Chapter 4 Pages 94-132 Outline

Chapter 4 Pages 94-132 Outline - AGRO 301 1/29/08 Chapter 4...

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AGRO 301 1/29/08 Chapter 4 Soil Architecture and Physical Properties I) Soil Color A Soil color has little effect on the behavior and use of soils B Soil color is important in classification and interpretation C Munsell color system -a chart that has color samples and is used to identify the color of the soil 1) Color samples are described by three components of color: (a) Hue ( in soils. Usually redness or yellowness) (b) Chroma (intensity or brightness) (c) Value (lightness or darkness) D Causes of soil colors 1) Color is influenced primarily by its content of organic matter and water and by the presence and oxidation state of iron and manganese oxides. 2) Organic matter is dark brown to black 3) Wet soils are darker than dry ones 4) Oxidation makes bright reds and browns 5) Minerals also add to the color of the soil II) Soil Texture (Size and Distribution of Soil Particles) A Mineral particle size is critical for understanding soil behavior and management B Nature of soil separates 1) Sizes: 2) Soil Separates - the manner in which scientist group the size of particles 3) Coarse Fragments are greater than 2 mm in diameter. (a) Include gravels, Cobbles, boulders (b) May affect the behavior of a soil 4) Fine Earth Fraction - the portion of the soil that passes through a 2mm diameter sieve opening 5) Sand (a) 2mm to 0.05mm in diameter (b) May be rounded or angular depending on how worn down they are (c) Can be rock fragments containing several minerals, but most sand grains consist of a single mineral, usually quartz (SiO2) (d) Has fewer plant nutrients than other separates Boulders 1m Gravel 2mm-75mm Sand 2.0-0.05mm Silt 0.05-0.002 Clay < 0.002mm Submicroscopic clays <10^-6 m
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(e) Sand has a large size and a small surface area thus it holds little water and are prone to drought (f) Sand particles are non-cohesive; that is, they do not tend to stick together in a mass 6) Silt (a) 0.05mm to 0.002mm in diameter: not visible to eye (b) Silt is essentially micro-sand particles with quartz generally the dominant mineral; composed mostly of quartz (c) Composed of weatherable minerals that weather rapidly enough to release significant amounts of plant nutrients (d) Feels like flour (e) Small pores between silt particles (f) Does not exhibit much stickiness or plasticity (g) Yet the cohesion and absorptive capacity some silt fractions exhibit is largely due to a film of adhering clay. 7) Clay (a) Smaller than 0.002mm (b) Have very large surface area, giving them a tremendous capacity to adsorb water and other substances (a) Coheres in a hard mass after drying due to SA (c) When wet clay is sticky and moldable (d) Colloids - organic and inorganic matter with very small particle size and correspondingly large SA per unit of mass (a) If suspended in water colloids do not readily settle out (e) Shaped like tiny flakes or flat platelets (f) Includes the soil properties: shrink-swell behavior, plasticity, water- holding capacity, soil strength, and chemical adsorption
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This note was uploaded on 03/26/2008 for the course AGRO 301 taught by Professor Dr.tomhallmark during the Spring '08 term at Texas A&M.

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Chapter 4 Pages 94-132 Outline - AGRO 301 1/29/08 Chapter 4...

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