Gypsic and Petrogypsic Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 179

Table 17.2 Classification of soils with gypsic and petrogypsic horizons in Soil Taxonomy
Order Suborder Great group Subgroups Total

Aridisols Argids Gypsiargids Typic (1), Ustic (1) 2

Gypsids Argigypsids Calcic (5), Typic (6), Ustic (3), Vertic (1), Xeric (1) 16

Calcigypsids Lithic (1), Petronodic (1), Typic (24), Ustic (11),
Xeric (1)

38

Haplogypsids Leptic (28), Sodic (2), Typic (8), Ustic (8), Xeric (4) 50

Natrigypsids Typic (5), Vertic (3) 8

Petrogypsids Typic (2) 2

Salids Aquisalids Gypsic (4) 4

Haplosalids Gypsic (2) 2

Subtotal 122

Inceptisols Ustepts Haplustepts Gypsic (3) 3

Xerepts Haploxerepts Gypsic (2) 2

Mollisols Ustolls Calciustolls Gypsic (1) 1

Vertisols Torrerts Gypsitorrerts Chromic (2), Typic (2) 4

Usterts Gypsiusterts Sodic (1) 1

Total 133

Soils with a gypsic horizon commonly have an aridic (92 % of soil series) soil-
moisture class, a mesic (50 %) or thermic (35 %) soil-temperature class, a mixed
(58 %) or gypsic (25 %) mineralogy class, a superactive (36 %) cation-exchange-
activity class, and a fine (24 %), fine-loamy (26 %), fine (21 %), or coarse-loamy
(20 % of soil series) particle-size class (Fig. 17.2 ).
Soils with a gypsic or petrogypsic horizon invariably contain an ochric epipedon
(98 % of pedon descriptions); subsurface horizons present in soils with a gypsic
horizon include the calcic (38 % of pedons), argillic (16 %), cambic (8 %), salic
(6 %), and natric (6 % of pedons).

17.4 Distribution of Soils with Gypsic and Petrogypsic
Horizons

Soils with a gypsic horizon comprise 9,062 km 2 , accounting for 0.1 % of the total soil
area of the conterminous USA (Fig. 17.3 ). Soils with a petrogypsic horizon occupy
only 30 km 2 . Gypsum-rich soils occur primarily in the Basin and Range and Colorado
Plateau provinces of western USA, including NM, AZ, CO, NV, UY, and WY.

17.5 Factors Influencing the Formation of Gypsic Horizons

Ninety-five percent of the gypsum-rich pedons were from well drained to exces-
sively drained. The parent materials were dominantly slope-wash alluvium (72 %),
with lesser amounts of residuum (12 %), and eolian materials (8 %; Table 17.3 ).