This question came up in another thread: What is the difference between ortstein and placic horizons?
Here is my general interpretation of these two diagnostic horizons. Please feel free to correct me if I am in error about any of these points. Below that is my opinion about the utility of having these two separate diagnostic horizons and suggestions for improving their use in taxonomy. Any and all responses are appreciated!
ROOT RESTRICTION: Both placic horizons and ortstein are made up of cemented materials that are potentially root-restrictive and can function the same way in determination of soil depth class (see 12th ed. of Keys to Soil Taxonomy, pg 330). However, ortstein is not necessarily continous or root-restrictive, but a placic horizon must be continuous and root-restrictive.
THICKNESS: From Soil Taxonomy (1999):
> "Ortstein is differentiated from a placic horizon within spodic materials solely on the basis of thickness. Placic horizons within spodic materials are less than 25 mm thick, and ortstein is 25 mm or more thick."
> "The placic horizon has a maximum thickness of 25mm where it is associated with spodic materials. Where not associated with spodic materials, it has no maximum thickness."
> "Where placic horizons occur within spodic materials, they are arbitrarily differentiated from ortstein by thickness."
CEMENTATION: By definition the ortstein consists of spodic material, which contains both organic matter and aluminum, and can exist with or without iron. This seems to imply that aluminum is the main cementing agent, but does not exclude iron. The placic horizon is cemented by iron with or without manganese, but can also contain other cementing agents. It is clear that iron is the main cementing agent in a placic horizon, but this definition does not exactly exclude aluminum.
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In my opinion, it would clarify things to say that ortstein occurs in spodic horizons, and placic horizons occur in non-spodic horizons, without overlap. The existing classification is clear, though I feel it introduces undue complexity. In short, I would propose this:
Continous or non-continuous cemented spodic materials = ortstein
Continuous cemented non-spodic materials = placic horizon
One has to ask why, in a spodic horizon, a thin cemented layer would be called placic when cementation specifically by iron (instead of aluminum) could not easily be proven in the field. The presence of a spodic horizon already implies the presence of aluminum as a likely cementation agent. Any cemented materials within this horizon could reasonably be considered ortstein, whether they are continous or not. Would there be any loss in interpretive value if this was instead referred to as thin, continuous ortstein? In this scenario the determination between placic and ortstein does seem arbitrary and, I would argue, unnecessary.
In pedons with no spodic horizon described, any continous dark cemented layers could be described as placic horizons, provided that they are at least 1mm thick. In this scenario, the horizon designation (non-spodic) implies that aluminum does not dominate the horizon, and the desgination as placic asserts that iron and/or manganese and organic matter are the suspected cementing agents.
Any thoughts on this? Should we just leave it alone? Does anyone have experiences where distinguishing between ortstein and placic horizons became problematic?
