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Soil Texture Soil Texture Soil and Site. Lindbo et al. 5/15/2012 DRAFT 1 NDWRCDP Disclaimer This work was supported by the National Decentralized Water Resources Capacity Development Project (NDWRCDP) with funding provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR827881-01-0) with Washington University in St. Louis. These materials have not been reviewed by the U.S. Environmental Protection Agency. These materials have been reviewed by representatives of the NDWRCDP. The contents of these materials do not necessarily reflect the views and policies of the NDWRCDP, Washington University, or the U.S. Environmental Protection Agency, nor does the mention of trade names or commercial products constitute their endorsement or recommendation for use. CIDWT/University Disclaimer These materials are the collective effort of individuals from academic, regulatory, and private sectors of the onsite/decentralized wastewater industry. These materials have been peer-reviewed and represent the current state of knowledge/science in this field. They were developed through a series of writing and review meetings with the goal of formulating a consensus on the materials presented. These materials do not necessarily reflect the views and policies of North Carolina State University, and/or the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT). The mention of trade names or commercial products does not constitute an endorsement or recommendation for use from these individuals or entities, nor does it constitute criticism for similar ones not mentioned. Citation - Stolt, M. H., Lindbo, D.L., R. Miles, D. L. Mokma, and S. Greene. 2005. 3. Field Description of Soils: Texture – Power Point Presentation. in (D.L. Lindbo and N. E. Deal eds.) Model Decentralized Wastewater Practitioner Curriculum. National Decentralized Water Resources Capacity Development Project. North Carolina State University, Raleigh, NC. Soil Texture Use texture to make inferences into pore size From pore size begin to estimate water movement and treatment Finer texture means slower water movement Finer texture means greater treatment Texture by itself is not enough information to determine site suitability Other factors that combine with texture Soil structure Organic matter and vegetation Soil mineralogy Land use Landscape position Parent material Soil wetness Soil Texture Mineral material only Material > 2mm are coarse fragments Material < 2mm only Sand - 2.0 - 0.05 mm Silt - 0.05 - 0.002 mm Clay - < 0.002 mm Soil Texture (mineral material only) Sand - gritty Silt - smooth, velvety Clay - slick, sticky Systems for Classifying Particle Size Distributions USDA System (ie. Sandy Loam) AASHTO: American Association of State Highways (ie. A-1) Unified Engineering (ie. SM) Wentworth (phi #) USDA Particle-Size Distribution Fine-earth fraction: Finer than 2 mm: Used for Soil Textural Class Rock fragments: 2 mm in diameter or larger. Particles less than 10” are many times called “coarse fragments”. Used to modify textural class. USDA Textural Classes (12) Sand Sandy Clay Loamy Sand Loam Sandy Loam Silty Clay Loam Loam Clay Loam Silt Loam Sandy Clay Silt Silty Clay Clay Examples Soil and Site. Lindbo et al. 5/15/2012 DRAFT 16 Sand + Silt + Clay = 100% 40 % Sand 40 % Silt 20 % Clay Texture = LOAM 25 % Sand ?? % Silt 45 % Clay 25 % Sand 30 % Silt 45 % Clay 25 % Sand 30 % Silt 45 % Clay CLAY 65 % Sand 20 % Silt ?? % Clay 65 % Sand 20 % Silt 15 % Clay 65 % Sand 20 % Silt 15 % Clay SANDY LOAM ?? % Sand 30 % Silt 30 % Clay 40 % Sand 30 % Silt 30 % Clay 40 % Sand 30 % Silt 30 % Clay CLAY LOAM Particle-Size Distribution Particle size distribution describes the abundance (by weight) of the various size particles that constitute the mineral portion of soil materials. Fine-earth fraction Sand - 2.0 - 0.05 mm Silt – 50 – 2 um Clay - < 2 um Sand Fractions Fraction Size ___________________________(mm)__ Very coarse sand.......................2.0 to 1.0 Coarse sand..............................1.0 to 0.5 Medium sand..........................0.5 to 0.25 Fine sand..............................0.25 to 0.10 Very fine sand........................0.10 to 0.05 Textural Groups for OSWW Group I: Sand, Loamy sand Group II: Sandy loam, Loam, Group III: Sandy clay loam, Silt loam, Clay loam, Silty clay loam, Silt Group IV: Sandy clay, Silty clay, Clay Textural Groups for OSWW Group I: Group III: Sand, Loamy sand Sandy clay loam, 2 1.2 – 0.8 gpd/ft Silt loam, Clay loam, Silty clay Group II: loam, Silt Sandy loam, Loam 2 2 0.6 – 0.3 gpd/ft 0.8 – 0.6 gpd/ft Group IV: Sandy clay, Silty clay, Clay 2 0.4 –0.1 gpd/ft LOAM 40 % Sand LTAR = 40 % Silt 0.8 – 0.6 gpd/ft2 20 % Clay CLAY 25 % Sand LTAR = 30 % Silt 45 % Clay 0.4 – 0.1 gpd/ft2 SANDY LOAM 65 % Sand LTAR = 20 % Silt 15 % Clay 0.8 – 0.6 gpd/ft2 CLAY LOAM 40 % Sand LTAR = 30 % Silt 30 % Clay 0.6 – 0.3 gpd/ft2 Determining Texture Soil and Site. Lindbo et al. 5/15/2012 DRAFT 37 Determination of Texture Field procedure Laboratory procedure Hydrometer Pipette Field Determination of Texture Soil and Site. Lindbo et al. 5/15/2012 DRAFT 39 Field determination of texture Soil must be moist, not saturated; moist enough to mold like putty when you try to form a ball in your hand. Soil Texture Class Key* Does soil form a ball or cast? No - the texture is SAND Soil does not form a cast: Textural class is SAND Field determination of texture Can the ball be handled No - the texture is LOAMY SAND. OR When pressing the soil between thumb and forefinger does the soil form a ribbon that extends beyond your forefinger? No - the texture is LOAMY SAND. Forms a cast of moist soil material. Textural class is LOAMY SAND The length of the ribbon will depend on mineralogy as well as clay content Making a ribbon Field determination of texture If the soil forms a ribbon that that extends past the forefinger, note the length of the ribbon. Next excessively wet a small sample in the palm and rub with the forefinger. Field determination of texture If the ribbon was < 1 inch long when it broke and the excessively wet sample feels: gritty, the texture is SANDY LOAM; smooth, the texture is SILT LOAM; neither gritty nor smooth, the texture is LOAM. Field determination of texture If the ribbon was between 1 and 2 inches long when it broke and the excessively wet sample feels: gritty, the texture is SANDY CLAY LOAM; smooth, the texture is SILTY CLAY LOAM; neither gritty nor smooth, the texture is CLAY LOAM. Field determination of texture If the ribbon > 2 inches long when it broke and the excessively wet sample feels: gritty, the texture is SANDY CLAY; smooth, the texture is SILTY CLAY; neither gritty nor smooth, the texture is CLAY. Laboratory Determination of Texture Soil and Site. Lindbo et al. 5/15/2012 DRAFT 50 Laboratory Determination of Particle-Size Distribution Percent sand and rock fragments are determined by a weight percent remaining in a sieve. Silt and clay fractions are determined based on how fast a particle of a given size falls in a sedimentation column. Determining Percent Silt and Clay (Hydrometer) Based on Stokes Law Uses a 100 gram sample Particles dispersed w/ calgon Measures concentration of solids in suspension by suspension density Less time consuming Less accurate Determining Percent Silt and Clay (Pipette) Based on Stokes Law Uses a 10 gram sample Particles dispersed w/ calgon Measures concentration of solids in suspension by weight More time consuming More accurate Stoke’s Law The settling time of a particle of a given density falling in a liquid of a given viscosity is proportional to the square of the particles radius. What does this mean? – Bigger particles fall faster!!! and If we know the viscosity of the liquid and the density of the particles we can figure out their size. Stokes’ Law: 2 V = 2r g(ps-pl)/(9n) V = velocity of fall r = particle radius g = acceleration due to gravity ps = particle density pl = liquid density n = fluid viscosity At 20oC how long does it take a 2um particle to fall 1 cm in a 0.5 g/l solution? At 20oC how long does it take a 2um particle to fall 1 cm in a 0.5 g/l solution? 8.41h/10h = .841h or 50.5 min. 1000 ml Sedimentation Column Plunger for mixing soil in column Pipette Setup: Lowy Pipette Pump Stand Bell Jar Sieve Funnel 25 ml Lowy Pipette for silt and clay determination Is there texture beyond just Sand, Silt and Clay? And so what if there is. Soil and Site. Lindbo et al. 5/15/2012 DRAFT 72 Textures Classes with Sand-Size Modifiers Coarse sand (CoS): >25% very coarse and coarse sand Fine sand (FS): >50% fine sand Very Fine sand (VFS): >50% very fine sand Loamy coarse sand (LCoS): >25% very coarse and coarse sand Loamy fine sand (LFS): >50% fine sand Loamy very fine sand (LVFS): >50% very fine sand Textures Classes with Sand-Size Modifiers Coarse sandy loam (CoSL): >25% very coarse and coarse sand Fine sandy loam (FSL): >30% fine sand Very Fine sandy loam (VFSL): >30% very fine sand Terms for describing rock fragments. Rounded, subrounded, and irregular: _____________________________________ Size (mm) Type Adjective 2 to 76...........Gravels.......gr.............gravelly 2 to 5.............Gravels......grf.......fine gravelly 5 to 20...........Gravels.....grm....med. gravelly 20 to 76.........Gravels......grc...coarse gravelly 76 to 250........Cobbles......cb...............cobbly 250 to 600......Stones.........st................stony >600..............Boulders.....by............bouldery Terms for describing flat rock fragments. _____________________________________ Size (mm) Type Adjective 2 to 150.........Channers....cn............channery 150 to 380......Flagstones...fl.................flaggy 380 to 600......Stones.........st................stony >600..............Boulders.....by............bouldery Using Rock Fragment Modifiers Less than 15 percent: No adjective or modifier terms are used.
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