Ce es ee ee ee 
USE OF RADIOACTIVE SUBSTANCES AS FERTILIZERS. 7 
Many experiments on the influence of radioactive matter on plant 
growth have also been made by Stoklasa.t_ 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 (d/elilotus albus) 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 cbtained with a concentration only 
one-eighth as great as the quantity 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? made a study of the change in 
rate of nitrogen fixation brought about by bacteria (azotobacter 
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 
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 concentraticn of emanation about 30 times as great as 
that given by Stoklasa, Fabre* 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). 
Aba. Cit. 
*The unit now generally used for expressing a quantity of radium emanation is 
called the cwrie, 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). 
