Spodic horizon (WRB 2022) — spodic • soilKey

Tests whether any horizon meets the spodic horizon criteria. The spodic horizon is an illuvial horizon with active Al + Fe oxalate- extractable material plus organic matter; diagnostic of Podzols.

Usage

spodic(
  pedon,
  min_thickness = 2.5,
  min_alfe = 0.5,
  max_ph = 5.9,
  min_oc_in_b = 0.5,
  engine = NULL
)

Arguments

pedon: A PedonRecord.
min_thickness: Minimum thickness in cm (default 2.5).
min_alfe: Minimum (Al_ox + 0.5 * Fe_ox) percent (default 0.5).
max_ph: Maximum ph_h2o (default 5.9).
min_oc_in_b: Minimum OC % in the candidate Bh / Bs layer for the v0.9.19 morphological inference path when Al / Fe oxalate are missing (default 0.5).
engine: One of "soilkey" (default; strict v0.9.19 morphological path requires Bh / Bs / Bhs designation + albic E above) or "aqp" (relaxed v0.9.84 path: any B* below E* with OC translocation peak). When NULL, reads getOption("soilKey.diagnostic_engine").

Value

A DiagnosticResult.

Details

Sub-tests:

test_spodic_aluminum_iron – (Al_ox + 0.5*Fe_ox) >= 0.5%
test_ph_below – ph_h2o <= 5.9
test_minimum_thickness – thickness >= 2.5 cm

v0.2 limitations: the WRB color criterion (hue 5YR or yellower with chroma <= 5, or specific dark colors) is not enforced. The (Al_ox + Fe_ox)/clay >= 0.05 alternative ratio test is not yet wired. Both deferred to v0.3.

v0.9.84 engine="aqp" relaxation

KSSL+NASIS Spodosols routinely use generic "B1" / "B2" / "Bw" designations rather than the specific Bh / Bs / Bhs that the v0.9.19 morphological-inference path requires. Of 14 KSSL+NASIS Podzol references, only 1 / 14 passes spodic via the v0.9.19 path; 7 / 14 have BOTH an E-designated albic-eligible horizon above AND an OC peak in a B horizon below (the canonical Podzol illuviation signature) but use generic B / Bw designations and so fail strict morph.

When engine = "aqp" (read from getOption("soilKey.diagnostic_engine", "soilkey") when engine is NULL) AND Al / Fe oxalate is unmeasured AND the v0.9.19 strict path did not fire, accept any B* designation below an E*-designated horizon when:

ph_h2o <= max_ph in the B horizon, AND
oc_pct >= min_oc_in_b in the B horizon, AND
OC in the B is greater than the maximum OC in any horizon above (the translocation signature).

Default engine is "soilkey" – canonical behaviour bit-for-bit preserved.

References

IUSS Working Group WRB (2022), Chapter 3, Spodic horizon.