Although it may not be flattering to be likened to a pig or a donkey, the fact remains that human beings are physiologically very similar to swine and burros. These animals are mammals with simple stomachs and have the same general size, shape, and placement of organs as do humans. Radiation studies with swine and burros, although slow and expensive, should give information more applicable to humans than the more rapid and inexpensive studies with small laboratory animals. What Else Can Radiation Tell Us? Two characteristics of soils besides fertility are vitally important and difficult to measure. These characteristics are moisture and density. Moisture must be determined frequently for efficient irrigation. Density controls the pore space available for water and oxygen; the possible damage to the soil from tillage and harvesting machines is revealed by before-and-after tests of density. Both soil moisture and density were formerly determined by laboratory methods, which had two drawbacks: the methods were laborious, and they tested soil in an unnatural state. Today a sort of double-barreled radiation method can be used to measure these two soil characteristics. Neutrons are readily scattered by water but not by soil; gamma rays are absorbed by both soil and water. In practice the experimenter drills two holes in the soil a few feet apart. Into one he puts a gamma-ray source; into the other, a radiation detector. The reading on his detector dial tells him the amount of gamma rays absorbed by both soil and water. Replacing the gamma-ray source with a neutron source, he obtains a reading on absorption by water only. The difference between the two readings is ascribed to the density, or degree of compaction, of that soil in its native state. Conclusion Radioactive tracers and radiation sources have become indispensable to all phases of agricultural research. They have helped answer questions that seemed unanswerable. But there will always be more questions to put to Nature. The physicists-philosophers of 1890 were confident that they had obtained all significant knowledge of the physical universe. Discoveries of the next twenty years revealed the immaturity of that conviction. The modern poet Archibald MacLeish has dramatized the meagerness of knowledge:[3] I will tell you all we have learned ... the lights in the sky are stars We think they do not see we think also The trees do not know nor the leaves of the grasses hear us.... Perhaps the most characteristic realization of the scientist today is that the universe is too complex to be fully described, that concepts must change repeatedly to absorb new findings, and that the recurring miracle of life is more majestic than any formula, any computer, or any rocket that man’s brain can devise. Suggested References