sw11 - Soil Texture Continued Soil texture refers to the...

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Unformatted text preview: Soil Texture Continued Soil texture refers to the relative amounts of three distinct size separates comprising the soil mineral component. Sizes classes of particles Sand Silt Clay Importance of Soil Texture (Distribution of particle sizes) Soil Porosity Particle Surface Area Water/Gas Movement Reactivity Texture, Pore Sizes, and Water Large particles yield large pore spaces Small particles yield small pore spaces Water moves rapidly and is poorly retained in Coarse-textured sandy soils. Water moves slowly and is strongly retained in Fine-textured, clayey soils. Sands Rapid Poor Retention Clays/iron Slow Water Retained Surface Area and Particle Size Surface Area Surface units Specific Surface Area = Surface Area cm2 g mass water nutrients Interface with the environment O.M. gasses microorganisms Soil Horizons Texture Clay Content Surface Area Potential Reactivity Soil Textural Classes Soil Textural Triangle Florida Soils clay Sand Loamy sand Sandy loam Sandy clay loam Sandy clay Clay <10% 10 – 15% 15 – 20% 20 – 35% 35 – 55% > 50% 60% sand, 10% silt, 30% clay 60% * Extra Credit 1. The ______ textural class dominates the textural triangle 2. Large particles yield _________ sized pore spaces 3. Surface area/unit mass is called ___________________ 4. The range in size of sand size particles is __________ mm 5. The size of clay size particles is less than __________ mm Measuring Soil Texture Determining Soil Texture Determining Moisten about 50g of soil to a uniform consistency, like putty. Texture-by-Feel yes No sand Grittiness Sand 1. Texture­by­Feel Smoothness Determining Soil Silt Does the soil form a ribbon? Plasticity Clay Texture Loamy yes No Sand Can the soil be formed into a ball that easily maintains its shape? Two Methods Ribbon length 0 – 2.5 cm Sand 2. The Hydrometer Method Gritty Gritty Gritty Sandy Clay Loam Smooth Silty Clay Loam Both Clay Loam Sandy Clay Smooth Silty Clay Both Clay Smooth Silt Loam Both Loam 2.5 – 5 cm > 5 cm Laboratory Analysis of Soil Texture Laboratory Analysis Laboratory Sedimentation – Sand, Silt, and Clay Fraction Drag/Friction Drag/Friction gravity Sedimentation Sand Sand Silt Silt Clay Clay silt sand Quantifying Sedimentation Rates Stokes’ Law Stokes’ g (dp-dL) D2 Velocity V(cm/s) = 18ų g = gravity dp = density of the particle dL = density of the liquid ų = viscosity of the liquid V = K D2 K = 11,241 cm-1 sec-1 1 cm · sec Stokes’ Law Stokes’ V = K D2 K = 11,241 cm-1 sec-1 Sand: D = 1 mm 1 cm · sec 0.1 cm V = 11,241 x (0.1)2 X cm2 = 112.4 cm/sec Stokes’ Law Stokes’ V = K D2 K = 11,241 cm-1 sec-1 clay: D = 0.002 mm 1 cm · sec 0.0002 cm V = 11,241 x (0.0002)2 X cm2 = 0.00045 cm/sec 500 450 400 350 300 250 200 150 100 50 0 0 Settling Velocity (cm/sec) clay silt sand 0.5 1 1.5 2 2.5 Particle Diameter (mm) Sedimentation Sedimentation The density of a soil suspension decreases as particles settle out. 1 minute 6 hours Sand settles out Silt settles out suspension 1 min. silt sand 6 hr. Hydrometer Method Stem Scale Bulb weight 0 g/L hydrometer t=0 Pure distilled water (18 C) = 0 g/L t = 1 min Hydrometer Method Hydrometer Add 40 g soil to 1 liter of water Time = 0 sec Time = 1 min. density = 40 g/L density = 10 g/L Sand settled = 40 g– 10 g= 30 g Sand (%) = 30 g sand = 0.75 = 75% 40 g soil Hydrometer Method Time = 4 hrs density = 4 g/L What is being measured? Clay content = 4 g Clay % = 4 g clay = 0.10 = 10% 40 g soil Silt % = 100% - (75% + 10%) 100% - 85% = 15 % Hydrometer Method Sand = 75% Silt = 15% Clay = 10% Sandy Loam Reactivity, Water and Gas Movement Next: Density, Structure, Porosity ...
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