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TIM: By now, you should understand that the massive ice sheets that covered Canada, New England, and New York, just 18,000 years ago, did not reach central Pennsylvania. Thus the glacial deposits on which the soils of New York are developed do not exist here. We differentiate soils developed on transported material from residual soils. That is soils developed insitu on bedrock native to a site.
I've come to Penn State's agronomy farm with Ashley Deer, a graduate student who teaches soil judging, to look at a residual soil and to introduce you to some of the aspects of field soil descriptions.
Hey, Ashley.
ASHLEY DEER: Hi, Tim.
TIM: What are you doing?
ASHLEY DEER: Trying to clear off some of this face here so we can get a good look at the soil pit and the cross-section of the soil that's here.
TIM: Why is this pit here?
ASHLEY DEER: It's actually for teaching purposes. They've come out with a backhoe, and several places throughout the Center County here, they've dug out so we can look at what's underneath.
TIM: Well I told the students that we'd have a look at a residual soil. I wonder if you could tell us a little bit more about what we're looking at here.
ASHLEY DEER: Here you've got limestone in the bottom of this soil profile. And all of this soil that you see in this valley that's being farmed, it's all forming from this rock as it weathers over the years. So it will keep weathering, keep forming more soil, get thicker and deeper. But in this spot it just happens to be shallow. In other areas it's thicker.
TIM: So you mean all this soil here is derived from this rock.
ASHLEY DEER: Exactly. All of it is forming from what you see at the bottom. And as the years go on it will get deeper and deeper as more of this rock weathers and forms the soil.
TIM: Well that's a great example of a residual soil.
ASHLEY DEER: It is. Makes for some very nice soil.
TIM: Well why don't we go ahead and describe it so the students can see how a soil scientist describes the soil in the field.
ASHLEY DEER: All right. I want to make sure we get the profile wet enough to see color differences. We'll look for the different horizons now. Soils tend to form in different layers and we can describe each of those layers to get a big picture idea of what's happening in the soil, how it forms, and what we can do with it.
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So now you can see that it was dry before, but now we have more striking color differences. Now that we've gotten the soil moist.
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So now we need to figure out where these distinct horizons occur. We have to measure that in centimeters. But look at where we see differences as the soils tend to form in different horizons and each have different properties that we can describe. And that will help us get the bigger picture of the soil.
So color is the most obvious difference of horizons. Here it's pretty clear now that we have moistened it.
TIM: Yeah. I can see an obvious change from this upper brown layer down into a redder orange here at about 24 centimeters. And then another change at about 40. And then down here obviously, at 63 at the limestone.
ASHLEY DEER: Right. Yep. And then down here we just assume, although we haven't dug any farther and really we can't because there's just rock here, we assume that whatever we see down here extends to 150 centimeters.
TIM: So I'm going to go ahead and start filling out this log form but. Maybe you could just briefly explain it for the students.
ASHLEY DEER: OK. We have this forum just to help document what we see. And we use a series of symbols, really they're just abbreviations for different properties, and systematically go through and piece-by-piece define these horizons. And describe each of them. And in the end we have this documentation of what the soil looks like.
So anyone can go back, who's familiar with this process, simply look at this sheet and get a mental picture of what is there. So it's really handy just to keep track of everything and for future reference. And it becomes a kind of language that explains what we have here.
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