Soil Texture - Jar Sedimentation Method

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Introduction

There are several sedimentation methods, some of which are more involved than others. The jar sedimentation method is a simple way of estimating soil texture by timing how long it takes for differently sized soil particles to settle in water contained by a clear jar. By separating soil particles of different size fractions, we can estimating soil texture.

The smallest particles are referred to as “clay”, the medium sized “silt” and the largest ones “sand”. This page can be referred to for a better understanding how different size fraction classes are categorized. The determination of soil texture in the laboratory is called particle size analysis or mechanical analysis. The amounts of silt and clay (+/- sand) are usually determined by a sedimentation procedure, which uses a basic principle of sedimentation called "Stoke’s Law": because soil particles are denser than water, they tend to sink, settling at a velocity that is proportional to their size. In other words, the bigger the particle, the faster it will fall. There are several different sedimentation methods for determining soil texture, of varying complexity. The method described on this page is considered to be one of the simpler methods.

Sedimentation Theory - Stoke's Law

Stoke’s Law states that the speed or velocity (V) of a particle falling through a fluid is directly proportional to the gravitational force g, the difference between the density of the particle (ρs) and the density of the fluid (ρL), and the square of the effective particle diameter (D squared). The settling velocity is inversely proportional to the viscosity or “thickness” of the fluid (η). Since velocity equals distance Z divided by time t Stoke’s Law can be written as:

Stoke's Law

Stoke’s Law is based on certain simplifying assumptions:

1. All soil particles have the same density

2. Particles are spherical, smooth, and rigid

3. The suspension is sufficiently dilute such that particles do not interact with each other or the container

4. There is no Brownian motion of fluid molecules

5. There is no turbulence, i.e. fluid flow around particles is laminar

The distance Z represents depth of fall by the particle in the time t. The depth Z is called effective depth of measurement. The diameter of the particle (D) corresponding to a known depth Z is calculated by rearranging the previous equation:

Materials

  • 3 clear jars with lids
  • Soil material from 3 different locations (the further apart, the better)
  • Water
  • Sharpie or masking tape

Method

  1. Fill each of the three jars about half full with soil, keeping your samples from different locations in separate jars. Mark the level of soil on each jar with a piece of tape of sharpie.
  2. Fill the remaining space in the jars with water, and shake vigorously until the soil-water mixture has been incorporated thoroughly for all three jars
  3. Allow the contents of your jars to settle for 40 seconds, then mark the level of soil on your jars. This roughly identifies the sand portion.
  4. Allow the jars to settle for 6 hours, then mark the new levels of soil on the jars. The difference between this mark and the sand mark identifies the silt portion. The difference between the highest mark (the first mark that was made) and the silt mark represents the clay fraction.
  5. The percentages of sand, silt and clay can be calculated by measuring the depth between each mark.

Refer to Figure 1 as a visual aid for this experiment method.

Figure 1: Jar sedimentation experiment example

Loamy soils will have over 60% silt

Sandy Soils will have over 60% sand

Clayey soils will have over 30% clay

References

  1. Food and Agriculture Organization of the United Nations. (n.d.). Soil Experiments for Children. Retrieved July 27, 2020, from http://www.fao.org/3/a-i7957e.pdf
  2. Soil texture. (2020, June 16). Retrieved July 21, 2020, from https://en.wikipedia.org/wiki/Soil_texture