Wednesday, February 19, 2014

Soil Particle Sizes - Sand, Silt, and Clay

One of the programs that I do with students is called "Soil Science".  In this program we discuss the different materials that make up soil (minerals, organic matter, water, air, etc.) and the idea that soil particles come in different sizes.  To show that soil particles come in different sizes, the students perform two different experiments.

In the first experiment, the students perform a sedimentation test.  In this test, a sample of soil is placed in a plastic test tube.  A small amount of ammonium aluminum sulfate (also called ammonium alum or just alum) is added to the test tube.  The alum bonds to any organic matter in the soil.  Then the test tube is filled with water and capped.  The students then vigorously shake the test tube for about 30 second to a minute before placing it in a rack for 15 to 20 minutes.  During this time the soil that was mixed with the water settles.  The larger (heavier) particles sink faster than the smaller (lighter) particles.  This means that the soil will settle into distinct layers based on size.  The bottom layer will consist of sand particles of varying sizes.  This will be topped by a layer of silt.  Finally, if we allow the sample to sit long enough a layer of clay particles will settle on top of the silt - the clay usually is still suspended in the water after 20 minutes and may take as long as 24 hours to fully settle out.  Any small particles of organic matter in the soil will float to the top of the test tube - the alum bonds to the organic matter and helps it float.

The second experiment that the students perform involves sorting soil particles with the use of a sieve.  The sieve that we use has a series of 4 mesh screens of decreasing diameter.  These screens allow the students to separate the soil into five different parts by size.  The first screen removes any large particles from the soil (pebbles/gravel/etc.).  The next three screens sort out sand particle of varying sizes - coarse, medium, and fine.  The soil that remains at the bottom of the sieve is a combination of  silt and clay (and possible some very fine sand particles).  The sand particles all have that familiar "gritty" texture that can be felt between your fingertips.  The combination of silt and clay, when just touched has a fluffy texture like flour, but if you take a small amount between your finger and thumb it will feel "slippery" when rubbed together.

So what do these particles look like?  When magnified the difference in size is quite astounding. 

First up is the coarse sand.  These particles are quite large.  The first image shows the coarse sand with only front lighting.  The sand is composed of particles of several different minerals of varying colors.

Coarse Sand - 2X magnification
The second image shows the same particles with back-lighting.  The particles are large enough that most of them are opaque - meaning you cannot see light shining through them.

Coarse Sand with back-lighting - 2X magification
The next two images are of a medium grain sand. These particles are about 1/4 to 1/2 the size of the coarse sand.  The first image shows the sand with front-lighting only.  Again the sand is composed of several different mineral types.  The second image shows the same sample with back-lighting.  In this image, most of the grains are small enough that they are now translucent - meaning light shows through them.  Another difference from the coarse sand is that while the coarse sand was quite angular, most of the edges on the medium grain sand have been rounded off.

Medium Grain Sand - 2X magnification

Medium Grain Sand with back-lighting - 2X magnification
The next two images are of a fine grain sand.  The grains here are about 1/4 to 1/2 the size for the medium grain sand and about 1/16 to 1/8 the size for the coarse sand.  Again the different minerals can be differentiated based on color.

Fine Grain Sand - 2X magnification
At this size, all but the darkest particles are translucent when back-lit.

Fine Grain Sand with back-lighting - 2X magnification

The final two images show a combination of silt and clay.  There are also a few very-fine grain sand particles in this sample (the largest particles).  The clay is best seen in the second image as very small black dots against the back-lighting.

Very-fine Sand, Silt and Clay - 2X magnification
Very-fine Sand, Silt and Clay with back-lighting - 2X magnification.  The clay appears as very small black dots in this image.
As you can see from these image (all taken at the same level of magnification) there is a great degree of difference in the size of soil particle from the largest to the smallest.  One way to think about this size difference is this:  Imagine if a particle of sand were the size of a basketball; at this scale a particle of silt would be the size of a softball; and a particle of clay would only be about the size of a golf ball.

UPDATE (12MAY 2015):

I recently did a follow-up to this post showing the results of the soil sedimentation experiment detailed above.  Check it out here.

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