physical properties of soil intro to soils ch 4. soil texture describes the proportion of soil...

Download PHYSICAL PROPERTIES OF SOIL INTRO TO SOILS CH 4. SOIL TEXTURE Describes the proportion of soil particle sizes: Sand Silt Clay Soil Texture influences

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  • SOIL TEXTURE Describes the proportion of soil particle sizes: Sand Silt Clay Soil Texture influences other traits such as: Water Holding Capacity Aeration
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  • EFFECT OF PARTICLE SIZE Smaller particles larger internal surface area
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  • EFFECT OF PARTICLE SIZE More pores micropores Larger pores macropores
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  • SOIL SEPARATES SAND 2.00 0.05 mm SILT 0.05 0.002 mm CLAY < 0.002 mm
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  • TEXTURAL CLASSIFICATION Twelve Textural Classes Textural Triangle
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  • SOIL DENSITY AND PERMEABILITY Two densities in Soil: Particle Density PD Bulk Density - BD
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  • SOIL DENSITY AND PERMEABILITY PD average soils ~ 2.65 gm/cu cm BD average range from 1.0 1.8 gm/cu cm Depends on amount of pore space BD = wt. dry soil= _g_ vol. dry soil cu cm e.g. BD = 650 g = 1.3 g/cu cm 500 cu cm e.g.
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  • SOIL POROSITY Usually expressed as a percentage; e.g. 50% Two means determining porosity: 1) Calculate ratio water volume to total core volume 2) Calculate from bulk density and particle density
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  • SOIL POROSITY Examples Water Volume to Core Volume Porosity = wet weight (g) dry weight (g) soil volume (cu cm) x 100%
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  • SOIL POROSITY Examples An oven-dry soil core, volume 500 cu cm, weighs 650g. When wet, it weighs 900g. Find its % porosity. Porosity = 900g 650g x 100% =250gx100%= 50% 500 cu cm500 cu cm Note: the unit cancellation is made possible by the metric system which defines 1 cu cm of water as weighing 1g.
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  • SOIL POROSITY Examples Bulk Density to Particle Density Defines the percentage of the soil that is solid matter The percent solid matter is subtracted from 100% to give percent porosity: Porosity = 100% - (BD/PD x 100%)
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  • SOIL POROSITY Examples An undisturbed oven-dry soil, BD of 1.3 g/cu cm, consists of average mineral composition (PD 2.65 g/cu cm). Find its % Porosity: Porosity = 100% - (1.3 g/cu cm/2.65 g/cu cm x 100%) Porosity = 100% - (0.49 x 100%) = 100% - 49% = 51%
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  • SOIL POROSITY Question: Which has greater porosity, Sand or Clay? Answer: Clay at about 50%; Sand is lower at about 30% Why?
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  • Why, then, does water move through sandy soil very rapidly, but moves s l o w l y into clay ?
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  • This is explained by the physical property Permeability
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  • PERMEABILITY Permeability the ease with which air, water, and roots move through soil - Depends on number, size, and continuity of pores - Liken to a maze
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  • Fine-textured soils would be impermeable if not for: Soil Structure
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  • STRUCTURE Structure the way soil particles clump together into large units called aggregates or peds
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  • STRUCTURE Structure can alter the effects of texture - e.g. a fine-textured silty clay with good structure can be permeable!
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  • STRUCTURE - Structure is classified by three groups of traits: 1) Type refers to shape of aggregates e.g. Granular, Platy, Blocky, Prismatic, Columnar 2) Class refers to size of peds e.g. very fine, fine, medium, coarse, very coarse 3) Grade refers to strength and distinction of peds e.g. weak/not visible vs. strong/easily distinguished
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  • FORMATION OF SOIL STRUCTURE Two-step formation: 1) Individual soil particles loosely aggregate 2) Weak aggregates are cemented to strengthen - clay - iron oxides - organic matter - microorganism gums
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  • ADDNL. SOIL PHYSICAL PROPERTIES Soil Consistence the behavior of soil when pressure is applied; measured at three different moisture levels: Wet, Moist, Dry (fig 4-15, p. 59) Soil Tilth ease of tillage, seedbed preparation, and seedling/root movement Compaction results from pressure applied at the soil surface
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  • ADDNL. SOIL PHYSICAL PROPERTIES Puddling, Clods, and Crusts: Puddling occurs when pressure is applied to very wet soils (esp. plowing) Crusts occur when bare soil is struck by raindrops; disperses soil then dries to a hardened crust
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  • ADDNL. SOIL PHYSICAL PROPERTIES Improving Tilth: Best accomplished by improving structure - tilth relates to texture, structure, permeability, and consistence; however, texture and consistence cannot be altered easily therefore, improve tilth by improving structure and avoiding compaction
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  • ADDNL. SOIL PHYSICAL PROPERTIES Soil Channels continuous macropores leading from surface to deep subsoil Soil Pans any layer of hardened soil; includes: - claypans - fragipans (clays) - plinthite (tropics) - caliche (Ca cemented)
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  • ADDNL. SOIL PHYSICAL PROPERTIES Soil Temperature (varies w/color, texture, O.M.) Soil Color Munsell soil color chart Hue, Value, Chroma e.g. 10YR 3/6


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