Tests whether any horizon meets the ferralic horizon criteria. The ferralic horizon is a subsurface horizon resulting from long and intense weathering, characterized by very low cation exchange capacity per unit clay – the canonical "low-activity clay" signal that defines the Ferralsol RSG.
Arguments
- pedon
A
PedonRecord.- min_thickness
Minimum thickness in cm (default 30).
- max_cec
Maximum CEC (1M NH4OAc, pH 7) per kg clay (default
NULL= 16 in soilkey engine, 20 in aqp engine; seeengine).- engine
One of
"soilkey"(default; strict 16 cmol_c/kg-clay threshold per WRB 2022) or"aqp"(relaxed 20 cmol_c/kg-clay – a regional tolerance that accommodates Brazilian / SOTERLAC Latossolos data, where Embrapa-style Mehlich/Ca+Mg+K+Al sum often reads ~17-20 on profiles that the canonical NRCS / WRB definition would accept as ferralic).NULLreadsgetOption("soilKey.diagnostic_engine"). The numeric threshold can also be overridden directly viaoptions(soilKey.ferralic_max_cec = ...).
Details
Sub-tests called:
test_ferralic_texture– texture sandy loam or finer.test_cec_per_clay– CEC / clay <= 16 (or 20 underengine = "aqp") cmol_c/kg clay.test_ferralic_thickness– thickness >= 30 cm.
v0.3.1 alignment with WRB 2022 Ch 3.1.10 (p. 44): the older "ECEC <= 12 cmol_c/kg clay" gate was removed because it is not in the canonical text – only CEC (1M NH4OAc, pH 7) <= 16 is required.
v0.9.67 regional tolerance: BDsolos RJ benchmark (n=722 perfis)
showed 88/115 Latossolos failing the strict 16-cmol gate because
Embrapa lab methodology often reads CEC at 17-20 on profiles that
are unambiguously Latossolos by every other criterion. The
engine = "aqp" threshold of 20 closes that gap without
redefining the WRB threshold itself; users targeting strict
WRB 2022 fidelity should keep engine = "soilkey".
The weatherable-mineral test (<= 10% by volume), water-dispersible-clay test, and stratification / rock-structure exclusions remain deferred (they need mineralogical data outside the canonical horizon schema) and are refinements rather than gates.