Mollic horizon (WRB 2022)

Tests whether any near-surface horizon meets the mollic horizon criteria. The mollic horizon is the diagnostic surface horizon of Chernozems, Phaeozems, Kastanozems, and several other RSGs; it indicates a thick, dark, base-rich, organic-matter-enriched topsoil formed under steppe or comparable vegetation.

Usage

mollic(
  pedon,
  min_thickness = 20,
  min_oc = 0.6,
  min_bs = 50,
  surface_top_cm = 5
)

Arguments

pedon

A PedonRecord.

min_thickness

Minimum thickness in cm (default 20).

min_oc

Minimum SOC % (default 0.6).

min_bs

Minimum base saturation % (default 50).

surface_top_cm

Maximum top depth (cm) for a horizon to be considered "surface-related" (default 5). v0.1 uses this as a proxy for the WRB rule that mollic must form continuously from the soil surface (after mixing of upper 20 cm if required).

Value

A DiagnosticResult.

Details

Sub-tests called:

• test_mollic_color – moist value <= 3, moist chroma <= 3, dry value <= 5.

• test_mollic_organic_carbon – SOC >= 0.6%.

• test_mollic_base_saturation – BS (NH4OAc, pH 7) >= 50%.

• test_mollic_thickness – horizon thickness >= 20 cm.

• test_mollic_structure – not simultaneously massive AND very hard when dry.

v0.1 limitations: cumulative thickness across contiguous mollic- qualifying horizons is not yet supported – this matters for profiles where mollic criteria are met by an A1+A2 sequence but no single horizon is >= 20 cm thick. Mixing of upper 20 cm before the test (per WRB) is also deferred to v0.2.

References

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