Thursday, May 7, 2015

Soil Sedimentation Experiment - Sand, Silt, and Clay

One of the programs that I do in classrooms is called Soil Science.  During this program, students perform two experiments that show to show that soil particles come in different sizes. One of the experiments involves sorting a soil sample with a screen sieve - I detailed this experiment a little bit in a post from February 2014.

The other experiment that the students perform is a sedimentation test.

Here are the materials that the students use for this experiment.  Each group of students receives a test tube rack with test tubes filled halfway with soil - any clear container with a water-tight lid will work.  They also receive a bottle of water (these bottles have been refilled with tap water), a plastic spoon, and a small amount of ammonium aluminum sulfate (also called ammonium alum or just alum).

The soil in each test tube is different - I collected soil from four different locations for this experiment.

Sample #1 is from a sandbar along the Chippewa River and consists mostly of medium-grain sand, with some course sand, and small amounts of silt, clay, and organic matter.

Sample #2 came from the field behind the Conservation District Office.  It consists mainly of a fine- to medium-grain sand with very small amounts of silt, clay, and organic matter.

Sample #3 consists of a topsoil with medium- and fine-grained sand, with lots of silt and organic matter.

Sample #4 was dug up in the native pollinator garden at Winn Elementary and consists almost entirely of clay.

It may seem obvious, but the first step to this experiment is to uncap the test tubes.  Next a small amount of alum is added to each test tube.  Use the spoon to add an amount of alum equal is size to an M&M candy (an amount equal to about 1/4 inch in diameter) to each test tube.  The alum bonds to any organic matter in the soil and helps it float.

The next step is to fill the test tubes with water and cap them.

I tell the students not to fill the water entirely to the top of the test tube, but rather to stop at the lip just below the cap.  This allows you to see all of the results of your experiment without having them hidden by the cap.  The water will quickly infiltrate the soil samples that are composed mainly of sand.  The infiltration rates will be much slower in soils with higher concentrations of silt and clay.  In this case, test tube may need to be capped, shaken to mix the soil and water, and then refilled up to the same line as the other test tubes.

Once the water has infiltrated all of the soil samples and the test tubes have been refilled up to the "fill line" the test tubes are capped tightly.  The students then vigorously shake the test tube for about 30 seconds to a minute before placing it back in the rack.  In the classroom, we then typically allow the contents to settle for 15 to 20 minutes before examining the results.  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.

Here are the results at various times.

At 15 seconds, the sand in the samples has already settled completely and the silt is also beginning to settle.

At 1 minute, Sample #1 has settled into distinct layers of sand and silt, with some clay still being in suspension.  Layer of sand can also be seen in Sample #2 and Sample #3.  Sample #4 remains almost entirely in suspension, with no visible settling.

Here are the samples at 2 minutes.  With back-lighting, it's easier to see how much has settled in each test tube.  Sample #1 is mostly clear.  Sample #2 still has a fair amount of clay in suspension.  Sample #3 has a large amount of organic matter floating at the top of the test tube.  Sample #4 still has not begun to visibly settle.

At 5 minutes, the clay particles in Samples #1, #2, and #3 have settled even more.  Sample #4 remains "as clear as mud".

At 10 minutes, Samples 1-3 are all clearer as clay settles out of them.  Organic matter can be seen floating in each of these samples.  Sample #4 still remains mostly in suspension.

At 20 minutes, Sample #4 has finally begun to settle out of solution.  The other samples continue to clear.

At 30 minutes, Sample #4 continues to settle out.  Sample #2 still has a significant amount of clay suspended in the water also.  Sample #1 and Sample #3 are mostly done settling at this time.

At 60 minutes, I moved the test tube rack to the window where it could sit undisturbed overnight.  As you can see Samples #1, #2, and #3 are mostly done settling at this point.  The test tube for Sample #2 has some discoloring to the plastic and is clearer than it looks.  Sample #4 has settled further.

The next photo shows the test tubes the following morning, 17 hours after starting the experiment.

As you can see in the above photograph, each sample has almost entirely settled out of suspension at this point.  The following four images are close-up photos of each sample showing the distinct layers that formed as the soil settled.

Here is Sample #1 after 17 hours of settling.  There is a distinct layer (bed) of coarse sand at the bottom of the test tube.  Above that is another layer consisting of medium- and fine-grained sand - these two are intermingles with some evidence of bedding.  Atop this layer is a thin layer consisting of silt and clay.  There is a very small amount of organic matter at the top of the test tube that is not visible in this photograph.

Sample #2 at 17 hours also shows distinct layers.  There is one large layer of intermixed fine and medium grain sand, topped with a thin layer of silt and clay.  There is a little bit of organic matter visible at the top of the test tube.

Sample #3 at 17 hours shows the most layers.  The bottom layer consists of a dark colored mix of medium- and fine-grained sand.  Above this bed is a layer of dark silt.  This is in turn topped by a bed of clay.  Finally there are two layers of organic matter - some of the heavier organic particles are resting atop the clay bed while others are floating at the top of the water.  The alum that was added to the test tube adheres to the organic matter and helps it float.  These organic particles are essentially neutrally buoyant and minor disturbances to the water cause some particles to sink and other to float upward.

Finally, Sample #4 at 17 hours shows no evidence of bedding.  Instead it consists of a single thick layer of clay particles.  These particles are still partially suspended in the water.  Two days after starting this experiment, this sample still looks as it did at 17 hours.

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