sw10 - Soil Structure Soil Texture Three separates: Sand,...

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Unformatted text preview: Soil Structure 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 Sandy clay Loam Silty clay Loam Silt Loam Loam Fine/Small Clay Loam Sandy Clay Silty Clay Clay Pore Size Reactivity Large Weak Medium Moderate Small Strong Texture and Civilization The earliest civilizations? Earliest Civilizations Mesopotamia Egypt Civilization: Before and After Development Paleolithic (old stone age) was the first period in the development of human technology of the Stone Age. Homo habilis ­2 million years ~12,000 years Slow alteration, refinement of tools Stone Tools Development was a period in the development of human technology that is traditionally the last part of the Stone Age Neolithic (new stone age) ­12,000 ­3500 years Domestication Pottery Weaving Hafted Axes Cause of the change? Agriculture 20,000 years ago Development Of Agriculture (systematic/irrigated) 8,000 years ago Development was slow and variable Neolithic Revolution food gatherers to food producers Fewer seekers of food Surplus Food Craftsmen Traders Technicians Specialized skills Diverse abilities Why Then? Forced Adaptation Climate and Water Homo sapiens Retreat of the Ice Italy Gibraltar Spain Africa Turkey Civilization Nile Jordan Tigris Euphrates Neolithic Founder Crops Wheat Barley Flax Chick Pea Lentil cows, goats, sheep, and pigs Other major feature? Flooding Floodplain: area along a river that is periodically flooded 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 Silt Clay River channel Sedimentation Sand Silts/Clays Flooding slows flow History and Soil Texture (knowledge from clay and stone) Stone and Clay Egypt Sumer Stone Clay Architecture and Sculpture Egypt Sumerian The Language of Power sacred, ceremonial, literary, and scientific language Extra Credit: 1. The Paleolithic was between _________ and ____ years ago 2. The human development leading to the Neolithic __________ 3. Area along a river that is periodically flooded _____________ 4. The two earliest civilizations __________________________ Two points Soil Structure 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, movement, roots Large pores Inter-aggregate pores (between) Small pores Intra-aggregate pores (within) Soil Structure is Desirable Poor soil structure can inhibit infiltration of water, water movement, growth of roots. Soil Structural Types Granular, Platy, Blocky, Prismatic, Columnar Granular Structure Characteristic of Surface soils Prominent in Grasslands Product of organic matter and organisms Very rounded edges Platy Structure Thin, horizontal peds Often inherited from parent material Blocky Structure Angular Blocky Sub-angular Blocky Subsurface horizons (B) Promotes drainage in B horizons Prismatic and Columnar Structure Prismatic Vertical orientation Subsurface horizons Common in semi-arid regions Columnar Classification type Spheroidal Platy Block-like Prism-like class Fine Medium Coarse V. coarse grade Strong Moderate weak 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, organic matter, 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. Next: Soil Density and Porosity End of Lecture 6 Disregard slides that follow 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 Density Density g Density = Mass ( cm3) Volume 2.65 g/cm3 Soil Bulk Density Soil y BD = mass OD soil volume solids + pores z Volume = xyz x 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 = Mass of particle Volume of particle z 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 Higher bulk density Aggregation can mitigate Some of these effects. 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 Bulk % Porosity = 1 - [ ( BD ) ] PD X 100 2.65 g/cm3 Porosity 100% Bulk Density (g/cm3) 0 2.65 Bulk density high Bulk density low porosity low porosity high A Bulk Density and Porosity E B A E B BD = 1.1 g/cm3 BD = 1.15 g/cm3 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 ...
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This note was uploaded on 02/04/2011 for the course SWS 3022 taught by Professor Bonczek during the Spring '11 term at University of Florida.

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