16 



magnesium brought about only slight changes in the strontium content of four pasture 

 species. However, potassium and sodium reduced the strontium content of blue- 

 grass as much as 34 per cent and that of redtop 51 per cent, whereas sodium addi- 

 tions increased strontium in Korean lespedeza. 



Similar observations have been made on the uptake of cesium- 137. When soil 

 potassium is low, additions of potassium reduce the cesium uptake, but additions of 

 stable cesium often increase cesium- 137 uptake, presumably by displacement of ex- 

 changeable cesium- 137 into solution (84). Rubidium, ammonium, and calcium in- 

 creased cesium uptake 8, 3, and 1-1/2 times, respectively, but when carrier 

 cesium- 137 was used, practically no effects of these ions were observed (128). 



C. Distribution Factors 



Because of the similarity in chemical behavior between certain fission products 

 and certain essential elements, fission-product uptake is often reported relative to 

 the uptake of the chemically similar essential element. The "Observed Ratio" 

 (OR) (18), or the "Distribution Factor" (DF) (66), for strontium is the ratio of stron- 

 tium to calcium in plant or plant part divided by the ratio of strontium to calcium in 

 the nutrient medium. The term "discrimination factor" is expressed in the same 

 manner but usually applies to a single step in the various successive processes that 

 determine the over-all relative distribution of the two elements between substrate 

 and tissue. 



In nutrient solution experiments, when only the plant discrimination processes 

 are measured, the strontium/calcium DF is close to 1. (67, 105). This indicates 

 little discrimination between strontium and calcium and is true for most of the plant 

 parts except the roots, where DF values as high as 6. were observed for low- 

 solution concentrations of strontium. The average DF values of rubidium/potassium 

 and cesium /potassium for millet, oat, buckwheat, sweetclover, and sunflower plants 

 were 0.85 and 0.20, respectively (67). This indicates some discrimination by the 

 plant against rubidium and more against cesium. 



Alfalfa and wheat grown on eight soils (98), wild plants and corn grown on soil 

 in a radioactive waste disposal area (6), and beans grown on a Sassafras sandy loam 

 with added calcium (105) had strontium /calcium DF values close to 1. 0, indicating 

 little discrimination in soil reactions. However, the DF values can vary within a 

 given plant, ranging from 2. 6 for corn flowers to 0. 5 for corn grain (6). 



The calculated DF will vary to some extent, depending upon the method of 

 extracting the cations from the soil. Based on the amounts of strontium-89 and cal- 

 cium-45 added to Cinebar soil, the DF for beans ranged from 0. 64 to 1.2 (43). 

 Another experiment with strontium-89 and calcium-45, using a dilute calcium chlo- 

 ride extract of the soil, gave an average DF of about 0. 7 (111). Discrimination 

 factors from 0. 8 to 1.6 were found for strontium /calcium in eight grasses and eight 

 legumes grown in three soils, using ammonium acetate for extraction (138). In a 

 study of soils and vegetation in a disposal area (33), the best soil index of strontium -90 

 uptcike by plants appeared to be concentrations of strontium-90 in the saturation ex- 

 tract. Others (112) have also suggested that a water extract may provide a better 

 measure of the availability of strontium and calcium in the soil than the exchangeable 

 fraction. 



