Lecture 4 Structure, Density, Porosity

Lecture 4 Structure, Density, Porosity - Exam Schedule Exam...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Exam Schedule Exam I: Friday, May 22nd Exam II: Friday June 5th Exam III: Friday June 19th Review sessions will be on the Thursday preceding the exam. Soil Texture Three separates: Sand, Silt, Clay Importance: indicator of pore size, surface area, water movement, reactivity There are 12 textural classes based on relative abundance of sand, silt, clay Florida soils tend to range from sandy to sandy clay textures Texture-by-feel assesses grittiness, smoothness, plasticity to estimate texture Laboratory analysis relies on sedimentation of soil particles in water Stokes’ Law determines the settling rates of particles from water Large particles (sand) settle quickly, small particles slowly. A hydrometer measures the density of a soil suspension Based on the density, the mass of particles remaining suspended is determined Texture Particle Size Large/coarse Sand Loamy Sand Sandy Loam Medium Fine/Small Sandy clay Loam Silty clay Loam Silt Loam Loam Clay Loam Sandy Clay Silty Clay Clay Pore Size Large Medium Reactivity Weak Moderate Small Strong Texture and Civilization? Earliest Civilizations Tigris-Euphrates Nile Civilization Year­round supply of water Enduring Sunlight Trustworthy harvests Building materials Nile Jordan Tigris Euphrates Neolithic Founder Crops Wheat Population increases Barley Flax Substantial homes Chick Pea Relative Safety, peace Lentil cows, goats, sheep, and pigs Periodic Flooding Flooding Flooding and Soil Texture Sand 2.0­.05 mm Silt 0.05 – 0.002 mm Clay < 0.002 mm Clay Stokes’ Law: V = kD2 K = 11,241 cm­1 sec­1 Sand 1 mm V = 112 cm/sec Silt 0.05 mm V = 0.281 cm/sec Clay 0.002 mm V = 0.0004 cm/sec Sedimentation Sand Clays/Silts River channel Flooding slows flow The Nile Blue Nile Kenya Uganda Tanzania Flood peaks in mid-September Mesopotamia Alluvial Plain Euphrate s Flood: March through June Agriculture and Irrigation Irrigation Canals Dikes Weirs Reservoirs channels History and Soil Texture (knowledge from clay and stone) Stone and Clay Egypt Stone Sumer Clay Architecture and Sculpture Egypt Sumerian The Language of Power sacred, ceremonial, literary, and scientific language Soil Structure, Density, Porosity Soil Structure Soil Arrangement or grouping of individual soil particles into secondary units. Soil Structure Soil Units of soil Structure Peds Aggregates Soil Structure Formation Chemical Processes – electrostatic attraction between clay particles and between organic particles. Biological Processes -macro-organisms (burrowing, tunneling,wastes) -roots, fungal hyphae (compression, fibers, exudates) -microorganisms (organic residues) Importance Effects on porosity, water retention, water movement Large pores Inter-aggregate pores (between) Small pores Intra-aggregate pores (within) Water moves easily and is poorly retained in inter-aggregate pores Water moves slowly and is strongly retained in intra-aggregate pores Soil Structure is Desirable Poor soil structure can inhibit infiltration of water, water movement, growth of roots. Summary Structure is the arrangement or grouping of individual soil particles into larger secondary units. Clays and organic matter possess natural electrical Charge which can electrostatically bind particles together. Macro-organisms and microorganisms can aid in binding of individual particles into larger aggregates.. Aggregation allows for both movement and retention of water via macropores and micropores, respectively.. Poor soil structure can inhibit infiltration of water, water movement, growth of roots. Soil Density Density Density g Density = Mass ( cm3) Volume 2.65 g/cm3 Soil Bulk Density Soil y BD = mass OD soil volume solids + pores x z Volume = xyz Density of soil including the particles and Density the pore spaces the B.D. ranges between 1.1 and 1.6 g/cm3 (for mineral soil with 1 – 5% organic matter) Bulk vs. Particle Density Bulk Bulk density measures the mass of the soil solids in relation to the volume of the soil solids and the soil pores. Bulk density = mass solids volume xyz y x Particle density = z Mass of particle Volume of particle No pores BD = 1.6 g/cm3 PD = 2.65 g/cm3 Known Volume Sampling for Density Known Volume Dry and Weigh: mass/volume = Bulk Density Factors Affecting Bulk Density Factors Porosity (pores are weightless) • organic matter • aggregation • arrangement of particles • compaction • depth in profile Factors Affecting Bulk Density Organic Matter Organic Typical Mineral Soil: 1-5% organic matter bulk density = 1.1 –1.6 g/cm3 Organic Soils: > 20% organic matter bulk density = 0.1 – 0.6 g/cm 3 Aggregation Aggregation One sand grain (Zero porosity) One aggregate of Clay sized particles (50% porosity) Aggregation generally increases overall porosity, decreases density Packing Arrangement Packing Discrete particle size classes Continuum of particle sizes Arrangement (compaction) Particle Size Sorting Depth in Profile Depth Lower organic matter => Fewer roots Compaction from above Aggregation can mitigate Some of these effects. Higher bulk density Summary Bulk densities of typical mineral soils range between 1.0 and 1.6 g/cm3. Organic matter increases porosity, decreases BD Organic soils can have BD as low as 0.1 g/cm . Compaction decreases porosity, increases BD3 Aggregation increases porosity, decreases BD Depth in profile decreases porosity, increases BD Porosity Bulk Density and Total Porosity [ % Porosity = 1 - ( BD ) ] PD X 100 2.65 g/cm3 Porosity 100% Bulk Density (g/cm3) 0 Bulk density high Bulk density low 2.65 porosity low porosity high A Bulk Density and Porosity E B A BD = 1.1 g/cm3 E BD = 1.15 g/cm3 B BD = 1.6 g/cm3 P = 59 % Pore Size Distribution Pore Macropores > 0.8 mm in diameter large, freely draining sands, inter-aggregate pores Micropores < 0.8 mm in diameter small, storage of water clays, intra-aggregate pores The effect of total porosity and of pore size distribution is largely related to the movement and retention of water as well as the movement of soil gases Knowledge of texture, structure, bulk density, and porosity allow deduction of the patterns of movement of water and gases in soils Next: Water in Soils ...
View Full Document

This note was uploaded on 07/31/2011 for the course SOS 3022 taught by Professor Staff during the Fall '08 term at University of Florida.

Ask a homework question - tutors are online