Introductory Soil Science Lab 5 Soil Texture and Density NRES 201 Page 1 of 5 RGD 1/07 LABORATORY 5. SOIL TEXTURE (HYDROMETER METHOD), BULK AND PARTICLE DENSITY. Texture, bulk and particle density are physical properties of soils that control many important soil processes. Texture affects the total water holding capacity of the soil, percentage of plant-available water, cation exchange capacity and many other soil properties and processes. Bulk and particle density are related to soil porosity, degree of compaction, movement of air and water into and through the soil, ease of root growth as well as other properties. 5.1 HYDROMETER METHOD FOR SOIL TEXTURE. The determination of the size distribution of soil particles is known as mechanical or particle size analysis. Soil texture is the composition of the soil particles expressed as the percent of particles in the sand, silt, and clay size separates after organic matter, carbonates, and iron and manganese oxides and other cementing or binding agents are removed. The hydrometer method1 is based on the change of density of a soil and water suspension upon the settling of the soil particles. Stokes' Law is used to predict the settling times for various sized particles. Stokes' law states that the rate which particles fall in a viscous medium (water) is governed by the radius of the particles and the force due to gravity. A special hydrometer, calibrated in terms of the grams of soil suspended, is used to measure density. The hydrometer is gently placed into the cylinder containing the suspension after predetermined periods of time and a reading taken by determining where the meniscus of the suspension strikes the hydrometer. 5.1.1 Stokes Law. The rate of fall (v) of a particle in a suspension can be predicted from Stokes' Law: v = 2r2(ρs –ρl)g/9n Where: v = velocity of particle falling in a liquid, cm/sec r = radius of particles, cm ρs = density of the solid particles, ~2.65 g/cm3 ρ1 = density of the liquid, g/cm3 g = acceleration due to gravity, 980 cm/sec2 n = viscosity of the liquid, poises ____________________ 1. Bouyoucos G.J. 1962. Hydrometer method improved for making particle size analysis of soil. Agron. J., 54:464-465.Introductory Soil Science Lab 5 Soil Texture and Density NRES 201 Page 2 of 5 RGD 1/07 The determination of texture by the hydrometer method is based on certain assumptions: 1. That soil particles are spherical and are large enough so that Brownian movement does not affect their settling. 2. That soil particles are of identical density. 3. That the particles fall independently, there is no interaction between particles. 4. There are no temperature gradients or currents to affect the density and viscosity of liquid. The first assumption that soil particles are spherical is not always valid since many soil particles are plate-like. Because of this Stokes' Law is used to calculate an approximate settling time and then the time is adjusted to match settling times for known soil size separates. The assumption of identical density is reasonable since most soil minerals are silicate minerals and have similar densities. To insure that particles fall independently, cementing agents, such as organic matter, carbonates and iron oxides are removed and then a chemical dispersant (sodium hexametaphosphate) in combination with mechanical dispersion is used to separate the soil aggregates into individual particles. And last, temperature control can be obtained by placing the sedimentation cylinders in a temperature controlled water bath. Laboratory directions - hydrometer method. Some soil samples require that organic matter be removed by oxidation with hydrogen peroxide (H2O2), that carbonates be dissolved using a pH 5 acetate buffer, and that iron compounds be removed by reduction with sodium dithionite. For the purpose of this laboratory these procedures are omitted. a. Weigh approximately 50 grams of air-dry soil (100 grams for sandy soils) that has passed a 2 mm sieve and quantitatively transfer into metal dispersing cup. Calculate the oven-dry weight of your soil sample using the air-dry water content (%) provided by your laboratory instructor. b. Add 20 mL of 2.5 N sodium hexametaphosphate, (NaPO3)6, fill to within two inches of the top of the dispersing cup with deionized water and let stand for 10 minutes. c. Carefully attach the dispersing cup to the mixer and stir for 5 minutes. d. Quantitatively transfer the dispersed sample (soil and solution) from the dispersing cup into a sedimentation cylinder. e. Fill the cylinder with deionized water to the 1000 mL mark. f. Calibrate the hydrometer used by placing it in a sedimentation cylinder that contains 20 mL 2.5 N sodium hexametaphosphate and 980 mL deionized water. This calibration procedure is necessary since not all of the hydrometers read 0 g/L when there is no soil in suspension. Subtract this value from the hydrometer readings if the calibration reading is greater than 0 and add this value to the hydrometer readings if the calibration reading is less than 0. Be certain to use the same hydrometer throughout the experiment or calibrate each new hydrometer that you use. g. Place a rubber stopper in the end of the cylinder and agitate vigorously by turning end to end. When all the soil material is resuspended set the cylinder down and record the exact time.Introductory Soil Science Lab 5 Soil Texture and Density NRES 201 Page 3 of 5 RGD 1/07 h. Immediately, very carefully, insert the hydrometer into the suspension. 40 seconds after the cylinder was set down, record the hydrometer reading. Repeat steps (g) and (h) three times, use the average of these values in your calculations. The 40-second reading gives the amount of silt and clay still suspended after the sand particles have settled. i. Measure and record the temperature of the suspension after both hydrometer readings (40 seconds and 2 hours). j. At the end of the 2-hour settling period carefully place the hydrometer into the suspension and record the reading. This reading gives the grams of clay per liter still in suspension. k. Pour the suspension into the crocks in the sinks (to avoid clogging the sinks) and clean the sedimentation cylinder and your work area. Data sheet for hydrometer experiment. Soil 1 Soil 2 Air-dry weight _____