Soil Physics PDF - Soil Formation, Soil Processes

👣 Soil Physics Tags Done? Parent item 👣 Soil Physics & Chemistry Priority High Status Not started Soil Formation 💡 How is soil formed?...

👣 Soil Physics Tags Done? Parent item 👣 Soil Physics & Chemistry Priority High Status Not started Soil Formation 💡 How is soil formed? Forming Factors Parent Material Differs in mineralogy, chemistry and grain sizes Determines colour, chemical properties, mineral content and permeability Granites tend to create soils that are sandy and high in quartz (which is resistant to weathering) Basalt contains more magnesium and calcium and produces clayey soils with low sand content Ultrabasic rocks produce soils with high content ferromagnesium minerals and form magnesium rich clay soils. Also contains heavy metals Soil Physics 1  Gneiss produces sand soils Slates produce clay soils Wind deposited material forms well-sorted aeolian deposits that consist of primarily of sand and/or silt-sized particles 💡 Which particular characteristics of primary minerals or rocks as „parent material“ interest soil scientists and why? They are interested in their genesis, chemical-mineralogical composition, weathering stability and physico-chemical properties, in order to gain a better understanding of the soils derived from them. We distinguish between natural substrates, loose materials, hard-rocks sediments, artificial substrates. Topography Location determines the accumulation of soil 💡 How does it influence? Do valleys have more soil than slopes Climate Determines the rate of weathering Temperature and precipitation exert a profound effect on chemical and physical processess Temperature speeds up reactions, water promotes translocation Climate affects soil horizon development Also influences the type of vegetation which influences the soil Organisms All organisms living on the soil determine the rate of humus formation Soil Physics 2  Plants affect soils through organic matter input, cycling of nutrients and water, and through root activity. Vegetation also reduces soil erosion. Organisms affect soils as consumers and decomposers of organic matter and through their earth-moving activities EG: Earthworms have a major effect on soil properties, promoting well aerated and aggregated soils. Human activities widely affect soil formation! Cattle, agriculture Time The length of time during which material has been subjected to weathering. Time determines the thickness of the soil profile If parent material is hard and resistant, then soil may take many thousands of years to develop. Horizons O horizon Leaf litter A Horizon Topsoil B horizon Subsoil C horizon Parent Material Soil Physics 3  💡 Which are the 3 main elements forming the earth’s crust? Oxygen (46,5%), Silicon (28%), Aluminium (8%), Iron (4,5 %) Minerals 💡 Why do micas play a major role in the formation of clay minerals in soils? 💡 Give a range of the following soil substances (NaCl, CaCO3 Soil Processes Additions Precipitation Flooding Flood waters often leave sediment behind Deposition Any sediment by wind, water, glaciers etc, can be deposited on the surface. Humans can also add soil material Leaf accumulation leaf litter in the fall adds organic mater to the soil Landslides once the landslide materials comes to rest it has been deposited on top of the existing soil Fertilization Soil Physics 4  Huge input of material in form of inorganic and/or organic material Losses Through the movement of wind or water or uptake by plants and microorganisms, soil particles or chemical compounds can be eroded, leached, or harvested from the soil, altering its chemical and physical makeup. Leaching this is as loss as leaching refers to materials being removed from the soil and into groundwater Erosion this can be from wind, water or ice Landslides the materials that moves down slopes off land surface is a loss Flooding although flooding can leave materials behind it can also remove material and move it downstream Transformations The chemical weathering of sand and formation of clay minerals, the transformation of coarse organic matter into decay-resistant organic compounds (humus). Physical weathering/fragmentation Freeze-thaw and drying-rewetting cycles, fire Chemical Weathering formation of secondary minerals from primary minerals Soil Physics 5  Decomposition of organic matter by microbial transformation processes Translocation Movement of soil constituents (organic or mineral) within the profile and/or between horizons. Examples: Animal digging animals from ants to moles to woodchucks to gophers dig holes and physically bring materials to the surface Mixing Humans disturb soil as do plants and burrowing animals Ploughing all forms of tillage move soil around. It may be far but soil below the surface is often brought up to the surface 💡 Which are the main soil structure formation mechanisms? Flocculation/dispersion Swelling and shrinking processes Cementation Adhesion Soil tillage Soil Phases Solid: Mineral and organic particles Soil Physics 6  Liquid: Soil water and solutes Gas: Air The content and spatial distribution of the 3 soil phases are highly variable 💡 How does the spatial distribution vary? 1 phase can fail completely and we have a 2 phase system which is extremely hostile 💡 Give examples for soil types or soil horizons for a 2-phase soil system The Gr-horizone of gleye soils or other hydromorphic soils (no air). Desert soils (no water). These are extreme conditions, where only specialised organisms can survive. Fully saturated soils Dry soils Only water and solid Oven fried; air and solid Mass, Volume and density Indication of phase fractions Mass fractions Volume fractions Related to the dry mass of the soil Related to the bulk soil volume mix To express results as vol% Generally used for expressing analysis results on a mass basis Soil Physics 7  Particle Density is the mass unit per volume of soil particles (no pores!) It is a relatively constant parameter For mineral soils it is assumes to be 2.65 Mg/m3 Is lower with high organic matter contents Soil Physics 8  💡 Give a definition of “particle density” and its formula. Which values does it usually show in soils? Particle density (dP) is solid soil mass (SM) related to the solid volume (SV). dP [Mg * m-3] = SM / SV dP is mostly also considered as dry density (105° C); Range in soils, depending on humus content, 2,4 - 2,7 Mg/m³. In soil science dP is normally not measured because it is very time consuming. Assumption in soil science: 2,65 Mg/m³ (if quartz is the main mineral component) Bulk Density Is the mass per unit bulk volume of soil that has been dried to a constant weight at 105ºC Is an indicator of soil compaction This volume includes the volume of soil particles and the volume of pores among soil particles Soil Physics 9  💡 What does overcompaction mean in agricultural soils? At overcompaction the horizontal stress is the highest and the vertical stress the lowest. It can be repaired by turbation (Bioturbation, arboturbation, Cryoturbation, Peloturbation, Technoturbation) The soil is much more compacted than the load at time of measurements would presume. Agricultural soils are generally overcompacted. Soil compaction leads to a decrease of soil porosity, especially of coarse pores. On the one hand this might be an advantage in terms of the higher amount of medium pores (higher water holding capacity, but there is a problem if the soil air content goes below 10 vol%. Bulk vs Particle density Factors affecting Bulk Density SOM Texture Density of soil minerals Soil Physics 10  Packing arrangement of aggregates Presence and amount of rock fragments Soil depth Soil management Loose, well-aggregated, porous, high organic matter soils have a lower bulk density Sandy soils have a higher bulk density because of less pore space 💡 define the bulk density of a soil and which values it usually takes Bulk density (dB) is solid soil mass (SM) related to total soil volume (TSV) dB [Mg * m-3] = SM/TSV In most of the cases the dry bulk density (105° C) is used; the moist bulk density is calculated at natural water content. dB ranges in soils 0.3 - 1.8 Mg/m3 Soil Texture 💡 Which processes can cause texture changes along a soil profile? Particle size organic substances are generally destroyed in the lab before measuring the particle size distribution because it is hard to classify them Soil particles are classed into sand, silt and clay The fraction limits are commonly on a logarithmic scale Soil Physics 11  The fine fractions of sand, silt and clay are further classified by dividing the scale sections in the middle of the logarithmic scale Equivalent diameter Measure for the size of irregular shaped particles which is used for the determination of particle sizes in soil and sediments 💡 How is the equivalent diameter in soil texture defined and why is it needed? Comparison of particles is complex, because of their very different shape. Therefore the concept of „equivalent diameter“ was introduced. It can be defined as an alternative value, which indicates the size of a theoretical regular shaped particle, a sphere, which behaves at determination in the lab the same way like the real particle under investigation. This makes the analyses of the particle size distribution easier. Analysis If particle sizes are determined by mechanical sieving, the equivalent diameter refers to the diameter of a sphere which barely passes through a defined sieve pore. This is commonly done for the sand fractions Smaller particle sizes are often determined by sedimentation in aqueous medium and the equivalent diameter refers to the diameter of a sphere with the same density and the same sedimentation velocity For determination of silt and clay fractions, sedimentation methods based on Stokes law are used to deduce particle size distribution Soil Physics 12  Soil particles settle in aqueous solution attaining terminal velocities proportional to their mass and size The amount of suspended soil after a given settling time is used to determine particle size fractions The amount of soil in suspension is determined by either extracting a sample by the pipette method or from a direct hydrometer reading 💡 Classify pore size distribution after their diameter and explain on which physical law it is based “Equivalent diameter” of a soil pore is needed defined as: diameter of theoretical cylindrical capillar with equivalent water retention force or capillary rise. The equivalent diameter of a soil pore to a straight circular capillary --> rises same hight. In nature the pores are a complex continuum of different shaped pores (not cylindric). The base for the calculation of pore size classes is the capillarity law: (only r variable!) h = pF = [(2*γ) / (r * D * g)] h = capillary rise, pf = log (h), γ = surface tension, r = capillar radius, D = water density, g = gravity Stokes Law Soil Physics 13  Limitation: Particles are large enough to be unaffected by the thermal motion of the fluid molecules All particles are rigid, spherical, and smooth All particles have the same density The suspension is dilute enough that particles do not interfere with each other Fluid flow around the particles is laminar In reality, soil particles are neither spherical nor smooth (therefore the equivalent radius is used) 💡 Should I add the methods Darcy’s Law describes the flow rate of a fluid through a porous medium. Q = K AΔh/L Soil Physics 14  Q = Discharge or volumetric flow rate (m³/s) This is the rate at which fluid flows through the porous medium. K = Hydraulic conductivity of the porous medium (m/s) a measure of how easily fluid flows through the material A = Cross-sectional area perpendicular to flow (m²) The area through which fluid flows, measured perpendicular to the direction of flow. Δh = Hydraulic head difference (m) Represents the energy available to drive the flow due to pressure or elevation differences. L = Length of the flow path (m) The distance over which the fluid flows through the medium. Saturated water flow : Unsaturated water flow: DARCY-equation: DARCY-equation: Q = ksat. grad ψ Qu = -ku. dψ/dz Q = flow amount (cm3) per flow area Qu = Flow amount (cm3) per flow area (cm2) in time (s) (cm2) in time (s) ksat = saturated hydraulic conductivity ku = Unsaturated hydraulic (cm/sec) conductivity (cm/sec) grad ψ = water potential along the flow dψ/dz = Potential gradient along the distance flow distance ku = f (ψm, φ) Particle size distribution curve Indicates if a soil is well-graded, poorly graded or gap-graded Well grated soil Poorly graded soil Gap graded soil Soil Physics 15  all particles of different The soil represented There is a gap between sizes are present in this can be called the distribution of soil uniformly graded soil. particles. Such soils miss means that there is a The curve is steep and the amount of soil good distribution of has very short range between two certain particles of particle sizes. particle sizes. 💡 How is the homogeneity factor U defined and what does it show? Give an example for “well” and “bad” sorted soil material The factor U expresses the sorting grade of the soil particle sizes. For the soil porosity it is of major importance whether the particle size is homogeneous (= badly sorted = all particles with similar size) or rather heterogeneous (= well sorted = many different sizes). The particle sizes are measured at 2 points (60% and at 10% of the y- axis) of the texture curve. U < 100: very homogeneous texture; U > 400: heterogeneous texture, well sorted Examples: detrital loam has U-factor of 465 and sand has a U-factor of 4. A well-graded soil with a high U-factor provides better compaction, structural integrity, and resistance to erosion, making it ideal for construction applications such as embankments and foundations. Atterberg limits Soil Physics 16  Relation between particle size distribution and soil water content The Atterberg limits tests establish the moisture contents at which fine-grained clay and silt soils transition between solid, semi-solid, plastic, and liquid states. 1. Liquid Limit (LL) is the water content at which soil changes from a plastic to a liquid state when the soil specimen is just fluid enough for a groove to close when jarred in a specified manner. 2. Plastic Limit (PL) is the water content at the change from a plastic to a semi- solid state. This test involves repeatedly rolling a soil sample into a thread until it reaches a point where it crumbles. 3. Shrinkage Limit (SL) is the water content where the further loss of moisture does not cause a decrease in soil volume Plasticity Index (PI): Index for the sensivity of soils against changes in water content and structure stability. PI=%H2O (LL)-%H2O (PL) They are needed to find the best consistency for soil tillage. The best consistency for soil tillage is at plastic limit, the worst the liquid limit. 💡 What are the plasticity limits after ATTERBERG and what are they needed for? Texture triangle Soil texture is the relative proportions of sand, silt, or clay in a soil. The soil textural class is a grouping of soils based upon these relative proportions. Soils with the finest texture are called clay soils, while soils with the coarsest texture are called sands. Soil Physics 17  A soil that has a relatively even mixture of sand, silt, and clay is called a loam. If the percentages of clay, silt, and sand in a soil are known, you may use the texture triangle to determine the texture class of your soil. Classification of soil textural class allows us to take advantage of research and experience at one location to predict the behaviour of similarly classified soils at another location. Why do we care about classifying soil types? To compress detailed particle size distribution information into an informative „index“. To develop predictive capabilities for hydrological and other applications. To aggregate and create map units with similar soil properties for land use planning–farming, irrigation, construction, etc. Sandy Soils Poor structure, grittyin natureand lack in cohesion Feeldry comparedtoloamand claysoils Soil Physics 18  Free draining, dry out rapidly Lack nutrients–washedout bydownwardmovementofwater Advantage: easy tocultivate(light in nature), warm upquicklyin spring (helpstoprovidea longergrowingseason) Management ofthesandysoilsdifficult–oftenignoredin agriculture Clay Soils Heaviestofthethreesoils, fineparticlesresultingin fewairspaces–givethesoila veryhigh levelofcohesion Clay soilsdrain poorlyand feellumpyand stickywhenitiswet Often sticks to footwear and tools in gardens Feels smooth not gritty, heavy to cultivate-forming clods that are difficult to separate Consists of 50% of clay particles, attcract positively charged particles– calcium, potassium& magnesium Slow draining can lead to water logging. Loamy Soils Has greater cohesion than sandy soils, hold together better when a handful is picked up Soft and rich in touch Comprisedof40%-40%-20% of sand, silt and clay The textureisgrittyand retainswaterveryeasily, yetthedrainageiswell. Therearevariouskindsofloamysoilranging fromfertile toverymuddyand thicksod. Containsmorehumus& nutrientsthanothers, betterinfiltrationand drainage Loamsmaybewetin winteraswatertableraisesbut arewelldrainedin summer Soil Physics 19  Silty Soils Silty soil feels slippery when wet, not grainy or rocky. Wind-blown silt deposits are known as loess More fertile than other types of soil Very good for growing crops Silt promoteswaterretentionand aircirculation High risk of erosion by wind and rainfall 💡 Which soil type can be considered a typical sandy soil and which one a silty soil? Podsol sandy soil, Chernozem is a silty soil Temporal Changes Considered a very stable soil property Over time, pedological processes can alter soil texture Dissolution Precipitation processes Clay illuviation (luvisols) Changes by soil management By mixing with another soil with a different textural class Mechanical desintegration Adding peat, compost, perlite, etc does not change soil texture since it only adds organic compounds, not mineral particles Importance Soil Physics 20  basic soil parameter that influences a wide range of soil properties Water-holding capacity–the ability of a soil to retain water for use by plants Permeability–the ease with which air and water may pass through the soil Soil workability–the ease with which soil may be tilled and the timing of working the soil after a rain Ability of plants to grow–some root crops like carrots and onions will have difficulty growing in a fine-textured soil. Erosion–soils rich in silt and clay are more susceptible to erosion than sandy soils Cation exchange capacity(CEC) –clay soils have a higher CEC Ecology–organisms have specific preferences of soil types Soil Organic Matter What is SOM Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by soil organisms. Components of hummus Non-humic substances Material with plant or animal origin Not or only slightly decomposed Morphology of tissues structures macroscopically distinguishable Humic substances Soil Physics 21  Strongly decomposed, high molecular-weight substances Usually dark colour More difficult to decompose than original substances Properties of hummus Chemically not clearly defined, highly polymeric organic compounds The tiny colloidal particles are composed mainly of C, H, and O. The collodial particles are negatively charged(-OH, -COOH, or phenolic groups) Large specific surface area Humic substances can(reversibly) bind water and ions (including nutrients) Cation exchange capacity(CEC) higher than of clay minerals! 4-5 times higher water holding capacity than that of silicate clay Humus has a very favourable effect on aggregate formation and stability Impart black colour to soils Classification of humus Soil Physics 22  Formation of SOM Soil Physics 23  Analysis of SOM Determination of C by loss-on-ignition Determination of total carbon by elemental analyses(EA) →Total C (dumas) Wet oxidation of organic substance TOC/NPOC (Non-purgable organic carbon) CO2emissionsbygas chromatography→ indicator for C decomposition Determination ofmicrobialbiomassC →back - calculationtoTOC CharacterizationofSOM byInfrared Spectroscopy Characterization o fSOM by15N-and 13C Nuclear Magnetic Resonance (NMR) spectroscopy Importance of SOM Soil Physics 24  Soil Colour Provides clues about other soil properties Diagnostic feature Munsell color chart Hue dominant spectral colour Soil Physics 25  Value indicates the lightness/darkness 10=white 0=black Chroma intensity /brightness of the colour a chroma of 0 is a neutral grey Colour and SOC Little change in hue Decreasing value Decreasing chroma Redoximorphic Features soil colours formed by the repeated chemical oxidation and reduction of iron and manganese compounds resulting from saturation Useful for predicting the presence and depth of seasonal high water tables in the soil Soil Physics 26  💡 Which substances influence the color of soils? Soil colors are the results of particular pedogenetical processes (e.g. weathering, water stagnation, etc.) where the Fe-oxides and humus play a major role, Fe-, (Al)-, (Mn)-oxydes/hydroxides give soils, together with humic substances, their typical color. Examples: Black or brown: Humus or magnetite White: Sodium salts, carbonates, quartz grains Light grey: reduced iron (Fe2+) Marbling grey zones indicate the removal of clay or a mixture of iron and manganese oxide. Indicate chemical reduction of iron resulting from saturation rusty or dark patches indicate accumulation of iron or manganese oxide Specific Surface Area Total surface area of soil particles per unit mass or per unit volume of soil Unit mass of soil particles: Am ​ = As /Ms [m /g] ​ ​ 2  Unit volume of particles: Av ​ = As /Vs [m /m ] ​ ​ 2 3  Unit bulk volume of soil: Ab ​ = As /Vt [m /m3] ​ ​ 2  💡 How does particle size affect surface area? Soil Physics 27  Importance Cation Exchange Capacity Adsorption and release of compounds Swelling Water retention Soil Plasticity, cohesion and soil strength Silt and sand are commonly idealized as smooth spheres: Clay usually idealized as: Measuring Methods 1. Adsorption of gas 2. Adsorption of polar liquids 3. Soil Physics 28  💡 What is specific surface of the soil solid phase and how is it related to soil texture? Write an increasing range for the clay minerals Kaolinite, Illite, Montmorillonite, Vermiculite Specific surface: Total amount of free surfaces, very reactive, generally electrically charged (especially the interlayer space of clay minerals); the specific surface increases with decreasing particle size = higher weathering intensity From low to high specific surface area: sand < silt < coarse clay < fine clay Especially important are boundary surfaces between solid, liquid and gasous phases = the walls of the pore space. Kaolinite 15m²/g Illite 90 m2/g Vermiculite 780 m2/g Montmorillonite 800 m2/g Particle Shape The shape of soil particles present in soil is equally important as particle size distribution because it has significant influence on the physico-chemical properties of a soil: Void ratio (pores), soil strength, compressibility, reactive surfacearea, etc. 💡 How does this relationship translate? Soil Physics 29  Estimation of roundness helps to understand whether a material has been deposited in situ or transported over longer distances. Roundness is particularly pronounced in fractions that have been transported for long time and distances by rolling, slipping, and abrasion (sand, silt). Clay is usually transported in suspension and much less rounded. Soil Porosity PORE SIZE DISTRIBUTION: Fine pores (< 0.2 µm) (Permanent Wilting Point) Medium pores ( 0.2-10 µm) (available Field Capacity) Narrow coarse pores ( 10-50 µm) (available Field Capacity) Big coarse pores (>50 µm) (Air Capacity) 💡 Classify soil pore size distribution after their diameter and explain on which physical law it is based “Equivalent diameter” of a soil pore is needed defined as: diameter of theoretical cylindrical capillar with equivalent water retention force or capillary rise. The equivalent diameter of a soil pore to a straight circular capillary --> rises same hight. In nature the pores are a complex continuum of different shaped pores (not cylindric). The base for the calculation of pore size classes is the capillarity law: (only r variable!) h = pF = [(2*γ) / (r * D * g)] h = capillary rise, pf = log (h), γ = surface tension, r = capillar radius, D = water density, g = gravity Soil Physics 30  Matrix Potential Is the potential energy of soil water due to the attractive forces (adhesion and cohesion) between water and the soil matrix. Matric potentialis expressed as energy per unit volume and equals the product of the height of rise in a capillary tube, the water density, and the gravitational constant. Matric head is expressed as energy per weight and is equal to the height of rise in a capillary tube. Soil Physics 31  💡 What does a negative or positive hydraulic potential gradient mean in water flow in soils? What is a watershed and at which hydraulic water potential does it occur? The hydraulic potential gradient describes the direction and magnitude of water movement in soils. Negative Gradient: Water moves upward or against gravity, typically due to capillary rise or strong suction forces in dry soils. Occurs in unsaturated soils when matric suction dominates, or during evaporation. Positive Gradient: Water moves downward, typically under gravitational forces. Common in saturated soils after rainfall or irrigation. A watershed (also called a drainage basin) is an area where all precipitation collects and drains into a common outlet, such as a river, lake, or ocean. Watersheds occur where hydraulic potential reaches a maximum or divide, with no flow across the boundary, forming the separation between different drainage basins. Soil Physics 32  💡 Define the gravity potential (+Ψg): The work, which is necessary to rise a unit water from a free water surface (ground water table) to a certain height in a soil pore (capillary) against the force of gravity! If this height lies above the Gravity potential, the water acquires a position energy from which a certain amount of work could be produced if it flows downwards (positive sign). If this level lies under the gravity potential, then the gravity potential has a negative sign. pF curve In soils a particular soil water potential ψis related to the soil water content θ. The potential is also expressed as pressure head h. The functional relationship h(θ) is typical for a given soil having its particular status of geometrical arrangement of particles (e.g. soil particle distribution). The function h(θ) is called a soil water retention curve or soil moisture characteristic curve.Because h extends over three or four orders of magnitude, it is frequently plotted in logarithmic scale(pF-curve). It is possible to compute the available water for a plant with the pF-curve. Soil Physics 33  Soil Structure Soil structure describes the spatial arrangement of particles to complex aggregations, pores, and channels. Sand, silt, clay, and organic particles become aggregated together due to various forces and at different scales to form distinct structural units called peds or aggregates. The term ped is most commonly used to describe the large-scale structure evident when observing soil profiles, involving structural units which range from a few mm to about 1 m. The attraction of soil particles in peds is mainly influenced by physical processes such as: freeze-thaw, wet-dry, shrink-swell, root growth, burrowing of soil animals, human activities. Soil Physics 34  💡 Give a definition of soil structure Soils are 3 dimensional phase systems: matrix, air, water!!! The soil texture alone does not explain the spatial distribution of particles and pores. There are different definitions, for example from BREWER (1976) physical composition of solid soil material, as given from size, shape and spatial distribution of particles and pores and their characteristics. The soil structure is a dynamic system and very sensitive to changes, this means that any changes in soil parameters influence other soil components and therefore the soil structure. Importance Soil structure influences the amount and nature of porosity. Structure controls the amount of water and air present in the soil. Structure controls run-off and erosion. Crumbly and granular structure provides optimum infiltration, water-holding capacity, aeration and drainage. Structure affects tillage practices. Structure influences the quality of the habitat for microorganisms and plant growth. Structure influences the supply of nutrients. Soil Physics 35  💡 Refer the most relevant soil structure-types at fine scale (morphologically) in soils and give examples, in which soil horizon they usually may occur. Morphological structure types: From the point of view of the scale of observation: large scale structure (whole soil profile) fine scale structure (visible with nacked eyes) microscopic scale (microscope tecniques) From the point of view of the aggregation: primary structure (no aggregates) C-horizon and hydromorph horizons secondary structure (aggregates) AB, Ah, B(weathered), Bg horizons anthropogenic structure (arable soils) Ah, AB – B horizons 💡 Write and explain the 4 main forces which influence the stability of soil structure 1. Weight of particles themselves (gravity) --> compacting 2. Static loads (weight of above layered horizons) and temporary loads (animals and machines) --> compacting 3. Flow pressure of soil water --> compacting 4. Cohesion/adhesion forces (including water menisci at contact points of the solid particles) --> acts neutral Classification Soil Physics 36  Structureless Particles are Decreased aeration and drainage Spheroidal Granular and crumb structures: Individual particles of sand, silt and clay grouped together in small, nearly spherical grains. Water circulates very easily through such soils. They are commonly found in the A-horizon of the soil profile. Plate-Like Block-like Angular Subangular Prism-like Soil Physics 37  💡 Refer the most relevant soil structure-types at fine scale in soils and give examples, in which soil horizon they usually occur From the point of view of the scale of observation: large scale structure (whole soil profile) fine scale structure (visible with nacked eyes) microscopic scale (microscope tecniques) From the point of view of the aggregation: primary structure (no aggregates) C-horizon and hydromorph horizons secondary structure (aggregates) AB, Ah, B(weathered), Bg horizons anthropogenic structure (arable soils) Ah, AB – B horizons Formation Physical-Chemical Processes Flocculation Soil clay particles can be unattached to one another (dispersed) or clumped together (flocculated) in aggregates. Soil aggregates are cemented clusters of sand, silt, and clay particles. Shrinking and swelling Capillarity The Soil Physics 38  Capillarity Law (or Capillary Law) refers to the principles governing the rise or depression of a liquid within a small tube or porous material due to surface tension and adhesive forces. h = 2γ cosθ/ρgr h = Capillary rise or depression (m) γ = Surface tension of the liquid (N/m) The cohesive force at the liquid's surface that causes it to behave as though it has a stretched membrane. θ = Contact angle between the liquid and the tube wall The angle between the liquid surface and the solid surface. For water on glass, θ is typically close to 0°, leading to a rise. ρ= Density of the liquid (kg/m³) g = Gravitational acceleration (9.81 m/s²) r = Radius of the capillary tube (m) Smaller capillaries lead to higher capillary rise due to stronger effects of surface tension. Soil Physics 39  💡 Write and explain the capillarity law and explain its relation to the pF-curve In soils, capillarity governs the movement of water in unsaturated conditions. The pF-curve describes the relationship between soil water tension (or suction) and water content. 1. Capillary Rise and Matric Potential: Capillarity creates negative pressure (suction) in the soil. The smaller the pores, the higher the capillary rise and matric potential, which corresponds to higher pF values. 2. Soil Texture Influence: Fine-grained soils (like clay) have higher capillarity and higher water retention, resulting in a steeper pF- curve. Coarser soils (like sand) have lower capillarity and lower pF values for the same water content. 3. Saturation Levels: At low pF values (around 0), soil is saturated, and capillary forces are minimal. As the soil dries, capillary forces dominate, and the pF value increases, showing the critical role of capillarity in water retention. Freezing and Thawing Effects of soil organisms Soil Physics 40  💡 Which bioturbations do you know? The mixing of soil material by soil organisms and plants. Deep turbations occur in continental steppe regions (Weinviertel, Pannon) through digging soil animals and earthworms. Earth worm activity: Earth worms form very stable dejection aggregates, with high content of organic matter, they further form stable channels (coarse pores) Influence of plants: Plants influence the soil formation especially on top soil, where their root system is highly developed, roots penetrate axially and radially. If the radial growth encounters different resistances on the bottom, the needed space is obtained through elevation of the soil surface. Burrowing Earthworms casts Hierarchy Soil Physics 41  Shearing stress 💡 Define “Shearing stress” and how it is influenced by structure type, content. Specific Heat Capacity 💡 Define Specific heat capacity and heat conductivity of soils. Which characteristics of soils do they influence? Specific heat capacity Cb = heat amount, which must be added/taken from a soil, in order to increase/decrease its temperature of 1° C. Thermal conductivity λ = energy amount [J], which flows in time [sec] through a surface [m2] at a temperature gradient of 1° C/m. They influence the soil temperature not only on the surface but also in the depth. Furthermore they influence the temperature-amplitude of the soil throughout the day and year. The temperature influences the activity of microorganisms. Matrix Potential Soil Physics 42  💡 Define matrix potential of a soil Matric potential – Ψm = "Saugspannung". The suction pressure can be understood as a mechanical pressure resulting from the capilarity of the grain skeleton in the soil and the surface tension of the wetting fluid. The finer the pores are, the higher the water can rise. The relation between water content (%Vol.) and soil matric potential describe the pF-curve. Gravity Potential 💡 What are „Druckzwiebeln (pressure distribution)“ in soils and what do they show? Which deduction can be done from their shape? 6) Define „permanent“ and „variable“ electric charge in soils and how they generate? All soil solid particles have negative or positive charged surfaces. Permanent charges: pH independent (eg: clay minerals) Variable charges: pH influenced due to variable positive/negative charge on the reactive group (eg: humic substances, metaloxides/-hydroxides). Positive variable charges: uptake of H+ protons at their reactive group Negative variable charge: discharge of H+ protons from their reactive group Zero point of charge (ZPC): pH-value, at which the total sum of positive and negative charges is = zero. 7) Which resistance forces can be released by soils at mechanical stress? Explain these forces. (sS75) Soil Physics 43  The resulting resistance force comprises 2 parts: shear stress (Tangentialspannung) τ: force tangential to surface normal stress (Normalspannung) σn: force perpendicular to surface Each normal stress causes shear stresses in other directions. Each mechanical impact on soils produces σn and also resistance (τ). Soil Physics 44