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III. RELATIVE AVAILABILITY OF FALLOUT CONSTITUENTS Single crops of plants may absorb about two per cent of the total radioactivity in a soil contaminated by a nuclear explosion, but usually they absorb less than 0. 1 per cent (85, 120). Strontium-89 and strontium-90 are the major nuclides absorbed (52) and may account for as much as 70 per cent of the absorbed activity from one- year-old, mixed-fission products (99). It is generally accepted that about one per cent of the applied strontium and less than 0. 1 per cent of the other elements are taken up by single crops of plants (47, 78, 94, 96, 104, 105). Higher amounts of strontium uptake, 4 to 8 per cent, have been observed in pot experiments (86, 105). The uptake of an element depends on its concentration in the external medium (17, 30, 67, 76). The ratio of plant-tissue concentration to the external-medium concentration, called a concentration factor, is used to indicate the relative uptake of the different elements. Results from solution culture studies have been based on the fresh tissue weight; oven-dry weight has been used for soil culture studies. There is relative agreement in the order of concentration factors for different isotopes in pot experiments using the Neubauer technique or other techniques and in field experiments (95). Some reported concentration factors for fallout constituents in soil culture are 0. 05 for the alkaline earth group, 0. 009 for the rare earths, 0. 05 for total beta activity in barley, and 0. 02 for total beta activity in beans (117, 120). Using soluble forms of isotopes in nutrient solutions (94), concentration factors from 0. 05 to 1. 0 have been found for strontium, cesium, iodine, and barium. The range was 0. 0001 to 0. 001 for ruthenium, yttrium, and cerium.
