PED talk – Soil: Texture, Clay, and Cation Exchange


[music plays] When you hear the word soil, you may imagine a big amorphous blob of brown… stuff. But there’s so much more to it. For starters, soil is made up of organic matter, and discrete mineral particles Those particles are grouped into three categories based on their size: Sand, silt, and clay. A sand particle is about the size of the head of a pin. Silt has a diameter similar to a strand of human hair. And a single grain of clay is about the size of a bacteria. Cool! But… so what? Well, the distribution of sand, silt, and clay in a soil, called soil texture, plays a huge role in determining how water, air, and even heat move through the soil; how easily compacted a soil is; how susceptible it is to erosion; how well plant roots can push through it; what nutrients can be stored in the soil; and, what contaminants can pass through it to our groundwater. Clay content plays an outsized role in many of these properties because of two key attributes: First, surface area. If you spread out the surfaces of all the grains of sand in a tablespoon, it would roughly cover your kitchen table. But, if you did the same with one tablespoon of clay, it could cover a football field. And all that surface area can hold on to water and nutrients and pollutants. Second, clays have charge. Clay minerals are formed in sheets of positive charged cations like silicon and aluminum surrounded by negative charged oxygen and hydroxyls. Through a process called isomorphous substitution, cations of similar size can replace each other in these structures when they form. And if an aluminum with a plus three charge is replaced by a magnesium with a plus two charge, there’s an extra negative charge left over that’s carried by the clay mineral. This charge gets balanced by cations that are held at the clay’s surface. And, they can be readily exchanged for other cations, since they’re not part of the clay structure. Generally speaking, the more of this type of isomorphous substitution, the greater the negative charge on the clay, and the greater its capacity to exchange cations, which, appropriately, we call cation exchange capacity. A lot of these exchangeable cations are nutrients used by plants, like potassium, calcium, and ammonium. Cation exchange can also slow the movement of pollutants like chromium or cesium. Multiply all that reactivity by and enormous surface area and it’s clear that clays have a huge impact on the environment, on agriculture, and on human health. So, next time you see some soil, erase the thought of a boring, brown blob and think about microscopic sheets of minerals storing nutrients and trapping toxins. and then remember that that’s just the tip of the iceberg for everything that’s going on in soil. And, the rest of the iceberg? Not only do we not understand all of the pieces yet, we still don’t really know just how big that iceberg is. [music plays]

29 thoughts on “PED talk – Soil: Texture, Clay, and Cation Exchange

  1. Thanks for the simple explanation! Thumbs down for the music throughout the whole video. It was terribly distracting.

    P.S. I did not actually give a thumbs down

  2. @Gordon Rees, have you ever thought about posting another video? I found this one super helpful. It would be great if you did another one on Sorption

  3. This could be a good video, but the background music with words makes it hard to focus on what you are saying.

  4. Excellent info & well-organized presentation, but I did find the background music distracting. Maybe limit it to bumper music at beginning and end?

  5. So, next time you see some soil, erase the thought of a boring round blob…

    …and think about boring microscopic sheets of minerals, storing boring nutrients and trapping boring toxins.

  6. Okay, It's not always background music alone that is bad, because many youtubers do it and it is fine. I believe one large part of it, is how loud the music is during talking, or if it has lyrics that make it distracting.

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