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

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• Slide 1
• PHYSICAL PROPERTIES OF SOIL INTRO TO SOILS CH 4
• Slide 2
• SOIL TEXTURE Describes the proportion of soil particle sizes: Sand Silt Clay Soil Texture influences other traits such as: Water Holding Capacity Aeration
• Slide 3
• EFFECT OF PARTICLE SIZE Smaller particles larger internal surface area
• Slide 4
• EFFECT OF PARTICLE SIZE More pores micropores Larger pores macropores
• Slide 5
• SOIL SEPARATES SAND 2.00 0.05 mm SILT 0.05 0.002 mm CLAY < 0.002 mm
• Slide 6
• TEXTURAL CLASSIFICATION Twelve Textural Classes Textural Triangle
• Slide 7
• SOIL DENSITY AND PERMEABILITY DENSITY: MASS per VOLUME D = M V
• Slide 8
• SOIL DENSITY AND PERMEABILITY Two densities in Soil: Particle Density PD Bulk Density - BD
• Slide 9
• 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.
• Slide 10
• 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
• Slide 11
• SOIL POROSITY Examples Water Volume to Core Volume Porosity = wet weight (g) dry weight (g) soil volume (cu cm) x 100%
• Slide 12
• 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.
• Slide 13
• 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%)
• Slide 14
• 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%
• Slide 15
• SOIL POROSITY Question: Which has greater porosity, Sand or Clay? Answer: Clay at about 50%; Sand is lower at about 30% Why?
• Slide 16
• TEXTURE AND SOIL PORES
• Slide 17
• Why, then, does water move through sandy soil very rapidly, but moves s l o w l y into clay ?
• Slide 18
• This is explained by the physical property Permeability
• Slide 19
• 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
• Slide 20
• Fine-textured soils would be impermeable if not for: Soil Structure
• Slide 21
• STRUCTURE Structure the way soil particles clump together into large units called aggregates or peds
• Slide 22
• STRUCTURE Structure can alter the effects of texture - e.g. a fine-textured silty clay with good structure can be permeable!
• Slide 23
• 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
• Slide 24
• 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
• Slide 25
• 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
• Slide 26
• 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
• Slide 27
• 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
• Slide 28
• 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)
• Slide 29
• 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