Oxic Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 207

19.2 Properties of Oxic Horizons
For the 64 soil descriptions in the SSURGO database for the USA, the depth to the
oxic horizon averages 39 cm, and the thickness of the oxic horizon averages 98 cm.
For the six soil series shown in Table 19.1 , the oxic horizon thickness averaged
greater than 90 cm; the mean clay content was 62 %, and the average bulk density
was 1.3 g/cm 3 . The SOC concentrations averaged 0.53 %, the pH 4.98, the CEC7
6.3 cmol(+)/kg soil, the Al saturation 59 %, the base saturation 25 %, and the
CDB-extractable Fe and Al 12.2 % and 0.9 %, respectively. These properties
illustrate the highly weathered nature of the oxic horizon. An Oxisol from Brazil
is shown in Fig. 19.1 . The oxic horizon is bounded by the shovel (Fig. 19.1 ).
19.3 Classification of Soils with Oxic Horizons
In Soil Taxonomy , there are 64 soil series with an oxic horizon; they occur in three
orders (Andisols, Inceptisols, and Oxisols), eight suborders, 18 great groups, and
34 subgroups. Eighty-eight percent of the soil series with an oxic horizon have fine
and very fine particle-size classes; 88 % have an isohyperthermic soil-temperature
class; and ten soil mineral classes are represented, with the kaolinitic (34 %) and
parasesquic (14 %) classes being dominant.
19.4 Distribution of Soils with Oxic Horizons in the USA
Although Oxisols account for 23 % of the soil cover worldwide, they account for
only 0.02 % of the land area (1,666 km 2 ) in the USA, primarily in HI, PR, and
Micronesia (Fig. 19.2 ).
19.5 Factors Influencing the Development of Oxic Horizons
All but seven of the soil series with an oxic horizon are well drained. The parent
materials are varied but include residuum from basalt (48 %), alluvium (19 %),
and residuum from limestone (11 %). The mean annual precipitation and
temperature average 2,200 mm and 24 C, respectively. Slopes generally range
between 5 % and 40 %. The vegetation is tropical forest and savanna. According
to the literature, most Oxisols receive between 1,000 and 3,000 mm/yr precipi-
tation (Table 19.1 ). The oxic horizon is best expressedinwell-drainedslope