Duripan Horizon and Durinodes - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 136

derived from old granitic alluvium, such as the Redding soil series, may have acidic
pH values as low as 4.3. The silica in duric soils is often accompanied by abundant
CaCO 3 which is involved in the cementation. The mean CaCO 3 concentrations of
the duripan horizon and horizon immediately above the duripan are 19 % and 14 %,
respectively. Because of CaCO 3 enrichment, Soil Taxonomy requires that less than
50 % of air-dried duripan fragments slake in 1 M HCl but that more than 50 % of
the fragments slake in KOH or NaOH (Soil Survey Staff 2010 ).
The clay content of duripans is substantially less (mean

7.5 %) than that of the
overlying B horizon (mean ¼ 25 %; Table 14.2 ). Duripans also have a wider water
retention ( 1,500 kPa water)/clay ratio than the overlying B horizon. From the
13 pedons included in this study, the mean water retention/clay ratios for duripans
and the overlying B horizon are 2.4 and 0.7, respectively (Table 14.2 ).

¼

14.3 Classification of Soils with Duripans and Durinodes

Duric soils occur in five orders, 14 suborders, 19 great groups, and 75 subgroups
(Table 14.3 ). There are 787 soil series with durinodes or duripans in the NRCS
dataset, which comprise about 4 % of the total soil series identified in the country.
The greatest number of soil series is in the Aridisols (523), which constitute 81 % of
the total identified in the USA, followed by the Mollisols (12 %) and Alfisols (7 %).
Vertisols and Inceptisols contain 0.7 % and 0.5 % of the total duric soil series,
respectively.

Table 14.3 List and areas of duric soils by taxa

No.
series

%of
total

Area
(km 2 )

%of
total Order Suborder Great group Subgroup

Alfisols Xeralfs Durixeralfs Abruptic 19

Abruptic Haplic 3

Aquic 1

Haplic 7

Natric 7

Typic 19

56 6.8 6,987 6.6

Aridisols Argids Calciargids Durinodic 3 232

Durinodic Xeric 8 927

Haplargids Durinodic 11 2,724

Durinodic Xeric 31 4,968

Natrargids Durinodic 13 3,826

Durinodic Xeric 9 970

Petroargids Duric 6 227

Duric Xeric 2 66

Calcids Haplocalcids Duric 2 126

Durinodic 20 3,746

Durinodic Xeric 24 5,115

(continued)