Soil Texture Hydrometer Method

Definition

• The hydrometer method involves dispersing soil particles with a substance such as sodium metaphosphate and then agitating the solution. The amount of sand, silt and clay in the soil sample is determined, after dispersion, by a hydrometer, which measures the particles in suspension. The amount of each type of particle is determined using Stokes' Law, which determines the amount of each particle type present by the rate at which each particle type falls out of suspension, based on its size.

DeForest Lab

• DeForest Lab at Ohio University conducts a soil texture hydrometer method, suitable for 100 samples, using 500 grams of sodium hexametaphosphate. A flask is used to hold air-dried soil and a dispersing agent, and both are placed in an orbital shaker overnight. The soil and distilled water are added to an electric blender, run for five minutes on high speed. The mix is then put into a graduated cylinder that contains tap water, along with the dispersing agent. Once the suspension is mixed for 30 seconds with a metal plunger, the hydrometer is eased into the cylinder and a reading can be taken after 40 seconds. Calculations are made using this number, a reading taken 12 hours after stirring, a blank reading to correct hydrometer readings, the percentage of each type of material and weight of the samples.

Hart Lab

• The hydrometer method adapted from Hart Lab protocol involves dispersing the soil into its primary particles for optimal particle-size analysis. Two people are needed, as the procedure goes most smoothly when readings are taken 30 and 60 seconds apart. This process involves using deionized water and sodium hexametaphosphate, and using an electric mixer to mix them with the soil. The sample is placed into a sedimentation cylinder once it is mixed, and the hydrometer is calibrated. Hydrometer readings and the temperature of the sample are taken after 30, 60 and 90 seconds, then 24 hours later.

Bouyoucos Method

• The Bouyoucos Hydrometer Method is one of the fastest ways to analyze soil particle size. The dispersed soil sample is mixed in a tall glass cylinder with water and, once it settles, the density of the suspension can be measured with the hydrometer. The time each sized particle takes to fall below an imaginary set plane on the cylinder can be measured, with readings are taken after 40 seconds to measure settling sand and after two hours to measure silt particles.

Calibration

• When a hydrometer is used to analyze soil texture, it must be calibrated by measuring the temperature of the instrument. All readings must be corrected for variations in temperature because this affects the viscosity and density of the water. If this isn't taken into account, incorrect values for the water's density could offset the readings and the readings for the particle sizes could be wrong.