Special Assignment for Wednesday: Watch video on soil texture analysis https://youtu.be/ioyabxj767s
Lecture 3: Soil Physical Properties
Important Physical Properties Soil Color: Most obvious physical property; provides clues about chemical conditions Soil Texture: Size distribution of soil particles Sand, silt, or clay size particles Proportions of each define 12 soil textural classes Soil Structure: Arrangement of sand, silt, clay, and organic matter in soils Spheroidal, platelike, blocklike, prismlike Soil Density: Particle and bulk density, controlled by texture and porosity
Color
Soils Display a Wide Range of Colors
Munsell Color System Hue: General color Value: Lightness or darkness Chroma: Intensity of color Determined by comparison with color chips in Munsell Soil Color Book
Hue: General color Value: Lightness or darkness Chroma: Intensity of color Munsell Color System
OM Produces Dark Soils
Soils are Typically Darker when Wet
Color Indicates Iron Oxidation State Fe +3 Fe +2 Fe +3 Fe +2
Gleyed Soils
Texture
Macropore Macropore
Size Classification of Soil Particles Sand, silt, and clay are the main size classifications Clay also has a mineralogical usage: Clay minerals are layered aluminosilicates that form via rock weathering
Relationship Between Mineralogy and Particle Size Primary Minerals: Inorganic compounds that make up rocks Examples: Quartz, Feldspar Secondary Minerals: New solid phases that form after rocks are weathered Examples: Clay minerals, iron and aluminum oxides, calcium carbonate See Table 2.1 for more information and examples Sand and silt size particles contain mostly quartz [SiO 2 ] The clay size fraction differs in mineralogy, containing mostly clay minerals like kaolinite and smectite
Specific Surface Area Smaller particles have more surface area per mass, and a greater ability to hold nutrients and water
12 Soil Textural Classes
Loams and Other Texture Classes Loams are defined as mixtures of sand, silt, and clay that exhibit properties of each in equal proportions Loams do not contain equal amounts of sand, silt, and clay!!!!! A small amount of clay has a big effect! Most soils are some type of loams Other soil textures reflect material with properties dominated by one particle type
Particle Size Distributions Vary with Soil Texture
Texture Influences Many Soil Properties Property Sand Silt Clay Water holding capacity Low Medium to high High Aeration Good Medium Poor Drainage rate Fast Slow to medium Very slow OM decomposition rate Susceptibility to water erosion Fast Medium Slow Low High Low to high Compactability Low Medium High Ability to store nutrients Pollutant leaching potential Low Medium to high High High Medium Low
Key Concepts in Soil Physical Properties: Part 1 Soils display a number of important physical properties that are often diagnosed Color, texture, structure, and density Soil color is classified using the Munsell system and reflects the composition of the soil Soil texture is primarily controlled by the soil mineralogy Soil textural classes are important components of soil classification
Soil Structure
Soil Structure
Soil Structure and Peds
Soil Structure Categories
Blocky Peds
Prismatic Peds
Ped Development The shape of a ped reflects the relative stresses placed on the material Platy: Vertical stress greater than horizontal Blocky: Equal stress Prismatic: Horizontal greater than vertical Stresses come from weight of material above and lateral pressure from swelling of clays Interped gaps form when soil dries Clays and organic matter fall into gaps, coating ped surfaces and preserving zone of weakness
Scales of Soil Aggregates
Influence of Organic Matter on Aggregation