USE OF EADIOACTIVE SUBSTANCES AS -FERTILIZERS. 7 



Many experiments on the influence of radioactive matter on plant 

 growth have also been made by Stoklasa. 1 In one set of experiments 

 there was observed the effect of adding varying amounts of uranium 

 in the form of uranium nitrate to a given quantity of soil. Using 

 plants of clover (MeUlotus aTbus) a maximum increase in growth of 

 24 per cent was obtained when 1 part of uranium was used to 1,310,000 

 parts of soil. But the presence of lead in the form of lead nitrate 

 was found to be even more stimulating in its action since a corre- 

 sponding increase in growth was obtained with a concentration only 

 one-eighth as great as the quantit}^ of uranium which gave best results. 

 Lead, however, is a rayless element and the effects observed with it 

 must have therefore been due to its chemical properties. As a soluble 

 salt of uranium had to be used to give the effects observed, it is rea- 

 sonable to conclude that these effects are likewise due, in a large 

 measure at least, to the chemical properties of the uranium rather 

 than to its radioactive properties. Further evidence of the truth of 

 this statement will be given later. 



In other experiments Stoklasa 2 made a study of the change in 

 rate of nitrogen fixation brought about by bacteria {azotohacter 

 chroococcum) when cultures of these bacteria were placed in an 

 atmosphere containing radium emanation. In carrying out the 

 experiments 2 liters of air having an activity of 150 Mache units 3 

 were passed daily into the vessel containing the cultures and there 

 resulted from this treatment a marked increase in the amount of 

 nitrogen fixed by the bacteria. It was further observed that the time 

 of germination of seeds was shortened and an increase in the de- 

 velopment of plants resulted when watered with water having an 

 activity of from 30 to 2,000 Mache units. 



Using a concentration of emanation about 30 times as great as 

 that given by Stoklasa, Fabre 4 likewise observed favorable results 

 in the germination and growth of seedlings. Many experiments on 

 the influence of radioactive matter on plants have also been made 

 by other investigators, but unlike the results just cited the effects 

 reported in the majority of cases were deleterious rather than 

 beneficial. 



As radium emanation is an inert gas, the results obtained with its 

 use can not be due to its chemical properties, as in the case of 

 uranium, but must be attributed to its property of being radioactive. 

 It is thus necessary to conclude that radioactive material does have 

 an effect on plant growth, and that when a certain concentration, 



1 Compt. rend., 155, 1096 (1912) ; 156, 153 (1913) ; 157, 879, 1082 (1913). 



2 Loc. cit. 



3 The unit now generally used for expressing a quantity of radium emanation is 

 called the curie, or the microcurie, and is the amount of emanation in equilibrium with 

 1 gram, or 1 microgram', of radium. One microcurie per liter equals a concentration of 

 about 2,700 Mache units. 



♦Compt. rend. soc. biol., 70, 187 (1911). 



