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Built-in classifications

Key Region Axes Classes Reference
USDA USA (global) sand, clay 12 USDA Soil Survey Manual (Handbook 18, 1993)
FAO International sand, clay 3 Verheye & Ameryckx 1984 (FAO grouping)
INTERNATIONAL International sand, clay 11 Leeper & Uren 1993
ISSS International sand, clay 12 ISSS / Verheye & Ameryckx 1984
AVERY UK sand, clay 12 Avery 1980 (Soil Survey of England and Wales)
JAMAGNE France sand, clay 13 Jamagne 1967
GEPPA France sand, clay 14 Baize & Jamagne 1995 (GEPPA-Aisne refinement)
HYPRES Europe sand, clay 5 Wösten et al. 1999
KA5 Germany sand, clay 31 Bodenkundliche Kartieranleitung 5, 2005
PTG Poland sand, clay 6 Polskie Towarzystwo Gleboznawcze 2008
NORTHCOTE Australia sand, clay 16 Northcote 1979 (Factual Key)
CHINA China sand, clay 6 GB/T 17296-2009
EMBRAPA Brazil sand, clay 5 Embrapa, SiBCS 5ª ed. 2018
KACHINSKY Russia / CIS physical_clay 9 Kachinsky 1965 (by particles < 0.01 mm)

Particle-size cutoffs

Most built-in classifications operate on the standard sand–silt–clay simplex, but the particle-size cutoffs differ between standards. See pedotri.psd for conversions between them.

Standard Sand–silt boundary Silt–clay boundary
USDA / FAO 0.050 mm 0.002 mm
ISSS / INTERNATIONAL 0.020 mm 0.002 mm
KA5 0.063 mm 0.002 mm

Convert before classifying when your data is on a different standard from the classification you want to use:

from pedotri.psd import convert

# Lab report in USDA cutoffs; classify with ISSS-based scheme
sand, silt, clay = convert(60, 30, 10, source="USDA", target="ISSS")

Localized class names

All built-in classifications ship with class names in en, fr, de, es, ru, and pt (where the translation is well established). Classifications native to a particular language default to that language:

  • GEPPA and JAMAGNE → fr
  • KA5 → de
  • KACHINSKY → ru
  • EMBRAPA → pt
  • PTG → pl
  • CHINA → zh
  • All others → en

pedotri.classify(..., locale=None) always returns the stable class key; pass a locale tag to get the localized name.

Polygon provenance

Built-in polygons fall into two tiers of fidelity:

Tier 1 — direct transcription from primary literature. Polygon vertices come from the original published reference and have been spot-checked against published reference points or against authoritative public implementations:

  • USDA — Soil Survey Manual, Handbook 18 (1993), Table 3.1
  • FAO / ISSS — Verheye & Ameryckx (1984), Pedologie
  • HYPRES — Wösten et al. (1999), Geoderma 90(3-4)
  • AVERY — geometric clone of USDA per Avery 1980 (which adopted USDA boundaries)
  • EMBRAPA — simplified 5-class grouping per SiBCS 5ª ed. (2018)

Tier 2 — interpreted from textbook descriptions. Boundaries reconstructed from class definitions stated in the cited reference; geometry is plausible but may need refinement once a domain reviewer cross-checks against authoritative reference points:

  • KA5 — full 31-class subdivision derived by converting the published (clay %, silt %) class ranges to (sand %, clay %) polygons
  • GEPPA / JAMAGNE — French texture triangle boundaries from Jamagne 1967 and Baize & Jamagne 1995; the AA / A split is the documented difference between the historical and modern variants
  • NORTHCOTE — Australian factual-key boundaries mapped onto the sand-clay plane (the Factual Key is primarily a field-test protocol, not a lab-data triangle)
  • PTG — six-class simplification of the Polish 2008 standard's finer class set
  • CHINA — six-class grouping per GB/T 17296-2009
  • KACHINSKY — 1-D interval definitions per Качинский (1965) (no geometric ambiguity since intervals are explicit)

The property-based test suite asserts that every Tier 1 and Tier 2 classification tiles the full sand-silt-clay simplex with no gaps; coverage holes are caught by tests/test_properties.py::test_tiling_classifications_cover_simplex.

Hierarchy

Where the underlying reference provides a coarser grouping, the class group field exposes it. Typical groups across the built-ins:

  • fine — clay-dominant
  • moderately_fine — clay loams
  • medium_fine — loams with elevated clay
  • medium — loams and silts
  • moderately_coarse — sandy loams
  • coarse — sands

The plotting backends use these groups to colour-code class polygons along the coarse-to-fine gradient.