Open brownag opened 2 years ago
pbuttigieg
commented
2 years ago Thanks @brownag - many thanks for your analysis and contributions. Please do leave an ORCID here so we can nanocredit you.
Yes, this is a complex continuum that labels fail at capturing. The above is very much appreciated - hopefully we can assemble a team of motivated soil experts and ontologists to get this to a state that's as accurate as possible while accounting for regional / authority-based variation.
brownag
commented
2 years ago Great! and a good reminder to put this in my profile: https://orcid.org/0000-0002-4565-533X. I would be happy to weigh in more as needed down the line.
I am interested in learning more about the ontology development--so I look forward to watching the progress. From my limited viewpoint I do think there is a path forward that will not require mirroring specific regions/authorities at the highest levels, while still providing flexibility to meaningfully describe them--but at this point I can't quite envision what it would be.
Hi, I just caught wind of this discussion: #1352 #1266 and #392. I have a few comments. It is really great to see the progress here and I hope that I can provide some useful info on the USDA system at least.
Sand/Silt/Clay
I am curious why the classes sand, silt and clay were removed from the USDA list in #392. and also this question about whether they are indeed "soil types"
From: https://github.com/EnvironmentOntology/envo/pull/1352#issuecomment-1216752076
These terms are used for both: 1) Names of "particle size classes" of soil fine earth separates (<=2mm diameter); and 2) "soil texture classes" (or composition of the <=2mm fraction of whole soil)
The sand/silt/clay extremes of the USDA triangle correspond to soil compositions with large amounts of sand/silt/clay, with thresholds defined by boundaries of the polygons within the diagram. The proportions on the diagram reflect inferences about at what point that particular size class dominates the behavior of the soil: so for instance you have "clay" textures once you have more than about ~40% clay, whereas to get a "sand" or a "silt" you are up around 90% for those constituents. These three ternary components are not theoretical constructs-- the textural classes sand, silt, and clay occur in nature
On "sandy" and "silty" subclasses of "loam", "clay loam" and "clay"
In the USDA system we would not consider "silty clay loam" and "sandy clay loam" to be subclasses of "clay loam", likewise for the "loam" and "clay" analogs. These classes have shared boundaries in terms of thresholds of sand/silt/clay, but are mutually exclusive, separate classes reflecting a continuum of their ternary composition in every system that I know of that uses those terms.
While the threshold for what is clay v.s. silt or silt v.s. sand (particle size class) will vary between systems, as well as some of the boundaries between classes, the superset/subset relationships in this case are not something that exists in the USDA textural classification at least. It may be a convenient abstraction, but it strikes me as a bit misleading considering the range covered by some of those (super)classes.
On "fine sand" and needed subclasses of sandy textures
This is a complicated one, especially in light of what I just said about the subclasses above. Technically "sandy" textures--which in USDA system includes: "sand", "loamy sand", "sandy loam" can be subdivided based on the dominant sand fraction if there is one (known from lab data, or inferred from hand texture). This is an added level of detail beyond the base "12 class" system, resulting in a total of 21 classes.
The sand fractionation terms are not applied to finer textures such as "sandy clay loam" or "sandy clay" (I have seen it done in the field for added context as a note, but it is not part of the USDA standard)
All of the "sandy" textures support the modifiers for their sand fractions. That is "sand" (particle size class) is subdivided into several mutually exclusive classes which include "very coarse", "coarse", "medium", "fine", and "very fine".
So you have a bit of a combinatorial explosion--given the fractionation of the sand size particles, plus percent clay and silt, in a sample you can determine that you have textures such as: "very fine sand", or a "loamy coarse sand", or a simple "sandy loam".
These first two examples are subclasses; that is: "very fine sand", "fine sand", and "coarse sand" are all subclasses of "sand" which are dominated by a specific sand fraction.
There are limits for the cutoff of very fine v.s. fine, as well as how much you need of the "fine" before the lab-determined texture is considered a "fine sand" rather than just a "sand". This can vary across the different national systems/standards to describe grain size
There are similar particle size subdivisions of silt and clay (fine, coarse) fractions that are not used in naming textural classes. You will sometimes see "light" or "heavy" modifiers used on loamy or clayey textures to distinguish ranges of total clay content within a class i.e. heavy loam would be a texture with higher clay content than a light loam, but both would be considered an a subclass of loam (subject to the same limits in total sand/silt/clay)
Some images from our USDA field book for describing soils for reference:
https://www.nrcs.usda.gov/resources/guides-and-instructions/field-book-for-describing-and-sampling-soils EDIT: update broken link