12 BULLETIN 149, U. S. DEPARTMENT OF AGRICULTURE. 



With a concentration of 1 part of lead, as lead nitrate, in 965,000 

 parts of soil Stoklasa 1 obtained in pot tests with oats (Avena sativa) 

 a maximum increase in growth for the grain and straw of 53 per 

 cent over that which took place in the control pot; but on increasing 

 the concentration of the lead only 2.5 times its toxic action became 

 apparent, and a decrease in growth resulted. Similar results were 

 also obtained, as already pointed out, in pot tests with clover using 

 uranium nitrate. With this compound the maximum stimulation 

 was obtained with a concentration of 1 part of uranium in 1,310,000 

 parts of soil, but as the concentration of the uranium was increased 

 its! toxic action became manifest, and the crop yield gradually 

 decreased. 



A corresponding series of experiments was also made by Loew and 

 his coworkers 2 using salts of both uranium and thorium. From the 

 results obtained it was concluded that " uranium and thorium com- 

 pounds differ widely in their effects on plants, uranium salts being 

 highly poisonous, thorium salts not." 3 It is known that thorium and 

 uranium both give off the same rays and of approximately the same 

 intensity. It would be expected, therefore, if the effects which these 

 elements produce on plants are due to their radioactivity, that the 

 effects would be approximately the same for each element. Since this 

 is not the case, and since the results obtained with uranium corre- 

 spond with those which follow the use of the so-called catalytic ferti- 

 lizers, it is necessary to conclude that the action of uranium on plants 

 is due to its chemical properties rather than to its property of being 

 radioactive. 



The material (B) of which an analysis is given in the table above 

 contains 1 per cent of uranium oxide. An application of this ma- 

 terial of about 175 pounds per acre would thus give to the first six 

 inches of the soil a concentration of uranium equal to that which 

 Stoklasa found, in the form of the nitrate, gave greatest stimulation 

 to clover plants. An effect would, therefore, be expected to follow 

 the addition to the soil of finely ground uranium ores, but whether 

 the result will be beneficial or otherwise will depend on the amount 

 applied and the kind of crops grown. 



In the various experiments which have been described on the use 

 of radioactive manure no account has apparently been taken of the 

 chemical action of the uranium present, and the conflicting results 

 obtained with radioactive material from different sources are no 

 doubt to be explained by the fact that the radioactivity of the 

 material was alone considered without regard to the presence or 



1 Compt. rend., 156, 153 (1913). 



2 Bui. Coll. Agr., Tokyo Imperial Univ., 5, 173 (1902) ; fi. 144, 161 (1904). 



3 Ibid, 6, 165. 



