I've been working on a validation script that evaluates "dark surface" epipedon (mollic, umbric) presence/absence and thickness for NASIS pedon records. For non-lab data, this assumes color is a reasonable surrogate for organic carbon and does not take into account base saturation for diff'n of mollic vs umbric.
This has made me question when exactly the ochric epipedon has the "18cm/plow layer" thickness versus when it "snaps" to horizon boundaries. To my knowledge, ochric boundaries aren't diagnostic for anything, so perhaps this is a bit nitpicky but it would be nice to know the rule.
I know that ochric epipedons include:
- Organic layers too thin for histic/folistic
- A/Ap horizons too thin, light or bright for mollic/umbric
- Eluvial horizons (E, transitional hz) above an illuvial
The part in the definition I'm having trouble understanding is:
"If the underlying horizon is a B horizon of alteration [cambic, oxic] and there is no surface horizon appreciably darkened by humus, the lower limit of the ochric epipedon is lower boundary of plow layer or equivalent [18cm] in an unplowed soil"
So, this suggests to me that if there is an "appreciably darkened" layer, and no underlying illuvial horizon, then the ochric boundaries are equal to the boundaries of that layer. Lower boundary of ochric is upper of illuvial, if present.
But what does "appreciably darkened" mean?
Does "appreciably darkened" mean 5-3-3 colors less than 18cm thick that do not mix over 18cm interval to produce a "dark" color? Does an organic layer (to thin for folistic) count as a "surface horizon appreciably darkened by humus"?
Scenario: the upper 18cm of mixed mineral soil material does not make 5-3-3 color requirement (no mollic or umbric), but underlies an organic layer.
Is the ochric epipedon:
1) just the OSM that is too thin for histic/folistic;
2) OSM + dark portion of MSM; or
3) interval from soil surface to 18cm?
Similarly, when a mollic or umbric makes the color/OM criteria by mixing of upper 18cm of mineral soil, what are the boundaries? 0 to 18 (relative to mineral soil surface)? or the boundaries of the darkened mineral horizon? If the required thickness of the epipedon is greater than 18cm due to one of the many modifying criteria, should the mixed color be determined over this minimum thickness? or over 18cm interval?
Another related question:
Do we use rounded integer values to determine whether requirements are met? For instance, a weighted average moist chroma of 3.16 would round to 3 given significant figures and ST rounding rules, but *technically* exceeds the limit given. I realize that this would also be within the old way of writing the limits using half-chip values (i.e. moist chromas less than 3.5)
Perhaps the answers to some of these questions are simpler than I realize. Thanks in advance for setting me straight.
