic products, such as organic acids and humus are synthesized from organic materials 8  Soil Forming Processes Transformations Weathering of large particles can cause the formation of smaller particles Particles can be arranged and built into aggregates 9  Soil Forming Processes Transformations Weathering Any of the chemical or physical processes by which rocks undergo chemical decomposition and physical disintegration. Weathering usually occurs at the earth's surface, but it can also occur at significant depths, for example, the percolation of groundwater through fractures in bedrock. It usually results in changes in color, texture, composition, or hardness of the affected rocks 10  Soil Forming Processes Transformations Weathering Controlled largely by climate More water available, the more likely that chemical processes can proceed In arid climates, weathering processes move very slowly Physical weathering will be the dominant process, but since it relies on chemical weathering, it will also be quite slow 11  Soil Forming Processes Transformations Weathering 1. Physical Weathering the breakdown of rock and mineral particles by physical forces This Photo by Unknown Author is licensed under CC BY-SA-NC 12  Soil Forming Processes Transformations Weathering 1. Physical Weathering Factors and Processes Associated with Physical Weathering a. Wetting and Drying – soils that are wetted up may be prone to swelling, then shrink when they dry out – these seasonal effects are termed shrinkage and swelling – subsidence is based upon such shrinkage and swelling of clays under foundations 13  Soil Forming Processes Transformations Weathering 1. Physical Weathering Factors and Processes Associated with Physical Weathering b. Freezing and Thawing/Melting – water expands when frozen and literally pushes the soil apart, breaking it down – when the ice thaws the soil can slump back again – the overall process is rather like a very slow “churning” 14  Soil Forming Processes Transformations Weathering 1. Physical Weathering Factors and Processes Associated with Physical Weathering c. Heating and Cooling – extremes of temperature can cause the soil to expand and contract – the effect is less pronounced than that of freezing and thawing but over time this can become significant 15  Soil Forming Processes Transformations Weathering 1. Physical Weathering Factors and Processes Associated with Physical Weathering d. Grinding or Rubbing – grinding of particles against each other leads to particle disintegration, most obvious on the beach (evidence of abrasion by water) – Windblown dust and sand can also wear down rocks by abrasion 16  Soil Forming Processes Transformations Weathering 1. Physical Weathering Factors and Processes Associated with Physical Weathering e. Unloading – when pressure is placed upon soil, i.e. glacier on soil, it becomes compressed – when ice melts a huge weight is lifted and the soils may react accordingly by uplifting and expanding 17  Soil Forming Processes Transformations Weathering 1. Physical Weathering Factors and Processes Associated with Physical Weathering f. Organisms – if plant roots can push through concrete, soil presents little obstacle – Worms burrow their way through soil, mixing and aerating it all through their lives 18  Soil Forming Processes Transformations Weathering 2. Chemical Weathering the process by which rocks are decomposed, dissolved or loosened by chemical processes to form residual materials This Photo by Unknown Author is licensed under CC BY 19  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Takes place in almost all types of rocks. Smaller rocks are more susceptible, however, because they have a greater amount of surface area Chemical weathering is much more common in locations where there is a lot of water 20  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering a. Hydration – process where minerals in the rock absorb water and expand, creating stress which causes the disintegration of rocks – softer, more stressed, and more easily decomposed mineral – E.g. CaSO4 + 2H2O → CaSO4.2H2O anhydrite gypsum 21  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering b. Hydrolysis – chemical reaction between the minerals in the rock and hydrogen in rain water – Water molecules split = H+ & OH- components; the H+ often replaces a cation from the mineral structure 22  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering c. Carbonation – dissolved carbon dioxide in rainwater or moisture in surrounding air forms carbonic acid and reacts with the minerals in the rock – this process weakens the rock thus breaking it down in the process. 23  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering d. Dissolution – process by which minerals in the rocks dissolve directly in water – when the rocks dissolve, become ions in solution in the water, and are carried away with it – Ex. Gypsum dissolving in water (CaSO4.2H2O + 2H2O → Ca2+ + SO42- + 4H2O) – Ex. Halite dissolving in water (NaCl + H2O → Na+ + Cl− + H2O) 24  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering e. Oxidation – takes place when oxygen combines with other elements in rocks to form new types of rock – these new substances are usually much softer, and thus easier for other forces to break apart – change in rock color is an indication of oxidation. One common example is oxidation rust 25  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering e. Oxidation Examples: 4FeO (Ferrous oxide) + O2 → 2Fe2O3 (Ferric oxide) 4Fe3O4 (Magnetite) + O2 → 6Fe2O3 (Hematite) 2Fe2O3 (Hematite) + 3H2O → 2Fe2O3 · 3H2O (Limonite) 26  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering f. Reduction - is the reverse of oxidation; it is equally important in changing soil color to grey, blue, as ferric iron is converted to ferrous iron compounds. Under the conditions of excess water, or water-logged conditions (less or no oxygen), reduction takes place. Fe2O3(s) + 6H+(aq) + 2e-→ 2Fe2+(aq) + 3H2O Fe(OH)3(s) + 3H+(aq) + e- → Fe2+(aq) + 3H2O Hematite Bernalite 27  Soil Forming Processes Transformations Weathering 2. Chemical Weathering Factors and Processes Associated with Chemical Weathering f. Reduction - Iron(III) oxide is reduced by hydrogen in a two-step process. 28  Soil Forming Processes Translocations Transfer of soil constituents Involve the movement of inorganic and organic materials laterally within a horizon or vertically (up/down)from one horizon to another 29  Soil Forming Processes Translocations The downward translocation of ions, clay and mineral fragments such as silica is important to the formation of subsoil horizons Over time, this process is one of the more visibly noticeable as alterations in color, texture, and structure become apparent Downward translocation stops where the wetting front stops. Maybe at the shallow depths in arid and semi-arid regimes and maybe mostly out of the profile in humid areas 30  Soil Forming Processes Translocations Upward translocation of salts is important to the formation of the saline and alkali soils of arid and semi-arid regions Upward translocation stops at the soil surface when water evaporates When the water stops moving, then evaporates, salts are left behind Soil layers with calcium carbonate or other salt accumulations, form this way If this cycle occurs several times, a calcareous hardpan can 31 form  Soil Forming Processes Translocations 32 https://www.fao.org/3/R4082E/r4082e08.htm  Soil Forming Processes Translocations Hardpan a hard, usually clay-rich layer of soil, lying at or just below the ground surface, in which soil particles are cemented together by silica, iron oxide, calcium carbonate, or organic matter that has precipitated from water percolating through the soil. Hardpans do not soften when exposed to water The duripan is a root-restrictive subsoil layer that is primarily cemented by illuvial silica. 33  Soil Forming Processes Translocations Soil organisms such as earthworms, incorporate organic litter into the A and B horizons of the soil Mound-building organisms, such as termites, transport materials from deeper in the soil (B and C horizons) to the surface Burrowing of rodents also transport materials through the soil 34  Soil Forming Processes Translocations Chelation also helps in translocation Metal ions such as Fe and Al combine with organic molecules released during humus formation The organic-metal chelate is more soluble than the Fe or Al minerals and is readily transported through the pedon 35  Soil Forming Processes Additions Inputs of materials to the soil profile from outside sources are considered additions Both human activity and natural processes could result in the addition of materials to the soil The most obvious is addition of organic matter 36  Soil Forming Processes Additions As soon as plant life begins to grow in fresh parent material, organic matter begins to accumulate Dead organisms, plant foliage, roots, etc., add organic materials to the soil 37  Soil Forming Processes Additions By causing rivers to flood, rainfall is indirectly responsible for the addition of new sediment to the soil on a flood plain On the average, rainfall adds about 5 pounds of nitrogen per acre per year 38  Soil Forming Processes Additions Minerals and other particles can be carried to and added to a developing soil (or any soil) by wind, as wind-blown or aeolian material 39  Soil Forming Processes Losses/Removals Most losses occur by leaching Leaching can remove water, salts or silica dissolved in water, and organic acids and materials Water moving through the soil dissolves certain minerals and transports them into deeper layers Erosion removes particles such as humus, clay, and silt from the surface of the soil Such losses can be serious because the material lost is usually the most productive part of the soil profile 40  Soil Forming Processes Losses/Removals Evaporation and transpiration causes the soil to lose water that is used by microorganisms and vegetation Decomposition by microbes can remove organic matter from the soil Grazing and harvesting by animals and humans causes nutrient losses and organic matter losses from the soil 41  Soil Forming Processes Losses/Removals Oxygen is released into the atmosphere by growing plants Carbon dioxide is consumed by growing plants, but lost to the soil as fresh organic matter decays When soil is wet, nitrogen can be changed to a gas and lost to the atmosphere. 42 Some processes of soil formation that are complexes of subprocesses and reactions Term Fourfold Brief Description Categorization* 1a. Eluviation 3 Movement of material out of a portion of a soil profile 1b. Illuviation 3 Movement of material into a portion of soil profile 2a. Leaching (depletion) 2 General term for washing out or eluviating soluble materials from the solum. 2b. Enrichment 1 General term for addition of material to a soil body. 3a. Erosion, surficial 2 Removal of material from the surface layer of a soil 3b. Cumulization 1 Aeolian, hydrologic and man-made additions of mineral particles to the surface of a soil solum. 4a. Decalcification 3 Reactions that remove calcium carbonate from one or more soil horizons 4b. Calcification 3 Processes including accumulation of calcium carbonate in Ck and possibly other horizons of a soil 5a. Salinization 3 The accumulation of soluble salts such as sulfates and chlorides of calcium, magnesium, sodium, and potassium in salty (salic) horizons 5b. Desalinization 3 The removal of soluble salts from salic soil horizons *The four categories are (1) additions, (2) losses, (3) translocation, and (4) transformation of material 43 Some processes of soil formation that are complexes of subprocesses and reactions Term Fourfold Brief Description Categorization* 6a. Alkalization (solonization) 3 The accumulation of sodium ions on the exchange sites in a soil 6b. Dealkalization (solodization) 3 The leaching of sodium ions and salts from nitric horizons 7a. Lessivage 3 The mechanical migration of small mineral particles from the A to the B horizons of a soil, producing in B horizons relative enrichment in clay (argillic horizons) 7b. Pedoturbation 3 Biologic, physical (freeze-thaw and wet-dry cycles) churning and cycling of soil materials, thereby homogenizing the solum in varying degrees 8a. Podzolization 3,4 The chemical migration of aluminum and iron and/or organic matter, resulting in the concentration of silica (i.e. silication) in the layer eluviated 8b. Desilication 3,4 The chemical migration of silica out of the soil solum and thus the concentration of sesquioxides in the solum (goethite, gibbsite, etc.), with or without formation of ironstone (laterite; hardened plinthite) and concretions 8c. Resilication 4 Formation of kaolinite from gibbsite in presence of excess Si(OH)4 in solution or formation of smectite from kaolinite in presence of large amounts of Si(OH)4 at higher pH values *The four categories are (1) additions, (2) losses, (3) translocation, and (4) transformation of material 44 Some processes of soil formation that are complexes of subprocesses and reactions Term Fourfold Brief Description Categorization* 9a. Decomposition 4 The breakdown of mineral and organic materials 9b. Synthesis 4 The formation of new particles of mineral and organic species 10a. Melanization 1,3 The darkening of light-colored mineral initial unconsolidated materials by admixture of organic matter (as in a dark A or mollic or umbric horizons) 10b. Leucinization The paling of soil horizons by disappearance of dark organic materials either 3 through transformation to light-colored ones or through removal from the horizons. 11a. Littering 1 The accumulation on the mineral soil surface of organic litter and associate humus to a depth of less than 30 cm 11b. Humification 4 The transformation of raw organic material into humus 11c. Paludization 4 Processes regarded by some workers as geogenic rather than pedogenic, including the accumulation of deep (>30 cm) deposits of organic matter as in muck and peats (Histosols) 11d. Ripening 4 Chemical, biological, and physical changes in organic soil after air penetrates previously waterlogged material. 11e. Mineralization 4 The release of oxide solids through decomposition of organic matter 45 *The four categories are (1) additions, (2) losses, (3) translocation, and (4) transformation of material Some processes of soil formation that are complexes of subprocesses and reactions Term Fourfold Brief Description Categorization* 12a. Braunification, 3,4 Release of iron from primary minerals and the dispersion of particles of iron Rubification, Ferrugination oxide in increasing amounts; their progressive oxidation or hydration, giving the soil mass brownish, reddish brown, and red colors, respectively 12b. Gleization 3,4 The reduction of iron under anaerobic “waterlogged” soil conditions, with the production of bluish to greenish gray matrix colors, with or without yellowish brown, brown, and black mottles, and ferric and manganiferous concretions 13a. Loosening 4 Increase in volume of voids by activity of plants, animals, and humans and by freeze-thaw or other physical processes and by removal of material by leaching. 13b. Hardening 4 Decrease in volume of voids by collapse and compaction and by filling of some voids with fine earth, carbonates, silica and other materials. *The four categories are (1) additions, (2) losses, (3) translocation, and (4) transformation of material 46  Thanks! ANY QUESTIONS? Message me. 47