Soil Physics 477 Manoj K. Shukla Agronomy and Horticulture
Introductory remarks on Soil Physics • Soil Mechanics • Soil properties, definitions, soil structure, surface tension, viscosity • Soil Hydrology- soil water, soil water potential, Darcy's law • Saturated/unsaturated flow through soil • Water infiltration into soil • Evaporation, evapotranspiration • Soil aeration, gas exchange • Heat flow and soil temperature • Solute transport Guest Lectures Field Visit Five laboratory practicals: soil bulk density, particle size distribution, saturated hydraulic conductivity, soil-water characteristic and solute transport
“Soil physics is just not an academic exercise. It involves applications for understanding present critical issues as food security, drinking water, pollution of waters, contamination of soils, air pollution, natural disasters as flooding and landslides …..” -Don Nielsen, Dean UCDavis
Precipitation Evaporation Soil-Air Interface Vadose Zone Portion of aquifer where pore spaces are occupied with water and air (unsaturated zone) Applications of soil physics are crucial to sustainable use of natural resources for agricultural and other land uses Soil-Water Interface Capillary fringe zone Ground water
Applications of soil physics to environment quality Greenhouse Effect: - Gaseous efflux of CO2, CH4, NOx - C sequestration aggregation Soil physical properties and processes Environmental Soil Physics Quality of Life Air quality Acid Rain: - Water quality - Vegetation cover - Biodiversity Particulate matter in air: - Wind erosion - Blowing salt Soil Physics and Environment Quality Fresh water resources and quality: - Suspended and dissolved loads - Biological and chemical O2 demand - Pathogens Soil quality Soil buffers and filters pollutants out of environment Water quality
Soil properties are highly variable at multiple scales • Molecules • Particles or Pore • Aggregate • Column or Horizon • Field or Watershed • Regional • Pedosphere
Soil • The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of plants. • The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics. • Soil Genesis: • The mode of origin of the soil with special reference to the processes or soil-forming factors responsible for development of the solum, or true soil, from unconsolidated parent material.
According to Jenny (1941) soil is a f (climate, organisms, relief, parent material, time) Therefore, similar soil forming factors produce similar types of soils.
Soil Classification is generally done to provide people (e.g., scientists, growers, and resource managers) with the information about the nature and properties of a soil found in a particular location. The principles of Soil Taxonomy are: to classify soils on the basis of properties, which are readily observable or measurable and should either affect soil genesis or result from soil genesis. Curtis F. Marbut (1930) NRCS: 11 soil orders: oxisols, aridsols, mollisols, alfisols, ultisols, spodsols, entisols, inceptisols, vertisols, histosols, and andisols.
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Definitions Soil Physics: • study of soil physical properties and processes, their interactions with one another and the environment, spatial temporal variations in relation to the natural, anthropogenic or management factors • Application of principles of physics for understanding the dynamic interactions between mass and energy status of components (inorganic, organic) and phases (liquid, solid, gas)
Soil Density: ratio of mass and volume • Particle density (rs) • Bulk density (wet and dry) (rb) • Relative density or specific gravity (Gs) • Dry specific volume (Vb)
Soil Mapping: Cartographic representation of actually occurring soil pedons or polypedons Pedon: A three-dimensional soil matrix where horizons shape and relations can be studied Polypedons: A group of contiguous similar pedons Map unit: A group of areas uniquely identified on a soil map. It consists of a collection of polypedons Soil map: A map showing the distribution and locations of a map unit in relation to the prominent geographical, physical and cultural features Reconnaissance map: A map containing some areas or features shown in greater detail than usual Consociations: mapped areas consist of similar soils or are under a single soil texon
Taxadjuncts: the properties are outside the range of a recognized soil series Soil taxonomy and Soil mapping units: Fundamentally different Soil texa: grouping of soil properties for the purpose of classification A soil mapping unit: pictorial representation of a pedon or polypedons actually occurring in an area.
Soil Solids • Inorganic (> 95%) • Organic • Soil is a storehouse of water and nutrients (N,P,K, Ca, Mg, Zn, Cu etc) Buffering Filtering -ability to withstand or adapt to sudden change -ability to leach out pollutants
Inorganic Component Primary Particles Secondary Particles Discrete units; cannot be further subdivided; also known as soil separates sand, silt, clay Consist of primary particles; can be further subdivided into its separates
Texture Particle size distribution Quantitative measure of particle size constituting the solid fraction Qualitative – based on feel method -coarse, gritty, fine, smooth Particle size is important soil physical properties: Total porosity, pore size, and surface area
Systems of Classification • United States Department of Agriculture (USDA) • International Society of Soil Science (ISSS) • American Society of testing materials (ASTM) • Massachusetts Institute of Technology (MIT) • US Public Road Administration (USPRA) • British Standard Institute (BSI) • German Standard (DIN)
Sand – mostly quartz, feldspar and mica (fragments) traces of heavy metal, low surface area Silt – mineralogical composition is similar to sand, intermediate surface area Clay – reactive fraction of soil, colloidal, large surface area, high charge density
Clay Alumino-silicate Secondary clay minerals • Also contain: Fine particles of • Iron Oxide Fe2O3 • Aluminum Oxide Al2O3 • Calcium Carbonate CaCO3 • Other salts
Important properties of clay fraction • Easy hydration because of high affinity to water • High swell/shrink capacity because of expanding nature of clay lattice • High plasticity as it can retain shape when moist • Develops cracks when shrinks • Forms a cake when swells (cohesive forces) • High density of negative charge, which leads to the formation of electrostatic double layer when fully hydrated
Process of determination of particle size fractions is known mechanical analysis Dispersion Fractionation Dispersion is removal of cementing materials to break secondary particles into primary
Fractionation is the process of physically separating the particles into different size fractions Methods of fractionation Approximate size range (mm) Sieving 100.0 - 0.05 Sedimentation 2.0 - < 0.002 Optical Microscope 1.0 - 0.001 Gravity sedimentation 0.1 - 0.0005 Permeability 0.1 - 0.0001 Gas absorption 0.1 - 0.0001 Electron microscope 0.005 - 0.00001 Elutriation 0.05 - 0.005 Centrifugal sedimentation 0.01 - 0.00005 Turbidimetry 0.005 - 0.00005
Sieving or Direct sieving: Dispersed soil suspension is passed through a nest of sieves of different seizes: 2 mm, 1mm, 0.5 mm, 0.25 mm, 0.10 mm Primarily suited for coarse fraction Sedimentation analysis: Based on rate of fall of particles through liquid and depends on particle size and properties of liquid G.G. Stokes (1851) law – “Resistance offered by a liquid to a falling rigid spherical particle varies with the radius of the particle and not with its surface”
Particle Size analysis: • Textural Classes • Frequency diagram • Summation Curve • Uniformity Coefficient F (r) r1 r2 Size distribution curve (schematics)
Uniformity Coefficient = D60/D10 % Finer 60 For uniform particle size UC = 1 UC>1 for nonuniform 10 0.1 D60 D10 10 Diameter, mm
Particle Shape (micrograph) • Depends on : • Size of particle (coarser more irregular) • Parent material • Degree of weathering Angularity (a shape having one or more sharp angles) reflects degree of weathering - Inverse relationship - Highly angular particles are less weathered - Become rounded with progressive weathering by water and wind Coarse fractions such as sand and silt are often angular or zigzag in shape Clay particles: plate or tubular shape
Indices for Particle Shape: • Roundness : measure of the sharpness of corners • Sphericity: how close to a sphere ri – radius of corner R- radius of maximum circle Dd – diameter of a circle with an area equal to that of the particle projection as it rests on flat surface Dc- diameter of smallest circumscribing circle
Dc r1 Soil Shapes: Well rounded rounded subrounded subangular angular very angular
Specific Surface Area Properties related to SSA are CEC, retention and movement of chemicals, swell-shrink capacity, plasticity, cohesion and strength • SSA is expressed as: • Surface area per unit mass (am) • Surface area per unit volume (av) • Surface area per unit bulk volume (ab)
SSA is expressed as: • Surface area per unit mass (am) • Surface area per unit volume (av) • Surface area per unit bulk volume (ab) As – total surface area Ms – mass of soil Vs – volume of soil solids Vt – total volume
SSA can be determined by: • For powdery substances such as clay • Adsorption isotherms • Using inert substances such as N2, water vapor ethylene glycol Amount adsorbed Solution concentration
Methods of measuring SSA • By Ethylene Glycol • Dry soil sample is saturated with ethylene glycol in a vacuum desiccator • Excess polar liquid is removed under vacuum • Surface area is calculated from weight of ethylene glycol retained
BET Method: Brunauer, Emmett, Teller (1938) • Assumptions: • Nonpolar gas molecules are adsorbed in multilayer on a solid surface • Amount of adsorbed gas in monolayer in contact with the surface can be determined by constructing an adsorption isotherm and analyzing it mathematically • Main assumption for BET equation • The molecules adsorbed on the first layer (directly on surface) are more energetically adsorbed than molecules on subsequent layers • Heat of adsorption of all layers after the first is equal to the latent heat of condensation of gas
Linear form of BET equation x = weight of gas adsorbed at equilibrium pressure p = equilibrium gas pressure po = saturation vapor pressure at temperature T xm = weight of gas in a complete monolayer c = exp(E1-L)/RTµ E1 = heat of adsorption in the first layer L = latent heat of condensation R = gas constant/mole (1,336 calories/mole) T = absolute temperature
Procedure • Conduct adsorption experiment by varying p and measuring x (0.05 < p/po < 0.35) • Plot p/x(po-p) against p/p0 Intercept = 1/xmc = value Slope =(c-1)/xmc = value p/p0 Solve these two equations for xm p/x(p0-p)
Total surface area of soil sample St = total surface area xm = experimentally determined weight of gas in an adsorbed monolayer M = molecular weight of the adsorbate (28.01 for N2) N = Avogadro’s Number (6.02 x 1023) (calculated value of the number of atoms, molecules, etc. in a gram mole of any chemical substance) Am = cross sectional area of gas molecule in the monolayer (16.2 x 10-20 m2 for N2) The specific surface area, am, is obtained by dividing the total surface area by the sample weight. Remember adsorption experiment must be conducted at or below the temperature of condensation of gas in order for significant adsorption to occur .
Clay Minerals • Inorganic component consists of : • crystalline and noncrystalline • Primarily- Si, Al, Fe, H and O • Also- Ti, Ca, Mg, Mn, K, Na, and P • Colloidal • Secondary minerals Influences various soil properties: SA, CEC, Nutrient and water holding capacities, buffering and filtering capacities, water transport properties, soil structure etc.
Basic Structural Units in Clay Minerals Tetrahedron (apyramid formed by four triangles ) Octahedron (an eight-sided geometric solid ) Silicon atom placed equidistant from four oxygen or hydroxyls Closely packed oxygen or hydroxyl with AL, Fe or Mg embedded Si4O6(OH)4 These two are joined in 1:1 or 2:1 to form clay minerals
Clay minerals are hydrous aluminum silicates Mg+2 and Fe+3- proxy for AL+3 Commonly observed secondary minerals
Geothite is rich in iron and weathers slowly to form oxide clays Hematite is an oxide mineral Fe2O3 Gibbsite is white crystalline mineral Al(OH)3 Dolomite is sedimentary rocks Ca or Mg(CO3)2 Calcite is mineral composed of CaCO3 Gypsum is natural crystalline mineral CaSO4.2H2O
Charge Properties of Clay minerals Total charge on mineral surfaces is called intrinsic charge density or permanent charge Independent of soil reaction or pH Variable charge is pH or proton dependent Imbalance of complex proton and hydroxyl charges on surface Most soils have a net negative charge Some weathered soils may have net positive
Electric double Layer Negative charge on clay particles is balanced by the cations in soil solution (due to Coulomb forces). Force that acts in two electrically charged bodies is proportional to the product of the module of their charges (q) divided by the square of the distance (d) between them + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Dry Fully hydrated
+ + Electric double layer is due to the negative charge on clay particles and positive on surrounding cations in solution + + Diffuse layer + + + + + + + Clay Particle + + + + + + + + Soil Solution + + + +
There are three models for explaining distribution of ion in water layer adjacent to clay Helmholtz Model: All balancing cations are held in a fixed layer between the clay surface and soil solution Gouy-Chapman Model: A diffuse double layer due to the thermal energy of cations causing a concentration gradient, which leads to a condition of maximum entropy or diffuse double layer Stern Model: Combines the two concepts and proposes condition of free energy. Double layer comprises a rigid region next to mineral surface and a diffuse layer joining the bulk solution Stern’s double layer Potential Potential Helmholtz layer (Fixed) Gouy’s layer (Diffuse) Distance
Stern double layer comprises of two parts: • single ion thick layer fixed to solid surface • diffused layer extending some distance into liquid phase Thickness of double layer is the distance from the clay surface at which cation concentration reaches a uniform or minimum value Nernst Potential or Total Potential Potential Zeta Potential Distance Zeta Potential: is the potential difference between the fixed and freely mobile diffuse double layer. It is also known as electrokinetic potential Nernst Potential: is the difference in cross potentials at the interface of two phases when there is no mutual relative motion. It is also called thermodynamic or reversible potential