Duripan Horizon and Durinodes - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 131

Fig. 14.1 ( a ) Argidurid in the Mojave Desert, California. The calcareous duripan begins at 30 cm
with several overlapping discontinuous layers and krotovina within the duripan. ( b ) Durixeralf in
the Sacramento Valley, CA. Duripan at 50 cm has clay films and MnO 2 coatings. ( c ) Thin section
of a duripan from a Durixeralf in CA. Crossed polarizers. Durinode, with incorporated sand grains,
occupies most of the right half of the image. Stringers of opal, probably mixed with clay coat the
durinode. Frame width represents about 1.7 mm (Images provided by Dr. Randy Southard)

Table 14.1 Silica stage in relation to taxonomic level for duric soils
Taxonomic level Si stage mean a No. of pedons Standard error

Durinodic (subgroups) 45 2.0 0.11

Durixeralfs, Durixerolls (great groups) 18 3.6 0.14

Durids (suborder) 74 3.8 0.036

a Harden et al. ( 1991 )

is a thin section under crossed polarizers of a duripan from the Durixeralf in
Fig. 14.1b . Most of the right side of the image shows a durinode with incorporated
sand grains. Stringers of opal, probably mixed with clay, coat the durinode.
The frame width represents about 1.7 mm.
From the NRCS dataset, the depth to the surface and base of the Bq or Bqm horizon
averages 42 and 108 cm, respectively; the average thickness of the duric materials is
65 cm (data not shown). However, duripans are commonly “several feet” or “several
meters” in depth; and standard soil descriptions rarely go beyond 150 cm (60 in.).
Fifty-five percent of the most common duric soils contain an argillic horizon
above the duripan; 28 % contain a calcic horizon either above or concurrent with
the duripan horizon; and 18 % have a cambic horizon above the duripan (data not
shown). Several soils, including the Arbidge, Handpah, Roseworth, and Sluka
series, have multiple duripans. The mean morphogenetic silica stage was 2.0 for
common soil series in durinodic subgroups, 3.6 for those in Dur- great groups,
and 3.8 for soil series in the Durid suborder (Table 14.1 ).
The silica-enriched horizon of 13 pedons representing the spectrum of duric soils
has an average pH of 7.7, which is a tenfold less concentration of protons than the
overlying Bw or Bt horizon (Table 14.2 ). However, duripans in some of the soils