Why point out that soil must occupy space?
February 15, 2018 2:31 PM   Subscribe

In this technical definition of soil (as referenced by clew in an excellent post about dirt), one of the necessary characteristics of soil is that it "occupies space". Why would they feel the need to make that qualification?

Here's the full definition:
Soil in this text is a natural body comprised of solids (mineral and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the original material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment. This definition is expanded from the 1975 version of Soil Taxonomy to include soils in areas of Antarctica where pedogenesis occurs but where the climate is too harsh to support the higher life forms.
From a really quick skim of the book on Google Books, I can't find any clarification of that clause. The word "space", again according to Google, never occurs anywhere else in the book. I'm having a lot of trouble imagining a natural body comprised of solids, liquid, and gases that occurs on the land surface and has layers or can support rooted plants - but does not occupy any space! Yet there must have been some reason to mention it.
posted by moonmilk to Science & Nature (12 answers total) 4 users marked this as a favorite
If I were to venture a guess, I think they're probably trying to imply something along the lines of taking up a continuous volume rather than sprinkled haphazardly in little bits over something else?
posted by Zalzidrax at 2:37 PM on February 15, 2018 [1 favorite]

Well, it's the kind of definition that you find in the sciences that is not really a definition: it's a brief brief description distilled from the author's experience, not a hard rule that you can evaluate in the mathematical sense. After all, almost everything occupies space (or almost nothing does, depending on your perspective).

They probably have a specific class of phenomena in mind - I guess maybe a thin layer sand over rock, or something like that? - and they're trying to say "soil looks like X except for that Y thing, that's just weird".
posted by Dr Dracator at 2:48 PM on February 15, 2018 [1 favorite]

Best answer: I'm an engineering geologist and actually own that USDA book (the 1999 2nd edition though), and I think what they are getting at is that soils are changed from the source material and develop in layers (horizons), which comes from the modern concept of soil first developed in 1870. So maybe this means like an in-place native soil that would intrinsically need to take up 3-D space as a side effect of its natural development.

It really reads to me like a weird turn of phrase that has just survived from edition to edition.

edited to fix a few typos
posted by cakebatter at 2:51 PM on February 15, 2018

Best answer: I read it as clarifying that the "horizons or layers" have some significant depth, and therefore volume, and aren't pseudo-2D surfaces.
posted by kickingtheground at 2:54 PM on February 15, 2018 [2 favorites]

I can imagine that they're trying to distinguish from the household concept of "this cloth is soiled" meaning it has particulates on it that make it visibly not clean (stained), but not necessarily talking about a real volume of actual stuff. But agreed it's weird phrasing.
posted by aimedwander at 3:36 PM on February 15, 2018 [1 favorite]

The main reason is to make sure that it’s a volume and thus it includes the gases. That’s the one part/aspect most lay people forget about.

The other points are also nice benefits, but not the key reason imo (based on conversations with some agronomy/weed science/soil science folk).
posted by SaltySalticid at 4:01 PM on February 15, 2018 [2 favorites]

Response by poster: Could you expand on that, SaltySalticid? I think I get the idea, but I'm not sure how "occupies space" conveys that. After all, solids (mineral and organic matter) and liquid necessarily occupy space, even if you disregard the gases.
posted by moonmilk at 4:27 PM on February 15, 2018

Best answer: Discontinuous horizons are consistent with soil taxonomy. The definition already specified that soil can be comprised of gases.

Maybe it's there to rule out a boundary between parent material and air that doesn't have any pedogenically altered material in it. Possibly it's a (strangely terrible) way to say that the geometric structure of soil in situ is important.
posted by clew at 5:25 PM on February 15, 2018

Best answer: To expand: I probably went too far with "the main" reason. There's probably lots of reasons. That's one that I think is important and some people miss, and maybe the authors had in mind.

In more detail: "occupies space", after the bit about "is comprised of [stuff]", seems to be highlighting the point that "soil" is defined to be the all of that stuff that is inside a certain space. This wording does also clarify (in my reading) that it's a 3D space that can encompass many horizons, similar to kickingtheground's point.

I also agree with Dr Dracator , this definition is not as formal as some. It's not math, but by adding more and more detail and clauses, the author can attempt clarify readings and interpretations, and to rule out misunderstandings. And they don't necessarily mind if there's some redundancy or overlap between clauses if it increases clarity. It's a good addition to the definition from where I'm standing: it's a clear and succinct statement that has lots of implications, and this can be used to check and verify whether something counts as soil. Though the fact your are asking proves that the wording is a bit award...

This is a hard question to answer definitively with references: outside of a few formal fields, researchers don't often give specific rationale for why certain words or concepts are the way they are in the definition. I did find some old scholarly discussion about definitions of soil that you might find helpful. Here, "Factors of Soil Formation" (Jenny, 1941). It has many sections about definition(s), but opens with:

It is problematic whether any definition of soil could be formulated to which everyone would agree. Fortunately there is no urgent need for universal agreement. For the purpose of presentation and discussion of the subject matter it is necessary only that the reader know what the author has in mind when he uses the word "soil." This common ground will be prepared in the following sections.

Hope that helps!
posted by SaltySalticid at 6:35 PM on February 15, 2018 [1 favorite]

Best answer: I think that’s referring to pore spaces in soil which can be filled with gasses and vapors can off gas through the pores. However some soil types may not contain very much much pore space such as caliche or hardpan.

You are looking at at usda reference, you might find it worth looking at other classification systems such as USCS or AASHTO!
posted by cakebatter at 7:20 PM on February 15, 2018

Best answer: The original 1975 version of the USDA soil taxonomy handbook gets into some pretty cool details that may shed some light on the strange use of the phrase "occupies space." Everything referenced below is from the first chapter.

The text opens with a discussion of current and past definitions of soil. It states that there is no One Definition to Rule Them All:
The word soil, like many common old words, has several meanings. This is true even in soil science ... [In a practical sense] soil covers land as a continuum, except on bare rock, in areas of perpetual frost, or on the bare ice of glaciers. In this sense, soil has a thickness that is determined by the depth of rooting plants.
In other words, soil is not just a matter of surface coverage, but also depth. Soil is characterized by volumetric properties (like soil horizons), not just surface properties. Saying that it occupies space is another way of expressing this idea. As cakebatter suggests above, it seems very plausible that "occupies space" is a strangely worded holdover of concepts from earlier editions of the text.

The handbook goes on to get into a history of scientific concepts of soil, beginning with some work in Russia in 1870. This work was the first scientific conception of soil that characterized it based on the nature of the soil itself, and not as just as a set of consequences arising from weathering and other transformative factors on the underlying parent material. It took soil from area (parent material, like bedrock, which can be mapped as a surface area) to a fuller concept including soil horizons (verticality). So, again, space-filling.

The text eventually gets into the idea that a minimum unit surface area is required to characterize the soil that covers a landscape. This is necessary when soil horizons occur intermittently in a soil type; it must be large enough to capture all the intermittent horizons that appear in the soil. The text describes how, without defining a minimum area, columns burrowed into the soil by animals could be considered "not-soil." This would not occupy space, and its mention is consistent with clew's speculation that "occupies space" is "there to rule out a boundary between parent material and air that doesn't have any pedogenically altered material in it."

The minimum area (and minimum depth) unit is called a pedon, and is here apparently a volumetric unit, but other sources refer to it as a 2-d/surface-area unit. Hence, I guess, another reason for clarifying that soil "occupies space."
posted by compartment at 7:24 PM on February 15, 2018 [3 favorites]

Response by poster: Thanks, everyone! I'm always impressed at how much cool stuff metafilter people know.
posted by moonmilk at 7:52 AM on February 16, 2018

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