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most of the Sr-90 deposited by tropospheric and stratospheric fallout tends
to remain in the upper 2 inches of undisturbed forest soils (Alexander
et al. 1960).
2. Plants. Most of the fission product in the upper few inches of fallout
contaminated soil is probably not available to deep-rooted plants, but the
preliminary results of experiments now in progress (Romney et al. 1962)
seem to indicate that such materials may be readily available to plants such
as wheat which have ‘‘feeder’’ roots near the surface. This possibility is
especially interesting because manydesert plants and a variety of cultivated
plants have root systems of this sort.
The availability of radionuclides in fallout contaminated soils for uptake
by the root systems of plants is dependent upon an impressive array of
biological and environmental factors (Hope 1959, Hansenet al. 1959, Menzel
1959, 1960, Nishita et al. 1960). Aside from the physical and chemical properties of the fallout and its distribution in and on the soil, most of these
factors are directly or indirectly related to the composition of the soil solution (Hansen et al. 1959), and/or to various aspects of plant metabolism
(Hope 1959). While it is not within the scope of this paper to discuss all of
these factors, it is well worth noting that the best criterion of availability is
actual uptake by plants.
Experimental studies have shown that the amountof radioactive material
absorbed bythe root systems of crop plants may varyconsiderablyin relation to: the kinds and concentrations of radionuclides in the soil, the physical
and chemical properties of different soil types, and the age and species of
the plants considered (Nishita et al. 1960). After a series of studies concerning the uptake of fission products by various crop plants from a variety
of soil types contaminated by fallout materials, Nishita et al. (1960) reported that, “The total uptake of fission products expressed as a percent
of total residual activity in the soil at harvest time ranged from 0 to 0.06%.”’
When solutions of radionuclides are added to soil, a larger percentage
of the total activity may be removed by plants. Menzel (1959) has reported
that from 0.5% to 5% of the Sr-90 added to soil as a solution maybe removed by a single crop of plants. Comparable figures for Ba-140 are 0.025%
to 0.25% ; but generally less than 0.1% of the Cs-137, Ru-106, and Ce-144
addedto soil as a solution can be removed by a single erop of plants. Subsequent crops remove smaller fractions of the residual activity.
In spite of variations related to soil type, species, and stage of development, the relative order in which major fission products are absorbed by
root systems from fallout contaminated soils is consistently as follows: Sr-89
Sr-90 >> I-131 > Ba-140 > Cs-137 > Ru-106 > Ce-144 (Nishita et al.
1960). The respective half-lives of these fission products are: 53 days, 28
years, 8.04 days, 12.8 days, 33 years, 1 year, and 282 days.