EARTH 530
The Critical Zone

Transcript: Soil Study with Ashlee Dere, Part 2

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Tim White: So, I'll begin at least by logging some of these initial depths. So we have 24, 40, 62, and the bottom at 72 centimeters.

Ashlee Dere: Right. Now that we've picked where these horizons transition, we need to also describe how quickly they transition from one to another. So we look at each individual boundary and decide how quickly it changes. So, here we're at 24.

Tim White: It looks pretty abrupt to me. I'd say less than two centimeters.

Ashlee Dere: Right. It changes from one to the other in less than two centimeters, so we would call that abrupt.

Tim White: Same at 40.

Ashlee Dere: Same at 40. It happens very quickly this transition and then obviously there's pretty strict boundary right there. And then we don't mark anything for the bottom because we can't actually see down below what's happening.

Tim White: All right. So what's next?

Ashlee Dere: So, next we need to look at the structure of the soil. And the structure is how the soil breaks apart. It has preferred planes of cleavage, just like rocks would. And so that is another property that we describe and it has impact for how water is going to move through the soil, and how roots can move, and even earthworms and other kinds of organisms. And then also air, for gases to move through the soil. So structure develops over time, you won't see it in very young soils but since this is an older soil we have really good examples of structure here.

So it's the same, you go systematically each horizon and describe what kind of shape the soil takes. The dryer the soil is, the more the structure is evident. As the moisture moves out and the peds is what they're called, they form into their own shapes. So the easiest way to do this is just to pull out a bit of soil and look at it in your hand and describe-- there's I would say eight general structure shapes. So what do you see here?

Tim White: Well, some of this is aggregated by roots so I don't think you count that.

Ashlee Dere: No, you want to look at what the soil's doing.

Tim White: I would look at these, right? So, I would call those granular.

Ashlee Dere: Right. These look kind of like granola. This is a pretty common structure you see near the surface of soils with all those roots there and all the microorganisms that are breaking down the organic material, they kind of form a cement that makes these nice particles. And this is really good for roots. They can easily move through the soil when it's granular like this.

We also describe how strong of a structure this is. And so 1 would be weak, 2 would be moderate, and 3 is strong. So this gives us an idea of how prominent the structure is.

Tim White: They look pretty strong to me.

Ashlee Dere: They are strong, right.

Tim White: So that's a 3. And is granular G?

Ashlee Dere: It's GR.

Tim White: GR.

Ashlee Dere: Yeah. So that will tell us that we've got good strong structure here in the surface horizon. Now we'll do the next horizon.

Tim White: Looks subangular to me.

Ashlee Dere: Yes. So it's a blocky shape, they're a bit bigger-- a lot bigger than these little granules up here, but they're not angled very sharply so they're subangular blocky.

Tim White: And that's SBK?

Ashlee Dere: SBK, right.

Tim White: And I guess I'd have to say that it's also strong.

Ashlee Dere: It is also strong. You can see some on the roots here being held together. And the majority of it comes out this strong, blocky shape so you go by whatever is dominant in the horizon. And it's definitely, although there are some smaller pieces, it's dominated by that blocky structure.

Tim White: So, let's look at this bottom see if we can get lucky. Some of these look kind of elongated.

Ashlee Dere: They are. You can see some of these vertical lines here, it's kind of--

Tim White: Like this?

Ashlee Dere: Exactly like that. These would be considered prisms. So we have these prismatic looking-- quite a few of these.

Tim White: I've never seen these, what's the symbol for prism?

Ashlee Dere: This is a really nice prism right there. That's a big one. The prism is a PR, that's how it's abbreviated. But you get these prisms often in pretty high clay soils usually down in the subsurface. And they're actually really good for water and for roots and air to get through. They just provide a lot of space in between these soil peds. So it's not compacted at all, it's got a lot of room to breathe so to speak. But these would definitely be prisms and this one is a very strong prism. And it's quite clear that it's breaking off in this fashion.

Tim White: Well, that's fun to see something new.

Ashlee Dere: Good examples. These are beautiful prisms actually.

Tim White: So now you want to take samples?

Ashlee Dere: So now we've done describing in place what we need to describe for the moment so we'll take sub samples from each horizon, take that out of the pit and look at the texture, the percent of sand, silt, and clay that we can estimate by feeling, describe the color more precisely, and then look at how firm the soil peds are.

Tim White: Well, I have your handy muffin tray.

Ashlee Dere: Right. So we'll just take a sub-sample of each. Make sure we get a representative sample from each horizon. There we go, the first.

Tim White: It's amazing how those peds hang on to the roots like that.

Ashlee Dere: They do, yeah, like strong glue.

Tim White: Is that enough?

Ashlee Dere: That looks good, we got plenty to work with. Let's go ahead and grab our equipment and go outside the pit.

Tim White: I'll keep taking notes.

Ashlee Dere: All right.