236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



Thus, we must find ways to increase the world's productivity not 

 only because we have found ways to increase the world's health and 

 because our world population is increasing at a rapid rate, but also 

 because so many factors are working against us. 



Radioisotopes are helping to provide some of the answers. They 

 have, for example, become an extremely useful tool in studying the 

 efficient use of fertilizers. Since food productivity is dependent to 

 a large extent on soil fertility, the replenishment of depleted and 

 overworked soils with fertilizers is a major problem. 



One of the most important group of fertilizers, the phosphate fer- 

 tilizers, can be readily studied with radioactive phosphorus (pi. 4, 

 fig. 2). Here, as in so many other tracer studies, the radioisotope 

 technique is used primarily because it provides the means for follow- 

 ing a specific batch of atoms. The radioactive phosphorus is incor- 

 porated in the fertilizer which is added to the soil being studied. 

 Later, radioactivity analyses of the plant show what parts of the 

 plant have taken up the radioactive atoms and hence the fertilizer. 

 Chemical analyses of the plant indicate the total amount of phosphorus 

 coming from the fertilizer plus that coming from the ordinary phos- 

 phorus previously present in the soil. 



From such studies investigators can determine not only how much 

 phosphorus is taken up by a plant and where it came from but also 

 the efficiency of the fertilizer, the best type of fertilizer to use, and 

 the most desirable place to put the fertilizer with reference to the 

 location of the plant. The U. S. Department of Agriculture, working 

 with various State agricultural experiment stations, has conducted 

 an extensive program of such tests during the past 5 years. Last 

 year the program included 94 field experiments in 26 States, Hawaii, 

 and Puerto Rico on 18 different crops including alfalfa, cotton, corn, 

 rice, peanuts, sugarcane, peaches, pineapples, and cantaloupes. 



The most fundamental of all tracer experiments, however, is the 

 use of radioactive carbon and other isotopes in man's effort to learn 

 the secret of photosynthesis. Chemical studies have shown that 

 plants combine water and carbon dioxide in the presence of sunlight 

 to form sugars and starches, but the details of how the synthesis takes 

 place are still unknown. By tagging with radioactive carbon 14 

 the carbon dioxide fed to plants and studying intermediate products 

 formed during this complicated synthesis, investigators are beginning 

 to achieve a more detailed understanding of the photosynthetic 

 process. 



Radioisotopes have also been used to supply new knowledge on 

 reactions between various soil elements, on insecticides and weed 

 killers, and on various types of blight and other plant diseases. Sim- 



