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Argic horizon (WRB 2022)

Source: R/diagnostics-horizons-wrb.R
argic.Rd

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,
  lithic_discontinuity = 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".

lithic_discontinuity

v0.9.220. Apply WRB 2022 criterion 2.a.i: a clay increase over a coarser layer of another material (Ap over 2Bt, read from the designations' material numbers) counts only with clay films of "common" or more (illuvial clay, criterion 2.b). NULL (default): TRUE for system = "wrb2022". The SiBCS B textural, whose item (j) accepts such an increase by its textural ratio, and the USDA keys pass FALSE.

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).

Until v0.9.219 a glossic, albeluvic or retic designation anywhere voided the argic horizon (the WRB 2014 Albeluvisol logic); in WRB 2022 retic properties belong to an argic or natric horizon.

v0.9.310, for system = "wrb2022" (WRB 2022 Ch 3.1.3, p. 35-36):

  • criterion 1: the layers have \(\ge\) 8% clay;

  • criterion 2.a.i (v0.9.317): the coarser-textured layer is not separated from the layer by a lithic discontinuity, read from the material numbers of the designations and from the data of each pair of layers in between (Ch 3.2.7, p. 76-77: the ratio sand to silt, criterion 2.a.iv; coarse fragments, criterion 6);

  • criterion 2.a.ii (v0.9.317): no clay increase counts below a plough layer (Ap) that directly overlies the argic horizon;

  • criterion 2.a.iii: a layer is compared with a coarser-textured layer above it only across a transitional horizon of \(\le\) 15 cm; layers already accepted and layers whose designation has the suffix t are part of the argic horizon, not transitional;

  • criterion 2.b.ii: clay films of "common" or more (clay coatings on \(\ge\) 15% of the aggregate surfaces) make a layer argic without a clay increase;

  • a subsurface horizon: the layer at the mineral soil surface and the A horizons are not part of it.

The USDA argillic and order keys and the SiBCS B textural keep the earlier reading.

References

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