Argillic, Kandic, and Natric Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 93

Table 11.2 (continued)
Order Suborders Great groups Subgroups a No. series

Oxisols Udox Eutrudox Kandiudalfic 0

Udox Kandiudox All 3

Ustox Eutrustox Kandiustalfic 3

Ustox Kandiustox All 1

Spodosols Aquods Alaquods Alfic Arenic (2), Arenic Ultic (2), Alfic (6),

Ultic (12)

22

Aquods Endoaquods Argic 17

Aquods Epiaquods Alfic (17), Ultic (2) 19

Orthods Alorthods Alfic, Ultic, Arenic Ultic 0

Orthods Fragiorthods Alfic (9), Alfic Oxyaquic (10), Ultic (1) 20

Orthods Haplorthods Aqualfic (3), Alfic Oxyaquic (30), Alfic (36),

Lamellic (9), Lamellic Oxyaquic (2),
Ultic (1), Oxyaquic Ultic (2)

83

Ultisols All All All 1,330

Vertisols Aquerts Natraquerts Typic 9

10,540

a Number of soil series within a subgroup given in parentheses

Alfic Humic subgroups) within nine great groups. Clay illuviation also is recog-
nized in Inceptisols at the subgroup level (Lamellic Eutrudepts, Lamellic
Haplustepts). However, the lamellae are considered part of the cambic horizon
and not the argillic horizon.

11.4 Distribution of Soils with Clay Enrichment

All of the Alfisols and Ultisols contain evidence of clay enrichment; these soils
contain 3,984 and 1,330 soil series and cover 1.2 million and 0.82 million km 2 of the
USA, respectively (Table 11.3 ). The distribution of these soils in the USA is shown in
Fig. 11.2 . Nearly half of the soil series in the Mollisol and Aridisol orders contain
diagnostic horizons or characteristics of clay enrichment. They comprise a large
number of soil series, 3,534 and 1,410, respectively, in the USA. Within the
Spodosols, there is a high number (161) and proportion (22 %) of soil series with
an argillic or kandic-like horizon. Andisols contain 109 soil series with an argillic
horizon, which constitutes 11 % of the total soil series in that order and an area of
about 17,150 km 2 . The remaining soil orders, Inceptisols, Oxisols, and Vertisols,
each contain eight soil series or less and collectively comprise about 4,275 km 2 in the
USA. A total of 44 % of the soil series in the USA contain taxonomically defined soil
horizons or characteristics reflecting clay illuviation. These soils account for an area
of 4.5 million km 2 , which is 56 % of the area of conterminous USA.