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? obtained in pot tests with oats (oan sativa) 
a’ maximum increase in growth for the grain and straw of 58 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? 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.”* 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 ee 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 Sco 
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 
1Compt. rend., 156, 153 (1913). 
2Bul. Coll. Agr., Tokyo Imperial Univ., 5, 173 (1902) : 6, 144, 161 (1904). 
$Tbid, 6, 165. 
