How to Do a Sedimentation Test on Soil

For studying sediments, or the sedimentary rocks made of them, geologists are very serious about their lab methods. But with a little care, you can get consistent, fairly accurate results at home for certain purposes. One very basic test is determining the mix of particle sizes in a sediment, whether thats soil, the sediment in a streambed, the grains of sandstone or a batch of material from a landscape supplier. Equipment All you really need is a quart-sized jar and a ruler with millimeters. First, make sure you can measure the height of the jars contents accurately. That might take a little ingenuity, like putting a piece of cardboard underneath the ruler so that the zero mark lines up with the floor inside the jar. (A pad of small sticky notes makes a perfect shim because you can peel off exactly enough sheets to make it precise.) Fill the jar mostly full of water and mix in a pinch of dishwasher detergent (not ordinary soap). Then youre ready to test sediment. Use no more than a half-cup of sediment for your test. Avoid sampling plant matter on the ground surface. Pull out any large pieces of plants, insects, and so on. Break up any clods with your fingers. Use a mortar and pestle, gently, if you have to. If there are only a few grains of gravel, dont worry about it. If theres a lot of gravel, remove it by straining the sediment through a coarse kitchen sieve. Ideally, you want a sieve that will pass anything smaller than 2 millimeters. Particle Sizes Sediment particles are classified as gravel if theyre larger than 2 millimeters, and if theyre between 1/16th and 2 mm, silt if theyre between 1/16th and 1/256th mm, and clay if theyre even smaller. (Heres the official grain size scale used by geologists.) This home test doesnt measure the sediment grains directly. Instead, it relies on Stokes Law, which accurately describes the speed at which particles of different sizes fall in the water. Big grains sink faster than small ones, and clay-size grains sink very slowly indeed. Testing Clean Sediments Clean sediment, like beach sand or desert soil or ballfield dirt, contains little or no organic matter. If you have this kind of material, testing is straightforward. Dump the sediment into the jar of water. The detergent in the water keeps the clay particles separate, in effect washing the dirt off the larger grains and making your measurements more accurate. Sand settles in less than a minute, silt in less than an hour and clay in a day. At that point, ​you can measure the thickness of each layer to estimate the proportions of the three fractions. Heres the most efficient way to do it. Shake the jar of water and sediment thoroughly—a full minute is plenty—set it down and leave it for 24 hours. Then measure the height of the sediment, which includes everything: sand, silt, and clay.Shake the jar again and set it down. After 40 seconds, measure the height of the sediment. This is the sand fraction.Leave the jar alone. After 30 minutes, measure the height of the sediment again. This is the sand-plus-silt fraction.With these three measurements, you have all the information needed to calculate the three fractions of your sediment. Testing Soils Soils differ from clean sediments in that they have organic matter (humus). Add a tablespoon or so of baking soda to the water. That helps this organic matter rise to the top, where you can scoop it out and measure it separately. (It usually amounts to a few percent of the total volume of the sample.) Whats left is clean sediment, which you can measure as described above. At the end, your measurements will let you calculate four fractions—organic matter, sand, silt, and clay. The three sediment size fractions will tell you what to call your soil, and the organic fraction is a sign of the soils fertility. Interpreting the Results There are several ways to interpret the percentages of sand, silt, and clay in a sediment sample. Probably the most useful for everyday life is characterizing a soil. Loam is generally the best kind of soil, consisting of an equal amount of sand and silt and a somewhat smaller amount of clay. The variations from that ideal loam are classified as sandy, silty or clayey loam. The numerical boundaries between those soil classes  and more  are shown on the USDA soil classification diagram. Geologists use other systems for their purposes, whether its surveying the mud on the seafloor or testing the ground of a construction site. Other professionals, like farm agents and groundskeepers, also use these systems. The two most commonly used in the literature are the Shepard classification and the Folk classification. Professionals use strict procedures and a range of equipment to measure sediment. Get a taste of the complexities in the U.S. Geological Survey:  Open-File Report 00-358.