Test the argic / argillic clay-increase criterion — test_clay_increase

Tests every horizon in the profile against the clay-increase rules of either WRB 2022 (default, system = "wrb2022") or USDA Soil Taxonomy 13th edition (system = "usda"). The two systems use the SAME structural rule (three brackets keyed on overlying eluvial clay percent) but DIFFERENT thresholds:

Usage

test_clay_increase_argic(h, system = c("wrb2022", "usda"))

Arguments

h: Horizons data.table (canonical schema).
system: One of "wrb2022" (default) or "usda". Selects the threshold set.

Value

Sub-test result list.

Details

Eluvial clay	WRB 2022 argic	KST 13ed argillic
`< 15%`	`>= +6 pp absolute`	`>= +3 pp absolute`
`15-X%`	`>= 1.4x ratio` (X=50)	`>= 1.2x ratio` (X=40)
`>= X%`	`>= +20 pp absolute`	`>= +8 pp absolute`

KST 13ed thresholds are taken from Chapter 3, "Argillic horizon" (p. 4); WRB 2022 thresholds from Chapter 3.1.3, "Argic horizon" (p. 36). v0.9.26 introduces the per-system switch – earlier versions used WRB thresholds for both systems, which under-detected the argillic horizon in KSSL profiles where clay increase is in the 1.2-1.4 ratio band or +3 to +6 pp absolute band.

Returns the indices of horizons that satisfy as argic candidates.

References

IUSS Working Group WRB (2022), Chapter 3.1.3, Argic horizon, criteria 2.a.iv-vi (p. 36); Soil Survey Staff (2022), Keys to Soil Taxonomy 13th ed., Chapter 3, Argillic horizon (p. 4).