Special Assignment for Wednesday: Watch video on soil texture analysis

Transcription

1 Special Assignment for Wednesday: Watch video on soil texture analysis

2 Lecture 3: Soil Physical Properties

3 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

4 Color

5 Soils Display a Wide Range of Colors

6 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

7 Hue: General color Value: Lightness or darkness Chroma: Intensity of color Munsell Color System

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9 OM Produces Dark Soils

10 Soils are Typically Darker when Wet

11 Color Indicates Iron Oxidation State Fe +3 Fe +2 Fe +3 Fe +2

12 Gleyed Soils

13 Texture

14 Macropore Macropore

15 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

16 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

17 Specific Surface Area Smaller particles have more surface area per mass, and a greater ability to hold nutrients and water

18 12 Soil Textural Classes

19 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

20 Particle Size Distributions Vary with Soil Texture

21 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

22 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

23 Soil Structure

24 Soil Structure

25 Soil Structure and Peds

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27 Soil Structure Categories

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30 Blocky Peds

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33 Prismatic Peds

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36 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

37 Scales of Soil Aggregates

38 Influence of Organic Matter on Aggregation