Updated: 4 December 2024
Remember, soil texture is the relative amount of sand, silt and clay particles. The USDA Soil Textural Triangle delineates the twelve soil texture classes. For a video with more information, click here.
- Notice, the clay texture class occupies the largest part of the triangle. Why is that?
- Sands have little surface area and essentially no charge, so they are essentially chemically inert; they do not react with elements and compounds in the soil.
- Silts have more surface area than sands, but much less than clay; they have very little charge and are mostly inert.
- Clays have tremendous surface area and usually have a negative charge, though the amount of charge varies with the type of clay mineral. Their huge surface area and charge causes them to control both the physical and chemical properties of the soil.
There are many types of clay minerals that vary in the shape and size of the particles, surface area, shrink-swell capacity, and charge properties that allow them to hold cations (positively charged ions), and on rare occasions to hold anions. Simply stated, clay particles are minerals in the mica family known as layer silicates and their properties are due to their unique structure. See below for more details.
The total amount of cations a soil can hold is called the cation exchange capacity – exchange because cations can freely move back and forth between clay particles and the soil solution (water in the soil always contains salts – anions and cations – and molecules in it).
This simple experiment uses a DC batterty to demonstrate the nature of the charge on soil clays resulting in the chemical filtering properties of soil. Keep reading to learn how to do it yourself.
Materials and Setup
- Setting up the battery and electrodes
- 6 V DC lantern battery
- Batteries have two terminals, positive and negative
- Don’t PANIC: Positive is Anode, Negative is Cathode.
- Electrons (negatively charged ions) flow from the cathode (negative terminal) to the anode (positive terminal).
- Opposites attract, so when electrodes attached to the battery terminals are placed in the soil slurry, the soil is attracted to the electrode with the opposite charge.
- Electrodes (I made my own, as you can see in the video linked above.)
- Colored wires to connect the electrodes to the battery
- 6 V DC lantern battery
- Preparing the clay slurry/saturated paste
- Container for the clay. Hint: Do not fill to the top. Dry bentonite clay can swell up to five times its original volume.
- A stir stick (metal or wood – not a glass rod)
- Bentonite clay (can be ordered online)
- Optional: native (local) clay, pottery clays
- Mix water into the clay gradually: Add water, mix, then repeat the process until you have a slurry, a saturated paste.
- Bentonite clays are smectite minerals. You may think you have achieved a saturated paste, then 5 minutes later, it is not. Why? The clay continues absorbing water into its mineral structure, which is the reason it swells. So, add more water and stir again.
- Do it!
- Wait at least 10 minutes, then gently lift the electrodes directly up and out of the slurry.
- If the slurry is at the right consistency (too wet or too dry does not achieve the desired result), the positive electrode will be encased in bentonite (clay), as in this video.
- If you use pottery clays or native, local clays, they may not have as strong a charge and may not cling to the electrode. If you have a very acid clay, it actually may cling to the negative electrode.
- Discussion points
- Bentonite clay yields the most striking result.
- Native clays and pottery clays may or may not be attracted to the electrode, demonstrating the difference in the properties of clay minerals.
- Pair this with Soil is a Filter and use deductive reasoning to determine the charge on the red and blue dyes in the purple Koolaid.
I first posted this activity on Dr. Dirt’s K-12 Teaching and Learning Activities. Through my work with the Soil Science Society of America K-12 Outreach Committee, it has been refined and demonstrated at the National Science Teacher’s Association conference and in teacher workshops as ready-to-go lesson plans. These documents will help you get started and be prepared to answer questions you or your students may have.
Updated: 4 December 2024
Clay minerals
Clay particles are minerals in the mica family known as layer silicates that have unique combinations of silica and oxygen molecules (silica-tetrahedral) and aluminum and hydroxide molecules (alumina octahedral) occurring in different combinations in repeating layers like a sandwich, or a stack of paper with repeating layers alternating colors in a 2:1 ratio (white-red-white) or a 1:1 ratio (white-red) representing two common configurations of soil clay minerals. (For detailed information on clay mineral structures, see the references below.) Sometimes a substitution of one element for another in the basic structure results in a net negative charge. Because opposite charges attract, cations (elements and molecules with a positive charge) are attracted to the clay particles. The number of negative charges on the clay determines the number of cations the soil can hold, the cation exchange capacity. The soil cannot hold more cations than it has negative charges, and the cations can freely move from the soil solution (water in the soil always contains salts – anions and cations – and molecules in it) to the clay particles and back. The different clay minerals have different amounts of substitution and therefore different amounts of charge and cation exchange capacity.
A single, thin sheet of a mica mineral 0.1 mm thick, may have as many as one million repeating layers.
References
Kumari, N. and Mohan, C. 2021. Basics of Clay Minerals and Their Characteristic Properties. Ch. 2 in (ed. Gustavo Morari Do Nascimento) Clay and Clay Minerals. DOI: 10.5772/intechopen.97672
LibreTexts GeoSciences, 13.7.2: Sheet Silicates, Dexter Perkins, University of North Dakota, accessed 20Nov2024 online at https://geo.libretexts.org/Bookshelves/Geology/Mineralogy_(Perkins_et_al.)/13%3A_Crystal_Structures/13.07%3A_Structures_of_the_Basic_Silicate_Subclasses/13.7.02%3A_Sheet_Silicates.
Updated: 4 December 2024