Argic horizon (WRB 2022) — argic • soilKey

Tests whether any horizon meets the argic horizon criteria per Chapter 3 of the WRB 2022 (4th edition). Argic is a subsurface horizon with distinctly higher clay content than the overlying horizon, qualified by three depth-conditional clay-increase rules; it must also have texture of sandy loam or finer, satisfy a minimum thickness, and not exhibit albeluvic glossic features (which would direct the profile to the Retisol path).

Usage

argic(
  pedon,
  min_thickness = 7.5,
  system = c("wrb2022", "usda"),
  engine = NULL,
  require_t = NULL
)

Arguments

pedon: A PedonRecord.
min_thickness: Minimum thickness in cm (default 7.5).
system: One of "wrb2022" (default) or "usda". Selects the clay-increase threshold set: WRB uses 6/1.4/20 pp/ratio/pp; KST 13ed uses 3/1.2/8 (looser). See test_clay_increase_argic for the table.
engine: v0.9.63+. One of "soilkey" (the hand-coded path, default for back-compat) or "aqp" (canonical NRCS dispatch via aqp::getArgillicBounds). When NULL (the new default) the function reads getOption("soilKey.diagnostic_engine", "soilkey") so a global options(soilKey.diagnostic_engine = "aqp") flips every argic() call without modifying call sites. See argic_aqp.
require_t: v0.9.63+. Forwarded to aqp::getArgillicBounds when engine = "aqp": TRUE requires a "t" suffix in the horizon designation (the strict KST 13ed text); FALSE accepts argic by clay-increase alone (more permissive on data-sparse profiles). NULL (default) auto-picks: TRUE for system = "usda", FALSE for system = "wrb2022". Ignored when engine = "soilkey".

Value

A DiagnosticResult.

Details

Sub-tests called (each a list with passed, layers, missing, details, notes):

test_clay_increase_argic – the three-pronged WRB 2022 clay-increase rule.
test_minimum_thickness – thickness >= 7.5 cm (configurable via min_thickness).
test_texture_argic – texture of sandy loam or finer (silt + 2 * clay >= 30).
test_not_albeluvic – excludes profiles with glossic tongues (Retisol path).

v0.1 limitations: clay-increase distance (<= 30 cm vertical, or <= 15 cm with abrupt textural change) is not yet enforced; that is scheduled for v0.2 and depends on horizon boundary descriptions.

References

IUSS Working Group WRB (2022). World Reference Base for Soil Resources, 4th edition. International Union of Soil Sciences, Vienna. Chapter 3 – Argic horizon.