Visualizing Soil Properties: Dispersion and Flocculation

(electronic jingle) – So today we’re gonna do a demonstration of dispersion and flocculation of soil. And we have here a sodic soil, a soil that’s dominated by sodium which causes the clay particles to push away from each other or disperse. So I’m gonna put just about a teaspoon of this soil in each of these beakers that just contain distilled water. Stir it up, and you’ll see that it looks like chocolate milk. What’s happening is the clay
particles are dispersing, and they won’t settle out because they’re too small for gravity to pull them out of solution. The water molecules are moving around and will keep the clay particles suspended because this soil is dominated by sodium. If we put a different salt
in, if we put calcium in, and here I have gypsum,
which is calcium sulfate, and if I put some calcium sulfate in and mix it into the soil and water, then what’s gonna happen
is the clay particles will clump together or flocculate. Now they’ll be big enough that they’ll act more like sand particles,
and they’ll settle out. Okay, so here I’m putting half a
teaspoon of gypsum in here, stirring it up, and then we’ll watch this over the next half hour, and what we should see is that this one, that just has soil, will stay suspended. This one, which has soil and gypsum, is going to start to settle out. (upbeat music) Okay, it’s been about a half an hour since we stirred this
up and let it settle. And you’ll see that the one
that just has water in it, this clay has stayed suspended. The clay particles are
acting independently. They’re dispersed. Where we’ve added calcium
in the form of gypsum, that’s allowed the clay particles to get close enough together to clump into aggregates or to flocculate. Now the aggregates are
much bigger than clay. They act more like sand particles,
and they’ve settled out. So if a farmer has a field of soil that has a lot of sodium in it, water won’t be able to soak in, and plants won’t be able to get the water. So what we typically do
with a high sodium soil is to add gypsum. That’s one of the treatments,
flocculate the soil, and then water will infiltrate and drain. We normally want a soil to be flocculated because the clumps in here can’t get very close to each other, so they’re big spaces between
the clumps or the aggregates. So, in a field of soil that’s aggregated, water can infiltrate and
drain though the soil through those big aggregates, and when the soil drains,
air can fill those pores between the aggregates
and supply the plant roots with air and water. The dispersed soil will
have very, very tiny pores. Water can’t penetrate very easily. Water can’t drain, and
roots can’t penetrate in through those tiny pores. So a dispersed soil is
great for making a pond. So if you wanted to make a pond with clay, we could disperse the soil. That would be great, but if
we want to grow plants in it, we want water to infiltrate and water to drain from the soil, then we want the soil to be aggregated. (upbeat music)

13 thoughts on “Visualizing Soil Properties: Dispersion and Flocculation

  1. Unfortunately you use a lot of very imprecise terms. Chemistry is a precise science that requires precise and appropriate terms.

  2. My professor missed this portion in class so I am going to give the whole class on flocculation and dispersion. Thanks 🙂

  3. I appreciate the scientific explanation of the flocculation v. dispersion aspect by Dr. Walworth but actually it can be misinterpreted. By addressing flocculation of sodic soil with gypsum as the "silver bullet" he used a demonstration of 1:1 volume to volume gypsum to soil to accomplish the flocculation and with distilled water, no less. This is impossible in a real world agronomic situation, either in agriculture or, especially, with topically applied gypsum for ornamental landscape soil management. I absolutely agree that calcium is the element to use in providing the solution for flocculating sodic soil, but there are much better calcium sources than gypsum.

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