286 BULLETIN OF THE TORREY BOTANICAL CLUB fVou. 91 The principal radionuclides in the earth’s igneous crust, and therefore in most soil types, are K-40, Rb-87, Ra-226, Th-232, U-238, and the daughter products of the last three (Alexanderet al. 1960). The C-14 content of soils is, of course, quite variable but amounts to approximately 1.0 x 10°” euries of C-14 per gram of carbon. The theoretically possible concentrations of most of these radionuclides in igneous rock or in soil derived from igneous rock are shown in Table 1. Estimates for other kinds of materials would vary in several respects. Potassium, for example, is less abundant in sandstone than in granite. Its abundance in shale and limestone is about the same as in granite. Rubidium, on the other hand,is generally absent from limestone, but its abundance in sandstone is lower and, in shale, about the same as in granite. The natural radionuclides in sea water are derived from the atmosphere and from the land. Using the abundance data given by Lowder and Solon (1956), the approximate activities of certain radionuclides in sea water have been caleulated and are tabulated in Table 2. Beeause of differences in their geochemistry, radionuclides which are relatively abundant in soil may be relatively rare in sea water. In soil, for example, K-40 is relatively more abundant than Ra-226, but in sea water this relationship is reversed. According to Love (1951) the average concentration of radium in fresh water is about 10x10¢/liter. This would be equivalent to 123 x 10% e/acre-foot which is about twice the concentration in sea water. The average ratio of H*/H? in rain water has been estimated as 1.0 x 108 and, in surface sea water, about 0.2 x 1078 (Lowder and Solon 1956). Since the specific activity of H® is given as 9.6 10° ¢/g (Kinsman 1957), this would amount to approximately 6.0 10° curies of tritium per acre-foot of rain water and 1.2 x 10° curies of tritium per acre-foot of sea water. 3. Internal emitters. The radionuclidesin air, soil, and water are readily incorporated in marine andterrestrial food-chains. Carbon-14 and H-3 are especially ubiquitous. The concentration of naturally produced C-14 in organism is about 1.0x 10e/gram of earbon while the concentration of H-3 is about 1 to 5 x 10°* ¢/gram of water. According to one estimate (Fry and Kuroda 1959) the above ground parts of green crop plants on an acre of cultivated soil may contain about 110 x 10° euries of natural radionuclides (Table 3). Considering only the radionuclides derived from the soil (Table 1), the plant/soil ratio on a unit area basis is approximately 23 x 10. In other words, a single crop of plants could remove about 0.23% of the natural radioactivity in soil.* Animals may acquire negligible amounts of radionuclides from the atmosphere. They regularly assimilate various fractions of the radionuclides * This estimate does not include the C-14 derived from air or the H-3 derived from soil water.