Particle Size Analysis Soils 206- Soil Ecosystem LabObjectives:Soil TextureDetermination of Soil TextureThe time required for a sand particle with a diameter of 0.05 mm or 0.00005 m to fall the distance of 15 cm or 0.15 m is calculated below.The time required for a silt particle with a diameter of 0.002 mm or 0.000002 m to fall the distance of 15 cm or 0.15 m is calculated below.Hydrometer method of PSADon’t forget to leave your labeled soil tin here so it can be oven dried for lab next week!Particle Size Analysis Soils 206- Soil Ecosystem LabObjectives:1. Understand the principles and assumptions of Stoke’s Law,2. Perform particle size analysis using the hydrometer method,3. Calculate the textural class using the results of the particle size analysis.Soil TextureSoil texture refers to the proportion (by weight) of sand, silt, and clay. The following chart is a generalization of the influences of soil separates on some properties and behavior of soils.Property Sand Silt ClayDiameter 2 – 0.05 mm 0.05-0.002 mm <0.002 mmSurface Area Low Moderate HighInherent Fertility Low Moderate HighMineralogy Usually Parent Often Different Often DifferentCharacteristic, Dry Soft, friable(if granular)Properties of Both Hard, brittleCharacteristic, Wet Non-plastic, Non-stickyProperties of Both Plastic and StickyWater Holding Capacity Low Moderate HighDrainage Rate Fast Moderate Slow (unless cracked)Porosity Low, Large Pores Moderate High, Small PoresCompactability Low Moderate HighShrink-Swell Potential Very Low Moderate HighSealing Potential Poor Poor GoodPollutant Leaching High Moderate Low (unless cracked)Organic Matter Level Low Moderate to High High to ModerateDecomposition of SOM Rapid Moderate SlowSpring Warm-Up Rapid Moderate SlowErosion, Wind Moderate High LowErosion, Water Low High ModerateDetermination of Soil TextureTwo methods exist for textural analysis. During a prior lab exercise you practiced using the texture by “feel” or hand-texturing method. Today we will use a mechanical analysis known as the Hydrometer Method. This method involves dispersing a soil sample in water and determining the sedimentation rate of the sand, silt, and clay particles. Sedimentation rates of suspended soil particles depend primarily on particle size. Large sand-sized particles settle faster than smaller clay-sized particles. This relationship can by quantitatively expressed bySpring 20061Stoke’s Law: V = d 2 g (Dp – Dl) 18Where:V = settling velocity (m/s)t = particle settling time (s)L = particle settling distance (m)d = diameter of the particle (m)g = acceleration due to gravity (9.81 N/kg)Dp = density of the particle (2.65x103 kg/m3)Dl = density of the liquid (1.00 x103 kg/m3) = viscosity of the liquid (1.0 x 10-3 Ns/m2)Stoke’s Law can be simplified for our use as gravity remains constant, the particle density and the density of water remain constant and the viscosity of water remains constant. These terms can be combined to form a constant, ‘k’. Entering the known values to the equation and leaving the variable of particle size the equation becomes:V = (9.81 N kg-1) (2.65x103 kg m-3 – 1.00 x103 kg m-3) * d2 18 (1.0x10-3 Ns m-2) solving,V = 9x105 s-1m-1 * d2 or V = kd2 Substituting for velocity and solving for time (t):V = L/ t so t = L/V or t = L/ kd2Units: t = seconds, L = m, d = m, k = 9x105 s-1m-1Assumptions of Stoke’s Law:1. Particles have the same density2. Particles are spherical, smooth and rigid3. Particles do not interact with each other or with the walls of the container4. No Brownian motion5. No turbulenceThe time required for a sand particle with a diameter of 0.05 mm or 0.00005 m to fall the distance of 15 cm or 0.15 m is calculated below.t = L/ kd2 t = 0.15 m = 67 seconds9x105 s-1m-1 * (0.00005 m)2 The time required for a silt particle with a diameter of 0.002 mm or 0.000002 m to fall the distance of 15 cm or 0.15 m is calculated below.t = L/ kd2 t = 0.15 m = 41 780 seconds or 11.6 hours9x105 s-1m-1 * (0.000002 m)2 Spring 20062Hydrometer method of PSAA hydrometer relies on buoyancy to calculate the number of grams of soil material in 1 liter of suspensionat the time of the reading (it has been calibrated to do the mathematics for you). The hydrometer method involves two basic principles; dispersion and sedimentation.Dispersion: Individual soil particles must be separated from each other and remain separated during the determination of particle size distribution. Since aggregates of solid particles are usually held together by some kind of binding agent, it is first necessary to remove these substances, or at least render them ineffective. Once the particles are separated they are said to be dispersed.Dispersion is achieved by chemical and mechanical means. Mechanical stirring is effective in dispersing larger aggregates, but is ineffective on small aggregated clay groups. Chemical dispersion is required in this case. Sodium hexametaphosphate is an effective chemical dispersing agent for two reasons:1. The sodium monovalent cation replaces polyvalent cations (predominately Ca2+) usually adsorbed on soils thereby breaking one type of interparticle linkage. The polyvalent cations are then reduced in activity by reacting with the phosphorus and precipitating.2. The adsorbed sodium cations are highly hydrated and raise the electronegativity of colloids until theseparticles repel each other and remain dispersed. The mixture of dispersed soil particles in water is called a suspension.Sedimentation: Large particles will settle out of suspension more rapidly than small particles because small particles present more specific area and therefore will experience greater frictional resistance. In practice the amount of material still in suspension at any one time is measured with a hydrometer, which indicates the density of the suspension at the hydrometer’s center of buoyancy. CalculationsPercent Silt and Clay = Corrected Silt and Clay Average Reading (g/l) * 100 Original Sample WeightPercent Sand = 100% - (Silt & Clay Percent)Percent Clay = Corrected Clay Reading (g/l) * 100 Original Sample WeightPercent Silt = 100% - (Sand Percent + Clay Percent)Use the percent sand, silt and clay to determine the textural class using the textural triangle.Spring 20063Procedure 1. Weigh 50g fine-textured soil (100 g of coarse-textured soil) into a labeled wide-mouth Erlenmeyer flask. The label should