Soil Texture in a Jar is a great low tech activity to do with audiences of all ages. In simple terms, the soil texture in a jar activity can be used to qualitatively or semi-quantitatively understand the proportion of sand, silt, and clay in a soil. The exercise provides an excellent visualization of the range of particle sizes that make up soil: sand, silt, and clay. While we come to intuitively know what sand or clay looks like as a separate fraction at the beach or in pottery class, we need to help to visualize soil’s complex mixture.
Soil Science Particle Size Definitions:
Sand: particles between 2mm-.053mm
Silt: particles between .053mm-.002mm
Clay: particles smaller than .002mm or 2µm
Clays are the most difficult fraction of soil to understand. In fact, clay is both a particle size definition and a mineralogical definition. Luckily, it turns out that most particles <2µm are also mineralogically clay.
Soil Texture in a Jar works by dispersing a soil sample into individual particles and letting the particles settle at different rates. If you are interested in having students determine the relative concentration of sand, silt, and clay, you should have them sieve their soil to 2mm. A cheaper alternative would be to get metal mesh that is as close to 2mm as possible. Mesh with 1/8″=3.175mm openings is cheap and available in any hardware store (1/12″ mesh would be even better). Here’s a cheap option for a 2mm soil sieve (12$).
Since clay naturally flocculates, soil texture analysis requires a chemical dispersant to disperse the soil into its individual particles. The soil science laboratory standard is sodium hexametaphospate. Sodium hexametaphosphate is effective at dispersing soil because the hexametaphosphate portion takes Ca2+ ions out of solution and the Na+ ions replace the Ca2+ ions on the exchange complex (Kettler, et al., 2001). Ca2+ and Mg2+ ions, both positively charged ions with two charges, help bridge clay particles together and hence contribute to clay flocculation. Whereas Sodium ions, Na+, contribute to the dispersion of clay particles. Sodium ions’ single charge and large hydrated radius cause them to have the weakest affinity towards negatively charged clays compared to the other predominant cations (positively charged ions) in the soil: K+, NH4+, Ca2+, and Mg2+. The large influx of sodium ions from the chemical dispersant knocks off other cations on the ion exchange complex and replaces them with Na+. Since sodium ions, Na+, are weakly attracted to negatively charged clays, they spread out in large swarms around the clay colloids and repulse other large swarms of Na+ around other clay particles. In the natural world there are soils with naturally high concentrations of sodium (sodic soils) that have poor/no structure due to dispersive properties of sodium.
Therefore it should be no surprise that sodium is a key ingredient in all the chemical dispersants listed below. Due to the negative environmental impact of phosphate based cleaning products, phosphorus has been replaced from cleaning products by other chemicals that take calcium out of solution. As a result, water softeners, such as Calgon (its name is derived from “calcium gone”), which used to be made up of sodium hexametaphosphate, have had to change their recipes. Although sodium hexametaphosphate is a standard in all soil labs, the other lower cost dispersant alternatives will do the trick for this activity.
When I trialed sodium hexametaphosphate alongside sodium bicarbonate and sodium carbonate, they all seemed to work equally. The solution was much lighter when I used sodium bicarbonate compared to the other soil dispersants (I’m still trying to understand why). Sodium carbonate is a common builder in cleaning products, which means that it serves to soften water by taking Ca2+ and Mg2+ out of solution. Powdered electric dishwater detergents like cascade may have an advantage because it also contains hydrogen peroxide, which oxidizes the organic matter that helps to aggregate soil particles together. From my research, I would recommend either sodium carbonate or a powdered electric dishwasher detergent as long cost alternatives to sodium hexametaphosphate.
Dispersants:
- Sodium Hexametaphosphate (soil lab standard for texture analysis); available at Humboldt Manufacturing in 1lb containers for texture analysis (9$).
- Sodium bicarbonate (baking soda)
- Sodium carbonate (washing soda);
- Powdered electric dishwasher detergent (e.g. cascade); The USGS curriculum calls for a powdered electric dishwasher detergent.
Materials:
- Quart Jar (Best if approximately cylindrical)
- Distilled water (Tap water may be hard: AKA have high concentrations of Ca2+ and Mg2+, which will make it more difficult to disperse the soil)
- Dispersant of your choice
- 1 cup of sieved soil
Protocal:
1) Fill quart jar ¾ full with distilled water
2) Add 1 teaspoon of dispersant
3) Add roughly a cup of sieved soil (remember to homogenize the sieved soil if you are planning on doing a couple replicates)
4) Shake well for a couple minutes (to disperse soil)
5) After 1 minute draw a line (this is sand)
6) After 2 Hours draw a line (this is silt)
7) The clay can take days to settle out
5) After 1 minute draw a line (this is sand)
6) After 2 Hours draw a line (this is silt)
7) The clay can take days to settle out
NOTE: If you are doing this activity in a set amount of time, it’s beneficial to let the soil sit for a couple days so that you have the total height of the settled soil. That way you can wait for the sand and silt to settle out in 2 hours and then you can have the clay by difference.
Below are five of the best explanations for this activity: Soil Texture in a Jar (USGS); Soil Texture in A Jar Colorodo State University; Soil Texture in a Jar New Mexico; Soil Texture in A Jar Utah; The Jar Test
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