Clearer differentiation between ortstein and the placic horizon in Soil Taxonomy is needed. The main issue seems to be that within Spodosols, the criterion appears to simply be thickness, with the placic being < 25mm, and ortstein being 25mm or more thick. This would seem to imply that ortstein and placic horizons are essentially the same thing. However, it is clear from reading the discussion in the 1st edition Soil Taxonomy (1975) as well as comments by Dr. Smith in the Guy Smith Interviews (http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051535.pdf) that the genesis of ortstein and the placic horizon were believed to be different and represented two different soil forming processes (although the genesis of the placic was less well understood and to some degree this is still true).
Ortsein forms in soils with a range of drainage classes as a result of the podzolization process common to Spodosols. Dissolved gels of humic material react primarily with aluminum (and to a lesser degree iron) to form alumino-humus compounds that accumulate to coat and infill around mineral grains, eventually forming a cementing agent. In some cases short-range ordered minerals such as allophane and ferrihydrite are also involved. Ortstein may begin as discrete nodules and expand to form columns in the layer and eventually coalesce into a continuous cemented layer that can be quite thick.
Placic horizons seem to occur mostly in poorly or somewhat poorly drained soils in areas with high precipitation of around 250 cm or more annually. Reduced iron is moved until it is restricted by a textural discontinuity, lithic contact, etc. where it combines with organic carbon to form a thin, continuous, often somewhat contorted, cemented layer. Aluminum is a lesser constituent in the process as compared to iron. Common forms of iron associated with the placic include ferrihydrite, goethite, or lepidococite. Rather than the result of the podzolization process, it is the result of redox processes. Guy Smith suggested that there seemed to also be an association of the placic horizon with manganese, again as a result of redox processes.
Given the difference in genetic process, the fact that the placic can form within a Spodosol makes differentiating ortstein from a placic difficult, especially in the field. Hence the seemingly arbitrary rule that when associated with spodics, a cemented layer <25mm is considered a placic, and thicker layers are considered ortstein. I wonder if manganese might also provide a clue to field identification as implied by Guy Smith? Manganese reacts with hydrogen peroxide, so possibly it could provide a clue, if the association is real and consistent.
It is interesting to note that the World Reference Base has accepted essentially the same definitions as Soil Taxonomy for these features in their definition of the qulaifiers used with their reference groups (note the inclusion of Mn for the placic). From WRB: "Ortsteinic (os): having a spodic horizon that has a subhorizon, ≥ 2.5 cm thick, that is cemented (ortstein) in ≥ 50% of its horizontal extension (in Podzols only). Placic (pi): having a layer, between ≥ 0.1 and ≤ 2.5 cm thick, within ≤ 100 cm of the mineral soil surface, that is cemented or indurated by a combination of organic matter, Fe, Mn and/or Al and is continuous to the extent that vertical fractures, if present, have an average horizontal spacing of ≥ 10 cm and occupy < 20% (by volume)".
What to do? Restricting the use of the placic to non-Spodosols would simplify things, but assuming they actually do occur in Spodosols and are different from ortstein, this might not be the right thing to do. Clearly Guy Smith and others believed the placic occurred in Spodosols because they created classes specifically for this (e.g. Placaquods, see description of this great group. pg. 338 in 1st edition ST). Unfortunately, placic horizons are not very common in the US so we don’t have a lot of information to go on. But some do exist, for example see the Isidore series. An effort to improve the definition and application of ortstein and placic horizons should be accompanied by observations in the field and evaluation of lab data collected for this purpose. Probably some coordination internationally with WRB would also be in order. With a bit more information to evaluate, perhaps we could then make some modifications to the definitions, to either restrict placics to non-Spodosols as suggested, or to continue as now, but make the differentiation of placic horizons from ortstein easier to understand and apply in the field.
