8153

• UCREFRP Catalog Number: 8153
• Author: Jacobs, e. L. W.
• Title: Editor
• USFW Year: Series
• USFW - Doc Type: 1989
• Copyright Material: YES

20 MAYLAND ET AL.
and 38 mg Se/kg as water-soluble selenate have been reported in selenifer-
ous areas of the world (Lakin, 1948; Swaine, 1955). Soils developed from
the Cretaceous shales of South Dakota, Montana, Wyoming, Nebraska, Kan-
sas, Utah, Colorado, and New Mexico tend to have high Se values ranging
from 1 to > 10 mg Se/kg. It is in these areas where natural Se toxicosis has
occurred historically. The Se content of some Hawaiian surface soils varies
from 1 to 20 mg/kg, but this Se is unavailable for vegetation because of it
complexation with the Fe and Al minerals (Anderson et al., 1961; Rosenfeld
& Beath, 1964).
Low-Se soils in the USA include those derived from sedimentary rocks
that predate the major Cretaceous period (northeastern USA). Other low-Se
soils include those that are formed from recent volcanic ash deposits or loes-
sal material derived from the ash (Pacific Northwest). Soils in the very low-
Se region of the eastern USA are formed from coastal deposits consisting
of weathered, recycled sediments (South Atlantic seaboard).
The soil parent materials of the low-Se areas in Montana and parts of
Idaho are mostly derived from granites and old metamorphic rocks. Low
total-Se concentrations occur in the tertiary volcanic rocks of Arizona and
New Mexico. Most of the soils from low-Se areas of the USA contain <0.5
mg Se/kg. Many of these low-Se areas correspond to areas where Se-
deficiency disorders such as white muscle disease occur.
As with other elements, the total concentration of Se in soils shows little
relationship to the concentration of Se in plants grown on those soils (Lakin,
1972). This is because Se in soil exists in several chemical forms that differ
widely in their solubility and availability to plants (Anderson & Scarf, 1983).
These chemical forms are:
Selenide, SeZ-, HSe-, H2Se (aq)
Elemental selenium, Se°
Selenite, SeO3-, HSeO3 , H2SeO3 (aq)
selenate, SeO4-, HSeO4 , HZSeO4 (aq)
Organic Se
The chemical forms of Se present in soils and sediments are closely related
to the oxidation-reduction potential and pH of the soil (van Dorst & Peter-
son, 1984, Fig. 2-3). Inorganic Se exists predominately as the oxyanions,
Selenite and selenate, in the aqueous solutions of well-aerated alkaline soils.
In poorly aerated acid soils, inorganic Se predominates as the relatively in-
soluble Selenide and elemental forms. Howard (1977) summarized the Se
geochemistry, as affected by Fe, and concluded that the geochemistry of Se
is largely controlled by that of Fe, with which Se is closely affiliated in both
oxidizing and reducing environments.
Soils will be high in biologically available Se if formed from parent
materials that were high in Se and maintained under alkaline conditions.
Plants growing on these soils will have sufficient Se (0.1 mg Se/kg forage)
for animal requirements. However, if the bioavailability of the Se is very
high, it may produce Se toxicosis in plants or the animals that ingest them
(Smith & Watkinson, 1984). Soltanpour and Workman (1980) indirectly ex-